Download Chapter 11 Notes: Plate Tectonics

Chapter 11 Notes: Plate Tectonics
Lesson 1 – Drifting Continents
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Alfred Wegener – German scientist who came up with the idea that the continents were once a single landmass
and have slowly drifted apart
o Published his evidence in a 1915 book called The Origin of Continents and Oceans
o His idea wasn’t accepted until after he died
Continental Drift – Wegener’s idea that the continents move slowly over the surface of the Earth
o Pangaea – supercontinent, one giant landmass, from about 300 million years ago
Evidence Supporting the Idea of Continental Drift – land features, fossils, climate
o Land features
 Shape of the continents seem to fit together like puzzle pieces
 Mountain ranges on the some continents lines up, specifically South America and Africa
 Coal fields in Europe and North America lined up where those continents would have once
touched
o Fossils – any trace of an organism that has been preserved in rock
 Glossopteris – fern-like plant found in Africa, South America, Australia, India, and Antarctica
 If found on all these continents, then they must have been connected
 Mesosaurus & Lystrosaurus – fresh-water reptiles found in Africa and South America
 Not able to swim well so could cross the ocean so 2 continents must have been
connected
o Climate – Arctic island, Spitsbergen, had evidence of a warmer climate 300 million years ago
 Island must have moved closer to poles to become colder
 If a place moves closer to the equator it will become warmer, like place in Africa with glacier
scratches in rocks
Wegener’s Hypothesis Rejected – because Wegener couldn’t explain how the continents moved, most geologists
didn’t accept his conclusions that continental drift occurred
Lesson 2 – Sea-Floor Spreading
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Mid-ocean ridges – form the longest mountain ranges that rise up from the ocean floor
o Mapped ocean floor with sonar – device that uses sound waves to measure the distance of an object
o Discovered steep-sided valleys split the top of some mid-ocean ridges
Sea-floor spreading – new magma/lava rises from inside the Earth, erupts, cools, and hardens inside the central
valley of a mid-ocean ridge to form a solid strip of rock; occurs at mid-ocean ridges; adds more crust to the
ocean floor
o Evidence from ocean-floor material – rocks shaped like pillows that only form when magma/lava
hardens quickly when erupting underwater
o Evidence from magnetic strips – as molten material erupts, magnetic minerals line up with Earth’s
magnetic poles; because the magnetic poles sometimes change, the magnetic minerals form in strips
with some facing one direction and others facing the opposite direction; pattern is the same on both
sides of the mid-ocean ridge
o Evidence from drilling for rock samples – rock that is further from a mid-ocean ridge is older than rock
right next to the mid-ocean ridge
Subduction - oceanic crust is turned back into magma when it gets pushed under continental crust; takes away
crust from the ocean floor
o Happens at Deep-ocean trenches – where older ocean floor plunges back into the Earth at deep
underwater canyons
o Oceanic crust is denser so sinks below continental crust
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Sea-floor spreading & Subduction
o Renew ocean floor about every 200 million years
o changes the size and shape of the oceans
 Size of oceans are determined by how fast new crust is made and how fast old crust is
subducted
 An ocean with many deep-ocean trenches will usually shrink
 An ocean with few trenches will usually grow (Atlantic Ocean)
Lesson 3: The Theory of Plate Tectonics
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Hypothesis – guess of what happened or will happen; has very little or no evidence (continental drift)
Theory – when we know what happened; has lots of evidence and can explain how it happened (plate tectonics)
Theory of Plate Tectonics – Earth’s plates are in slow, constant motion, driven by convection currents in the
mantle
o Plates – pieces of Earth’s lithosphere (all the crust and the upper part of the mantle)
o Plate motion – occurs because the plates are the top part of a large convection current in Earth’s mantle
 During subduction, denser material (at the edge of a plate) is pulled down into the mantle by
gravity which pulls the rest of the plate over
 Measured precisely using satellites – plates move from 1-12 cm per year so it is slow & constant
 N. American and Eurasian plates move apart at 2.5 cm per year
 Over millions of years, the plates have moved far distances and changed location of the
continents and the size & shape of oceans
 Evidence shows that Pangaea was not the first supercontinent
 Causes changes such as formation of volcanoes, mountain ranges, and deep-ocean trenches
o Plate boundaries – where plates meet
 Faults – breaks in the Earth’s crust where rocks have slipped past each other; form at plate
boundaries
 Divergent boundary – plates diverge, or move away, from each other
o Usually along mid-ocean ridges, where sea-floor spreading occurs
o Can happen on land (Iceland, East African Rift Valley)
 Forms a rift valley – deep valley formed from a divergent boundary on
land
o Can be associated with volcanic activity as magma/lava erupt to form new crust
 Convergent boundary – plates converge, or come together
o Oceanic crust collides with oceanic crust – the denser oceanic crust is
subducted, or sinks, under the less dense oceanic crust
o Continental crust collides with oceanic crust – because the oceanic crust is
denser, it moves under the continental crust
 Sometimes this pushes up continental crust to form mountains (Andes)
 Sometimes water enters magma and lowers the melting point causing
the magma to partially melt and rise up to form a volcano
o Continental crust collides with continental crust – because the densities are
similar, both push up to form high mountain ranges
 Transform boundary – plates slip past each other in opposite directions
o Plates are rocky and jagged so get caught, or lock, on each other as they try to
slip past each other
 When the plates unlock, or get uncaught, they move and release built
up pressure, causing an earthquake (San Andreas Fault in CA)