Plate Tectonics and Plate Tectonic theory

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This exam covers Sedimentary rocks, Early Earth, Igneous Rocks and volcanoes, Plate tectonics, Earthquakes and Other Hazards, Geochronology and Geologic time!

*Continental Drift

Hypothesis of German meteorologist Alfred Wegener that all continents were once joined as a single continent, which he named Pangea, from where they drifted to their current positions.

Antonio Snider

(1858) published maps depicting continental drift.
Anticipated Wegener's theories concerning Pangaea by several decades. In 1858, Snider-Pellegrini published his book, La Création et ses mystères dévoilés ("Creation and its Mysteries Unveiled"). He proposed that all of the continents were once connected together during the Pennsylvanian Period. He based this theory on the fact that he had found plant fossils in both Europe and the United States that were identical. He found matching fossils on all of the continents. He also attributed the cause of the fragmentation of the supercontinent to the Great Flood of the Bible.

Alfred Wegner

-German Meteorologist who proposed the theory of coninental drift (the idea that the continents were moving)
-Evidence: fossils, rocks, climate, coal, glacial and mineral deposits.
-Not a good mechanism(not plausible): earth is trying to spin water off and moon pulls water a little.

Pangea (Gondwanaland and Laurasia)

Ancient super continent (Pangea meaning all of earth)
Panthalassa: seas surrounding it (meaning: all ocean)
Gondwanaland

Gondwanaland

a hypothetical continent that (according to plate tectonic theory) broke up later into India and Australia and Africa and South America and Antarctica, (Southern half of Pangaea)

Laurasia

a hypothetical continent that (according to plate tectonic theory) broke up later into North America and Europe and Asia (The northern section of Pangea after it broke apart)

Climatic evidence

Paleoclimatic Evidence
-Glacial till of the same age is found in southern Africa, South America,
India and Australia — areas that it would be very difficult to explain the
occurrence of glaciation.
-Pangaea with S. Africa centered
over the South Pole could account
for the conditions necessary to
generate glacial ice in the
southern continents.

*Fossil Evidence (Working Definition of P.T. Theory)
-Lithosphere
-Asthenosphere

Wegener found that identical fossils were located directly opposite on
widely separated continents.
This had been realized previously but the idea of "land bridges" was the
most widely accepted solution.

Lithosphere

Outer strong shell of earth consisting of the crust and the uppermost mantle. (rigid and brittle)

Asthenosphere:

A week layer of Earth's mantle below the lithosphere that may contain small amounts of magma. (plastic mantle and not liquid)

*Physical behavior of rocks

Rocks get older as you move away from ridges.

*Sea Floor spreading

The process whereby oceanic lithosphere is pulled apart along the crests of mid-ocean ridges as new lithosphere forms and fills the gap where the plates separate.

Hezz and Dietz

Harry hess: (ship captain) professor of Geology
-> Phathometer (measures depth) in was
-> Sea floor not flat (mountain ranges on sea floor)
-> mid-ocean ridges (mid atlantic ridges)(iceland formed from mid atlantic ridges)
-> sea floor spreading (oceans were growing)
-> why doesnt the earth expand?
+Oceanic crust digest back into planet (subduction zones)
Dietz: Paleomagnetism: normal, reverse, normal, reverse polarity.

Paleomagnetism:

The ancient orientation of Earth's magnetic that is recorded in rocks. The Earth's magnetic field produces invisible lines of force that extend from
one pole to the other.

Curie Temperature:

•The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetisation.(the temperature above which a ferromagnetic substance loses its ferromagnetism and becomes paramagnetic)

Paleomagnetic reverals

Geomagnetic reversals: A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. Earth's magnetic field periodically reverses polarity — the north and south poles switch. Rocks crystallizing during one of these periods of magnetic reversal will be magnetized with a polarity opposite of rocks the crystallize today.

Vine and Matthews:

Scientists who observed paleomagnetism. The "Vine-Matthews-Morley hypothesis", also known as the "Morley-Vine-Matthews hypothesis" was the first key scientific test of the seafloor spreading theory of continental drift and Plate tectonics.

Geophysicist Frederick John Vine and the Canadian geologist Lawrence W. Morley independently realized that if the seafloor spreading theory was correct, then the rocks surrounding the mid-oceanic ridges should show symmetric patterns of magnetization reversals, a record of the Earth's geomagnetic reversals, captured in the cooling volcanic rocks. Morley's letters to Nature February 1963 and Journal of Geophysical Research April 1963 were both rejected, so Vine and his adviser Drummond Hoyle Matthews were first to publish in 1963. Later geomagnetic surveys found the patterns are in fact present, providing strong confirmation of the theory.

Transform Faults

a fault that runs across a mid-ocean ridge, faults where there is no loss or gain in plate material, plates slide past each other and are areas of earthquakes.

Lynn Sykes

-seismic reflections
-map sub-surface
-faluts around mid ocean ridges that could only be explained where new crust is built

*Plate Boundaries.

Divergent plate boundaries
•􀀁 Established mid-ocean ridge
•􀀁 New rift zone, new ocean

Mid-Ocean Spreading Centers (examples)

-Estimating Spreading Rate:Typical spreading rates are ~5 cm/year,
however, range from 2 to 20 cm/year.
-Red Sea and Gulf of California — new
spreading ridges developing

Continental Rift zones (examples)

1)-Continental crust can begin to rift from upward movement of hot rock from mantle.
-Uplift results in extension that stretches the crust.
2)-Extension of the crust is accompanied by episodes of faulting and volcanism.
-This results in rift valley like the East African Rift Valleys.
3)-As the spreading continues, the rift valley will widen and deepen,
extending out to the sea.
-The valley then becomes a narrow linear sea — Red Sea, Gulf of California.
4)The rifting will continue until a full blown ridge system is created forming a large ocean basin —-- ex. Atlantic Ocean.

*Plate Bondaries

Convergent Plate boundaries

Subduction Zones

-oceanic-oceanic convergence
-continental-oceanic convergance
-continent- continent Convergence

oceanic-oceanic convergence (examples) -Volcanic Island Arc

When a convergant boundary forms between plates of oceanic lithosphere, the plate that is older, thicker, and denser subducts the less dense plate.(Similar to ocean-continent collision except that volcanoes can form
islands — volcanic island arc.
Ex. Aleutian Islands, Mariana, Tonga Islands.)

Continental-oceanic convergance (example)
- volcanic- magmatic range (continental arc)

When oceanic and continental plates convergence, the oceanic plate must subduct beneth the continental plate because the density of the thick continental crust is too low to permit it to sink into the asthenosphere.
(Denser oceanic crust is subducted — trench, accretionary wedge,
volcanism.
Examples: Andes, Cascade Range
Sierra Nevada represents an inactive continental arc.)

Continent- Continent Convergance (examples)

When subduction brings two continents together limited subduction may occur but the the bouyancy of continental crust eventually stops the subduction. The contraction of crust in the collision zone doubles the thickness of continental crust and creates high mountains. Silvers of oceanic crust are commonly uplifted in the mountain range and record the basin consumed by subduction prior to collision of the continents.
( Usually preceded by subduction until continents smash into one
another.
Results in thicken of crust and jumbled mess of many rock types
superimposed on one another.
Example: Himalayan Mtns.)

Hotspots

An area of in tense volcanic activity not explained explained by plate-boundary processes. Hot spots form when form where asthenosphere rises beneath lithospheric plates.

Estimating Rates of Plate motion by mantle hotspots

-Plate motion in Hawaii: 10cm/year

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