Title: Powerpoint Presentation Physical Geology, 10/e
1Plate 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
2Plate 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
3Early Case for Continental Drift
- Puzzle-piece fit of coastlines of Africa and
South America
4Early 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
5Early Case for Continental Drift
6Early 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
7Early 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
8Paleomagnetism 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
9Paleomagnetism and Continental Drift Revived
- Rocks with increasing age point to pole locations
increasingly far from present magnetic pole
positions
10Paleomagnetism 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
11Seafloor 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
12Seafloor 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
13Plates 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
14Evidence 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
15Evidence 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
16Divergent 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
17Transform 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
18Convergent 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
19Movement 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
20What 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
21Mantle Plumes and Hot Spots
- Mantle plumes narrow, rising columns of hot
mantle - Stationary with respect to moving plates
22Mantle Plumes and Hot Spots
- Mantle plumes may form hot spots of active
volcanism at Earths surface - Approximately 45 known hotspots
23Mantle 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
24Mountain Belts and the Continental
CrustPhysical Geology 12/e, Chapter 20
25Introduction 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
26Characteristics 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)
27Characteristics 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
28Rock 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)
29Rock 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
30Rock Patterns in Mountain Belts
- Horsts and grabens can produce mountains and
valleys
Fault-block mountains with movement along normal
faults
31Evolution 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
32Evolution 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
33Evolution 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
34Evolution 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
35Evolution 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
36Evolution 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
37Growth of Continents
- Continents grow larger as mountain belts evolve
along their margins - Accumulation and igneous activity add new
continental crust
38Geologic ResourcesPhysical Geology, Chapter 21
39Energy 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
40Energy 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
41Energy 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
42Energy 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
43Energy 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
44Energy 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
45Energy Resources(Other Energy Sources)
- Uranium
- 10 energy for US
- Geothermal
- Renewable Energy Sources
- Solar
- Wind
- Wave (tidal)
- Hydroelectric
46Metallic 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
47Metallic 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
48Metallic 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
49Metallic 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
50Metallic 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