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PLATE TECTONICS

Continental Drift

E.B. Taylor (1910) and Alfred Wegener (1915) published on Continental Drift. Later Alexander du Toit (1937)gave more supporting evidence.

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Continental Drift

Wegener and duToit’s evidence

1. Fit of the Continents 2. Fossil Evidence 3. Rock Type and Structural Similarities 4. Paleoclimatic Evidence

Continental Drift

Du Toit’s evidence

Fossils - Mesosaurs

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Continental Drift – Wegener’s Evidence,

fossils

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Continental Drift – Wegener’s Evidence

Rocks and Geologic Structures

Same rock types

Same order

Same faults and folds

Same age

300my old orogeny

Continental Drift- Wegener’s Evidence

Climates

Evidence of glaciation

Unlikely places

Better explained by fit

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Continental Drift – Du Toit’s Evidence

Climate

Coal Deposits

Should be in warm temperate areas or equatorial areas

Better explained by fit

Continental Drift

Wegener lacked a mechanism for continental movement through oceanic crust.

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Evidence: Polar Wandering – Keith Runcorn

Plate Tectonics

Evidence from exploration of the ocean floor Global oceanic ridge system and guyots

Paleomagnetic stripes

Deep ocean trenches associated with earthquakes – Benioff zones

All seafloor less than 180my old

Sediments thin where expected to be thick

Ocean deepest away from center, near edges

Rocks youngest near center, ageing toward edges

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Plate Tectonics

Harry Hess (1906-1969)

Surveyed ocean floor

Discovered guyots

Hypothesized sea-floor spreading

Plate Tectonics – the ocean floor

The mid-ocean ridge system is a nearly continuous volcanic ridge found in all oceans.

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Sea Floor Spreading

A process known as seafloor spreading occurs where

magma from the mantle wells up into the divergent

boundary - forming new basaltic seafloor.

Spreading rates average ~5 cm/year.

Sea Floor Spreading

The mid-ocean ridge has an elevated position on the seafloor because it is formed from relatively hot igneous rocks.

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Plate Tectonics- Sea Floor Spreading

As the seafloor moves

away from the ridge, it

cools and contracts — thus

the seafloor generally is at

a greater depth as you

move away from the mid-

ocean ridge.

Sea Floor Spreading

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Plate Tectonics – Sea Floor Spreading

Fred Vine and D.H. Mathews

Interpreted “magnetic stripes”

Earth has experienced periods of reverse polarity

As magma solidifies it is magnetized according to the polarity at the time

Stripes of magnetic polarity are formed at mid-oceanic ridges

Magnetic Stripes

In the figure, regions of

normal polarity are indicated

in white where magnetic

north is coincident with the

geographic north.

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Magnetic Stripes

Stripes are parallel to mid-oceanic ridge

Stripes are mirror images across ridge

Dates of stripes indicate sea floor spreading

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PLATE TECTONICS

Sea-floor spreading provided the missing mechanism for Continental Drift

Plate Tectonics

If the sea floor is spreading, and the earth is not expanding, where does the excess crust go?

Subduction zones

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Subduction Zones

Studies of crust around deep sea trenches showed zones of earthquakes beneath the crust called “Wadati-Benioff Zones”

Heat and heat flow studies around trenches revealed a “cold slab”

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Plate Tectonics – Subduction Zones

Earthquake epicenters revealed plate boundaries.

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Plate tectonics is a theory about how

the surface of the Earth evolves due to

strong internal forces. The surface of

the Earth is composed of rigid plates

that are mobile and move relative to one

another.

Plate tectonics is a unifying theory in Geology — different geologic

phenomena such as mountain building, earthquakes, volcanoes, and the

distribution of fossils and organisms can be explained through plate

tectonics.

Earth’s Major Plates

The Earth’s surface is composed of a strong, rigid layer known as the

lithosphere. The lithosphere is broken into pieces known as tectonic

plates.

Lithospheric plates are thinnest in the oceans (<100 km thick) and may

be more than 250 km thick on the continents.

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The lithospheric plates

overlie a weaker region

of the mantle known as

the asthenosphere.

The rocks in the

asthenosphere are near

their melting point and

are relatively weak and

ductile.

The asthenosphere

allows the plates to

move above it. Plates

move slowly but

continuously - generally

on the order of a ~5

cm/year.

Note that the plates generally include a continent or a portion of a

continent AND a portion of the ocean floor.

Each plate moves as a coherent unit relative to others

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Plate Tectonics- Plate Boundaries

Plate Boundaries are where earthquakes, volcanoes, and crustal deformation take place.

There are three general types of boundaries:

Divergent –Plates move away from each other

Convergent- Plates move toward each other

Transform – Plates move past each other

Divergent Boundaries

Most divergent boundaries are located along mid-ocean ridges.

Divergent plate boundaries are known as constructive margins because

they are the site where new oceanic crust (lithosphere) is generated.

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Some divergent boundaries occur under continental crust

The East African Rift represents a modern example of a continental rift. If this rift is successful, eastern Africa may split off from the rest of the continent and a new ocean basin may form between the two “Africas.”

Convergent margins are also known as destructive margins since oceanic crust is destroyed or consumed.

Most convergent margins are associated with a subduction zone

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The map shows the world’s oceanic trenches. Note that the Pacific is

Nearly encircled in deep-ocean trenches.

Convergent Boundaries: Oceanic-Continental Convergence

Oceanic-continental

convergence occurs when

leading edge of one plate is

composed of continental

rocks (granitic) and the other

is oceanic (basaltic).

The denser oceanic plate dives beneath (subducts) the lower-density

continental plate. Lower density granitic rocks tend to float in the

asthenosphere, i.e., via isostacy

Dewatering of the subducted slab causes melting in the wedge of the

asthenosphere above it. The magma that is produced is buoyant and

rises through the mantle toward the Earth’s surface.

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The magma results in volcanic activity along a line parallel to the

subduction zone known as a continental volcanic arc.

Examples of continental volcanic arcs include

the Cascade volcanoes such as Mt. Rainier and

Mt. St. Helens and the volcanoes of the Andes

mountains along the west coast of South

America.

Mt. St. Helens

Convergent Boundaries: Oceanic-Oceanic Convergence

Oceanic-oceanic convergence

occurs when the leading edge

of both plates consists of

oceanic crust. These plate

boundaries have many of the

same features as in oceanic-

continental convergence.

In oceanic-oceanic convergence,

the line of volcanoes forms a

string of islands parallel to the

subduction zone known as a

volcanic island arc.

Examples of island arc systems

include the Aleutian Islands,

Tonga, Indonesia, and Japan.

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Ocean to Ocean Convergence NOTE: Oceanic crust subducts other oceanic crust Volcanoes form islands

Continent-continent convergence usually begins as oceanic-

continental convergence (ex. Andes). As the oceanic crust is

subducted, a continental block on the subducting plate may approach

the continent.

The Himalayan mountains were formed by the collision of the Indian

subcontinent into the Asian mainland.

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These figures show the convergence of

India into Asia over the last 71 million

years.

Continent to Continent Collision

Uplifted continental crust No volcanoes Folding and faulting

NOTE:

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Transform Fault Boundaries

Transform plate boundaries

are where plates slide past one

another.

Most transform boundaries are

associated with mid-ocean

ridges where they form linear

breaks in the ridge system.

The active transform boundary

exists between the two offset

ridge segments

Transform Faulting

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The Breakup of

Pangaea

Now that we

understand plate

tectonics, we can use

geologic data to

reconstruct Pangaea

and model the

movement of the

continents during the

last 200 million

years.

Continental Drift animation

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What Drives Plate Motions?

This is an active area of research

and there is a diversity of opinions.

Most geologists agree on the

following points about the driving

forces for plate motion:

1. The Earth’s mantle is convecting - hotter rocks rise buoyantly and

cooler denser rocks sink. This motion helps drive plate motion.

2. Mantle convection and plate tectonics are part of the same system.

3. Density differences due to the unequal distribution of heat within

the Earth’s mantle ultimately drive the mantle convection cells and

plate motion.