PLATE TECTONICS
A. DRIFTING CONTINENTS
1. Alfred Wegner proposed the Theory of
Continental Drift in early 1900’s.
•Wegner’s theorized that all the
continents were once a single landmass
about 245 million years ago that
gradually drifted apart (are still
drifting).
Called this supercontinent Pangea
which is Greek for “all Earth”
The land masses Split again –
Laurasia & Gondwana 180 million
years ago
See Page 16
2. Evidence for Pangaea
a.Continents fit together like puzzle
pieces
b.Fossils of the same plants and
animals are found on different
continents
**climate evidence supports this
c.Mountains that appear on one
coastline had similar formations and
rocks on opposite coastlines
BUT>>>>>Wegner couldn’t explain
why the plates moved
See Page 15
Matching Mountain Ranges
Glacier Evidence
3. How did plates move then?
1950’s-mapping of the seafloor found Midocean Ridges-huge underwater mountain
chains found in every ocean around the
Earth
a. They formed along cracks in the crust
where molten rock pushed up and out in a
process called Sea Floor Spreading
 younger rock found closer to ridge than on
continents.
 Earth DOESN’T get larger because oceanic
crust is destroyed along deep-ocean
where why
the oceanic
plates
sink into
If trenches,
this is true,
isn’t the
Earth
the asthenosphere
getting bigger?
Sea Floor Spreading
4. How do plates move then?
• Due to tremendous heat, rock in the
asthenosphere is like hot taffy
• This allows plates to ride on top of hot,
flowing rock.
• Plates move because heat is being released
from deep inside the earth.
• Convection currents causes hot material to
rise and expand (plates diverge) and cooler
material to sink and contract (plates
converge).
https://www.youtube.com/watch?feature=player_embedded&v=ryrXAGY1dmE
See Page 18
See Page 19
All of this new information helped to
create the Theory of Plate Tectonics
Section 1.3
C. Plates Move Apart
See Page 35
Boundaries are
formed when
tectonic plates
move. The
direction of
the movement
determines
the type of
boundary.
1. Divergent Boundary: plates move apart
• See Page 23
a. Usually found in the ocean
b. Mid-ocean ridges and rift valleys occur
• Mid-ocean ridges form the longest mountain ranges on earth.
• Most contain a rift valley along their center.
See Page 26
Continued
c. Earth’s magnetic poles have switched places
several times at divergent boundaries.
See Page 24
See Page 24
These magnetic reversals are caused by changes in Earth’s magnetic fields.
Bands of rock record periods of magnetic reversals. As molten material cools,
magnetic minerals line up with the magnetic field. When it hardens, the
minerals act like tiny compass needles.
d. Hot spots can be used to
track plate movements.
• Hot Spot: an area of volcanic activity
that develops above where magma
rises in a plume from the mantle.
– They don’t move but plates do
– A fixed point to measure speed and
direction of plates
The Hawaiian islands are located in the middle
of the Pacific Plate. The largest island,
Hawaii, is still over the hot spot.
See Page 28
When the plate moves on, it carries the first volcano
away from the hot spot. Heat from the mantle
plume will then melt the rock at a new site, forming
a new volcano.
2.Convergent Boundary: plates push together.
• They form three
types of
convergent
boundaries.
• One plate is
subducted or
pushed below the
lighter plate
a. Continental-Continental
2 continental plates
collide crumpling
and folding the
rock between them.
Mountains could
form.
Continued
Oceanic-oceanic subductions: two oceanic
plates collide and the older, denser plate sinks
beneath the top plate, forming deep-ocean
trenches and island arcs.
Continued
Oceanic-continental subductions: an oceanic
plate sinks beneath a continental plate, forming
a deep-ocean trench and volcanic coastal
mountains.
See Page 32
3. Tectonic plates scrape past each
other at transform boundaries.
• Two plates move
past each other in
opposite directions.
• No crust is formed
or destroyed.
• Occurs on the sea
floor and on land.
The San Andreas Fault is a transform
boundary and moves about 1 inch per year.
Comparing Boundaries
Divergent
*plates move apart *in ocean and on
land *produce mid-ocean ridges, rift
valleys, volcanoes, earthquakes
Transform
*plates move past each other in
opposite directions *in ocean and on
land
*crumples and folds crust * produces
mountains, earthquakes
Convergent
C-C Collision
Convergent
O-O Subduction
Convergent
O-C Subduction
*older, denser plate sinks *produce
deep-ocean trenches, island arcs
*oceanic plate sinks under
continental plate * forms deep-ocean
trench, volcanic coastal mountains
The theory of plate tectonics helps
geologists today.
• The plate tectonics theory
enables geologists to
understand how Earth’s
continents and ocean
basins formed.
• Helps scientists predict
earthquakes and volcanic
activity.