Ch 10
Plate Tectonics
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Continental Drift
Alfred Wegener (1912)
• German Meteorologist
• Continents moved slowly to their
current locations.
• Pangaea- the single landmass of
Earth that broke apart about
200mya . “All Land”
 Evidence
 Puzzle-like Fit
• Coastlines of
continents fit together
like puzzle pieces.
• Examples:
S.America & Africa

Fossil Clues
• Certain fossils of ancient animals &
plants are found on distant
landmasses.
 Freshwater/land-dwelling reptile
Mesosaurus fossils have been found
on South America & Africa.

Fossils of the ancient plant Glossopteris
were found in Africa, South America,
Australia, India, & Antarctica.

Climate Clues
-Fossils of warm climate plants were found in
present-day cold climates. (Coal)
-Rocks deposited by glaciers from the ice
ages have been found in present-day warm
climates.
 Rock Clues
• Similar rock structures are found on
different continents separated by oceans.
-Appalachian mountains of the USA are
similar to mountains found in Greenland
& western Europe
-Rocks of South America match those in
Africa
Main objection to Wegener’s hypothesis
was its inability to provide a mechanism for
the movement of the continents.
Seafloor Spreading
 Hypothesis
Radio waves were used to map the ocean
floor.
Mid-ocean Ridges- underwater mtn
ranges
• Less dense magma flows sideways
dragging the seafloor along.
• As the seafloor spread apart, magma flows
up through cracks in at the ridge forming
new rock.
Rift Valley
Midocean Ridge
Evidence
•
Drilling
– Ocean floor rock samples show that the
age of the rock gets older the further the
rock is from the mid-ocean ridge
 Magnetism
– Changes in the magnetic alignment of
the Earth are found in alternating bands
in the seafloor rock.
Theory of Plate Tectonics
 Theory
Combined theory of Continental Drift &
Seafloor Spreading.
Earth’s crust & upper mantle
(lithosphere) are broken into plates and
move around on a plastic-like layer of
the mantle (asthenosphere.)
Seven major plates:
Pacific (largest)
S. American
Eurasian
Antarctic
N. American
African
Australian

Plate Boundaries
• Divergent Boundaries
– Two plates spread apart.
– Where seafloor spreading occurs.
– New crust is formed.
Ex: Mid-Atlantic Ridge
•
Convergent Boundaries
– Two plates come together (collide)
– Oceanic plate will sink under a continental
plate (subduction) producing a deep
trench.
Ex: Andes & Cascades
– Two oceanic plates will converge
resulting in a volcanic island arc.
Ex: Mariana Islands
– Two continental plates will converge &
crumple forming folded mountains.
Ex: Himalayas, Appalachians, & Alps
• Transform Fault Boundary
– Two plates slide past each other.
– Move at different rates.
– Also known as strike-slip faults.
Ex: San Andreas Fault
 Causes (Hypothesis)
• Convection Currents
– Hot magma rises in the mantle.
– Magma hits the crust & cools.
– Cooling magma sinks dragging
the lithospheric plate with it
(centimeters.)
– Magma sinks into the mantle &
reheats.
Features Formed by Tectonics
• Faults
– Cracks in the tectonic plates caused
by stress.
– Plate boundaries are also faults.
– Movement along faults cause
earthquakes.
• Volcanoes
• Magma produced by friction during
subduction oozes up through the
crust, erupts, & forms volcanoes.
Mountains
• Convergent & Divergent boundaries
form mountains.
• Process of mountain building is called
orogenesis.
-Volcanic Mountains
• Folded Mountains
oHorizontal rock layers are squeezed
from opposite sides, causing them to
buckle & fold. EX: Himalayans &
Appalachians
 Fault-block Mountains: Huge, tilted
blocks of rock that are separated by
faults.
EX: Sierra Nevada
•
Upwarped Mountains- blocks of Earth’s
crust are pushed up by forces inside the
Earth. (Domes) EX: Rocky Mountains