Download Plate Tectonics and Continental Drift

Plate Tectonics and
Continental Drift
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
• 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
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
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
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
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 reversals
– Earth’s magnetic field polarity has
reversed through time
• Normal polarity – Nmagnetic = Ngeographic
• Reversed polarity - Nmagnetic = Sgeographic
• At least 12 reversals in last 4 my
• Magnetic chronology is established by
combining polarity chrons with radiometric
age dating
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
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
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
– Japan
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
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
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
Intraplate volcanism
Rising mantle plumes can
produce localized hotspots
and volcanoes when they
produce magmas that rise
through oceanic or
continental crust. Hawaii
is an example
Composition?
Diamonds ascend to the
Earth's surface in rare
molten rock, or magma, that
originates at great depths.
Carrying diamonds and
other samples from Earth's
mantle, this magma rises
and erupts in small but
violent volcanoes. Just
beneath such volcanoes is a
carrot-shaped "pipe" filled
with volcanic rock, mantle
fragments, and some
embedded diamonds. The
rock is called kimberlite
after the city of Kimberley,
South Africa,
Ants, erosion and Namibian diamonds
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