EESA06 – Textbook Notes
Chapter 1 – Introduction to Physical Geology and the Environment
What is Geology?
­ Geology derived from the Greek word geo and logos and means “the study of the Earth”
­ Geology came into being in the late eighteenth century
o Industrial Revolution in northern Europe created a growing demand for energy
and minerals (coal, limestone, iron, water)
 Construction of mines, railways, tunnels, canals, brought to light details
of how rocks are arranged below the ground, distance that they varied,
and how they can be matched from one place to another (fossils)
­ Earliest Detailed geologic maps – England, 1815 – William Smith
o William Smith – Referred to as “Father of English Geology”
­ In North America, geological mapping began in mid­nineteenth century
o Sir William Logan (1842) – Founding director of Geological Survey of Canada;
first to describe the geology of Canada
­ Age of the Earth has been determined to be at least 4 500 billion years
Moving Continents
­ Alfred Wegner – Suggested the movement of continents on the Earth’s surface (early
twentieth century) – Wrote in the 1912 Continental Drift
o Recognized today’s continents had previously been clustered together in a large
land mass, but subsequently moved apart (Pangea)
o Development of Plate Tectonics Theory
 Canadian Geophysicist; J. Tuzo Wilson – 1970s; responsible for
bringing together several key elements in “pate tectonics theory”
Time and Geology
­ Geology involves vastly greater amounts of time – “deep time”
o Some geological processes occur quickly (Ex. Great landslide, volcano
eruptions); stored energy is suddenly released
­ Earth is estimated to be a least 4.55 billion years old (4 550 000 000)
­ Fossils in rocks indicate complex forms of animal life have existed in abundance on
the Earth for about the past 545 million years
­ Reptiles become abundant about 230 million years ago
­ Dinosaurs evolved from reptiles and became extinct about 65 million years ago
­ Humans have only been here only about the last 3 million years
What Do Geoscientists Do?
­ Traditional geologists – Spent most of their time in the filed looking for tell­tale signs of
minerals (prospecting)
o Now referred to as exploration geologist
o May work for exploration company (gold, silver, other metals, diamonds)
o Getting around in four­wheel­drive trucks, helicopters, being in touch with
financiers, market analysts, other business professionals
­ Geoscientists; Specialize in a number of areas
o Geochemists; Comfortable working in the ordered environment of the
laboratory, using high­technology equipment to analyze the chemistry of
rocks/minerals
Environmental Geology; New Challenges for Geoscientists
­ New challenge – Finding and managing of drinking water (groundwater) and dealing
with wide variety of wastes (radioactive waste to household (municipal) waste);
environmental geoscientists
What is the Scientific Method?
­ Scientific Method – Problem or Question Methodology/Data Collection
Analysis/Interpretation Hypothesis/Hypotheses Testing Theory?
­ Hypothesis – Theoretical explanation
How did the Earth Form?
­ Earth is a solid body, with oceans, an atmosphere and life
­ Universe was formed by the clumping together of gas and debris in the aftermath of the
Big Bang that is thought to have occurred some 15 billion years ago
o Billions of galaxies; The Milky Way contains our own solar system and planet
Earth
­ Solar system consists of the sun and nine planets and space debris, orbiting the sun
o Created from a cloud of gas and dust particles; nebula
­ Acceration – Process of building large bodies of matter through collisions and
gravitational attraction
o Differing densities of the planets of the solar system show planets differ in
composition
o Terrestrial Planets – Plants formed close to the sun, small, dense, and rocky
(Mercury, Venus, Earth, Mars)
o Jovian Planets – Low density planets (Jupiter, Saturn, Uranus, Neptune)
What was the Early Earth Like?
­ After the Earth was formed, it collided with a planetismal; Earth’s moon was created
form the debris that was flung off into space
­ Differentiation – Process of zonation of different materials within a planet
o Heavier materials (Iron, Nickel) settled toward the planet’s center, lighter
materials (silica, oxygen) rose toward the earth’s surface
­ Early heating of planetary interiors generated large quantities of magma, rising to the
surface along fractures produced by meteorite impacts or by tectonic processes
Internal Structure of the Earth
­
­
­
­
­
Earth is differentiated – intense heat and pressure within the body of the planet gives rise
to “onion­like” layers of different chemical composition and physical behaviour
No drill has penetrated the crust, the understanding of the Earth’s interior is based on
indirect evidence
o Layers form an innermost core; composed of iron alloy (iron with nickel and
silicon
o A mantle; Composed of Fe­Mg silicates
(Forms a rock – peridotite)
o Outer crust; composed of lighter rocks
(basalt, granite)
Lithospheric Plates – Mantle convection
breaking the crust and uppermost rigid mantle into
large pieces
Lithospheric plates are pushed around the surface
of the planet over a weak layer called
asthenosphere
Movement of rigid lithospheric plates over the
more mobile asthenosphere is the fundamental
process involved in plate tectonics
Formation of the Early Atmosphere
­ Earth’s early atmosphere derived predominantly
from water and gaseous elements released during
volcanic eruptions in a process called outgassing
­ Most outgassing occurred within the first billion
years of Earth’s history, forming extensive oceans
and lakes, allowing sediments to be eroded and
deposited
­ Oldest known sedimentary rocks known are
around 3.8 billion years old
­ Oxygen rich atmosphere created by evolution of
photosynthetic life forms that could separate
carbon dioxide into carbon and free oxygen
Early Life Forms
­ Earliest life forms preserved in geologic record are microorganisms; Prokaryotes
­ Prokaryotes were able to trap sediment to grow organic structures; Stromatolites
­ Oldest known fossil on Earth are 2.5 billion year old stromatolites found in the rocks of
Australia’s Pilbara shield
­ Cambrian explosion Life forms about 600 million years ago with the appearance of
more complex organisms with backbones and hard shells
o Rich repositories of Cambrian­age fossils Burgess Shale
What is the “Earth System”?
­ Earth System is a small part of the larger solar system but has its own component
parts/subsystems
o Subsystems/Spheres ­­ Includes atmosphere (gases that envelop Earth),
Hydrospehre (Water on or near the Earth’s surface), biosphere (living or once
living materials), geosphere (rock or other inorganic Earth materials)
­ The entire Earth system is fuelled by two major sources of energy
o Earth’s External Energy Source – Sun, drives atmospheric and hydrologic
­
­
­
processes and controls weather, climate, weathering, ocean circulation, erosion,
deposition
o Earth’s Internal Energy Source – Geothermal heat, remaining from our planet’s
formation and generated by radioactive decay of minerals within the Earth
 Drives plate movements, volcanic eruptions, earthquakes
Internal and external forces continue to interact, forcing substances out of equilibrium
o The Earth has a highly varied and ever­changing surface
Rock Cycle – Conceptual model that links rock­forming process that operate in the
Earth’s crust
o Three major rock types – Igneous, metamorphic, sedimentary
o Magma – Molten rock
o Igneous Rocks – From when magma solidifies
 Extrusive igneous rock – Magma solidifying when brought to the surface
by a volcanic eruption
 Intrusive igneous rock – Magma solidifying very slowly beneath the
surface, may be exposed later after uplift and erosion remove the
overlying rock
o Igneous rock being out of equilibrium, may undergo weathering and erosion and
debris produced is transported and deposited as sediment
o Unconsolidated sediment becomes lithified (cemented or consolidated into a
rock), becomes sedimentary rock
o As the rock is buried by additional layers of sediment and sedimentary rock, heat
and pressure increase, if the increase becomes high enough, the original
sedimentary rock is no longer in equilibrium and recrystallizes into
metamorphic rock
o If temperature gets very high, the rock
partially melts producing magma,
completing the cycle
Chapter 2 – Plate Tectonics
Plate tectonic theory suggest that the surface of
the Earth is divided into several large plates that
change in position and size
o Intense geologic activity (volcanic
eruptions, earthquakes) occur at plate
boundaries
What is Plate Tectonics?
­ Tectonics is the study of the origin/arrangement of the broad structural features of the
Earth’s surface (including; mountain belts, continents, earthquake belts)
­ Plate Tectonics – Earth’s surface is divided into a few large, thick plates that move
slowly and change in size
o Intense geologic activities occur at plate boundaries where plates move away
from one another, past one another or toward one another
o These plates make up the outer shell of the Earth (the crust and upper part of the
mantle)
­ Plate tectonics explains a lot; Earthquake distribution, origin of mountain belts, origin of
sea­floor topography, distribution and composition of volcanoes, and etc.
­
­
­
Concept of plate tectonics was developed by combining two pre­existing ideas –
Continental drift and sea­floor spreading
Continental Drift – Idea that continents move freely over the Earth’s surface, changing
their positions relative to one another
Sea­floor Spreading – Hypothesis that the sea floor forms at the crest of mid­oceanic
ridges, then moves horizontally away from the ridge crest toward an oceanic trench
o Two sides of the ridge are moving in opposite directions like slow conveyor belts
How did the Plate Tectonics Theory Evolve?
­ 1900s Alfred Wegener (German meteorologist) made a strong case for continental drift
o Noted South America, Africa, India, Antarctica, Australia had almost identical
late Paleozoic rocks and fossils
Plant Glossopteris is found in Pennsylvanian and Permian­age rock on
fall five continents, fossil remains of Mesosaurus (freshwater reptile) is
found in Permian­age rocks only in Brazil and South Africa, fossil
remains of land­dwelling reptiles (Lystrosaurus, Cynognathus) are found
in Triassic­age rocks on all five continents
o Wegener reassembled the continents to form a giant supercontinent; Pangea
o Pangea initially separated into two parts
 Laurasia was the northern supercontinent (North America, Eurasian
(excluding India)
 Gondwanaland was the southern supercontinent (All present­day
southern­hemisphere continents and India)
o Wegener also reconstructed old climate zones
 Glacial features indicate a cold climate near the North or South
 Coral reefs indicate warm water near the equator
 Crossbedded sandstones indicate ancient deserts near 30 degrees North
and 30 degrees South latitude

Skepticism about Continental Drift
­ Much of Wegener’s evidence was not clear­cut and had no good mechanism to account
for continental movement
­ Wegner’s ideas received little support in North America or much of the northern
hemisphere in the first half of the twentieth century
­ Wegener’s concept of
continental drift implied that
less dense continents drifted
through oceanic crust,
crumpling up mountain ranges
on their leading edges as they
pushed against oceanic crust
Renewed Interest in Continental
Drift
­ 1940s and 1950s – New ideas
about sea­floor spreading, into
the concept of plate tectonics
­ New Investigations
o (1) Study of sea floor
rock
o (2) Geophysical
Research, in relation
to rock magnetism
Study of the Sea Floor
­ Oceans cover more than 70
percent of the Earth’s surface
­ Samples of rock and sediments can be taken from the sea floor in several ways
o Rocks can be broken from the sea floor by a rock dredge (open steel container
dragged over the ocean bottom at the end with a cable)
o Sediments can be sampled with a corer (weighted steel pipe dropped vertically
into the mud and sand of the ocean floor)
o Both rocks and sediments can be sampled by means of sea­floor drilling
­
To indirectly study the sea floor – single­beam echo sounder; measures water depth and
draws profiles of submarine topography (sound signal is sent downward form a ship
bounces off the sea floor, returning back to the ship)
o Depth is determined form the time it takes the sound to make the round trip
­ Sidescan Sonar measures the intensity of sound reflected back to the tow vehicle from the
ocean floor and provides detailed images of the sea floor and information about
sediments and bedforms
­ Seismic reflection profiler works on the same principles of echo sounders but uses a
louder noise at lower frequency
o Records water depth and reveals internal structure of the rocks and sediments of
he sea floor
Geophysical Research
­ Polar wandering came form the study of rock magnetism
­ Wegener’s work dealt with the wandering of Earth’s geographic poles of rotation
­ Magnetic poles are located close to the geographic poles, magnetic poles move form year
to year, but magnetic poles stay close to the geographic poles as they move
­ Many rocks record the strength and direction of the Earth’s magnetic field at the time the
rocks formed
­ Magnetite in a cooling basaltic lava flow acts like a tiny compass needle, preserving a
record of Earth’s magnetic field when the lava cools below the Curie point
­ Rocks (and their paleomagnetic records) have moved as part of migrating tectonic plates
o Apparent polar wandering paths are now used to reconstruct continental
movements over time
Recent Evidence for Continental Drift
­ Most convincing evidence for continental drift came form greatly refined rock matches
between now­separated continents
­ Matches between South America and Africa are particularly striking
o Distinctive rock contacts extend out to sea along the shore of Africa
o Two continents fitted together, identical contacts are found in precisely the right
position on the shore of South America
o Isotopic ages of rocks also match between these continents
History of Continental Positions
­ Rock matches show when continents were together, one the continents split, the new
rocks formed are dissimilar
­ Pangea split up 200 million years ago, the continents were moving much earlier
­ Continents have been in motion for at least the past 2 billion years (some geologists say 4
billion years)
What is Sea­Floor Spreading?
­ Harry Hess (geologist @ Princeton University) proposed that the sea floor might be
moving too
o Contrasted with the ideas of Wegener, who though the ocean floor remained
stationary as the continents ploughed through it
­ Hess’s 1962 proposal was quickly named sea­floor spreading, suggests the sea floor
moves away form the mid­oceanic ridge as a result of mantle convection
­
Initial concept of sea­floor spreading
o Sea floor is moving like a conveyor belt away form the crest of the deep­ocean
basin, finally to disappear by plunging beneath a continent or island arc
­ Spreading Axis – Ridge crust, with sea floor moving away form it on either side
­ Subduction – Sliding of sea floor beneath a continent or island arc
­ Hess’s original hypothesis was that sea­floor spreading is driven by deep mantle
convection
­ Convection is a circulation pattern driven by deep mantle rising of hot material and/or
sinking of cold material
o Convection drives sea­floor spreading, hot mantle rock must be rising under mid­
oceanic ridges
o Mantle rock moves horizontally away form ridge crests on each side of the ridge;
this movement creates tension at the ridge crest, cracking open the oceanic crust
to form rift valleys and associated shallow­focus earthquakes
­ Downward plunge of cold rock accounts for the existence of the oceanic trenches as well
as their low heat flow values
o Large negative gravity anomalies associated with trenches, for the sinking of the
cold rock provides a force that holds trenches out of isostatic equilibrium
­ Sea floor moves downward into the mantle along a Subduction zone, it interacts with the
stationary rock above it
o Interaction between moving­sea floor rock and stationary rock can cause the
Benioff zones of earthquakes associated with tranches
o Also producing andesitic volcanism; forming volcanoes either on the edge of a
continent or along an island arc
How Old is the Sea Floor?
­ Marine geologists determine the age of sea­floor rocks (isotopic dating) and sediments
(by fossils)
­ All rocks and sediments of the sea floor proved to be younger than 200 million years old
o Only true for rocks and sediments form the deep sea floor, not those from
continental margins
­ Earth is estimated to be 4.5 billion years old
o Every continent contains rocks formed during the Paleozoic Era and the
Precambrian
o Some Precambrian rocks on continents are more than 3 billion years old, and a
few are almost 4 billion years old
­ Young age of sea­floor is neatly explained by Hess’s sea­floor spreading
o Young sea floor is continually being formed by basalt eruptions at the ridge crest
o Basalt is then carried sideways by convection and is subducted into the mantle at
an oceanic trench
o Old sea floor is continually being destroyed at trenches, while new sea floor is
being formed at the ridge crest
o Young sea floor at the ridge crest has little sediment because the basalt is newly
formed
­ Sea floor spreading implies that the youngest sea floor should be at the ridge crest, with
the age of the sea floor becoming progressively older toward a trench
o Increase in age away form the ridge crest was not known to exist at the time of
Hess’s proposal but was an important prediction of his hypothesis
What Are Plates And How Do They Move?
­ Plate – Large, mobile slab of rock that is part of Earth’s surface
o Surface of the plate may be made up entirely of sea floor, or it may be made up
of both continental and oceanic rock
­ Plates that are part of a relatively rigid outer shell of the Earth; Lithosphere
o Lithosphere includes rocks of the crust and uppermost mantle
­ Young lithosphere near the ridge my be 10km thick, old lithosphere far form ridge crest
may be 100km thick
­ Continental lithosphere is thicker, varying from 125km to as much as 200km/250km
thick
­ Below the lithosphere is the asthenosphere; a zone of low­seismic­wave velocity that
behaves plastically (increased temperature and pressure)
o Made of upper­mantle rock, low­velocity zone
o May extend from 70 to 200km beneath oceans
­ Plates move away form the mid­
oceanic ridge crest; some plates
move toward oceanic trenches
­ If the plate is made mostly of sea
floor, the plate can be subducted
down into the mantle
o If the leading edge of the
plate is made up of
continental rock, that
plate will not subduct
­ Continental rock, being less dense
than oceanic rock, is too light to
be subducted
­ 3 general types of Plate Boundaries
o Divergent Plate Boundary; Boundary between plates that are moving apart
o Convergent Plate Boundary; Lies between plates that are moving toward each
other
o Transform Plate Boundary; One at which two plates move horizontally past
each other
How Do We Know That Plates Move?
Paleomagnetic Evidence
­ Magnetic Reversals – Changes in the polarity of the magnetic field
­ Normal Polarity – Magnetic lines of force flow form the south pole to the north pole and
our compass needles point to the north
­ Reversed Polarity – Lines of magnetic force run the other way and our compass needles
would point toward the south
­ Paleomagnetism – Study of ancient magnetic fields recorded in rocks
­ Lava flows contain abundant magnetic minerals and can be isotopically dated
­ Stacked continental lava flows have been used to construct a magnetic polarity time
scale that records the pattern of magnetic reversals over time
­ Earth’s magnetic field reverses about every 500 000 years, taking about 10 000 years for
a reversal to develop
­ Deviation of magnetic strength form average readings; anomaly
­
Magnetometer – Instrument that measures the strength of the Earth’s magnetic field; it
may be carried over the land surface or flown over land or sea
Marine Magnetic Anomalies
­ Most magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift
valley of mid­oceanic ridges
­ Alternation positive, negative anomalies form a stripelike pattern parallel to the ridge
crest
The Morley­Vine­Matthews Hypothesis
­ Working independently, Lawrence Morley (Canadian Geophysicist), Fred Vine (British
geologists), Drummond Matthews made several observations about anomalies
o Pattern of magnetic anomalies was symmetrical about the ridge crest (mirror
image of the pattern on the other side)
o Same pattern of magnetic anomalies exists over different parts of the mid­
oceanic ridge
 Pattern of anomalies over the ridge in the northern Atlantic Ocean is the
same as the ridge in the Southern Pacific Ocean
o Magnetic anomalies at sea matches the pattern of magnetic reversals already
known form studies of lava flows on the continents
o There is continual opening of tensional cracks within the rift valley on the mid­
oceanic ridge crest
 Cracks on the ridge crest are filled by basaltic magma from below, cools
to form dikes
 Cooling magma in the dikes records Earth’s magnetism at the time the
magnetic minerals crystallize
How Fast Do Plates Move?
­ Two important points about the Morley­Vine­Matthews Hypothesis of magnetic anomaly
origin
o (1) Allows us to measure the rate of sea­floor motion
 Magnetic reversals have already been dated form lava flows on land,
anomalies caused by these reversals also dated can be used to discover
how fast the sea floor has moved
o (2) Predicts the age of the sea floor
Predicting Sea­Floor Age
­ Distinctive pattern of anomalies through time allows them to be identified by age
­ Sea­floor age is usually measured by fossil dating of sediment in the cores rather than by
isotopic dating of igneous rocks
Another Test: Fracture Zones and Transform Faults
­ Earthquakes do occur along facture zones, but only in those segments between offset
sections of ridge crest
o Motions of the rocks on either side of the fracture zone during an earthquake is
exactly opposite to the motion
­ Portion of a fracture zone between two offset portions of ridge crest; Transform fault
(J.Tuzo Wilson)
What Happens at Plate Boundaries?
Divergent Plate Boundaries (Plates moving away from each other)
­
Divergent plate boundaries can occur in the middle of the ocean or in the middle of a
continent
o Result of divergence at plate boundaries is to create, or open, new ocean basins
­ Divergent boundaries are marked by rifting, basaltic volcanism, and uplift
o During rifting; continental crust is stretched and thinned. Producing shallow­
focus earthquakes on normal faults, and a rift valley forms as a central graben
o Faults act as pathways for basaltic magma, rising up form the mantle to erupt on
the surface as cinder cones and basalt flows
­ Shallow earthquakes and basalt eruptions occur in this rift valley, which has high heat
flow
­ Salt precipitation increases if the continent is in one of the desert belts or if one/both ends
of the new ocean become temporarily blocked
o Not all divergent boundaries contain rock salt
­ Divergent boundary on the sea floor is located on the crest of mid­oceanic ridge
o Slow spreading rate, the crest has a rift valley
o Fast spreading rate, prevents a rift from forming
­ Divergent boundary at sea is marked by the same features as a divergent boundary on
land – tensional cracks, normal faults, shallow earthquakes, high heat flow, basaltic
eruptions
Mid­Oceanic Ridges
­ Mid­oceanic Ridges – Giant undersea mountain ranges that extend around the world like
the seams on a baseball
o Made mostly of basalt, more than 80 000km long, 1 500 – 2 500 km wide; rising
2­3 km above the adjacent ocean floor
­ Rift Valley –Tensional origin commonly runs down the crest of each ridge
o 1 ­2km deep, several kilometres wide – dimensions of the Grand Canyon in
Arizona
o Present in the Atlantic and Indian oceans, being absent in the Pacific Ocean
Geologic Activity on Ridges
­ Shallow­focus earthquakes – 0­20 km below the sea floor
­ Measurements of heat loss from the Earth’s interior through the crust have shown a very
high heat flow on the crest of mid­oceanic ridges
­ Basalt eruptions occur in and near the rift valley on ridge crests
o These eruptions may build up volcanoes that protrude above sea level as oceanic
islands
Biologic Activity on Ridges
­ Exotic organisms (mussels, crabs, starfish, giant white clams, giant tube worms) are able
to survive toxic chemicals, high temperatures, high pressures, and total darkness at depths
of more than 2km
o These organisms live on bacteria that thrive by oxidizing hydrogen sulphide from
the hot springs
Ridges and Ore Deposits
­ Metals released by rift­valley hot springs are predominately iron, copper and zinc
Transform Boundaries
­ Transform Boundaries; Where one plate slides horizontally past another place, the plate
motion can occur on a single fault or on a group of parallel faults
o Marked by shallow­focus earthquakes in a narrow zone for a single fault, or in a
broad zone for a group of parallel faults
­
Transform Fault; comes from the fact that the displacement along the fault abruptly ends
or transforms into another kid of displacement
­ Most common type of transform fault occurs along fracture zones and connects two
divergent plate boundaries at the crest of mid­oceanic ridge
­ Not all transform faults connect two ridge segments
o A transform fault can connect a ridge to a trench (a divergent boundary to a
convergent boundary) or can connect two trenches (two convergent boundaries)
­ Offset appears to be the result of irregularly shaped divergent boundaries
o Ridge crest align perpendicular to the spreading direction, and the transform
faults align parallel to the spreading direction
Convergent Plate Boundaries
­ Convergent Plate Boundaries; two plates move toward each other
­ Oceanic plates converging with a plate capped by a continent, the dense oceanic plate
subducts under the continental plate
­ Two approaching plates both carrying continents, the continents collide and crumple but
neither is subducted
Ocean­Ocean Convergence
­ Two plates capped by sea floor converge, one plate subducts under the other
o Subducting plate bends downward, forming the outer wall of an oceanic trench,
usually forming the outer wall of an oceanic trench
­ Oceanic trench a narrow, deep trough parallel to the edge of a continent or an island arc
(a curved line of islands)
­ Deepest spots on Earth, more than 11km below sea level, are in oceanic trenches in the
southwest Pacific Ocean
­ One plate subducts under another, a Benioff zone of shallow­, intermediate­ and deep­
focus earthquakes is created within the upper portion of the downgoing lithosphere
o As descending plates reaches depths of at least 100 km, magma is generated in
the overlying asthenosphere
­ Magma works its way upward to erupt as an island arc, to the oceanic trench
o Beneath the volcanoes are large plutons in the thickened arc crust
­ Inner wall of a trench consists of an accretionary wedge of thrust­faulted and folded
marine sediment and pieces of oceanic crust
Ocean­Continent Convergence
­ Plates capped by oceanic crust is subducted under the continental lithosphere,
accretionary wedge and forearc basin form an active continental margin between the
trench and the continent
­ Magma created by ocean­continent convergence forms a magmatic arc (island arcs at sea,
and belts of igneous activity on the edges of continents)
­ Beneath volcanoes are large plutons in thickened crust
o Plutons are batholiths on land when they are exposed by deep erosion
­ Crustal thickening causes uplift, so a young mountain belt forms as the thickened crust
rises
o Growth of mountain belt also occurs as it is the stacking up of thrust sheets on
the continental (backarc) side of the magmatic arc
Continent­Continent Convergence
­ Continents must be separated by ocean floor that is being subducted under one continent
and lacks a spreading axis to create new oceanic crust
­
As sea floor is subducted, the ocean becomes narrower and narrower until the continents
collide destroying/closing the ocean basin
­ One continent may slide a short distance under another, but will not go down a
subduction zone
Backarc Spreading
­ Regional extension occurs within or behind many arcs
o Extension can tear an arc in two, moving the two halves in opposite directions
 Occuring behind an arc, can move the arc away form the continent
 Can split the edge of a continent, moving a narrow strip of the continent
seaward
­ Backarc oceanic crust is the type of crust found in most ophiolites
Oceanic Crust and Ophiolites
­ Oceanic crust differs significantly from continental crust
o It is thinner and a different composition
­ Oceanic crust is 7km thick, divided into 3 layers
o Layer 1; Marine sediment, may consist of several kilometers of terrigenous
sediment
o Layer 2; 1.5m thick, consists of pillow basalt overlying dikes of basalt
o Layer 3; 5km thick, thought to consist of sill­like gabbro bodies
­ Ophiolites – distinctive rock sequences found in many mountain chains on land
Convergent Boundaries and Ore Deposits
­ Sea floor spreading carries metallic ores away form ridge crests, perhaps to be subducted
beneath island arcs or continents at convergent plate boundaries
­ Volcanism at island arcs can also produce hot­spring deposits on the flanks of the
andesitic volcanoes
­ Island arc ores usually contain more lead and gold
How Do Mountain Ranges Form?
Orogenies and Plate Convergence
­ Orogeny – An episode of mountain building, often characterized by intense deformation
of the rocks in a region. Mountain belts form along plate margins, particularly where play
convergence compresses the crust, causing uplift and deformation
Ocean­Continent Convergence
­ The Andes, where the South American plate is overriding the Nazca plate, is an example
of a mountain belt formed by ocean­continent convergence
­ An accretionary wedge develops where newly formed layers of marine sediment are
folded and faulted as they are snowploughed off the Subducting oceanic plate
­ Magmatic arc is at a high elevation, because the crust is thicker and composed largely of
hot igneous and metamorphic rocks
Arc­Continent Convergence
­ As the arc and continent converge, the intervening ocean is destroyed by subduction.
When collision occurs, the arc, like a continent, is too buoyant to be subducted.
Continued convergence of the two plates may cause the remaining sea floor to break
away form the arc and create a new site of subduction and a new trench seaward of he arc
o Direction of new subduction is opposite to the direction of the original
subduction (flipping subduction zone)
Continent­Continent Convergence
­
­
Mountain belts that we find within continents (with cratons on either side) are
hypothesized to be products of continent­continent convergence
o Ex. India colliding with Asia to create the Himalayans
o Ex. Appalachian Mountains are also continent­continent convergence
 However, arc­continent convergence was also involved and the mountain
belt was later split apart by plate divergence
The cycle of splitting of a
supercontinent, opening of an ocean
basin, closing of the basin, and collision
of continents is known as the Wilson
Cycle
o Canadian geophysicist J. Tuzo
Wilson proposed the cycle in
1960s
How Do Plates Change Over Time?
Plate Boundaries
­ Plates can move, along with late
boundaries
­ Plates can move away from each other
at a divergent boundary on a ridge crest,
they can also migrate across Earth’s surface and jump to new positions too
Plate Size
­ Plates can change in size
o North American plate is growing in size as it moves slowly westward
The Wilson Cycle
­ The opening and closing of an ocean basin is called a Wilson Cycle; subdivided into six
stages
­ Stage A; A stable craton under which a hot spot develops, causing the crust to dome
upward and fracture creating a rift valley or graben
­ Stage B; Reached when a sea­floor spreading centre with an erupted basaltic ocean floor
is flooded to form a new ocean. The separated continental margins on each side of the
new ocean cool, become denser, and subside beneath sea level
­ Stage C; Ocean basin widens through addition of igneous mantle material via a mid­
ocean ridge system characterized by young volcanoes and steep slopes
­ Stage D; Ocean basin is enlarged sufficiently to allow the basaltic crust to cool and sink
down into the asthenosphere a subduction zone is formed
­ Stage E; Most of the original oceanic crust has been subducted, the two continents that
were originally separated begin to collide
­ Stage F; Collision of the two continents follows complete subduction of the remnant
ocean
What Causes Plate Motions?
­ Several reasons for plate motion
o 1) Mid­oceanic ridge crests are hot and elevated, while trenches are cold and
deep
o 2) Ridge crests have tensional cracks
o 3) The edges of some plates are Subducting sea floor, while the edges of other
plates are continents (which cannot subduct)
­
­
­
Recent models sggest lowermost part of the mantle does not mix with the upper and
middle mantle, acts as a “lava lamp” turned on low, fuelled by internal heating and heat
flow across the core­mantle boundary
Basic Question: Why do plates diverge and sink?
o 1) Ridge­push – As a plate moves away from a divergent bouondary, it cools and
thickens, cooling sea floor subsides as it moves, and this subsidence forms the
broad side slopes of the mid­oceanic ridge
 A slope also forms between the lithosphere and the asthenosphere, 80­
100 km
o 2) Slab­pull – Cold lithosphere sinking at a steep angle through hot mantle
should pull the surface part of the plate away from the ridge crest and then down
into mantle as it cools
 Hypothesized to cause rapid plate motion
o 3) Trench­Suction – Subducting plates fall into the mantle at angles steeper than
their dip, then trenches and the overlying plates are pulled horizontally seaward
toward the Subducting plates
 Possibly a minor force, regardless important in moving continents apart
Plate motions are controlled by variations in lithosphere density and thickness, which are
controlled largely by cooling
How are Mantle Plumes and Hot Spots Related?
­ Mantle Plumes are narrow columns of hot mantle rock that rise through the mantle from
thermal boundary layers at the base of the mantle (or upper mantle)
­ Plumes may form ‘hot spots’ of active volcanism at the Earth’s surface
o Many hot spots are located in volcanic regions
­ Not all hot spots are fed by mantle plumes; some geoscientists suggest that a few plumes,
underlying hot spots are enough to drive plates apart
­ Continued radial flow outward from the rising plume eventually separates the crust along
two/three fractures, leaving the third fracture inactive
o Two active fractures become continental edges as new sea floor forms between
divergent continents
o Third fracture is a failed rift (aulacogen), an inactive rift that becomes filled with
sediment
­ Most aseismic ridges on the sea floor appear to have active volcanoes at one end, with
ages increasing away from eruptive centers
Seamounts, Guyots and Aseismic Ridges
­ Conical undersea mountains that rise 1 000m or more above the sea floor are called
seamounts
o Some seamounts rise above sea level to form islands
­ They are scattered on the flanks of the mid­oceanic ridge and on other pats of the sea
floor
­ Guyots are flat­topped seamounts found mostly in the western Pacific Ocean
­ Many guyots and seamounts on the sea floor are aligned in chains
o Volcanic chains, together with some other ridges on the sea floor are given the
name aseismic ridges (submarine ridges that are not associated with earthquakes
o Aseismic is used to distinguish these features from the much larger mid­oceanic
ridge where earthquakes occur along the rift valley
Why Is It Important to Understand Plate Tectonics?
­ Plate tectonics processes also appear to have been responsible for the repeated creation
and breakup of “supercontinents” such as Rodinia and Pangea
­ Valuable mineral resources are commonly formed at plate margins where geological
processes are most active
­
Chapter 3 – Earthquakes
December 24, 2004, magnitude 9.3 earthquake (Second largest recorded since 1900)
o Severely deformed the Indian Ocean floor off the western coast of Northern
Sumatra
o Earthquake, slightly changed the shape of the planet, reduce the length of the day
by almost 3 microseconds, moved the north pole by several centimeters
What Causes Earthquakes
­ Earthquake – A trembling or shaking of the ground caused by the sudden release of
energy stored in the rocks beneath Earth’s surface
o Great forces acting deep in the Earth may put a stress on the rock,
bending/changing in shape
­ Rock can deform only so far before it breaks, when broken, waves of energy are released
and sent out through the Earth
­ Seismic Waves; waves of energy produced by an earthquake
o These casue the ground to tremble and shake during an earthquake
­ Fault – Break between two rock masses
­ Elastic Rebound Theory – Sudden release of progressively stored elastic strain energy
in rocks, causing movement along a fault
­ Most earthquakes are associated with movements on faults, but in some quakes the
connection with faulting may be difficult to establish
­ Earthquakes also occur during explosive volcanic eruptions as magma forcibly fills
underground magma chambers prior to many eruptions; these quakes may not be
associated with fault movements at all
­ Deep earthquakes; (100 – 670km below the surface) All of which are found on cold,
Subducting plates sliding down into the mantle
Indonesia/Sumatra Earthquake and Tsunami, December 26, 2004
­ Magnitude 9.3 earthquake was caused by the sudden released of large amounts of energy
along a portion of the boundary between the India plate and the Subducting Burma plate
­ Possibility of up to 1 500 km of the plate boundary slipped as a result of the quake
­ Ocean floor above the thrust fault was uplifted by several metres as a result of the
‘megathrust’ quake and created a massive tsunami wave that spread out in all directions
­ Power of tsunami was documented by measuring two important factors
o Inundation – The distance from the shoreline to the limit of tsunami penetration
o Run­up Elevations – Elevation above sea level of the water surface at the inland
limit of inundation
Why Do Earthquakes Cause So Much Damage?
­ Focus (hypocenter) – point within the Earth where seismic waves first originate
o Centre of the earthquake, point of initial breakage and movement on a fault
­ Epicentre – The point on the Earth’s surface directly above the focus
­ Two types of seismic waves are generated
o Body Waves – Seismic waves that trawl through the Earth’s interior, spreading
outward from the focus in all directions
o Surface Waves – Seismic waves that travel on Earth’s surface away from the
epicenter
Body Waves
­ Two types of body waves
o P­wave – Compressional (or longitudinal) wave in which rock vibrates back and
forth parallel to the direction of wave propagation
 First (or primary) wave to arrive at a recording station following an
earthquake)
o S­wave – Slower, transverse wave that travels through near­surface rocks at 2 to 5
km per second
 Propagated by a shearing motion like a stretched, shaken rope
 Rock vibrates perpendicular to the direction of wave propagation
­
Both waves easily pass through solid rock
o P­waves can also pass through fluids (gas/liquid) but S­waves cannot
Surface Waves
­ Surface waves are the slowest waves set off by an earthquake ; causing more property
damage than body waves
o Produces more ground movement and travels more slowly, taking longer to pass
­ Love waves – Like S waves with no vertical displacement
o Ground moves side to side in a horizontal plane that is perpendicular to the
direction the wave is travelling or propagating
o Do not travel through liquids and would not be felt on a body of water
­
Rayleigh waves – Behave like rolling ocean waves
o Causes the ground to move in an elliptical path opposite to the direction the wave
passes
o Tend to be destructive to buildings; producing more ground movement and
taking longer to pass
How Do We Know Where Earthquakes Occur?
­ Instrument used to measure seismic waves – seismometer
o Keep a heavy suspended mass as motionless as possible
o Usually placed in clusters of three to record the motion along the x, y, and z axes
of three­dimensional space
­ Seismograph – a recording device that produces a permanent record of Earth motion
detected by a seismometer, in digital format that can be processed and displayed on
computer terminals
­ Seismogram – Paper record of the Earth’s vibration
o Can be used to measure the strength of the earthquake
­ Network of seismograph stations is maintained all over the world to record and study
earthquakes
­ Different types of seismic waves travel at different speeds, they arrive at seismograph
stations in a definite order; first P­waves, then the S­waves and finally the surface waves
Determining the Location of an Earthquake
­
On a seismogram, the first arrival of the P wave is separated from the first arrival of the
S­wave by a short distance on the paper record
­ Increase in P­S intervals is regular with increasing distance for several thousand
kilometres and can be graphed in travel­time curve (plots seismic­wave arrival time
against distance)
­ Modern three­component seismographs now record the intensity of earthquake vibrations
in three orthogonal directions
o Allows scientists to determine the direction of the incoming wave and the
distance form the epicenter
­ Maximum depth of focus – The distance between focus and epicenter—for earthquakes
is 670 km
o Shallow Focus – 0­70km deep (Most common)
 Accounts for 85% of the total quake energy released
o Intermediate Focus – 70 – 350 km deep
 Accounts for 12% of total quake energy released
o Deep Focus – 350 ­670 km deep
 Accounts for 3% of total quake energy
 Rarer; most deep rocks flow plastically when stressed or deformed
Measuring the Size of an Earthquake
­ Size of earthquake is measured in two ways
o Intensity – Find out how much and what kind of damage the quake has caused
 Measure of an earthquake’s effect on people and buildings
 Expressed as roman numerals ranging from I to XII on the modified
Mercalli scale (higher numbers, higher damage)
 Big advantage of intensity ratings – no instruments required; allows
seismologists to estimate the size of earthquakes that occurred before
seismographs were available
o Magnitude – Amount of energy released by the quake
 Usually calculated by measuring the height (amplitude) of one of the
wiggles on a seismogram
• Larger the quake, the more the ground vibrates and the larger the
wiggle
 Magnitude has been reported on the Richter scale, numerical scale of
magnitudes
• Open ended scale, no earthquakes too large or too small to fit on
the scale
­ New method of calculating magnitude – Use of seismic moment of a quake, determined
form the strength of the rock, surface area of the rupture, and the amount of rock
displacement along the fault
o Moment magnitude the most objective way of measuring the energy released by a
large earthquake
Location and Size of Earthquakes in North America
­ Most large earthquakes occur in western North America
­ British Columbia is the most seismically active region of Canada and lies close to active
plate margins along the Cascadia Subduction Zone
­ Southwestern corner of B.C. experiences more than 200 earthquakes a year and nine
moderate to large earthquakes have occurred in the region during historic times
­
Large quakes are extremely rare in central and eastern North America
o When they do occur they can be very destructive and widely felt, Earth’s crust is
older, cooler, and more brittle in the east than in the west; seismic waves travel
more efficiently
What Kinds of Damage Can Earthquakes Cause?
­ Ground motion – Trembling and shaking of land that can cause buildings to vibrate
o Small quakes can cause windows and walls to crack
o Large quakes, ground motions may be visible; strong enough to topple large
structures (bridges, office, apartment buildings)
 Buildings built on soft sediment are damaged more than buildings on
hard rock
­ Fire; particularly serious problem after an earthquake because of broken gas and water
mains and fallen electrical wires
­ Landslides; can be triggered by the shaking of the ground
­ Liquefaction; special type of ground failure caused by earthquakes, occurs when a water­
saturated soil or sediment turns form a solid to a liquid as a result of earthquake shaking
o May occur several minutes after an earthquake
­ Permanent displacement of land surface; result of land movement along a fault
o Rocks can move vertically, those on one side of a fault rising while those on the
other side drop
o Rocks can also move horizontally, those on one side of a fault sliding past those
on the other side
­ Scarp – Trace of a fault on Earth’s surface may appear as a low cliff
Earthquakes in Canada, Eh?
­ Most activity is focused along the Cascadia subduction zone off the western coast of
British Columbia
­ Earthquakes also occur frequently along the St. Lawrence and Ottawa river valleys
­ Eastern Canada lies far from any active plate boundaries, the majority of earthquakes are
caused by movement along ancient basement faults in Precambrian rocks buried below
younger sedimentary strata
Earthquake Engineering
­ Buildings constructed of strong, flexible, and light materials (steel, wood, reinforced
concrete) are the most resistant to damage by seismic shaking
Tsunami
­ Sudden movement of the sea floor upward/downward during a submarine earthquake
displaces the entire water column, can generate very large sea waves called tidal waves
­ Tsunamis also called seismic sea waves
o Caused by great earthquakes (magnitude 8+) disturbs the sea floor, also resulting
from submarine landslides or volcanic explosions
o Large section of sea floor suddenly rises of falls during a quake, all water over
the moving area is lifted or dropped for an instant
o Most tsunamis are associated with subduction­zone earthquakes; tend to be some
of the strongest quakes
­ Tsunamis can have wavelengths of 160 km, moving at possibly 725 km per hour
o In deep water, the wave height may be only 0.6 to 2m, but near shore the tsunami
may peak up to heights of 15 to 30 m
­
Tsunamis travel at high speeds, there is usually sufficient time to warn low­lying coastal
communities of the impending wave
Where Do Earthquakes Occur on a Global Scale?
­ (most) Earthquakes are concentrated in narrow geographic belts, (some) earthquakes
have occurred in most regions on Earth
­ Most important concentration of earthquakes is in the circum­Pacific belt; encircles the
rim of the Pacific Ocean
o 80% of the world’s shallow­focus quakes, 90% of intermediate­focus quakes,
nearly 100% of deep­focus quakes
­ Another major concentration of earthquakes is in the Mediterranean­Himalayan belt,
runs through the Mediterranean sea, crosses the Mideast and the Himalayas, and passes
through the East Indies to meet the circum­Pacific belt north of Australia
­ Number of shallow­focus earthquakes also occur in significant locations on Earth
o Along the summit or crest of the mid­oceanic ridges; huge underwater mountain
ranges that run through all the world’s oceans
o Few earthquakes recorded in isolated spots associated with basaltic volcanoes
­ Inclined seismic activity – Benioff zones
­ Benioff zones slope under a continent or a curved line of islands island arc
o Andesitic volcanoes may form the islands of the island arc, or be found near the
edge of a continent that overlies a Benioff zone
­ All of the world’s intermediate and deep­focus earthquakes occur in Benioff zones
What is the Relationship Between Earthquakes and Plate Tectonics?
­ Plate tectonics have the ability to explain the distribution of earthquakes and the rock
motions associated with them
o Plates change not only position but also size and shape
­ Earthquakes occur commonly at the edges of plates (interplate earthquakes),
occasionally in the middle of a plate (intraplate earthquakes)
­ Corresepondence between plate edges and earthquake belts can be seen by comparing the
map of earthquake distribution with plate maps
­ According to plate tectonics, earthquakes are caused by the interactions of plates along
plate boundaries Narrow bands of earthquakes are used to outline plates on plate maps
­ Earthquakes on the western border of the Nazca plate are shallow­focus quakes
Earthquakes at Plate Boundaries
­ Three types of plate boundaries
o 1) Divergent Boundaries – Plates move away from each other
o 2) Transform Boundaries – Plates move horizontally past each other
o 3) Convergent Boundaries – Plates move toward each other
Divergent Boundaries
­ Earthquakes are shallow, restricted to a narrow band, much lower magnitude than those
that occur at convergent or transform boundaries
­ Divergent boundary on the sea floor is marked by the crest of a mid­oceanic ridge and the
rift valley that is often found on the ridge crest
­ Earthquakes are located along the sides of the rift valley and beneath its floor
­ Divergent boundary within a continent is usually also marked by a rift valley, shallow­
focus earthquakes and normal faults
Transform Boundaries
­
Two plates that move past each other along a transform boundary, earthquakes are
shallow
­ Most transform faults occur on the ocean floor and offset ridge segments, some are found
in continental crust
Convergent Boundaries
­ Convergent boundaries are of two general types
o Collision of two continents
o Subduction of the ocean floor under a continent/piece of sea floor
­ Collision boundaries characterized by broad zones of shallow earthquakes on a complex
system of faults
o One continent usually overrides the other slightly (continents are not dense
enough o be subducted) creating thick crust and mountain ranges
­ During subduction, earthquakes occur for several different reasons
o Dense oceanic plate bends to go down a trench, it stretches slightly at the top of
the bend, and normal faults occur as the rocks are subjected to tension
o Quakes are caused by shallow­angle thrust faulting Underthrusting
Waiting for the Big One in British Columbia
­ Three types of damaging earthquakes could affect south­western British Columbia
o 1) Crustal earthquakes may originate within the North American plate
o 2) Intraplate earthquakes may originate within the Juan de Fuca plate
o 3) Subduction or plate boundary earthquakes at the boundary between the two
plates
­ Earthquake damage in the Vancouver area will be particularly severe due to the thick
cover of unconsolidated surficial sediments prone to seismic wave amplification,
presence of waterlogged coastal sediments prone to liquefaction, and the steep slopes in
surrounding areas prone to land sliding
Subduction Angle
­ Horizontal and vertical distribution of earthquakes can be used to determine the angle of
subduction of a down going plate
­ Subduction angle is probably controlled by plate density and the rate of plate
convergence
­ Older oceanic lithosphere (Southeast Pacific) tends to be colder and more dense,
therefore subducts at a steeper angle
­ Younger oceanic plates in close proximity to the oceanic ridge are warmer and more
buoyant and subducts at a shallower angle
­ Faster Rate of convergence may also result in a shallower angle of subduction
Earthquakes Away from Plate Boundaries
­ Many earthquakes also occur within plates, accounting for about 5 percent of earthquake
energy released in a year (intraplate earthquakes)
­ (Most) Intraplate earthquakes occur in areas of thinned/weakened crust such as
continental margins, aulacogens, or ancient suture zones
­ Triggered by stress between the lithosphere and the underlying asthenosphere as plates
move, or by isostatic adjustments of the crust due to loading and unloading by ice or
sediment
­ Intraplate earthquakes can be particularly damaging as they occur on areas of relatively
rigid plates that allow the efficient transmission of seismic waves
Can We Predict When Earthquakes Will Occur?
­
Several techniques are being explored for scientifically forecasting a coming earthquake
o Monitoring slight changes or precursors, that occur in rocks next to a fault before
the rock breaks and moves
 This method assumes large amounts of strain are stored in rocks before it
breaks
 Rocks may give warning signals that it is about to break; before a large
quake, small cracks may open within the rock causing small tremors
(microseisms) to increased
 Properties of the rock nex tot the fault may be changed by the opening of
such cracks
 Changes in rock’s magnetism, electrical resistivity or seismic velocity
may give warnings of an impending quake
 Opening of tiny cracks changes the rock’s porosity Water levels in
wells often rise of fall before quakes; increase in radon emission form
wells may be a prelude to an earthquake
o Local method of predicting quakes – Time the interval between eruptions of old
faithful geyser in Yellowstone National Park
o Surfaces of Earth tilts and changes elevation slightly before an earthquake (some
areas)
o Japanese and Russian geologist were the first to predict earthquakes
successfully, and Chinese geologists have made some very accurate predictions
o Fundamentally different method of determining the probability of an earthquake
occurring relies on the history of earthquakes along a fault and the amount of
tectonic stress building in the rock
 Geoscientists look at the geologic record for evidence of past
earthquakes using the techniques of paloseismology
o Recurrence interval and likelihood of future earthquakes are also determined by
measuring the slip rate along plate boundaries
Chapter 4 – The Earth’s Interior
Only rocks geoscientists can study directly in place are Earth’s crust
o Earth’s crust is a think skin of rock, making less than 1% of the earth’s total
volume
­ Mantle rocks brought to Earth’s surface in basalt flows, diamond­bearing kimberlite
pipes and by tectonic attachment of lower parts of the oceanic lithosphere to the
continental crust give geoscientist a glimpse of what underlying mantle might look like
­ Evidence of geophysics suggest that the Earth is divided into three major layers
o Crust on the surface
o Rocky mantle beneath the crust
o Metallic core at the centre
­ The crust and the uppermost mantle can be conveniently divided into the brittle
lithosphere and the plastic asthenosphere
­ Earth has a radius of about 6 370 km
­ Geophysics – The application of physical laws and principles to a study of the Earth
o Includes the study of seismic waves and Earth’s magnetic field, gravity and heat
Deep Drilling on Continents
­
­
­
­
Surface mapping and seismic reflection and refraction suggest continents are largely
igneous and metamorphic rock (granite and gneiss), overlain by a veneer of sedimentary
rocks
Sedimentary cover is generally thin, but may thicken to 10km or more in giant
sedimentary basins, where underlying “basement rock” has subsided
Igneous/metamorphic basement averages 40km thick, making up most of the continental
crust (rarely sampled deeper than 2 or 3km)
o Uplift and erosion have exposed some rocks widely thought to have been formed
much deeper in the crust
What Can We Learn from the Study of Seismic Waves?
­ Seismic waves from a large earthquake may past through the entire Earth
­ Seismic Reflection – The return of some of the energy of seismic waves of the Earth’s
surface after the waves bounce off a rock boundary
o If two rock layers of differing densities are separated by a fairly sharp boundary,
seismic waves reflect off the boundary just as light reflects off a mirror
­ Reflected waves are recorded on a seismogram, showing the amount of time the waves
took to travel down to the boundary, reflect off it and return to the surface
­ Seismic Refraction – The bending of seismic waves as they pass from one material to
another
o Similar to the way that light waves bend when they pass through the lenses of
eyeglasses
o Seismic wave strikes a rock boundary, much of he energy of the wave passes
across the boundary
 Wave crosses from one rock layer to another, changing directions
 Change of direction/refraction occurs if the velocity of seismic waves is
different in each layer
Canadian Lithoprobe Project
­ Groundbreaking Canadian scientific effort investigating the composition/structure of the
Canadian Shield and surrounding organic belts since 1984
­ Aim – Develop a comprehensive understanding of the geological evolution of the North
American continent
­ Shield is formed of distinct geological terranes that were once separate land masses but
brought together by the forces of plate tectonics
­ Seismic Reflection – Sending waves into the ground and recording them when they
bounce back up
o Lithoprobe Project has used a series of large vibroseis trucks to generate
sufficiently large vibrations
o Vibroseis trucks (4­5) termed “dancing elephants” – work together to stamp in
unison along a road bed or road shoulder
 Energy waves created pass through Earth & reflect/refract when they
encounter a boundary between rocks with different physical properties
­ Geophones (cup sized motion sensors) are laid out on the ground to detect the reflected
sound waves
­ Since 1984, Lithoprobe project has collected more than 10 000 km worth of seismic
reflection data from the Canadian Shield, that are now being used to create
multidimensional maps of he Earths crust
What is Inside the Earth?
­ Three main zones of the Earth’s Interior
o Crust – Outer layer of rock, forms a thin skin on Earth’s surface
o Mantle – A thick shell of rock that separates the crust above from the core below
o Core – The central part of earth. Probably metallic and the source of Earth’s
magnetic field
Crust
­ Studies of seismic waves have shown
o 1) The crust is thinner beneath the oceans than beneath the continents
o 2) Seismic waves travel faster
in oceanic crust than in
continental crust (Difference in
velocity; assumption of two
types of crust made up of
different kinds of rock)
­ Seismic P waves travel through
oceanic crust at about 7km per second,
speed traveled through basalt and
gabbro
o Oceanic crust
averages 7km in
thickness, varying
from 5 to 8 km
­ Seismic P waves travel more
slowly through continental
crust – about 6km per second,
same speed as travelling
through granite and gneiss
o Continental crust
often called “granite”
– Usually called felsic
by geoscientists
o Felsic – Rocks high
in feldspar and
silicon – for
continental crust and
mafic – rocks high in
magnesium and iron
(ferric) – for oceanic
crust
­ Continental crust is much thicker than oceanic crust, averaging 30 to 50km in thickness,
varies from 10 to 70 km
­ Seismic waves show, crust is thickest under geologically young mountain ranges (Andes,
Himalayas) bulging downward as a mountain root into the mantle
­ Continental crust is less dense than oceanic crust
­ Mohorovičić discontinuity – Boundary that separates the crust from the mantle beneath
it (Moho for short)
­ Mantle lies closer to the Earth’s surface beneath the ocean than it does beneath continents
­
Ambitious project – Project Mohole (Early 1960s) was to use equipped ships to drill
through the oceanic crust and obtain samples from the mantle
The Mantle
­ Geoscientists interpret it to be made of solid rock based on the way seismic waves pass
through
­ Magma chambers of melted rock may occur as isolated pockets of liquid in both the crust
and the upper mantle, however, most of the mantle seems to be solid
­ P waves travel at about 8 km per second in the upper mantle, the mantle is a different
type of rock from either oceanic crust or continental crust
o Hypothesis – Consists of ultramafic rock (Ex. Peridotite)
o Ultramafic rock is dense igneous rock made up chiefly of ferromagneian
minerals (olivine & pyroxene)
­ Crust and Uppermost mantle together form the lithosphere (Outer shell of earth that is
relatively strong and brittle)
­ Lithosphere makes up the plates of plate tectonics theory
o Averages 70km thick beneath oceans and may be 125 to 250 km thick beneath
continents
­ Seismic waves increase in velocity with depth as increasing pressure alters the properties
of the rock
o At depth of 70 to 125km, seismic waves travel more slowly than they do in
shallower layers, zone is referred to as low­velocity zone
o Zone extending to a depth of (perhaps) 200km is called the asthenosphere
­ Rocks in asthenosphere may be close to their melting point, two reasons
o 1) It may represent a zone where magma is likely to be generated
o 2) Rocks here may have relatively little strength and therefore are likely to flow
A CAT Scan of the Mantle
­ New technique of looking at the mantle similar to the medical technique of CAT
(Computed Axial Tomography) scanning
­ Seismic tomography uses earthquake waves and powerful computers to study planar
cross­sections of the mantle following large earthquakes
­ Deeper CAT scans of the mantle indicate that some mantle plumes emanate from the
core­mantle boundary and are fed by heat loss from the core
­ New tomographic images also reveal high­velocity areas, interpreted as cold sinking
slabs of subducted plates, also extend all the way to the core­mantle boundary
­ Other plates stop descending at the 670km boundary within the mantle; perhaps depth of
sinking is controlled by plate density
The Core
­ Seismic­wave data provide the primary evidence for the existence of the core of the Earth
­ Seismic P waves spread out form a quake until 103 degrees of arc (11 500km) from the
epicenter, they suddenly disappear from seismograms; at more than 142 degrees (15
500km) from the epicenter, P waves reappear
o Region between 103 degrees and 142 degrees that lacks P waves, is called the P­
wave shadow zone
­ P­Wave Shadow Zone
o Refraction of P waves when they encounter the core boundary deep within
Earth’s interior
o P waves can travel through solids and fluids, S waves can travel only
through solids
o S wave shadow zone indicates S waves do not travel through the core at all
 Implies the core of Earth is a liquid, or at least acts like a liquid
o Way P waves are refracted within the Earth’s core suggests that the core has two
parts – a liquid outer core and a solid inner core
Diamonds ­ A Window Into the Mantle
­ Mantle materials are found in diamond­bearing igneous rocks called kimberlites;
forming carrot­shaped bodies (up to 200m across, more than 1km deep – referred to as
kimberlite pipes)
How Do We Know That Diamonds Form in the Mantle?
­ Diamonds are made of high­pressure form of crystalline carbon
o Produced experimentally at extreme temperatures (1500c) and pressures (55
kilobars)
­ These conditions found in nature in depths more than 150km below the Earth’s crust, in
the mantle
How Do Diamonds Form?
­ Thought to have originated from carbon­bearing rocks on oceanic plates that were
subducted at collisional plate margins
­ Carbon transformed into diamond under extreme heat and pressure, trapped in the mantle
below continents
­ Eruptions of kimberlite magmas through volcano­like vents of kimberlite pipes brought
diamonds to surface of continents
­ Diamonds are not stable at the Earth’s surface and will eventually over geologic time,
break down to form graphite
When Did Diamonds Form?
­ Kimberley diamonds are 3.3 billion years old
­ All diamond­bearing kimberlites are found only on the oldest parts of continents
­ Kimberlite pipes range in age from Precambrian to Cretaceous
o Formation of diamonds was peculiar to the early Earth, sitting in the mantle
below continents until major continental rifting events released them, bringing
them to the surface
Where Can We Find Diamonds in North America?
­ Diamonds are found on most of the ancient continental cratons – southern and central
Africa, Siberia, India, Australia and the Canadian Shield
­ First commercial mine in North America – Ekati mine; lies 300km northeast of
Yellowknife in Northwest Territories
o Total of 121 kimberlite pipes have been identified in the Ekati claim area
o Projected lifespan of 25 years, producing more than 2 million carats of diamonds
each year
Composition of the Core
­ Earth’s core is made of metal – not silicate rock – metal is probably iron (along with
minor amount of oxygen, silicon, sulphur or nickel)
­ How did geoscientists arrive at this conclusion?
o Overall density of Earth is 5.5 gm/ cm3
 Calculations from Newton’s law of gravitational attraction
­ Choice of iron as major component of the core comes from looking at meteorites
o Thought of by some scientists to be remnants of the basic material that created
our own solar system
o Estimated 10% of meteorites are composed of iron mixed with a small amount of
nickel
Existence of Earth’s magnetic field, also suggests a metallic core
The Core­Mantle Boundary
­ Boundary between the core and mantle is marked by great changes in seismic velocity,
density and temperature
­ There is a transition zone up to 200km thick, known as the “D” layer, at the base of the
mantle where P­wave velocities decrease dramatically
­ Ultra­low­velocity zone (ULVZ) forms the undulating border at the core­mantle boundary
may be due to hot core partially melting overlying mantle rock or could be part of the
liquid outer core reacting chemically with the adjacent mantle
­ Both mantle and core undergo convection, a circulation pattern in which low­density
material rises and high density material sinks
o Heavy portions of the mantle sink to its base, unable to penetrate the denser core
o Light portions of the core may rise to its top, and may be incorporated into the
mantle above
­ Continent sized blos of liquid/liquid crystal slush may accumulate at the core­mantle
boundary, (perhaps) interfering with or helping cause heat loss from the core to help drive
mantle convection and transfer of heat to the surface & causing changes in Earth’s
magnetic field
­
How Does the Elevation of Continents Change?
­ Isostasy is a balance or equilibrium of adjacent blocks of brittle crust “floating” on the
upper mantle
o Crustal rocks weigh less than mantle rocks, crust can thought of as floating on
the denser mantle
o Crustal rocks can be thought of as tending to rise or sink gradually until they are
balanced by the weight of displaced mantle rocks
 Concept of vertical movement to reach equilibrium, referred to as
isostatic adjustment
­ Crustal blocks have come into isostatic balance, a tall block (mountain range) extends
deep into the mantle (mountain root), column thick continental crust (mountain and its
root) has the same weight as a column containing thin continental crust and some of the
upper mantle
o Column containing sea water, thin oceanic crust, and thick section of heavy
mantle weights the same as the two other columns
­ Crustal Rebound – Rise of the crust after the removal of the ice
What Can Gravity Tell Us About the Earth’s Crust?
­ Force of gravity between two objects varies with the masses of the objects and the
distance between them
o Force of gravity between A and B = Constant (MassA x Mass B / Distanceº)
­ Useful tool for studying the crust and upper mantle is the gravity meter; measures the
gravitational attraction between the Earth and a mass within the instrument
­ Geoscientists use gravity meters to identify relative changes in gravity that may indicate
local variations in rock density (mass=density x volume)
­ Gravity meter also useful to discover whether regions are in isostatic equilibrium
­ Positive gravity anomaly – Gravity reading higher than the normal regional gravity
o Indicates that tectonic forces are holding a region up out of isostatic equilibrium
­
Negative gravity anomaly – Gravity reading lower than the normal regional gravity
o Indicates that a region is being held down or that local mass deficiencies exist for
other reasons
How Does The Earth’s Magnetic Field Change Through Time?
­ A region of magnetic force – a magnetic field – surrounds the Earth
­ Invisible lines of magnetic force surrounding Earth deflect magnetized objects that are
free to move
o The field ahs north and south magnetic poles, one near the geographic North
Pole, the other ear the geographic South Pole
 The Earth’s field is called dipolar (two poles)
­ Magnetic poles are displaced about 11.5 degrees from the geographic poles
­ Changes in the position of magnetic poles have been well documented, since the time of
the great explorations of the globe
o Earth’s field is not 100% dipolar, magnetic poles appear to be moving slowly
around the geographic poles
­ Rate of the poles’ changes in position, together with the strength of the magnetic field,
strongly suggest the magnetic field is generated within the liquid metal of the outer core
rather than within the solid rock of the crust or the mantle
­ How is Earth’s magnetic field generated? (Numerous hypotheses)
o Magnetic field is created by electric currents within the liquid outer core; outer
core is extremely hot and flows at a rate of several kilometers per year in large
convection currents, about one million times faster than mantle convection above
it
o Convecting metal, in the presence of existing magnetic field, creates electric
currents, which could sustain Earth’s magnetic field
Magnetic Reversals
­ Magnetic Reversals – Earth’s magnetic field has periodically reversed its polarity in the
past
­ Time of normal polarity, magnetic lines of force leave Earth near the geographic South
Pole and re­enter near the geographic North Pole
o “Normal Polarity” – It is the same as present polarity
­ Reversed Polarity – Magnetic lines of force run the
other way, leaving Earth near the North Pole and
entering near the South Pole
­ Many rocks contain a record of the strength and
direction of the magnetic field at the time the rocks
formed
­ Other rock types, (sedimentary rocks stained red by
iron compounds) also record former magnetic­field
directions
­ Paleomagnetism – Study of ancient magnetic fields
o Studies a series of staked lava flows often show that some of the lava flows have
a magnetic orientation directly opposite to Earth’s present orientation
o Many periods of normal and reverse magnetization are recorded in continental
lava flows
­ Reversals tend to be randomly
o Earth’s field reverses on average about once every 500 00 years, present normal
orientation has lasted for the past 700 000 years
o Most geoscientists think it takes 10 000 years for a reversal to develop
­
What causes magnetic reversals? (Difficult to answer)
o Magnetic field is generated by convection currents in the liquid outer core; if the
field is caused by convection currents within the liquid outer core, perhaps a
reversal is caused when the currents change direction or by a temporary current
building up and then dying out
Earth’s Spinning Inner Core
­ Gary A Glatzmaier of Los Alamos National Laboratory in New Mexico and Paul H.
Roberts of the University of California, Los Angeles developed a sophisticated computer
model of convection in the outer core that has been successful in simulating a magnetic
field similar to that measured on Earth
­ Model predicts Earth’s solid inner core spins faster than the rest of the planet, gaining a
full lap on the rest of the planet every 150 year
­ Glatzmaier and Roberts’ model produced a magnetic reversal on its own without any
additional input from the experimenters after about 35 000 years of simulated time
Magnetic Anomalies
­ Magnetometer – An instrument used to measure the strength of Earth’s magnetic field
o Can be carried over the land surface or flown over land or sea (towed behind
ships)
­ Anomaly – Deviation from average readings
o Very broad regional magnetic anomalies may be due to circulation patterns in
the liquid outer core or other deep­seated causes
­ Positive magnetic anomaly – A reading of magnetic­field strength that is higher than the
regional average
Magnetotellurics: A New Tool for Investigating the Earth’s Interior
­ Magnetotellurics – New geophysical approach that is being used in remote regions of
the Canadian Arctic to investigate and map structure within the underlying crust and
mantle
­ Earth’s magnetic field is roughly 50 000 nano Tesla and the equipment used in
Magnetotellurics can measure changes smaller than 0.03 nano Tesla
­ Maps produced by magnetotelluric surveys are used for a variety of purposes
o Economic ore bodies, finding geothermal energy, look for fluids, groundwater
contamination, aid in assessment of earthquake risk
How Hot Is The Earth’s Core? What is the Origin of the Earth’s Heat?
Geothermal Gradient
­ Geothermal Gradient – Rate of temperature increase with depth into Earth
o Can be measured on land in abandoned wells of on the sea floor by dropping
specially designed probes into the mud
o Average temperature increase is 25°C per kilometer of depth
o Temperature at the centre of the Earth may be 6 900°C
o Some regions have a much higher gradient, indicating concentrations of heat at
shallow depths; potential of generating geothermal energy
o Seismic evidence indicates a solid, not molten, mantle, therefore geothermal
gradient must drop to values as low as 0.3°C/kilometer within the mantle
Heat Flow
­
­
­
­
Small but measureable amount of heat from the Earth’s interior is being lost gradually
through the surface
Heat Flow – The gradual loss of heat through Earth’s surface
What is the ‘origin of the heat’?
o Original heat from the time that the Earth formed; if the Earth formed as a mass
of planetesimals that coalesced and compressed the inner material
o OR heat could be a by­product of the decay of radioactive isotopes inside Earth
(Radioactive decay may actually be warming up the planet)
Heat loss per unit area from continents and oceans is about the same, perhaps because of
convection of hot mantle rock beneath the oceans