8.1 Plate Tectonics: On the move…
pg 172
Continental Drift
• In 1912, Alfred Wegener proposed the theory of “continental drift.”
• He proposed that the continents were once part of a single land mass and they drifted to
their present locations.
• Wegener named the supercontinent Pangea.
• The vast ocean that surrounded it was named Panthalassa.
Evidence used to support the Continental Drift Theory
1. The Continents shapes match: The shorelines of the continents appear to fit together like
a jigsaw puzzle.
2. The Plants & Animal fossils match: Fossils of the same kind were found on several different
continents. Ex: Mesosaurus
3. The Rocks match: Rocks of the same age, type, and structure are found on widely
separated continents. When the continents are reassembled, the mountain chains form a
continuous belt.
Ex: The Appalachian and Caledonian mountain ranges align.
4. The Ice matches: Glacial striations on rocks show that glaciers moved across continents.
5. The Climates don’t match: The discovery of fossils of tropical plants in the form of coal
deposits in Antarctica. Glacial till & striations found in deserts.
Although plenty of evidence supports Wegener’s Continental Drift theory, it was not accepted
by the scientific world at the time.
Beginning in the 1950’s new evidence emerged to support the old
theory and helped develop a new one…
Plate Tectonics: in general
The Earth’s solid surface is broken up into pieces called ‘plates’ that
“float” on top of the “gooey” part of the mantle.
Crust – 2 types
 Ocean crust
– thin, dark in color, more dense, basaltic
 Continental crust
– thick, light in color, less dense, all types of rock
Tectonic Plates are made from the Earth’s broken
lithosphere. The lithosphere consists of the crust and
the upper solid part of the mantle.
The lower pliable part of the mantle is the
asthenopshere.
The lithospheric plates move over the slow flowing
asthenosphere.
New Evidence used to develop Plate Tectonics
Mid-Ocean Ridges
Chains of volcanic mountains on the ocean floor with a deep central valley called a rift zone.
Mid-ocean ridges are the sights of Seafloor spreading.
Seafloor Spreading occurs when ocean plates move away from each other.
 Magma rises through the rift and spreads out at the surface forming a rift zone and a ridge.
 As magma cools & hardens it pushes ocean floor away from the ridge forming new ocean
crust.
 Old rock at the end of the plate is destroyed as it is forced back into the mantle forming a
trench.
 The age of rocks increases away from the mid-ocean ridge.
Magnetic reversals recorded in rocks along the mid-ocean ridges support Seafloor Spreading
and Plate Tectonics.
 The direction of the Earth’s magnetic field is preserved in the new rock formed at the
ridges.
 The poles switch and are seen in a pattern of bands paralleling mid-ocean ridges.
Locations of Earthquakes & Volcanoes
 Do not occur randomly
 Actually mark the locations of plate boundaries.
 Occur when plates are pushing towards, pulling away or sliding past each other.
Plate Boundaries are where two plates meet. There are 3 types:
1. Divergent Boundaries
• Plates are moving away from each other and new crust is formed.
• Continental or Ocean crust splits.
–A
rift valley forms over land forms when continental crust splits.
–A
mid-ocean ridge forms when ocean crust splits.
Examples:
Continent splits: rift valley
Ocean splits: mid-ocean ridge
The island broken in two along the Mid-Atlantic Ridge is Iceland.
2. Convergent Boundaries
• Plates are moving towards each other.
Ocean  Continent
• Subduction occurs when one plate dives under another forming a trench.
• Ocean crust is carried down into the mantle, where it begins to melt.
• The magma rises to form volcanoes along the edge of the continent.
Ex: Andes Ex: Cascades
Ocean  Ocean
• The deep sea trench is the site of a subduction zone.
• The subducted ocean crust melts and rises to the surface forming an island arc
or volcanic islands. A trench always borders volcanoes – Ring of Fire!
Ex: Japan Ex: Marianas Trench (deepest ocean point)
Ex: Aleutian islands (Chain of volcanic islands off the coast of Alaska)
Continent  Continent
 Continental crust collides with and slides over other continental crust, forming
high mountains.
 The continuous collision of India and Asia, which began about 45 million years
ago, produced the Himalayas
3. Transform Fault
• Ocean or Continental crust slide past each other.
• Forms an off set crack in the ocean or land.
Ex: The San Andreas Fault in California.
Ex: Found perpendicular to the mid-ocean ridges
What drives plate movement? Mantle convection