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Chapter 7 Plate Tectonics
Section 7-1 Earth’s Interior
The Earth is composed of 4 layers:
Video How do we know what’s
inside Earth?
Earth’s Interior
Earth’s Interior
Crust- Earth’s “outer skin” 3-40 miles thick. Made
of solid rock. Thickest on continents (continental
crust), thinnest below ocean (oceanic crust)
Mantle- largest layer, plastic-like and able to flow
Outer core- liquid iron and nickel.
Inner core- solid iron and nickel because of
extreme pressure.
History of Plate Tectonics
Alfred Wegener- German scientist, believed continents were
once joined and moved to present locations.
Proposed theory of Continental Drift in
1912.
Stated Pangaea broke apart over
200 million years ago.
Wegener and Pangaea
Evidence of Wegener
1. Puzzle-like fit of continents.
South America and Africa look to fit.
2. Fossil Clues- Same fossils
found on South America and
Africa
Mesosaurus- alligator-like
fossil
Glossopteris- fernlike fossil
Evidence Con’t
3. Rock Clues- Similar rock types of same age found in
mountains of England and Eastern U.S.
4. Glacial Clues- Evidence of glaciers exist in areas that are
too warm for them now
Wegener died in 1930, scientific community made a joke of his work.
Seafloor Spreading
1960- Harry Hess took Wegener’s work and proposed
theory of Seafloor Spreading.
Hot, molten material is forced upward and pulls apart
the crust. Lava fills in rift to create new seafloor.
Seafloor spreading
Mid-Atlantic Ridge
Mid-Atlantic Ridge
discovered during
WWII while Navy was
mapping ocean floor
How do we know seafloor
looks like this?
Sonar waves, satellites,
GPS
Evidence for Seafloor Spreading
1. Age of rocks. Glomar Challenger, a seafloor drilling ship
sampled rocks along ocean ridge. Rocks get older moving
away from the Mid-Atlantic ridge.
Evidence Con’t
2. Magnetic Clues- a magnetometer showed that the
rocks along the mid-atlantic ridge had a reversal in
magnetic polarity.
Magnetic evidence
Earth’s Magnetic Field
• Generated by the circulation of the core of
the Earth
• Protects us from harmful rays of the Sun
• Rocks on seafloor record past events of
magnetic reversal.
Theory of Plate Tectonics
Plate Tectonics-states that Earth’s crust and upper
mantle are broken into sections called plates.
12 major plates, move at an average of 1 inch per year.
Lithosphere-crust and upper mantle plates are composed of.
The lithosphere floats on top of the a plastic-like layer
called the Asthenosphere
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What causes the Plates to Move?
Convection Currents- A heating cycle in the
mantle in which mantle material rises and cools
Plate Boundary Types
Divergent- 2 pieces of continental crust
Causes EQ’s, rift valleys, volcanoes
Examples are Iceland, Great rift
valley, and Red Sea
Divergent- 2 pieces of oceanic crust
Examples are Mid-Atlantic Ridge,East Pacific
Rise. EQ’s and mid-ocean ridges occur
Plate Boundaries
Convergent- 2 pieces of continental crust
Causes Earthquakes, mountain building. Forms the
Himalayan Mts.
Convergent- 1 piece of continental crust, one piece
of oceanic crust
Magma generated
along subduction
Zone. Area where
crust melts into
mantle.
Causes Earthquakes, volcanic mountains, trenches
Andes mountains, Mt St. Helens
Convergent- 2 pieces of oceanic crust
Formed Japan,
Philippines,
Marianna Trench
Causes EQ’s, volcanic island arcs, trenches
Transform boundary
Causes Earthquakes
Formed San Andreas
Fault