Download Plate Tectonics and Continental Drift

Plate Tectonics and
Continental Drift
There are numerous ‘seams’ on the surface of the Earth
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?
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
Plate Tectonics
• Tectonics
– Movement of
Earth’s crust
• Plate tectonics
– Movement of
discrete
segments of
Earth’s crust in
relation to one
another
5
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
Continental drift maps by Wegner (1915)
Wegener’s
Pangea
Modern
reconstruction of
Pangea
Continental drift maps by Wegner (1915)
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
What is the evidence for
Continental Drift?
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
Paleontological evidence
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!!!!!!!!!
Rock type & structure evidence
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
Geologic Time Scale
Eon
Era
Period
Quaternary
C
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o
z
o
i
c
Next
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to recreate
this figure
P
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a
n
e
r
o
z
o
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c
0.01
1.8
Neogene
5.3
23.8
Tertiary
33.6
Paleocene
54.8
65
M
e
s
o
z
o
i
c
Cretaceous
144
Jurassic
Triassic
Permian
P
a
l
e
o
z
o
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c
Pennsylvanian
Mississippian
Devonian
Silurian
Ordivician
Cambrian
P
r
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c
a
m
b
r
i
a
n
Age (Myrs)
206
248
290
323
354
417
443
490
543
Proterozoic
2500
Archean
3800
Hadean
Age of the Earth 4600 Myrs (4.6 Byrs)
Source: Geological Society of America (1999)
Epoch
Holocene
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Reconstruction from glacial deposits
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
Fig. 17.6. Paleoclimate evidence
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
20
Reconstruction from paleomagnetic data
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
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
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
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
Crustal Properties
Crust
Density
continental ~2.8 g/cm3
oceanic
~3.2
g/cm3
Composition Thickness
Felsic
Thick:
20-70 km
Mafic
Thin:
2-10 km
Age
Old:
up to
4 Byrs
Young:
<200 Mys
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
Fig. 17.21. Models of plate tectonic motion
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
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
Fig. 17.10. Patterns of magnetic reversals
Age of the sea floor
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
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
Major tectonic boundaries
Divergent Plate Margins
• Oceanic-Oceanic Crust
• Mid-oceanic ridge with central rift
valley
• Shallow earthquakes, less than
100km
• Basaltic lavas
The Mid Atlantic
Ridge
Line of fissures and volcanoes in Iceland
Rates of Seafloor Spreading
FAST
SLOW
(East Pacific Rise)
(Mid Atlantic Ridge)
~10-20 cm/year
~1-2 cm/year
Life of a person
100 years
10 meters
1-2 meters
Civilization
10,000 years
1 km
100-200 m
Modern Humans
100,000 years
10 km
1-2 km
Stone tools
1,000,000 years
100 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.
East Pacific Rise
Fast spreading
Ridges subside and are covered with sediment
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
Ju an d eF uc aS mo k er. av i
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
East Africa
• East African system extends 3000
km from Ethiopia to Mozambique
– Red Sea rift extends from Ethiopia to the
Sinai and Dead Sea
– Complex volcanism throughout
– Oceanic crust present in SE Red Sea
– Lakes form in isolated down-dropped
blocks – Several areas below sea level
Fig. 19.33. The Red Sea
A transform fault in the Gulf of Aqaba
The parting of the Red Sea
The Red Sea
Fig. 17.15. Divergent plate margins
Convergent Plate Margins
• Oceanic-Oceanic
– Seafloor Trench
– Shallow and deep earthquakes, 0-700
km deep
– Andesitic volcanoes in an island arc
– Example: Japan
Ocean-Ocean convergence
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
The MarianaTrench
http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
Oceanic-Oceanic and Oceanic-Continental Subduction
Convergent Plate Margins
• Oceanic-Continental
– Subduction Zone
– Shallow and deep earthquakes, 0-700
km deep
– Andesitic volcanoes in a continental arc
– Example: Cascade range
Ocean-Continent convergence
Major tectonic boundaries
Mt. Vesuvius
The people of Pompeii; mummified in 5-8 meters of
hot ash in A.D. 79
The smoldering city of Pierre, Martinique
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
Continent-Continent Collision
Convergent plate boundaries
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
Fig. 19.29. Basin & Range Province
Juan de
Fuca plate
Fig. 20.12b. Landforms along the San Andreas
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
Fig. 17.20. Rates of plate motion around the world
Where do we see deep earthquakes? What is happening there?
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
Fig. 17.21. Models of plate tectonic motion