Plate Tectonics
BIG Idea:
 Most geologic activity occurs
at the boundaries between
plates.
I. Drifting Continents
• MAIN IDEA: The shape and
geology of the continents
suggests that
they were
once joined
together.
Continental Drift
theory proposed by
Alfred Wegener
(VAY guh nur) in 1912
stated that the continents
were once a single landmass
the continents have moved...
-Pangaea (“all the earth”):
supercontinent; single landmass
-Panthalassa (“all seas”): huge ancient
ocean
A. Evidence of Continental Drift
1. Puzzle-Piece Fit
2. Rock Formations
Ages, types and layers of
coastal rocks of widely
separated regions matched…
Western Africa and
Eastern Brazil
Appalachians
(along eastern
U.S.) fit
Greenland and
Europe
3. Fossils
Mesosaurus: freshwater reptile
that lived 270 million years ago
• remains (fossils) found in South
America and Africa
• impossible for these reptiles to
have crossed the
Atlantic
4. Climate
(a) Glossopteris: fern
that grew in
temperate climates
 places where fossils had been
found were once closer to the
Equator
 rocks containing these fern fossils
had once been joined
(b) Coal Deposits:
 Coal forms from ancient swamp
plants
 Coal beds found in Antarctica
indicated that this frozen land
once had a tropical climate
 Antarctica
must have
been closer
to the Equator
(c) Glacier Deposits:
 found in Africa and South
America
 climates are much warmer
there today
A Rejected Notion…
• Wegener’s Continental Drift
Hypothesis was never
accepted by the scientific
community
• He could not explain what
forces could cause such
massive movement
• Wegener died in 1930, on expedition
in Greenland, while collecting
evidence to further support his
theory
II. Seafloor Spreading
• MAIN IDEA: Oceanic crust forms at
ocean ridges and becomes part of the
seafloor.
1.Mid-Atlantic Ridge:
undersea mountain range
with a steep narrow valley
down the center
• the middle of the ocean
floor is very young
2. Renewal of the Ocean Floor:
seafloor spreading - floor moving
away from the center
Harry Hess and Robert Dietz
3. Paleomagnetism: as magma solidifies
the iron-rich minerals align with
magnetic north like a compass
reverse polarity: magnetic
orientation pointing south
III. Plate Boundaries
• MAIN IDEA: Volcanoes, mountains,
and deep-sea trenches form at the
boundaries between the plates
Theory of Plate Tectonics
• Plate Tectonics:
theory that the
lithosphere is made
of rigid plates that
‘float’ on the
asthenosphere
• Tectonics: the
study of the
formation of
Earth’s features
What are the inferred
properties of Earth’s
interior?
Zones of Earth
1. Crust: thin,
solid outermost
layer; surrounds
Earth like a shell
surrounds an
egg.
Two Types of Crust:
Continental Crust: makes up the
continents (land); thicker, less
dense; granitic rock
Oceanic Crust: under the oceans;
thinner, MORE dense; basaltic
rock
2. Moho: boundary between the
crust and mantle.
3. Mantle:
 thickest layer
 sits below the crust
 makes up 2/3 of the Earth’s mass
•Lithosphere (‘rocky
sphere’): upper part of
mantle, cool and rigid
• Asthenosphere (‘weak
sphere’): lower (‘plastic’)
mantle; flows like hot tar
• Plasticity: the ability of a
solid to flow
4. Core: center of the Earth;
two parts:
• Inner Core: ball of hot,
solid metals (iron and
nickel)
Outer Core: only true liquid
zone
If we’ve never been there,
how have we learned about
the interior of Earth?
By studying seismic waves…
Seismic Wave:
vibration that
travels through
the Earth;
results from
earthquakes or
huge
explosions
Types of Plate Boundaries
1. Divergent Boundaries:
• two plates that are moving AWAY
from each other
• as plates move apart, the
asthenosphere flows up to fill the
empty space
What forms when plates
DIVERGE?
a. Mid-Ocean Ridges
Mid-Atlantic Ridge
b. Rift Valleys:
(1) narrow valleys in the
middle of ocean ridges
(2) where continents
separate
East African Rift Valley
2. Convergent Boundaries
• direct collision of one plate with
another
Three types of converging
boundaries:
• Types of Convergent
Boundaries
Oceanic vs. Continental
Subduction Zone: – oceanic crust
moves (dives) under continental
crust
 forms an oceanic trench and
volcanic mountain range
 oceanic crust is more dense than
continental, thus it dives down or
subducts
Andes Mountain Range
Mount Saint Helens - Cascade
Mountain Range
Oceanic vs. Oceanic
 older, more dense oceanic crust
subducts
 forms trenches and arcs of
volcanic islands
Aleutian Islands
Mariana Trench
Continental vs. Continental
 no subduction
 two continental plates collide
pushing up and creating folded
mountains
3. Transform Boundaries
• Plates slide horizontally past
each other
IV. Causes of Plate Motions
• MAIN IDEA: Convection
currents in the mantle
cause plate
motions.
cool material sinks as hot
material rises, due to
DENSITY DIFFERENCES.