Plate Tectonics Flashcards
Eduard Suess
Late nineteenth century.
– Postulated that southern continents were once part of a giant continent called Gondwanaland
Alfred Wegener
Early twentieth century. – Fitted together separate continents to form giant supercontinent of Pangaea. – First proposed his continental drift hypothesis in 1915 – Published The Origin of Continents and Oceans
Continental drift hypothesis
– Supercontinent called Pangaea broke apart and the “pieces” drifted to their present positions to become modern continents
The Great Debate
• Strong opposition to the continental drift
hypothesis from the scientific community
• Hypothesis lacked a viable mechanism for
moving continents:
–Wegener incorrectly suggested that
continents broke through the ocean crust,
much like ice breakers cut through ice
• Wegener’s hypothesis was correct in principle,
but contained incorrect details
• A few scientists considered Wegener’s ideas
plausible and continued the search for more
evidence
Evidence that supported the Continental Drift
–Fit of the continents –Fossil evidence – Rock type and structural similarities –Paleoclimatic evidence –Paleomagnetism
Fossil Evidence for Continental Drift
A. L. Du Toit (early twentieth century) identified ancient fossils on widely separated continents: • Some fossils from different continents were remarkably similar to one another • Wondered how similar animals could have crossed entire oceans • Concluded that the now separated continents have at one time been joined
Paleoclimatic Evidence
• Glacial deposits found in South America, Africa, India and Australia: – Some in places where the climate is now tropical • Grooves in bedrock at these locations may have been carved by ancient glaciers
What type of ancient rocks and fossils do not match the climate at their present location? What is this an example of and what does it suggest?
• Coal in Antarctica • Reef-building corals in frozen Greenland • Glacial deposits in Sahara Desert Example of Paleoclimate Evidence for Continental Drift. Suggests that continents were at one time at different latitudes and "drifted" to present positions.
What is paleomagnetism?
Study of the magnetic properties of ancient rocks. This provides further evidence
that continents drifted over time.
When and how was the magnetic field discovered.
discovered by the Chinese in the 13th century by floating a lodestone on a piece of wood – it pointed north.
Compass directions define two components of magnetism:
– Declination: Horizontal swing
– Inclination: Tilt of magnetization
Declination
Compass direction points to the magnetic north pole
Inclination
Magnetic inclination is the
angle a compass needle
makes with Earth’s surface and is a function of latitude
What can magnetic materials in volcanic rocks record?
The direction of Earth’s magnetic field at the time the lava flow crystallized.
Assembling continents to their former positions does what to the Polar Wandering Paths?
Aligns them
Magnetic polarity reversals throughout Earth history are recorded as?
Magnetic stripes in oceanic crust.
Presently magnetic lines of force travel from?
The south to north magnetic poles
What happens to the oceanic crust during normal and reverse magnetism?
During normal magnetism, the oceanic crust acquires normal polarity and during reverse magnetism the oceanic crust acquires reversed polarity.
The crust can split apart in opposite directions, forming stripes on either side of a mid-ocean ridge.
Youngest oceanic lithosphere occurs along?
Mid-ocean ridges. Oceanic lithosphere is progressively older away from these ridges.
How does the thickness of sediments change with distance from a ridge crest.
The thickness increases.
Oldest oceanic lithosphere occurs along where?
The edge of the ocean basin adjacent to a subduction zone and/or continental margin.
Stages in the formation of an ocean basin
- Hot upwelling mantle material impinges on the base of continental lithosphere.
- There is a continental rift (splitting of the continental lithosphere)
- Linear sea develops between the now separated continents
- Linear sea evolves into a mature ocean basin, with a mid-ocean ridge at the origin of the rift and passive continental margins.
e. g. gulf coast and east coast of North America
What are deep-sea fans?
Deposits of sediments in deep water that have been carried offshore by currents.
Divergent plate boundary
- constructive
- two plates move apart from one another
Convergent plate boundary
- destructive
- two plates collide with one possibly subducting beneath the other.
Subduction
The sideways and downward movement of the edged of a plate of the earth’s crust into the mantle beneath another plate.
Transform plate boundary
- lithosphere is conserved
- two plates slide laterally past one another
Mantle convection cells sink at?
Convergent plate boundaries
Convection cell
a self-contained convection zone in a fluid in which upward motion of warmer fluid in the center is balanced by downward motion of cooler fluid at the periphery.
Parts of a subduction zone
- Trench
- Accretionary wedge of prism
- Forearc Basin
- Volcanic Arc
Trench
Deep depression on seafloor where oceanic plate bends downward
Forearc Basin
Where volcanic material and sediment accumulate in subduction zone
Accretionary wedge/prism
Leading edge of overriding plate in subduction zone where sediment and pieces of oceanic crust scraped off of down-going plate accumulate.
Volcanic arc
Formed when the subducted plate reaches a depth hot enough to melt it and the mantle and magma rises to erupt at the surface.
The three type of convergent plate boundaries
- Ocean-ocean convergence
- Ocean-continent convergence
- Continent-continent convergence
Ocean-ocean convergence
Oceanic lithosphere subducts beneath another oceanic plate; characterized by linear chain of volcanoes called island arcs
Ocean-continent convergence
Oceanic lithosphere subducts beneath continental lithosphere; causes continental volcanic arc
Continent-continent convergence
Two continental plates collide; causes mountain ranges like the Himalayas
collage (collage tectonics)
an assemblage of micro-continents, arcs, and other crustal fragments accreted along the edge of a continent adjacent to a subduction zone.
Driving forces of plate tectonics
- Slab pull
- Ridge push
- Whole-mantle convection
- Layer-cake convection
Slab pull
Results from the sinking of a cold, dense slab of oceanic lithosphere and is the major driving force of plate motion; major driving force of plate motion
Ridge push
a gravity driven force that results from the elevated position of the ridge
Convection in the mantle enhances plate motion when?
When the velocity of the asthenosphere exceeds that of the overlying plate.
Whole-Mantle Model
Sinking slabs of cold oceanic lithosphere are the downward limbs of convection cells, while rising mantle plumes carry how material from the core-mantle boundary toward the surface.
Layer-Cake Convection Model
Has two largely disconnected layers. A dynamic upper layer driven by descending slabs of cold oceanic lithosphere and a sluggish lower layer that carries heat upward without appreciably mixing with the layer above.
Bathymetry
“submarine topography,” or the depths and shapes of underwater terrain.
Feature of seafloor bathymetry
- Deep trenches
- Immense fracture zones
- Countless submerged volcanoes
- Enormous submarine mountain ranges (mid-ocean ridges)
Mid-ocean ridges extend the length of?
Entire ocean basins; can be hundred or thousand of kilometers long.
Sites of numerous submarine hot springs where heated seawater is discharged
Mid-ocean ridge
Ophiolites
Slices of oceanic crust found on land
Pillow basalts
result of the rapid cooling of hot, fluid magma that comes into contact with water
Sheeted Dyke Complex
consists of swarms of basaltic dykes, the feeder channels for the overlying pillow basalts
Gabbros
usually banded or layered resulting from the crystallization in the magma chamber at the base of the crust
Convection cells spread apart at?
Mid-ocean ridges, carrying plates in opposite directions
Transform faults do what to mid-ocean ridges?
These faults break the mid-ocean ridges into segments causing an offset(s) along the ridge.
Mantle plume
- Long-lived structures
- Some originate at the core-mantle boundary
- Hot plumes relatively buoyant and rise towards the surface
Rising plumes of mantle material create?
Hot spots in the overlying crust; volcanoes can form over them
