Introduction to Plate Tectonics A Revolution in the Earth Sciences Peter Wyllie GLY 2010 – Summer 2013 Lecture 3 1

Alfred Wegener, 1880-1930 German meteorologist Published a book in 1915 whose (translated) title is The Origin of Continents and Oceans Wegener proposed the idea of Continental Drift 2

Continental Drift •

Pangaea

– supercontinent containing all land • Based on evidence available in early 1900’s 3

Wegener’s Evidence • Fit of continents when map cut apart and rearranged • Climate similarities in adjacent areas 4

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 5

Fossil Evidence • Glossopteris had left leaf remains in large areas of Southern Hemisphere • Wegener concluded that southern continents must have been joined 6

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 7

Oceanographic Exploration • Ocean floors were mapped to add submarine navigation • Knowledge gained revitalized Wegener’s ideas 8

What Does “Plate Tectonics” Mean?

• Plate = Large, Rigid slab of rock • Tectonics comes from Greek root meaning “to build” 9

Plates • The earth’s surface is divided into about a dozen major plates • Composed of

lithosphere -

the extreme outer mantle crust plus • Lithosphere comes from

lithos

, meaning stony, and sphere - hard and rigid • Lithosphere – extends from the surface to the top of the mantle 10

Map of Major Tectonic Plates 11

Mid-ocean Ridge Map 12

MOR Video 13

Alvin • Jan Morton entering Alvin 14

East Pacific Rise Segment Computer Generated Image • Yellow to red shows high elevation • Green to blue shows lower elevation • Latitude 9° north 15

Asthenosphere • Behaves as a

plastic

deform slowly - a solid that may • Plastic because it is hot and under pressure • Extends a few hundred kilometers below the lithosphere • It is entirely in the upper mantle 16

What Supports the Plates?

• Lithospheric plates float on the asthenosphere, which is denser than the lithosphere 17

Sea-floor Spreading • Concept came from oceanographic investigations • Uses Convection cells, an idea Wegener would have been familiar with 18

Convection Cell • Heat beaker • Water expands and rises • It spreads and cools at the top • Cool water sinks 19

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 20

Hess in WWII • Keenly interested in geology of ocean basins • Used time between battles to collect data • Collected echo-sounding surveys of ocean depths 21

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 22

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 23

Hess-Dietz Hypothesis, Cont.

• Cooling magma sinks, completing convection cell • Mobile sea-floor helped to answer several puzzles 24

Mid-Ocean Ridge • Click to start 25

Spreading Center • Click to start 26

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 27

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 29

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 31

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 32

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 33

Subduction 34

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 35

Asthenosphere Density • The density of the asthenosphere is about 3.3 g/cm 3 • Density increases with depth below the surface 36