Powerpoint Presentation Physical Geology, 10/e

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Plate Tectonics

• Basic idea of plate tectonics -
  Earths surface is composed of a few
  large, thick plates of lithosphere that move
  slowly and change in size

2
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

3
Early Case for Continental Drift

• Puzzle-piece fit of coastlines of Africa and
  South America

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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

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Early Case for Continental Drift

• Glaciation patterns

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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

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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

8
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

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Paleomagnetism and Continental Drift Revived

• Rocks with increasing age point to pole locations
  increasingly far from present magnetic pole
  positions

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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

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Seafloor Spreading

• In 1962, Harry Hess proposed seafloor spreading
• Seafloor moves away from the mid-oceanic 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

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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
  lt200 million years) is explained by this model

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Plates and Plate Motion

• Tectonic plates are composed of
  the 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

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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

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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

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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

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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

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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

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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

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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 ridge-push and/or
  slab-pull

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Mantle Plumes and Hot Spots

• Mantle plumes narrow, rising columns of hot
  mantle
• Stationary with respect to moving plates

22
Mantle Plumes and Hot Spots

• Mantle plumes may form hot spots of active
  volcanism at Earths surface
• Approximately 45 known hotspots

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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

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Mountain Belts and the Continental
CrustPhysical Geology 12/e, Chapter 20
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Introduction Mountain Belts and Earths Systems

• Major controlling factors during a mountain
  belts history
• 1. Intense deformation
• Mainly compression
• Folds faults
• Foliation metamorphism
• Orogeny is an episode of intense deformation
• 2.Isostasy
• Vertical movement before after an orogeny
• Continental crust floats on mantle
• 3. Weathering erosion
• Depends on climate, rock type, elevation, etc.

Major Mountain Belts
Andes
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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 dont.
• 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

The mountain belts and craton of North America
Schematic cross section through part of a
mountain belt (left) and part of the continental
interior (craton)
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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
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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

Recumbent folds in the Andes
False-color satellite image of part of the Valley
and Ridge province of the Appalachian mountain
belt, near Harrisburg, Pennsylvania
Cross section of an Andean type mountain belt
(oceanic-continental convergence)
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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
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Rock Patterns in Mountain Belts

• Horsts and grabens can produce mountains and
  valleys

Fault-block mountains with movement along normal
faults
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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
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Evolution of Mountain BeltsOrogenies Plate
Convergence

• Orogenies Ocean-Continent Convergence
• Accretionary wedge
• Igneous and metamorphic processes
• Fold thrust belts on craton (backarc side)
• Gravitational collapse spreading

Cross section of an Andean type mountain belt
(oceanic-continental convergence)
Schematic cross section of a mountain belt in
which gravitational collapse and spreading are
taking place during plate convergence
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Evolution of Mountain BeltsOrogenies Plate
Convergence

• Orogenies and Continent-Continent 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

34
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
35
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 strain
• Isostatic vertical adjustment
• Bounded on both sides by normal faults or tilted
  fault blocks

Development of fault-block mountain ranges
The Teton Range, Wyoming, a tilted fault-block
range
36
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
37
Growth of Continents

• Continents grow larger as mountain belts evolve
  along their margins
• Accumulation and igneous activity add new
  continental crust

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Geologic ResourcesPhysical Geology, Chapter 21
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Energy ResourcesCoal

• Photosynthesis
• CO2 H2O ? CH2O O2
• Peat
• Lignite (brown coal)
• Subituminous coal
• Bituminous coal (soft coal)
• Anthacite (hard coal)

Layer of peat being cut dried for fuel on the
island of Mull, Scotland
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Energy ResourcesCoal

• BTU British Thermal Units
• Amount of heat energy to raise one pound of water
  from 62 to 63F
• 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

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Energy ResourcesPetroleum 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), 82C (180F)
• Crack into petroleum
• Deeper burial
• 4,600 meters (15,000 ft)
• Natural gas

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Energy ResourcesPetroleum 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

43
Energy ResourcesPetroleum 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
44
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

45
Energy Resources(Other Energy Sources)

• Uranium
• 10 energy for US
• Geothermal
• Renewable Energy Sources
• Solar
• Wind
• Wave (tidal)
• Hydroelectric

46
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

47
Metallic ResourcesOres 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) dont 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

48
Metallic ResourcesOres 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
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Metallic ResourcesOres 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

50
Metallic ResourcesOres 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