Download Plate Tectonics and Continental Drift

Plate Tectonics and
Continental Drift
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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?
Plate Tectonics
• Tectonics
– Movement of
Earth’s crust
• Plate tectonics
– Movement of
discrete
segments of
Earth’s crust in
relation to one
another
4
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)
Continental drift maps by Wegner (1915)
Continental Drift
• Wegner proposed a mechanism for
drift
– Less dense silicic rocks plowed through
more dense ocean floor
– Earth’s rotation was driving force
• Although supporting evidence
existed, the theory was not widely
accepted
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
Continental Drift
• DuToit’s evidence
– Expanded
Wegener’s ideas
– Mesosaurus fossils
• Found on
Gondwana
continents
• Freshwater
– Could not swim
across Atlantic
11
Continental Drift
• Glossopteris flora
present only in
southern
hemisphere
continents
12
Additional Evidence
• Lystrosaurus
– Heavyset
herbivore
– Found in
Antarctica
• 1969
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Evidence for Continental Drift
• Rock type & structures
– Distinct rock type 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
Continental Drift
• Geologic
Similarities
– Brazil and South
Africa have nearly
identical geologic
sequences
• Similar in
Antarctica and
India
– Glacial sediments
– Coal
15
Rock type & structure evidence
Evidence for Continental Drift
• Glaciation
– Late Paleozoic glaciation
• Covered large portions of the southern
continents
• Distinct glacial deposit
• Glacial striations indicate direction of
movement
• No evidence for glaciation on northern
continents at this time
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
• Apparent Polar
Wander
– First studies
indicated poles
had moved
– Instead, plates
had moved
– North American
and European
paths met
23
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
– Ridge system 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
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
30
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
Geology of the Ocean Floor
• Polar wandering
– Earth’s north magnetic pole was shown
to have moved through time
• Systematic change in position
– Polar wandering paths varied by
continent
– Multiple magnetic poles are not possible
Reconstruction from paleomagnetic data
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
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
The Mid Atlantic
Ridge
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
Fig. 17.15. Divergent plate margins
Passive Continental Margin
Size comparison of various volcanic features
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
• East African Rift Valley
Convergent Plate Margins
• Oceanic-Oceanic
– Seafloor Trench
– Shallow and deep earthquakes, 0-700
km deep
– Andesitic volcanoes in an island arc
– Japan
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
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
– Cascade range
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
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
– In oceanic crust, deep valleys are
formed
– Transform faults may extend onto
continents
– San Andreas fault
Juan de
Fuca plate
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.
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
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
More evidence….
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
The World’s Hot Spots
Plate Motion
• GPS
– Global Positioning
System
– Earth-orbiting
satellites identify
motion
• Transmitter on
satellite
• Ground-based
receiver
• Average rate
– 5 cm/year
81
Tectonic setting
Lavas and pyroclastics
Rock/sediment type
Felsic
Granites and Rhyolite
Turbidites, clays,
silts, sands
Marine sediments
(cherts, limestones,
red clays)
Basalts (Ophiolites)
Mafic
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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