1 introduction to plate tectonics a revolution in the earth sciences - peter wyllie gly 2010 –...
TRANSCRIPT
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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