Chapter 8 Plate Tectonics
Earth’s Drifting Continents
I. Theory of Continental Drift (pg 220)
A. proposed by Alfred Wegener (1912)
B. Earth’s continents were once joined together as one large piece of land (Pangaea) that
broke into smaller pieces that drifted apart.
C. Earth’s plates are moving 1-4 inches every year because of convection currents in the
mantle.
D. Most scientists did not believe Wegener’s theory until evidence was found to support
it (late 1950’s and 1960’s).
II. Evidence from Fossils (pg 221)
A. Identical fossils found on different continents support Wegener’s Theory of
Continental Drift.
B. Scientists believe that the fossils were once located on Pangaea and then drifted
apart.
1. Glossopteris, an extinct plant who’s fossils are found in South Africa, Australia,
India, and Antarctica.
2. Mesosaurus fossils found in South America & Africa supported Wegner’s theory
III. Evidence from Rocks (pg 221)
A. Rocks on different continents line up like puzzle pieces to support Wegener’s theory.
1. Ancient folded mountain chains in Africa and South America.
2. Coal fields in Africa and South America
B. Some rocks are evidence of the climate change that would have occurred when
continents moved.
1. Glacial deposits (from ancient glaciers) are found in South America, Africa,
India, Australia, and Antarctica.
2. Salt, coal, and limestone deposits
Earth’s Spreading Ocean Floor
I. Ocean Floor Spreading
A. caused by volcanic activity at midocean ridges
1. lava spills out from rift valleys between ridges, hardens, and forms new ocean
floor.
2. as it hardens, it pushes old ocean floor away, causing the ocean floor to spread
B. As the ocean floor moves, it takes any attached continents with it. This helps to
explain why continents drift!
C. Subduction occurs at trenches.
1. old ocean floor sinks back into the mantle and melts back into molten rock.
2. some magma may rise back up to form volcanoes
Earth’s Moving Plates
I. Theory of Plate Tectonics (pg 224-225)
A. combines theories of continental drift and ocean floor spreading to explain how the
Earth has changed over time
II. Lithospheric Plates (pg 224 )
A. usually contain both oceanic and continental crust
B. 7 major plates: Pacific plate is the largest
III. Plate Boundaries (pg 225)
A. most plate boundaries are on the ocean floor
B. divergent boundaries – where plates move apart (mid-ocean ridges)
C. convergent boundaries – where plates come together (trenches)
1. earthquakes and volcanoes occur
D. strike-slip boundaries – plates move horizontally past one another
1. formed by lateral faults (strike-slip faults)
2. earthquakes often occur
3. ex. San Andreas fault in California
IV. Plate Motion (pg 222)
A. convection currents in the mantle cause the lithospheric plates to move at different
speeds in different directions
B. plates collide!
1. collision of oceanic and continental plates: oceanic plate is subducted, volcanoes
and/or mountains may form.
2. collision of two continental plates: crust is pushed up to form mountains
3. collision of two oceanic plates: denser plate is subducted,
volcanoes may form an island arc.