Mt. Fuji, Japan

The Theory of Plate Tectonics

Steps in Development

1. Benjamin Franklin (late 1700s)Recognized that crust of Earth was a shell.Surface could be broken and parts moved about.

2. Alfred Wegener (1912)German meteorologist-geophysicistProposed theory of Continental Drift.Proposed continents float on a denser

underlying interior of the Earth.CONTINENTS periodically break up and

DRIFT apart.

The Theory of Plate Tectonics

Wegener believed all continents were joined together.Supercontinent of Pangaea existed about 200 myBP.

Pangaea covered 40% of the Earth’s surface.

The Theory of Plate Tectonics

Majority of Pangaea was in Southern Hemisphere.Pangaea was surrounded by a single ocean, thePanthalassic Sea.Pangaea broke up ~ 180 million years ago.

The Theory of Plate Tectonics

Wegener first published his theory 1912.

The Theory of Plate Tectonics

Evidence in support of Continental Drift

1. Continental Fit

Sir Francis Bacon (1620)noted that the continentsmight fit together.

Made observation afterseeing some newly mademaps.

The Theory of Plate Tectonics

2. Habitats of ModernOrganisms

Hippopotamus found inAfrica and Madagascar.

Marsupials in Australia.

Indicate some migrationand evolution took place before and after drift began.

The Theory of Plate Tectonics

3. Fossil Record

Wegener used thefossil record.

Found fossils of plants andanimals that werefound on severalcontinents.

Included animals, Cynognathus, Lystrosaurus, Mesosaurus,and plants Glossopteris.

The Theory of Plate Tectonics

4. Similar Rock Types Across Ocean BasinsMountains of Northern Hemisphere similar in Greenland, NA, andEurope.Also similar rocks between South America and Africa.

The Theory of Plate Tectonics

5. Ancient ClimatesGlacial striations found in India, Australia, South America

and Africa.Radiate from a point in southern Africa.Also coal deposits found in presently cold regions,

such as Norway.

The Theory of Plate Tectonics

Wegener could not explain how the continents drift.He could provide no mechanism for the theory.

Wegener seemed to have a great idea, and lots of supporing evidence.

However, Plate Tectonics was not widely accepted until the early 1970’s.

Why?

Captain Harry Hammond Hess

Mapped the mid-ocean ridges (3000 m high and 2000 m wide)

Conducted echo-soundings of the oceans during WWII

(later Professor of Geology at Princeton)

Located deep-sea trenches (10,000 m deep) associated with large continental mountain belts (the Andes) and island arcs

(Aleutians, Japan)

Hess presented sea-floor spreading as a mechanism in 1960’s

Believed in Wegener’s hypothesis

The Theory of Plate Tectonics

Led to the discovery ofOceanic Ridges and Trenches.

Still no explanation.

Research vesselsfound alternatingbands of rock onthe sea floor oneither side of theridges.

Rocks possessediron that indicatedmagnetic field reversals.

The Theory of Plate Tectonics

Basic Plate Tectonics Concepts

1. The outer portion of the Earth, the lithosphere (the uppermantle and crust), is composed of rigid units called plates.

2. Plates move slowly.

3. Most of the Earth’s large-scale geologic activity, such asearthquakes and volcanic eruptions, occur at or near plate boundaries.

4. Interiors of plates are geologically quiet, with fewer and usually milder earthquakes and fewer volcanoes than at plate margins.

The Theory of Plate TectonicsEarth’s Lithospheric Plates

The Earth's surface is broken into ~15 crustal plates

The Theory of Plate TectonicsEarth’s Lithospheric Plates

Continental plates are composite.Include both continental and

oceanic crust.North American Plate - continental portions are thicker, lower density lithosphere. - ~ 120 km thick in continental regions. - oceanic portions are thinner, higher density lithosphere.

The Theory of Plate Tectonics

When plates move, everything on them moves with them.North American Plate moving west, Eurasian Plate moving east.Rate is about 5-10 cm/yr.Over long periods of time, movement is large.

The Theory of Plate Tectonics

So….

Has the size of the Earth increased?

Why?

Where plates move apart or diverge, at some other place on the Earth, they must come together orconverge.

This is where the Earth gets exciting.

The tectonic plates are continually moving

Convection currents in the mantle drive plate movements

The source of heat is radioactivity deep in the Earth

Divergent (plates move away from each other)

Convergent (plates collide with each other)

Transform (plates move past each other)

The Theory of Plate Tectonics

Types of Plate Boundaries

convergent divergent

transform

convergent divergent

hot spot

The Theory of Plate Tectonics

The Theory of Plate TectonicsDivergent Plate Boundaries

Occur along spreading centers where plates are moving apart

The Mid-Atlantic Ridge:Spreading at ~2.5 cm/yr

Youare

here

BRUCE visits the Mid-Atlantic Ridge

The Theory of Plate TectonicsDivergent Plate Boundaries

East Pacific Rise

“Alvin!”

ALVIN visits the East Pacific Rise in January, 2002

East Pacific Rise Sea Floor at 3000 m

East Pacific RiseBlack Smokers

Divergent BoundariesThe East-African Rift Zone

Oldoinyo Lengai erupting