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Introduction to Plate Tectonics

A Revolution in the Earth Sciences -

Peter Wyllie

GLY 2010 – Summer 2015

Lecture 3

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Alfred Wegener, 1880-1930

German meteorologistPublished a book in 1915 whose(translated) title isThe Origin of Continents and OceansWegener proposed the idea of

Continental Drift

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Continental Drift• Pangaea – supercontinent containing all

land

• Based on evidence available in early 1900’s

Wegener’s Evidence

• Fit of continents when map cut apart and rearranged

• Climate similarities in adjacent areas

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Lithologic (Rock) Evidence

• Unusual rocks found only where continents fit together

• Cratons – cores of continents, strongly showed this pattern

• Karoo (South Africa) and Santa Catarina (Brazil) formations appear identical

Fossil Evidence

• Glossopteris had left leaf remains in large areas of Southern Hemisphere

• Wegener concluded that southern continents must have been joined

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Fate of Continental Drift Hypothesis

• Biggest objection: How to move a continent?• Wegener died in Greenland in 1930 – before most

people accepted his ideas• Wegener’s ideas languished until the end of WWII• Use of submarines during the war spurred research

after the war• This lead to oceanographic exploration

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Oceanographic Exploration

• Ocean floors were mapped to add submarine navigation

• Knowledge gained revitalized Wegener’s ideas

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What Does “Plate Tectonics” Mean?

• Plate = Large, Rigid slab of rock

• Tectonics comes from Greek root meaning “to build”

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Plates• The earth’s surface is divided into

about a dozen major plates• Composed of lithosphere - crust plus

the extreme outer mantle• Lithosphere comes from lithos,

meaning stony, and sphere - hard and rigid

• Lithosphere – extends from the surface to the top of the mantle

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Map of Major Tectonic Plates

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Mid-ocean Ridge Map

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MOR Video

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Alvin

• Jan Morton entering Alvin

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East Pacific Rise SegmentComputer Generated Image

• Yellow to red shows high elevation

• Green to blue shows lower elevation

• Latitude 9° north

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Asthenosphere

• Behaves as a plastic - a solid that may deform slowly

• Plastic because it is hot and under pressure

• Extends a few hundred kilometers below the lithosphere

• It is entirely in the upper mantle

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What Supports the Plates?

• Lithospheric plates float on the asthenosphere, which is denser than the lithosphere

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Sea-floor Spreading

• Concept came from oceanographic investigations

• Uses convection cells, an idea Wegener would have been familiar with

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Convection Cell

• Heat beaker

• Water expands and rises

• It spreads and cools at the top

• Cool water sinks

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Harry Hess, 1906-1969

• In Navy during WWII• Rear Admiral in Naval

intelligence• Commented that

geologists make good intelligence officers because they can work with incomplete data sets

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Hess in WWII

• Keenly interested in geology of ocean basins

• Used time between battles to collect data

• Collected echo-sounding surveys of ocean depths

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Hess at Princeton• After WWII, Hess became Professor of

Geology at Princeton University

• Used WWII data to publish a paper called “History of the Ocean Basins” in 1962

• Paper outlined idea of sea-floor spreading

• Robert Dietz, working independently, proposed a very similar concept

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Hess-Dietz Hypothesis

• Asthenosphere contains numerous convection cells

• Cells cause molten rock (magma) to rise

• Some magma erupts on surface• Most magma stays beneath the surface

and spreads, carrying lithospheric plates with it, and slowly cooling

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Hess-Dietz Hypothesis, Cont.

• Cooling magma sinks, completing convection cell

• Mobile sea-floor helped to answer several puzzles

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Mid-Ocean Ridge

• Click to start

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Spreading Center

• Click to start

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Mantle Convection Cells

• New crust created by magma hardening at the Mid-Ocean Ridge (MOR)

• From other data, we know the earth is not expanding

• Crust must be destroyed somewhere

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Puzzles Solved

• Why is there so little sediment on ocean floor?

• What are the rock ages so young?

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Seismic Evidence

• In 1935, K. Wadati showed earthquakes occur at greater depths toward the interior of the Asian continent

• Earthquakes further toward the Pacific Ocean occurred at shallower depths

• H. Benioff later observed the same distribution in other regions

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Age of Ocean Fossils

• Continental fossils are at least 3.5 billion years old

• Oldest marine fossils are about 180 million years

• Since life is though to originate in the oceans, why aren’t ocean fossils older?

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Subduction Zones

The key to subduction is the density of the rock types involved

Density = mass/unit volume

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Rock Densities

• Continental lithosphere is about 3.00 grams/cubic centimeter

• Oceanic lithosphere gradually increases in density as it ages, reaching a maximum value of about 3.28 grams/cubic centimeter

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Converging Plates

• When two plates collide, the denser plate will sink (subside) beneath the less dense plate

• Density differences as small as 1% are enough to cause subduction

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Subduction

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Plate Movement

• Plates move slowly (up to 15 cm/yr)

• Plates may collide, move apart, or slide past each other

• Friction during plate movement often generates earthquakes

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Asthenosphere Density

• The density of the asthenosphere is about 3.3 g/cm3

• Density increases with depth below the surface