earth science standards 3.a - students know features of the ocean floor (magnetic patterns, age, and...
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Earth Science Standards 3.a - Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.3.b - Students know the principal structures that form at the three
different kinds of plate boundaries.
Plate Tectonics and
Plate Boundaries
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Wegener’s Evidence
• Matching Coastlines• Rock Evidence
• Fossil Evidence• Climate Change
Evidence
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Hess’s Evidence
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Two Types of Crust1. Oceanic Crust• Mainly Basalt (more dense rock – sinks under
continental crust)• Thickness: 3-5 miles (thinnest at the ocean
ridges)• Mineral composition: low in silicates; dark-
colored rock (high in magnesium and iron)
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2. Continental Crust• Mainly Granite (less dense rock – floats on top of
the mantle)• Thickness: 18-30 miles• Mineral composition: lots of silicates; light-
colored rock (quartz and feldspar)
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• The lithosphere (crust and solid upper mantle) of Earth is broken into fragments called plates
• Vary from 62-125 mi in thickness
• Plates move on top of the hot asthenosphere
Earth’s Plates
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• There are about 8 major plates and nine minor ones.
• The major plates include both continents and oceans.
• The number of plates and their sizes, shapes, and motions have changed throughout Earth's history ... and continue to do so today.
• Different plates move at different speeds.
What plate are we
riding on?
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• Because continental crust is less dense and thicker than oceanic crust, it "floats" higher on the underlying mantle rock.
• Oceanic crust is made of dense basalt; continental crust consists of less dense granite.
Why do continents rise above the ocean floor?
Notice how high the continental crust rises above the oceanic crust in this cross section of the North American Plate.
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• Plate Boundaries – places where 2 broken plates meet
• Most earthquakes, volcanoes, and mountains occur at plate boundaries
Plate Boundaries
Plotting earthquakes
and volcanoes help scientists
define plate boundaries!
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3 Types of Plate Boundaries1. Divergent – Separation2. Convergent - Collision3. Transform – Sliding Past
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• Plates move apart• Mostly on seafloor but
can occur on land• Occurs when 2 oceanic
plates separate or when 2 continental plates separate
• Hot, rising material causes the plates to separate (convection currents)
1. Divergent
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• On seafloor:• Oceanic crust separates• Creates ocean ridges
and rift volcanism
• On land:• Continental Crust
separates• Creates Rift Valleys
and rift volcanism• Ex. African Rift Valley
and Iceland
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• Plates move towards each other• Cooler, sinking material pulls plates together
(convection currents)• 3 types of convergent boundaries (classified by
the type of crust involved)
2. Convergent
a) Oceanic-oceanicb) Oceanic-Continentalc) Continental-
Continental
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• 2 ocean plates collide• Oceanic plate subducts underneath oceanic plate &
a trench forms• Subducting plate melts forming magma• Magma rises to surface forming a chain of volcanic
islands (island arc)
a) Oceanic-Oceanic Convergence
Subduction – one plate goes
underneath another
Ex. - JAPAN
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Why is Japan known for its earthquakes
and tsunamis?
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• An ocean plate and a continental plate collide• Oceanic plate subducts underneath continental
plate which forms a trench at coastlines• Subducting plate melts forming magma• Magma rises to surface forming a volcanic
mountain range on the continents
b) Oceanic-Continental Convergence
Ex. - CASCADES
Ocean Crust
denser than
continental crust!
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Subduction ALWAYS forms Volcanoes!
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• Continental crust collides with continental crust• Crust same density – NO SUBDUCTION• Folded Mountains form as rock gets crumpled
c) Continental-Continental
Ex. – Sierra Nevadas & Himalayas
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Old Rocks Young Rocks
Evidence of Moving Plates
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• Strike-Slip• 2 plates slide past
each other• Crust deformed or
fractured as giant slabs of crust grind past each other
• Causes earthquakes
• Ex. San Andreas
3. Transform
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• Ridge Push – heat rising pushes crust at a ridge toward a trench.
• Slab Pull – cooler material sinking pulls oceanic crust down into a subduction zone (trench)
Ridge Push and Slab Pull
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