plate tectonics and continental drift - montana state ... 2009/suits/geo 101... · what are the...
TRANSCRIPT
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Plate Tectonics and Continental Drift
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There are numerous ‘seams’ on the surface of the Earth
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Questions and Topics
1. What are the theories of Plate Tectonics and Continental Drift?
2. What is the evidence that Continents move?
3. What are the forces that drive plate tectonics?
4. What happens at the boundaries between plates?
5. How do the different types of plate boundaries impact the regional geology and geomorphology?
6. How has continental drift affected the positions of the continents over time?
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Answers
1. Large crustal plates at the Earth’s surface move about, colliding with one another.
2. There is geographic, geomagnetic, paleontologic and other evidence that this occurs
3. Convection in the mantle is the main driver of plate movement
4. Neighboring plates move relative to one another, causing earthquakes and volcanic eruptions
5. Active plate boundaries produce mountains and trenches
6. Continents have changed position
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Plate Tectonics• Tectonics
– Movement of
Earth’s crust
• Plate tectonics
– Movement of
discrete
segments of
Earth’s crust in
relation to one
another
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Continental Drift
• Alfred Wegener (1880-1930)
– Proposed that all of the continents were once part of a large supercontinent -Pangaea
– Based on:
• Similarities in shorelines
• Distinctive rock and fossil groups found in Africa & South America
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Continental drift maps by Wegner (1915)
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Continental drift maps by Wegner (1915)
Wegener’s
PangeaModern
reconstruction of
Pangea
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Continental Drift
• Wegner mechanism for drift was not
credible
– Less dense silicic rocks (the continents)
plowed through more dense ocean floor
– Earth’s rotation was driving force
• Other scientists didn’t buy it
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Evidence for Continental Drift
• Paleontological
– Similarity of fossils on opposite sides of
the Atlantic Ocean
• Plants and land dwelling animals
• No mechanism to transport across ocean
• Glossopteris on all southern continents
• Divergence of species following break-up
What is the evidence for
Continental Drift?
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Paleontological evidence
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Evidence for Continental Drift
• Rock type & structures
– Distinct and similar rock types and
geologic structures on both sides of the
Atlantic Ocean
• Cape fold belt and equivalent – S.Africa &
Argentina
• Appalachian Mtns and equivalent – U.S.,
Canada, Scotland & Norway
• Only occur in rocks > 145 mya!!!!!!!!!
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Rock type & structure evidence
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Evidence for Continental Drift
• Glaciation
– Late Paleozoic glaciation
• Covered large portions of the southern continents
• Distinct glacial deposit
• No evidence for glaciation on northern continents at this time
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Era Age (Myrs) Epoch
0.01Holocene
1.8Pleistocene
5.3Pliocene
23.8Miocene
33.6Oligocene
54.8Eocene
65Paleocene
144
206
248
290
323
354
417
443
490
543
2500
3800
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Eon
Proterozoic
Archean
Hadean
Period
Quaternary
Tertiary
Neogene
Paleocene
Mississippian
C
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Cretaceous
Jurassic
Age of the Earth 4600 Myrs (4.6 Byrs)Source: Geological Society of America (1999)
Geologic Time Scale
Devonian
Silurian
Ordivician
Cambrian
Triassic
Permian
Pennsylvanian
Next
homework is
to recreate
this figure
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Reconstruction from glacial deposits
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Evidence for Continental Drift
• Paleoclimate
– Evidence of extreme changes in climate
as compared to the present
• Coal deposits in Antarctica
• Evidence from evaporite deposits, eolian
deposits & coral reefs
• Paleoclimate reconstruction shows strange
patterns unless continents are moved
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Fig. 17.6. Paleoclimate evidence
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20
Paleomagnetism• Magnetization of
ancient rocks at the
time of their formation
• Declination
– Angle that a compass
needle makes with the
line running to the
geographic north pole
• Rocks lock in this
orientation at formation
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Reconstruction from paleomagnetic data
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Geology of the Ocean Floor
• Paleomagnetism
– Fe rich rocks are weakly magnetized by
the Earth’s magnetic field as minerals
form
– Orientation of magnetic field is
preserved
– Magnetic field orientation varies with
position on Earth’s surface
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Modern Plate Tectonic Theory
• Original evidence for continental drift was from continental rocks
• Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor
• Geophysics & paleomagnetism provided new data
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Geology of the Ocean Floor
• Topography of the ocean basins
– Basins are divided by a large ridge
system, which is continuous around the
entire globe
– Central rift valley within the ridge
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Geology of the Ocean Floor
• Physical properties
– Composed of basalt
– Younger in age than most continental
rocks
– Oceanic crust is thinner than continental
– No evidence of crustal deformation –
folded mountains
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Crustal Properties
Crust Density Composition Thickness Age
continental ~2.8 g/cm3 FelsicThick:
20-70 km
Old:up to
4 Byrs
oceanic ~3.2 g/cm3 MaficThin:
2-10 km
Young:
<200 Mys
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Geology of the Ocean Floor
• Seafloor spreading proposed by Hess (1960)
– Considered new data on ocean floor
– Proposed mechanisms of:
• Mantle convection
• Rifting and volcanism along ridge system
• Continents pushed along w/ spreading seafloor
• Recycling of oceanic crust by subduction
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Fig. 17.21. Models of plate tectonic motion
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Geology of the Ocean Floor
• Vine & Matthews (1963) tested
Hess’s hypothesis using
magnetism
– Magnetic polarity reversals recorded
in ocean floor basalt
• Magma cools forming new crust
• Polarity at time of cooling preserved
• Old crust pushed aside
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Geology of the Ocean Floor
• Magnetic polarity stripes in ocean
crust parallel ridges
– Symmetrical on either side of the ridge
– Polarity chrons give age of seafloor
• Increases away from ridge
• Rates of plate motion may be calculated
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Fig. 17.10. Patterns of magnetic reversals
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Age of the sea floor
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Geology of the Ocean Floor
• Seafloor sediments support plate
tectonic theory
– Youngest sediments resting directly on
basalt near the ridge
– Sediment just above the basalt gets
older moving away from the ridge
– Accumulation rates of ~3 mm/1000 yr
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Plate Geography
• Lithosphere is divided into individual
plates
– Boundaries based on structural
features, not land and ocean
– Plates are outlined by ridges, trenches
and young mountain belts
– Plates are not permanent features
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Major tectonic boundaries
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Divergent Plate Margins
• Oceanic-Oceanic Crust
• Mid-oceanic ridge with central rift
valley
• Shallow earthquakes, less than
100km
• Basaltic lavas
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Fig. 17.15. Divergent plate margins
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The Mid Atlantic
Ridge
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Passive Continental Margin
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Size comparison of various volcanic features
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Divergent Plate Margins
• Continental-Continental Crust
– Rift Valley
– Shallow earthquakes, less than 100km
– Basaltic and Rhyolitic volcanism
• New material rising from the mantle produces basaltic lavas
• Thinning continental crust melts to produce rhyolitic lavas & instrusions
• Example: East African Rift Valley
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Convergent Plate Margins
• Oceanic-Oceanic
– Seafloor Trench
– Shallow and deep earthquakes, 0-700
km deep
– Andesitic volcanoes in an island arc
– Example: Japan
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The Aleutian Island Chain
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Seismic activity in the Aleutian Islands
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Oceanic-Oceanic and Oceanic-Continental Subduction
Zones
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Convergent Plate Margins
• Oceanic-Continental
– Subduction Zone
– Shallow and deep earthquakes, 0-700
km deep
– Andesitic volcanoes in a continental arc
– Example: Cascade range
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Convergent Plate Margins
• Continental-Continental
– Intensely folded and thrust faulted mountain belts
– Metamorphic rocks dominate
• Sediments accumulated along continental margin are squeezed
– Igneous rocks commonly included
• Granitic magmas
• Example: The Himalayas
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Convergent plate boundaries
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Transform Fault Margins
• Transform faults are large vertical fractures or faults in the crust
– Movement along faults is side to side
– May extend for long distances
– Example: San Andreas fault and transform faults in the ocean
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Juan de
Fuca plate
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Rates of Seafloor Spreading
FAST
(East Pacific Rise)
~10-20 cm/year
SLOW
(Mid Atlantic Ridge)
~1-2 cm/year
Life of a person 100 years
Civilization 10,000 years
Stone tools 1,000,000 years
Modern Humans 100,000 years
10 meters
1 km
10 km
100 km
1-2 meters
100-200 m
1-2 km
10-20 km
Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide.How long would it take to create this much ocean crust.
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Rates of Plate Motion
• Two ways to look at plate motion
– Relative velocity – the movement of
one plate relative to another
• Age of seafloor / distance from ridge
– Absolute velocity – compares plate
movement to a fixed position
• Use hotspots as fixed points of
reference
• Rates vary from 1 to 20 cm/yr
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Fig. 17.20. Rates of plate motion around the world
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Where do we see deep earthquakes? What is happening there?
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Tectonic Mechanisms
• Convection of heat from the core and
mantle drives tectonics
– Convection cells bring new material to
the surface
– Old crust is pushed away from ridges
– Subduction carries cool crust back into
the mantle
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Fig. 17.21. Models of plate tectonic motion
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Tectonic Mechanisms
• Plates are active participants in the convection process
– Slab pull – dense ocean crust descends under its own weight
– Ridge push – gravity pulls lithosphere down & away from ridge
– Friction – resistance to movement from various sources
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Mantle Plumes and Hot Spots
• Mantle plumes may form “hot spots”
of active volcanism at Earth’s surface
– Approximately 45 known hotspots
• Hot spots in the interior of a plate
produce volcanic chains
– 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 are a good example
More evidence….
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Fig. 22.1. Hawaiian Island chain
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The World’s Hot Spots
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Fig. 22.21. Cenozoic features of NW U.S.
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81
Plate Motion• GPS
– Global Positioning
System
– Earth-orbiting
satellites identify
motion
• Transmitter on
satellite
• Ground-based
receiver
• Average rate
– 5 cm/year
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Basa
lts
(Oph
iolite
s)
Marine
sedim
ent
s (c
hert
s, lim
est
ones,
re
d c
lays)
Tur
bidites,
clays,
silts,
sand
s
Gra
nite
s, R
hyolite a
nd
pyro
clast
ics
Lava
s and
pyro
clast
ics
Ro
ck
/se
dim
en
t ty
pe
Te
cto
nic
se
ttin
gMafic Felsic
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Composition of the Ocean Crust
• Seismic surveys suggest oceanic crust is ~7
km thick and comprised of three layers
– First layer is marine sediment of various
composition and thickness (extensively sampled)
– Second layer is pillow basalt overlying basaltic
dikes (extensively sampled)
– Third layer is thought to be composed of sill-like
gabbro intrusions (not directly sampled)
• Ophiolites are rock sequences in mountain
chains on land that are thought to represent
slivers of ocean crust and uppermost mantle
– Composed of layers 1-3 overlying ultramafic rock
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JuandeFucaSmoker.av i
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Smoker.mov
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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http://www.scotese.com/earth.htm
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Rates of Seafloor Spreading
FAST
(East Pacific Rise)
~10-20 cm/year
SLOW
(Mid Atlantic Ridge)
~1-2 cm/year
Life of a person 100 years
Civilization 10,000 years
Stone tools 1,000,000 years
Modern Humans 100,000 years
10 meters
1 km
10 km
100 km
1-2 meters
100-200 m
1-2 km
10-20 km
Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide.How long would it take to create this much ocean crust.
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Rock types and tectonic setting
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Ocean-Ocean convergence
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Ocean-Continent convergence
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Continent-Continent Collision
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Fig. 21.13. Structure of western NA
Oregon/Washington Idaho Montana
Cascades/Olympics Rockies
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