Plate Tectonics
• Basic idea of plate tectonics Earth’s surface is composed of a few large, thick plates
of lithosphere that move slowly and change in size
Plate Tectonics
• Intense geologic activity is concentrated at
plate boundaries where plates move away,
toward, or past each other
• Combination of continental drift and seafloor
spreading hypotheses in late 1960s
Early Case for Continental Drift
• Puzzle-piece fit of coastlines of Africa and South America
Early Case for Continental Drift
• In early 1900s, Alfred Wegener noted South America, Africa,
India, Antarctica, and Australia have almost identical late
Paleozoic rocks and fossils
• Wegener reassembled continents into the supercontinent Pangaea
Early Case for Continental Drift
• Glaciation patterns
Early Case for Continental Drift
• Pangea initially separated into
Laurasia and Gondwanaland
– Laurasia - northern supercontinent
containing North America and Asia
(excluding India)
– Gondwanaland - southern supercontinent
containing South America, Africa, India,
Antarctica, and Australia
• Late Paleozoic glaciation patterns on
southern continents best explained by
their reconstruction into
Gondwanaland
Early Case for Continental Drift
• Coal beds of North America and Europe
support reconstruction into Laurasia
• Reconstructed paleoclimate belts
suggested polar wandering, potential
evidence for Continental Drift
• Continental Drift hypothesis initially
rejected
– Wegener could not come up with viable
driving force
– continents should not be able to “plow
through” sea floor rocks while crumpling
themselves but not the sea floor
Paleomagnetism and
Continental Drift Revived
• Studies of rock magnetism allowed
determination of magnetic pole locations
(close to geographic poles) through time
• Paleomagnetism uses mineral magnetic
alignment direction and dip angle
“frozen” into the rocks to determine the
direction and distance to the magnetic
pole when rocks formed
– Steeper dip angles indicate rocks formed
closer to the magnetic poles
Paleomagnetism and
Continental Drift Revived
• Rocks with increasing age point to pole locations
increasingly far from present magnetic pole positions
Paleomagnetism and
Continental Drift Revived
• Apparent polar wander curves for
different continents suggest real
movement relative to one another
• Reconstruction of supercontinents
using paleomagnetic information
fits Africa and South America like
puzzle pieces
– Improved fit results in rock units (and
glacial ice flow directions) precisely
matching up across continent margins
Seafloor Spreading
• In 1962, Harry Hess proposed
seafloor spreading
– Seafloor moves away from the midoceanic ridge due to mantle convection
– Convection is circulation driven by
rising hot material and/or sinking
cooler material
• Hot mantle rock rises under
mid-oceanic ridge
– Ridge elevation, high heat flow,
and abundant basaltic volcanism
are evidence of this
Seafloor Spreading
• Seafloor rocks, and mantle rocks beneath them, cool and become
more dense with distance from mid-oceanic ridge
• When sufficiently cool and dense, these rocks may sink back into
the mantle at subduction zones
– Downward plunge of cold rocks gives rise to oceanic trenches
• Overall young age for sea floor rocks (everywhere <200 million
years) is explained by this model
Plates and Plate Motion
• Tectonic plates are composed of
relatively rigid lithosphere
– Lithospheric thickness and age of
seafloor increase with distance
from mid-oceanic ridge
• Plates “float” upon ductile asthenosphere
• Plates interact at their boundaries, which are
classified by relative plate motion
– Plates move apart at divergent boundaries, together at
convergent boundaries, and slide past one another at
transform boundaries
the
Evidence of Plate Motion
• Seafloor age increases with distance
from mid-oceanic ridge
– Rate of plate motion equals distance
from ridge divided by age of rocks
– Symmetric age pattern reflects plate
motion away from ridge
Evidence of Plate Motion
• Mid-oceanic ridges are offset
along fracture zones
– Fracture zone segment between offset ridge
crests is a transform fault
– Relative motion along fault is result of
seafloor spreading from adjacent ridges
• Plate motion can be measured
using satellites, radar, lasers and
global positioning systems
– Measurements accurate to within 1 cm
– Motion rates closely match those predicted
using seafloor magnetic anomalies
Divergent Plate Boundaries
• At divergent plate boundaries, plates
move away from each other
– Can occur in the middle of the ocean
or within a continent
– Divergent motion eventually creates a
new ocean basin
• Marked by rifting, basaltic
volcanism, and eventual ridge uplift
– During rifting, crust is stretched and thinned
– Graben valleys mark rift zones
– Volcanism common as magma rises through
thinner crust along normal faults
– Ridge uplift by thermal expansion of hot rock
Transform Plate Boundaries
• At transform plate boundaries, plates
slide horizontally past one another
– Marked by transform faults
– Transform faults may connect:
• Two offset segments of mid-oceanic ridge
• A mid-oceanic ridge and a trench
• Two trenches
– Transform offsets of mid-oceanic ridges
allow series of straight-line segments to
approximate curved boundaries required
by spheroidal Earth
Convergent Plate Boundaries
• At convergent plate boundaries, plates
move toward one another
• Nature of boundary depends on plates
involved (oceanic vs. continental)
– Ocean-ocean plate convergence
• Marked by ocean trench, Benioff zone, and
volcanic island arc
– Ocean-continent plate convergence
• Marked by ocean trench, Benioff zone,
volcanic arc, and mountain belt
– Continent-Continent plate convergence
• Marked by mountain belts and thrust faults
Movement of Plate Boundaries
• Plate boundaries can move over time
– Mid-oceanic ridge crests can migrate
toward or away from subduction zones
or abruptly jump to new positions
– Convergent boundaries can migrate if
subduction angle steepens or overlying
plate has a trenchward motion of its own
• Back-arc spreading may occur, but is
poorly understood
– Transform boundaries can shift as slivers
of plate shear off
• San Andreas fault shifted eastward about five
million years ago and may do so again
What Causes Plate Motions?
• Causes of plate motion are not yet fully
understood, but any proposed
mechanism must explain why:
– Mid-oceanic ridges are hot and elevated,
while trenches are cold and deep
– Ridge crests have tensional cracks
– The leading edges of some plates are
subducting sea floor, while others are
continents (which cannot subduct)
• Mantle convection may be the cause or
an effect of circulation set up by ridgepush and/or slab-pull
Mantle Plumes and Hot Spots
• Mantle plumes – narrow, rising columns of hot mantle
• Stationary with respect to moving plates
Mantle Plumes and Hot Spots
• Mantle plumes may form “hot spots” of active
volcanism at Earth’s surface
– Approximately 45 known hotspots
Mantle Plumes and Hot Spots
• Hot spots in the interior of a
plate produce chains of
volcanoes
– Orientation of the volcanic
chain shows direction of plate
motion over time
– Age of volcanic rocks can be
used to determine rate of plate
movement
– Hawaiian islands
Mountain Belts and the
Continental Crust
Physical Geology 12/e, Chapter 20
Introduction: Mountain Belts and
Earth’s Systems
• Major controlling factors during a
mountain belt’s history
• 1. Intense deformation
•
•
•
•
Mainly compression
Folds & faults
Foliation & metamorphism
Orogeny is an episode of intense deformation
Major Mountain Belts
• 2.Isostasy
• Vertical movement before & after an orogeny
• Continental crust “floats” on mantle
• 3. Weathering & erosion
• Depends on climate, rock type, elevation, etc.
Andes
Characteristics of Mountain Belts
• Mountain belts are very long compared to
their width
– The North American Cordillera runs from
southwestern Alaska down to Panama
• Mountain belts in North America tend to
parallel coast lines. Others, e.g. Himalayas
don’t.
• Older mountain ranges (Appalachians) tend
to be lower than younger ones (Himalayas)
due to erosion
– Young mountain belts are tens of millions of
years old, whereas older ones may be hundreds
of millions of years old
Schematic cross section through
part of a mountain belt (left) and
part of the continental interior
(craton)
The mountain belts and craton
of North America
Characteristics of Mountain Belts
• Ancient mountain belts (billions of
years old) have eroded nearly flat to
form the stable cores (cratons) of the
continents
– Shields - areas of cratons laid bare by
erosion
Schematic cross section through part of a
mountain belt (left) and part of the continental
interior (craton)
Satellite image of part of a
craton in Western Australia
Rock Patterns in Mountain Belts
• Fold and thrust belts (composed of
many folds and reverse faults) indicate
crustal shortening (and thickening)
produced by compression
– Common at convergent boundaries
– Typically contain large amounts of
metamorphic rock
False-color satellite image of
part of the Valley and Ridge
province of the Appalachian
mountain belt, near
Harrisburg, Pennsylvania
Recumbent folds in the
Andes
Cross section of an
“Andean type” mountain
belt (oceanic-continental
convergence)
Rock Patterns in Mountain Belts
• Erosion-resistant batholiths may be left behind as
mountain ranges after long periods of erosion
• Localized tension in uplifting mountain belts can
result in normal faulting as a result of vertical
uplift or horizontal
Schematic cross section of
a mountain belt in which
gravitational collapse and
spreading are taking
place during plate
convergence
Rock Patterns in Mountain Belts
– Horsts and grabens can produce mountains and valleys
Fault-block
mountains with
movement along
normal faults
Evolution of Mountain Belts
Orogenies & Plate Convergence
• Mountains are uplifted at convergent
boundaries during the orogenic stage
– Result of ocean-continent, arc-continent, or
continent-continent convergence
– Subsequent gravitational collapse and spreading
may bring deep-seated rocks to the surface
Schematic cross section
of a mountain belt in
which gravitational
collapse and spreading
are taking place during
plate convergence
Evolution of Mountain Belts
Orogenies & Plate Convergence
• Orogenies & Ocean-Continent Convergence
–
–
–
–
Cross section of an
“Andean type” mountain
belt (oceanic-continental
convergence)
Accretionary wedge
Igneous and metamorphic processes
Fold & thrust belts on craton (backarc side)
Gravitational collapse & spreading
Schematic cross section of a
mountain belt in which
gravitational collapse and
spreading are taking place
during plate convergence
Evolution of Mountain Belts
Orogenies & Plate Convergence
• Orogenies and ContinentContinent Convergence
–
–
–
–
–
Figure 20.13 (Alps)
Figure 20.14 (Himalayas)
Continent crust too buoyant to subduct
Suture zone
Appalachian mountains (Alleghenian
Orogeny)
– Wilson Cycle is the opening & closing
of ocean basins and continental
collisions
Evolution of Mountain Belts
Post-Orogenic Uplift & Block Faulting
• After convergence stops, a long
period of erosion, uplift and
block-faulting occurs
– As erosion removes overlying
rock, the crustal root of a
mountain range rises by isostatic
adjustment
Isostasy in a mountain belt
Development of fault-block mountain ranges
Evolution of Mountain Belts
Post-Orogenic Uplift & Block Faulting
– Tension in uplifting and
spreading crust results
in normal faulting and
fault-block mountain
ranges
– Horizontal extensional
Development of fault-block mountain
strain
ranges
– Isostatic vertical
adjustment
– Bounded on both sides
by normal faults or
tilted fault blocks The Teton Range, Wyoming, a tilted fault-block range
Evolution of Mountain Belts
Post-Orogenic Uplift & Block Faulting
• Basin-and-Range province
of western North America
may be the result of
delamination
– Overthickened mantle lithosphere
beneath old mountain belt may
detach and sink into asthenosphere
– Resulting inflow of hot
asthenosphere can stretch and thin
overlying crust, producing normal
faults
Upwelling, hot,
buoyant mantle
(asthenosphere)
causes extension,
thinning, and
block-faulting of
the overlying
crust
Delamination and
thinning of
continental crust
following orogeny
Growth of Continents
• Continents grow larger as
mountain belts evolve along their
margins
– Accumulation and igneous activity
add new continental crust
Geologic Resources
Physical Geology, Chapter 21
Energy Resources
Coal
• Photosynthesis
– CO2 + H2O → CH2O + O2
•
•
•
•
•
Layer of peat being cut & dried for
fuel on the island of Mull, Scotland
Peat
Lignite (brown coal)
Subituminous coal
Bituminous coal (soft coal)
Anthacite (hard coal)
Energy Resources
Coal
• BTU: British Thermal Units
– Amount of heat energy to raise one
pound of water from 62° to 63°F
• Strip mining
• Shaft & tunnel mining
• Resource: total amount of any
geological material of potential
economic interest
– Size of nonrenewable resource is
fixed and theoretically determinable
• Reserve: that portion of a
resource discovered and
economically & legally
extractable
– Size can change in time
Energy Resources
Petroleum and Natural Gas
• Petroleum (oil)
• Nutrient desert versus nutrient trap
– Large rivers, less-evaporative climate
• Buried hydrocarbons heated to break down (crack)
– Anoxic environment
– Isostatic subsidence
– Sapropel
• Burial
– 2,300 meters (7,400 ft), 82°C (180°F)
– Crack into petroleum
• Deeper burial
– 4,600 meters (15,000 ft)
– Natural gas
Energy Resources
Petroleum and Natural Gas
• Exploration
• Source rock
• Original sapropel
• Reservoir rock
• Usually sandstone or limestone
• Permeable and porous
• Structural (oil) trap
• Anticline, pinchout, fault, unconformity, patch reef,
sandstone lens, salt dome
• Cap (trap) rock
• Impermeable rock prevents further upward migration
Energy Resources
Petroleum and Natural Gas
• Oil reservoir
• Fluid pressure
• Secondary recovery
methods
• Energy return on
energy invested
(EROEI)
• Oil field: Regions
underlain by one or
more oil reservoirs
Major oil fields of North
America
Energy Resources
(Other Sources of Hydrocarbons)
• Coal bed methane
– Problem with salt water contamination
– 700 tcf US (100 tcf recoverable)
• Heavy crude & oil (tar) sands
– Heavy crude is dense, viscous petroleum, uneconomical
– Oil (tar) sands are asphalt cemented sand or sandstone deposits
• Oil shale
– Black/brown shale with high content of organic matter
– Extracted by distillation
– Green River Formation, 300-600 billion barrels recoverable
Energy Resources
(Other Energy Sources)
• Uranium
– 10% energy for US
• Geothermal
• Renewable Energy Sources
–
–
–
–
Solar
Wind
Wave (tidal)
Hydroelectric
Metallic Resources
• Ore: Naturally occurring material that can
be profitably mined
• Types of ore deposits
–
–
–
–
–
–
Crystal settling within cooling magma
Hydrothermal deposits
Pegmatites
Chemical precipitation as sediment
Placer deposits
Concentration by weathering and ground water
Metallic Resources
Ores Formed by Igneous
Processes
• Crystal Settling: Early-forming minerals
crystallize & settle to the bottom of a cooling body
of magma (differentiation)
• Hydrothermal Fluids
– Most important source of metallic ore (except Fe & Al)
– Hot water & other fluids injected into country rock during last
stages of magma crystalliztion
– Atoms of Au & Cu (for example) don’t fit into crystals in cooling
pluton concentrate in water-rich magma which is injected along
with quartz into the country rock
– Most are metallic sulfides mixed with milky quartz
•
Metallic Resources
Ores Formed by Igneous
Processes
Four types of
Hydrothermal ore
deposits
– (1) Contact metamorphic
deposits
• Iron, tungsten, copper, lead,
zinc, & silver
• Country rock may be
completely or partially
removed
– (2) Hydrothermal veins:
narrow ore bodies formed
along joints & faults
• Lead, zinc, gold, silver,
tungsten, tin, mercury, and
copper
Hydrothermal quartz veins in
granite
Metallic Resources
Ores Formed by Igneous
Processes
• Four types of Hydrothermal ore deposits
– (3) Disseminated deposits: metallic sulfide ore
minerals are distributed in very low concentration
through large volumes of rock above & within a pluton
• Copper, lead, zinc, molybdenum, silver & gold
– (4) Hot-spring deposits
– Pegmatites
Metallic Resources
Ores Formed by Surface Processes
• Chemical precipitation in layers
– Iron & manganese some copper
– Banded iron ores
• Placer deposits
– Streams concentrated heavy
sediment grains in a river, waves
on a beach
– Gold, platinum, diamonds,
titanium, & tin
• Concentration by weathering
– Aluminum (bauxite)
– Supergene enrichment of
disseminated ore deposits
2,250 million year old banded iron ore