Study Sheet – GEOL 1104 – Exam 3
Relative geologic time
Principles – know what they say and how they apply
Uniformitarianism
- ‘the presesnt is the key to the past”. Processes that act today acted in the same way in the
past.
Original horizontality
- All sedimentary sequences originally form as flat, horizontal beds. Any tilting or folding
must occur AFTER the rock formed.
Superposition
- The oldest rocks are always on the bottom and the youngest are on the top.
Lateral continuity
- Sedimentary deposits will extend out laterally in all directions. Eventually, they will thin out
or be terminated by the edge of a basin, but they will never just end abruptly.
Cross-cutting relationships
-any structure that cuts another must be younger than what it cuts
Faults
Intrusions
Unconformities
Contact metamorphism
Inclusions
- applies to any rock unit containing pieces of another rock. A rock found included within
another rock must be older than the rock it is inside.
Fossil succession
- fossils can be fairly predictable over geologic time. They will typically be found in
sedimentary rocks and most will be restricted to a few specific rock types.
-criteria: (a) life has varied through time
(b) fossil assemblages are recognizably different
© relative ages are known or can be determined
-Index (Guide) fossils: fossils of organisms that are known to have existed for only limited
periods.
Unconformities – know rock types and orientations
- where part of the rock record is missing, either through erosion or non-deposition
Disconformity
- occurs between two parallel rock beds, usually with rocks being sedimentary
Angular unconformity
- also typically between two sedimentary rocks, but in this case, both rock units are not
parallel. The upper units will be horizontal, and the lower units will be titled.
Nonconformity
- occurs between two different types of rock. It doesn’t matter which two, as long as they are
not the same.
Sequence diagrams – like the Daily Work quiz
Absolute dating
Decay
- When an isotope decays, it loses particles. As a result, it can start as one element and end up as a totally
different element. The original element is referred to as the Parent atom and the resulting element is the
Daughter atom.
half-lives
- the specific rate decay occurs. The time it takes for half of the radioactive parent atoms to be converted
into stable daughter atoms. Decay will continually convert half of what is left.
Requirements
A. the parent must be serperated from the daughter atoms at the beginning of the event to be dated.
Essentially, the original rock must have 100% parent atoms and 0% daughter atoms. If daughter
atoms are present at the start, it will make the resulting date too high/old.
B. The system in which decay occurs must be closed. This means that neither the parent nor the
daughter atoms can escape. If by some chance daughter atoms leak out, the calculated age will be too
low. If parent atoms leak out, the calculated age will be too high. With most rocks, this is not a
problem, but this condition is extremely important when dealing with decay from Potassium (K) to
Argon (Ar) 40 as this occurs in a gaseous state.
Best rock types
- igneous are the best to use. As rocks cool, the parent and daughter atoms are naturally segregated
from one another during the crystallization and then locked in as the rock cools completely. Either
intrusive or extrusive may be used.
- Only a few sedimentary rocks meet this conition. Galuconite can be used with care, but most
sedimentary rocks show the age that the pieces formed, not the age the rock formed.
- Metamorphic rocks typically show a partial or complete resetting of the parent/daughter ratio from
when the rock originally formed. If the ratio is completely reset, the rock would be good to use for
age dating, but since it is impossible to tell is a complete resetting has occurred or if it was just a
partial resetting, the rocks are typically not used.
Types –
Alpha: the isotope will lose 2 protons and 2 neutrons. As a result, the atomic number will change by
2 and the atomic mass (protons + neutrons) will change by 4.
Beta: the isotope will lose an electron. This electron is given off by a neutron, which causes it to
change into a proton (0 –[-1] = +1). This will result in a positive change of one to the atomic number,
but no change in the atomic mass.
electron capture: works the opposite of beta decay. The isotope gains an electron, converting a
proton to a neutron (+1 +[-1] = 0). This results in a negative change of one to the atomic number, but
no change to the atomic mass.
Geologic time scale – different units
Eons
- largest blocks of time and are shown on the very left side of the scale. PreCambrian and the
Phanerozoic
Eras
- the Phaneorozoic is further broken into these: Paleozoic, Mesozoic, and Cenozoic
Periods
- each era is further broken into these
Epochs
- the periods in the Cenozoic are further divided into these units as we have much more information
that allows for further refinement of the scale.
Breaks
- defined by a number of different events
1. Changes in life forms as identified using fossils. As one species dies out and another rises to
prominence, this will be shown on the time scale.
2. Unconformities
3. Correlations between continents with respect to life and geology. Similar creatures, rocks and
structures can be found on currently separated continents, allowing us to compare more features to
each other.
4. Major events, such as extinctions. These tend to form the major breaks in the time scale, such as
between eons or era.
Relative Time
- In geology, a million years is considered a very short period of time, since it is being compared to
the total.
Structures
Stress
- a force applied to rock, which results in a strain, or the changing of a rock.
Types: know motion and result (shortening, lengthening, etc)
Compression
- pushes the rocks together. Generally causes a shortening and thickening of the rock.
(similar to convergent boundaries in Plate Tectonics)
Tension
- pulls the rock apart. Generally causes lengthening and thinning of the rock. (similar
to divergent plate boundaries in Plate Tectonics)
Shear
-two opposing forces act parallel to one another to displace rock. The rock just pushes
past each other with no change in shape.
Effect on rocks – elastic vs. permanent (brittle and plastic)
- Elastic: once the stress is removed, the rock returns to its original shape with no changes.
Ex. Rubber band – you can stretch it, but when you let go, it goes back to original shape.
- Plastic: the rock is BENT permanently. Typically found deep in the Earth, where
temperatures are high enough for the rocks to flow (ductile). The result is Folds.
- Brittle: the rock is BROKEN permanently. Found at shallow depths, near the surface, where
temperatures are low. The result is Faults and Fractures.
Tools –
Strike
- the compass direction of the intersection of a bed with the horizontal plane defined by the
Earth’s surface ( the surface exposure of the rock unit). It is always measured from the north.
On a map, the longish thick line will show the strike.
Dip
- the angle between the horizontal and the top of an inclined bed. Always measured
perpendicular to strike. The direction water would flow down the rock. Always described
with an angle and a direction. On a map, a shorter, thicker line and a number will show the
dip.
block diagrams
- show a block of the Earth. The top is a map view and the sides show cross-sections or slices
into the Earth. Extremely useful for showing geologic structures.
Ex. Layer cake – from the top you see the icing and from the side you can see all the layers
and filling.
Folds: characteristics, stress type, specifics of each type, axial plane and limbs
Anticlines
- a hill shaped, upward-bowing fold. Essentially, the limbs are being bent down in respect to
the axial plane. The older units will be in the center of the fold. On a map view, the dips will
point away from the center. Forms the “A” shape.
Synclines
- a basin/bowl shaped, downward bowing fold. The limbs are being bent up in respect to the
axial plane. The younger units will be in the center of the fold. On a map view, the dips will
point in towards the center. It forms a “sink” (bowl) shape.
Tilted axial planes
Asymmetrical and Plunging folds
- the axial plane can be titled in any direction and sometimes even tilted in a combination of
directions. If it is tiled away from vertical, this can create over-turned or asymmetrical folds.
If it is titled away from horizontal/level, this can create a plunging fold.
Domes and basins
large scale deformation can sometimes result in the formation of domes or basins.
- Domes occur through up warping of the underlying rock. This can also be caused by
intrusions such as laccoliths. Domes can be thought of as a double anticline and will show the
same characteristics. The oldest rocks will be found in the center of the fold and the dips will
point away from the center.
- Basins occur through the down warping of the underlying rock. This can be caused by a
buildup of sediment or through asteroid impacts. Basins can be thought of as a double
syncline and will show the same characteristics. The youngest rocks will be in the center and
the dips will point in towards the center.
Monoclines
- just a small bend or flexure in the rocks. It is essentially only half a fold (mono = one).
Instead of both limbs bending, only one will. Typically theses folds form due to a fault that
doesn’t reach the surface. The fault will push up one side, causing the overlying rocks to be
pushed up as well.
Breaks – characteristics of each, stress type
Joints – no motion
Faults – (motion) hangingwall vs. footwall, specifics of each type, thinning versus thickening,
examples, related features (fault breccia, etc.)
Dip-slip – normal and reverse
- motion is basically up and down. Movement occurs along the dip of the fault. If you
were to stand on the fault, one block would be above your head, the hanging wall, and
the other below your feet, the footwall.
Normal: caused by tensional stress-the rocks are being pulled apart. This creates
space and the rocks drop down to fill the space. The hanging wall moves DOWN
relative to the footwall, resulting in extension and thinning.
Ex. Basin Range in the western United States
Reverse: caused by compressional stress- the rocks are being pushed together. This
causes one block of rock to pop up. The hanging wall goes UP relative to the
footwall, resulting in shortening and thickening. A very low angle reverse fault is
called a thrust fault.
Ex. Himalayas
Strike-slip
- motion is basically back and forth, occurring along the strike of the fault. These are
smaller versions of transform boundaries. Shear stress is involved, which just moves
the rocks past each other, with no change in topography (no thinning, shortening,
extension or thickening). Motion is defined as either right-lateral or left-lateral. (Is the
block moving to the right or left?
Ex. San Andreas Fault in California
Oblique slip
- combination of dip-slip and strike-slip. The movement is both upward and bachforth.
Related Features
(a) Fault breccia
- resembles sedimentary breccia, but is formed through movement instead of weathering. The rocks
are crushed and shattered by the fault. Heat from friction helps to cement these pieces together to
form a rock.
(b) Fault scarp
- associated with dip-slip and oblique-slip faults. As the rocks move up (reverse) or down (normal),
an offset will form at the surface. The resulting cliff from this offset is the fault scarp.
© Fault striations
- as rocks move past each other, they will scratch and scrape each other, which causes grooves and
smoothing to occur. Sometimes, it will make the rocks look polished. Grooves help geologists
determine the direction of the movement.
Earthquakes
Definition:
- occur when energy stored in the rocks is released suddenly
elastic rebound theory: as rocks try to move past each other, they get caught and can’t move, but the
stressing does not stop and continue to push on the rock and strain will build up until levels are high
enough to force the rocks to move. All the energy is released from the rock in a sudden burst.
fault creep
- when rocks can move past each other, no stress or strain builds up and energy related to the
earthquake is released gradually, so there is no earthquake
Seismic waves
- how energy is released through the earth
Types – characteristics, including vibration direction
(A) Body waves: spread out from an earthquake in all directions, traveling both along
the surface and through the Earth
1. P-waves: primary waves because they are always the first to occur and they
occur in a push-pull motion. They are compressional because the vibration
occurs parallel to the overall motion of the waves. They first push/compress
and then pull/expand.
2. S-waves: occur in secondary waves because they are always the second to
occur and occur in a shaking (up/down) motion. Longitudinal because the
vibration occurs perpendicular to the overall motion in an up and down
movement. *Cannot travel through liquids
(B) Surface waves: travel only on the surface of the Earth. Also referred to as Lwaves because they are larger and there is longer time between waves (slower, bigger
waves). They typically have much more damage.
1. Love waves: occur perpendicular to the overall motion, but are side to side
with no vertical movement. Shake back and forth and do not travel through
liquids
2. Raleigh waves: longitudinal because the vibration occurs perpendicular to
the overall motion in an up and down movement. Huge, slow, rolling waves
that take a long time to pass and cause lots of destruction. These cause much
of the damage associated with earthquakes
Seismometer – how works, what shows, seismogram (when waves arrive)
Records seismic waves. A suspended mass inside a box that responds to shaking of the
ground. The mass is still while the box moves around because of the earthquake.
Almost all are digital and results are automatically sent to computers vs. pen and paper setup.
Seismogram: records arrival times for each of the seismic waves
Arrival times
1. P-waves: move fast and have small amplitudes
2. S-waves: higher amplitudes and will be felt more
3. Surface waves: biggest waves and will be felt the most
Location – how determine
Seismometers and seismograms
Seismometers:
Seismograms:
Triangulation and travel time graphs
Travel time graph: plots the time between the arrival of the P-waves and the arrival of
the S-waves at the seismometer
Triangulation: to narrow down a specific location, information from at least three
different seismometers must be used.
Focus vs. epicenter
Focus: actual position where the earthquake occurs. Located between below the
ground and energy dissipates in all directions
Epicenter: point directly above the focus. Given in media reports because it is easier
for public to relate to. (10 miles from..)
Where typically occur – in U.S. and world
- clustered around the Pacific Ocean and in central Asia due to convergent boundaries
Intensity and magnitude
Scales: characteristics, what show, problems, benefits
Mercalli
- based on intensity and language. Type and amount of damage caused determines the
size of the earthquake. Purely visual, but only useful in populated areas, because it is
based on existing structures/how people feel. Extremely subjective
Richter
- based on magnitude (energy released during earthquake). Plots difference of arrival
time and the size/amplitude of the S-wave to determine size of the earthquake.
Moment magnitude
- related to amount of energy released, taking into account the particulars (strength,
surface area, etc)
Effects – characteristics and effects of each
Ground motion
- seismic waves cause shaking which can shake building and structures. If the frequency
matches that of the building, it can shatter.
Debris
- from falling buildings, materials shaken loose, cars, etc.
liquefaction
- when wet sediment is forced apart like quicksand
fire
- when gas mains, electric cables, water mains, etc. shake, it is easy for fires to occur
landslides
- the ground shakes sediment on slopes, loosening large amounts of sediment which will
travel downhill very rapidly
ground rupture
- the movement of the rock can cause a break or bend to occur at the surface
tsunamis
- occur when undersea earthquakes display large blocks of rock on the sea floor, causing
hazardous waves
aftershocks
- after the earthquake, the area continues to shake for a period of time. Can last several days
Prediction – short term versus long term (how each work, reliability?), Parkfield example
Earth’s interior
Short term: look for precursors the the earthquake. NONE have proven reliable.
(1) uplift, subsidence and strain buildup: looks at the properties of the rock
(2) anomalous animal behavior: animals act strangely before an earthquake
(3) groundwater levels: may raise or lower due to changes in the porosity in the rock or to
offsets in the aquifier as the rocks move
(4) electrical conductivity or magnetism of the rocks
Long term: based on historical records
(1) identification of the earthquake history for an area
(2) identification of “seismic gaps”, or areas that haven’t had an earthquake in a long time
(3) studying long term offsets and material responses
A little more reliable, but can only narrow an earthquake to month or years
Parkfield example
- an experiment was conducted to study what actually happens on the fault and in the surrounding
region before, during and after an earthquake
Seismic evidence - characteristics of waves:
Speeds through composition and density
Boundaries – refraction vs. reflection
-Refraction:
-Reflection:
Shadow zones: P and S – where and why
Layers: characteristics of each (including, density, thickness, composition, etc)
Crust
- solid earth, least dense material, 2 types
(1) Oceanic: richer in Fe, Mg – more dense, 3-15 km thick (basalt)
(2) Continental: more silicate- less dense, avg. 35 km thick (granite/gneiss)
Mohorovicic discontinuity
Mantle
- 82% of the total volume of the Earth, several layers with different physical properties, solid
material flows like silly putty, composition=periodite=Fe, Mg silicates
Lithosphere
- upper most rigid part of the mantle plus the crust, broken up into plates which move around
the Earth’s surface dictated by plate tectonics
Asthenosphere
- the level in the mantle below the low velocity zone, rocks begin to melt and behave
plastically (flow), only reach a semi-molten state – not complete melting, floats above the
lithosphere
Core
- very thick layer, (3486 km), extreme temperatures, (6700C) and pressures (three million
times the pressure at the surface of the Earth), high seismic velocities (10 g/cm3 at the coremantle boundary to 14g/cm3 at the center)
Inner: solid, compression keeps in solid state, mostly iron, 10-20% nickel, densest section
Outer: liquid, proven by shadow zone, Fe with some sulfur and/or Si, Ni, K to reduce
measured density
Plate tectonics
History
Wegener
- first to propose the idea that all the continents were once part of a giant supercontinent and
that they drifted apart to their current locations. This idea is referred to as continental drift.
Observations: fit of continents
-several of the continents seemed to fit together like puzzle pieces.
(especially true for Africa and South America) For a more
perfect fit, you need to measure along the continental slope. This is
much more stable than sea level, which can fluctuate greatly
over time.
rock sequences and mountain ranges
- There are numerous features on different continents that all seemed
very similar and possibly related. These features all lined up to form
continuous units (rock types: marine, non-marine, and glacial units;
structures such as mountain ranges and processes) glacial flow lines
that made much more sense when all the continents were together –
apart the glaciers would be flowing uphill, together they were flowing
downhill.
paleoclimatic evidence
- With the continents in their current positions, there are some climate
features that don’t make much sense. Glacial rocks have been found at
the equator, where it is much too hot for ice to have formed. Coalforming plants and ferns have been found at high latitudes, where it is
too cool and dry for them to grow. Wegener’s supercontinent could
account for these climate issues, as the current continents would all
have been found at different latitudes than where they are currently
fossil evidence
- Organisms are usually restricted to specific regions based on climate
and conditions. Ex. cacti in the desert. If you find fossils in a different
climate than what they needed to survive, you know something has
changed, either the environment (wetter or drier, hotter or colder, etc)
or the position of the continent. Two examples:
1. Ferns require warm, moist climates; however, fossil ferns have been
discovered in Antarctica. Not a climate known for being warm and
moist.
2. The mesosaurus was a fresh water swimmer and as such would not
have been able to cross the ocean. However, identical fossils have been
discovered on South America and Africa
continental drift hypothesis – Pangea
- all the continents started as one supercontinent called Pangea and drifted to their
current positions, but Wagner couldn’t figure out how the continents were able to
move. He theorized that they bulldozed through the ocean. but his theory was not
widely accepted.
Paleomagnetism – characteristics of each and how relate to development of Plate Tectonics
Earth’s magnetic field
- functions very similar to a bar magnet, with a “positive” end and a “negative” end,
causes the magnet in compasses to align with magnetic north
Preservation in igneous rocks – Curie Point, etc.
-As igneous rocks cool, they will preserve the magnetic direction and field of
their location. It is set when the rock reaches a temperature of 585C, referred to as
the Curie Point. Prior to this temperature, any changes to the magnetic direction,
field, or location will also change in the rock, particularly true with ironcontaining minerals, especially the mineral magnetite. Since the minerals preserve the
magnetic direction and field during crystallization, they will show the location of the
Earth’s magnetic poles at that time. They will also show a dip based on the latitude
where they form so it is possible to use the crystals to determine the location of the
continent at that time.
Polar wander – showed similar to Pangea
- recently formed rocks pointed to the present magnetic north, while older rocks
pointed in other directions. By plotting these locations and “connecting the dots”,
APPARENT polar wander curves were discovered. (This would mean that the north
magnetic pole was wandering around the Earth over time, instead of remaining in a
relatively set location.) rocks of the same age, on different continents, had parallel
paths
Could mean one of two things: either there were two magnetic poles at these times
(and somehow only one now) or that the continents had moved from their original
positions. When the two curves were overlapped, the result showed a supercontinent
very similar to Pangaea.
Sea-floor spreading – Hess and Deitz
Ocean Ridges
- Using the presence of the linear ridges, Harry Hess proposed a theory for the driving force
behind continental drift. He suggested that new oceanic crust is being created at the midocean ridges. Since the Earth is not expanding, oceanic crust must also be destroyed
elsewhere on the planet. This occurred through subduction of the crust at the oceanic trenches.
Magnetometer and magnetic reversals
- The magnetometer was developed by the military in the1960’s to look for submarines. It is
essentially a big metal detector. The military failed to recognize that many of the rocks on the
ocean floor were composed of metals, just like the submarines, so the device ended up having
no military benefits and was turned over to scientists. (Imagine the test run of the device –
thinking that only submarines will show up on the results, someone flips the switch and
discovers readings coming from everywhere. They probably thought they were under attack.)
Geologists already knew about reversals in the Earth’s magnetic pole. Every-so-often, the
poles shift 180˚ from their current positions (North becomes South and South becomes
North). See figure below. Geologists know this happens, but have yet to determine why or
how. Some ages had already been determined using these reversals. (Incidentally, some
scientists believe that we might be overdue for a reversal and that it could occur at any time.
It is not know if this is a gradual switch or a sudden one, but it will definitely wreck havoc on
anyone using a compass when it happens.)
- Using magnetometers, geologists discovered symmetrical magnetic “stripes” on the seafloor.
Fred Vine and D. H. Matthews finally determined what these stripes meant. The stripes
alternated – one high-intensity with normal polarity (enhancing the Earth’s magnetic field)
and the next low-intensity with reverse polarity (weakening the Earth’s magnetic field).
Around the mid-oceanic ridges, the stripes formed identical patterns. Vine and Matthews
proposed that this proved Hess’s sea-floor spreading theory. New crust would form at the
ridge, preserving the Earth’s polarity at that time. As more crust formed, the older crust
would move away from the ridge on both sides (see figures below).
Boundaries – characteristics of each
Divergent – motion, characteristics, rock types, formation and triple junctions, rates, etc
“sea-floor spreading” areas defined by Hess and Dietz Another name for them is MidOceanic Ridges (MOR).
Plates are being pulled apart at these boundaries (tensional forces). This causes space to open
between them.
a. New oceanic crust is created at the oceanic ridge to fill the open space.
Most divergent boundaries are found in the ocean; one exception is Iceland. Iceland is a
divergent boundary that occurs on land rather than in the ocean. The island has a divergent
boundary running right through the middle of it. As a result, Iceland is growing in size as
new crust is created between the two pieces. This makes Iceland a very geologically active
location, marked by volcanism and faulting.
b. Characteristics of these boundaries
1. Rocks are basaltic in composition (oceanic crust).
2. Linear volcanoes can be found along the top of the ridge, creating the new crust.
3. Shallow earthquakes (<100 km deep) occur parallel to the ridge, due to the tensional
stresses pulling the rocks apart
4. Normal faults are associated with the ridges (see figure below – B). These are due to the
tensional stresses and the extension of the rocks as they move apart.
5. Spreading rates will vary from 1 to >9 centimeters per year. At slower rates, the ridges
and rift valley between them will be much more defined and rugged (similar to figure B
below). At faster rates, the topography will be much smoother, and typically the rift
valley will be absent.
6. As the rocks move away from the ridge and the heat source (pushed away by the creation
of new crust), the lithosphere will cool. This causes it to become denser, making it sink in
elevation (see figure C below). Rocks near the ridge are warm, young, thin, and higher in
elevation. Further away, they are older, colder, thicker, and lower in elevation.
c. Formation
initiate beneath continental lithosphere. Either the lithosphere thins from direct heat from
below, possibly from a Hot Spot, or thinning of the crust causes heat to rise due to decreased
pressure in the area. Both thinning and heat occur, it is just not known which comes first.
The heat causes the plate to upwarp, thin, and tear apart
(pushing your finger through bread dough – a bump forms, the dough gets progressively
thinner, before eventually tearing. Continental tearing will occur as triple junctions – three
cracks will form. Several triple junctions can be active at once, forming a linear trend. The
resulting long linear crack is believed to be the mechanism that causes the splitting of the
continent.
With triple junctions, all three parts will not remain active. Two of the cracks will form the
long linear crack that results in a new ocean basin as the continent splits. The failed third arm
will later form a passive margin, along which no geologic forces will be at work.
Currently, active triple junctions can be found in eastern Africa. The lower area, around Lake
Victoria, may open up into a new divergent margin, as the Somali subplate moves away from
the main African Plate, but geologists are still debating that. North of that area, the Red Sea
marks another active zone. The Arabian Peninsula is moving away from the African Plate in
an active divergent boundary. New crust has been created here for the last 5 million years.
Looking at the figure below, you can see evidence of the triple junction – Red Sea, Gulf of
Aden, and as the failed third arm, the Afar Lowlands.
Convergent – motion, which sinks
- plates are being pushed together (compressional forces), not every convergent boundary will
be the same. Characteristics will vary depending on the type of plates that are involved.
Oceanic plates are composed of basalt, a relatively dense rock. Continental plates, averaging
to granite, are less dense. These density differences play an important role in the type of
boundary.
Ocean-continent – location (EQ’s), volcanism, examples, accretionary wedge, etc
- Since oceanic crust is denser than continental crust, it will sink/get pulled down into the
Earth (see figure below).
1. The oceanic plate is subducted beneath the continental plate. This causes the oceanic
plate to be destroyed. A trench marks the point of subduction The location of the plate as
it descends into the Earth can be determined using seismic imaging and earthquakes. A
combination of shallow and deep earthquakes occurs along the junction of the two plates.
As the plate descends, the associated earthquakes will get progressively deeper moving
away from the trench. This can be seen in the figures below. By plotting depth versus
distance from the trench, geologists are able to track the location of the plate This is
referred to as the Wadati-Benioff zone
2. A volcanic/magmatic arc will form on the continental plate as the oceanic plate subducts
beneath it. The oceanic plate releases water and volatiles as it descends, causing the
surrounding asthenosphere to melt. This may even cause some melting of the over-riding
continental plate. The combination of mafic melt from the asthenosphere and felsic melt
from the continental plate will produce volcanoes of intermediate composition – andesite.
3. As the oceanic plate subducts, sediment will be scraped off of it, accumulating at the edge
of the trench. This is referred to as an accretionary wedge. The thick sediment deposit
will experience lots of folding and faulting due to the compressional forces acting on it.
EXAMPLES:
Cascades (Juan de Fuca Plate beneath the North America Plate)
Andes (Nasca Plate beneath the South American Plate)
Ocean-ocean – accretionary wedge, island arc, location (EQ’s), examples, etc
1.
- Since both plates are oceanic and dense, one will “lose” and is destroyed. A trench
marks the point of subduction.
2. As the plate subducts, sediment will be scraped off of it, accumulating at the edge of the
trench, forming an accretionary wedge. The thick sediment deposit will experience lots of
folding and faulting due to the compressional forces acting on it.
3. An island arc will form on the over-riding plate as the other plate subducts beneath it. (It
is referred to as an arc because it forms on a sphere – the Earth – and a flat layer cutting a
sphere will form a curved shape. Try slicing into an apple to see this.) The subducting
plate releases water and volatiles as it descends, causing the surrounding asthenosphere to
melt. This may even cause some melting of the over-riding plate. Even though both
compositions are generally basaltic, the composition of the volcanoes will be basaltic
andesite due to impurities and variations in composition.
4. The location of the plate as it descends into the Earth can be determined using seismic
imaging and earthquakes along the Wadati-Benioff zone. As with ocean-continent
convergence, a combination of shallow and deep earthquakes will mark the position of the
plate, with the deeper earthquakes occurring progressively further from the trench.
EXAMPLES:
Japan (Pacific Plate beneath the Eurasian Plate)
Philippines (Philippine Plate beneath the Eurasian Plate)
Continent-continent – mountains, suture zone, EQ’s, folds and faults, examples, etc
-Since both plates are low-density, neither is heavy enough to sink into the Earth. Instead,
they are crunched together and pushed up into mountain ranges
1. The oceanic plate attached to the continental plate is completely subducted before the
continent-continent convergence occurs. Once the oceanic plate is completely subducted,
volcanism will stop
2. Where the continental lithosphere meets, it will suture and weld together.
3. Movement does not stop once the plates first meet. Compressional forces will continue to
push the plates together. This causes the crust to try to “get out of the way” of each other
through faulting and folding. As a result, mountains will be continually pushed upwards
and the region will experience many tectonic events. The figure below shows the region
surrounding the Himalayas. Note how many faults have formed – these are all different
types of faults. The drawings to the right show what happened during an experiment
using a clay version of the boundary.
4. Earthquakes occur as energy is built up and released. These are usually shallow to
intermediate in depth and develop parallel to the suture zone. (It is because of India
pushing up into Asia that Pakistan, Afghanistan, and that region has so many
earthquakes.) Deep earthquakes will cease to occur once the originally attached oceanic
plate is completely destroyed.
5. Thick sedimentary basins form on both sides of the mountains due to active weathering
and erosion occurring in the mountain range.
6. Younger ranges will be higher in elevation and more jagged than older ones. Younger
ones may still be actively rising (such as the Himalayas which grow ~1 cm per year) and
older ones have experienced much more weathering and erosion (such as the Rocky
Mountains which are relatively young at 50-100 million years old or the Appalachian
Mountains which are old at over a billion years old).
EXAMPLE:
Himalayas (Indo-Australian Plate pushing into Eurasian Plate)
Transform – motion, linear valleys, EQ’s, connect different boundaries, examples, etc
1.
2.
3.
4.
-
At these boundaries, plates simply slide past each other (shear forces). As a result, crust is
neither created nor destroyed.
Friction causes intense, shallow earthquakes (think about the San Andreas Fault).
Motion along the fault will be roughly parallel to the plate motion.
Most transform boundaries connect sea-floor spreading ridges (divergent boundaries) on the sea
floor (see top figure below). However, transform boundaries can connect any type of boundary –
two ridges, a ridge and a subduction zone/trench, or two subduction zones/trenches (see lower
figures below).
The San Andreas Fault is one of the very few transform boundaries that occur along continental
lithosphere – typically these boundaries are oceanic. It connects spreading centers in the Gulf of
California to the Juan de Fuca Ridge subduction zone (see figure below).
Hot spots – definition, where occur, how related to Plate Tectonics
stationary, super-hot plumes of mantle material that rise to the surface to create a volcano, typically
NOT associated with plate boundaries, produced by irregularities in the boundary between the outer
core and the mantle, resulting volcanoes tend to be isolated and activity tends to be sporadic.
Very helpful in regards to Plate Tectonics. As the plate moves over the stationary hot spot, the
sporadic pulses of melt will form a line of volcanoes. By studying the ages and locations of the
volcanoes, it is possible to determine the motion and history of the plate.
Ex. by studying the Emporer Seamount-Hawaiian Islands volcanic chain, it can be determined that
the plate changed direction at one point, from a generally northern direction to a more northwestern
one
Mechanism
One aspect of Plate Tectonics that has yet to be determined is the mechanism that drives it – what
makes it happen? Three main theories:
1. Convection of heat in the mantle
Heat in the Earth moves in convective currents – heat rises up at divergent boundaries and
descends at convergent boundaries . This heat most likely originates from the core and
moves up towards the surface. There are different theories as to how convection works in
the Earth. However, none so far is without some problems and irregularities. Some
geologists think this heat flow is an effect of Plate Tectonics, not a cause.
2. Ridge (slab) push
the gravitational force of new cooling crust subsiding as it moves away from an elevated
ridge causes the plates to move, crust is pushed away from the ridge, relatively minor
effect.
3. Slab pull
the gravitational pull of plates being subducted into the Earth causes the plates to move
and the downgoing portion of the plate pulls the rest of the plate along with it. Seems to be
more of a factor than ridge push, but there is still some questions as to whether or not this
is enough energy to drive movement on such large plates.