Download chpt 7Plate Tectonics

Plate Tectonics
CONTINENTAL DRIFT
SEAFLOOR SPREADING
THEORY OF PLATE
TECTONICS
Continental Drift
 Evidence for Continental Drift
 Pangea: Alfred Wegener (1912) suggested that all the
continents were joined together at some time in the
past

Continental Drift was suggested that this huge land mass
(called Pangea) began to break apart about 200 mya
 Idea wasn’t accepted as he was unable to explain how
they drifted apart
 Long after his death (1930) was there enough
evidence to accept Continental Drift
Pangea Videos
http://www.5min.com/Video/Learn-about-The-Continental-Drift---PlateTectonics-117505893
http://www.pond5.com/stock-footage/128542/pangea-earth-s-continentsseparate.html
Clues to support Continental Drift
 Fossil clues: fossils found (Mesosaurus) in S
America and Africa that were freshwater land
animals that couldn’t have swum sea water to both
sides, therefore lived together before the split
 Widespread Plants: Glossopteris fossil found on
Africa, Australia, India, Antarctica, S America
suggested that these plants thrived once when
continents were attached
Clues to support Continental Drift
 Rock clues: similar rock structures found on
different continents
 Climate Clues: glacier deposits found in tropical
climates and tropical plants found on island in Arctic
Ocean
How could continents drift?
 Wegener’s radical idea of continental drift provided
no evidence as to how the continents plowed through
higher density ocean floor material
 His proposal suggested it was caused by Earth’s
rotational spin, but no evidence supported that
 Only after discovery of seafloor spreading was his
idea re-introduced as explanation for continental
drift
Seafloor Spreading
 Mapping the Ocean Floor: sound and radar waves
were used to map the ocean floor in 1940’s and 50’s
 Noticed that in middle of oceans were huge
mountain ridges
 This discovery caused scientists to ask why?
Seafloor Moves
 Harry Hess suggested that the seafloor moves
 Proposed that hot, less dense material below Earth’s
crust rises toward the surface at mid ocean ridges ,
lows sideways carrying the seafloor away from te
ridge in both directions
Evidence for Spreading
 Research ship Glomar Challenger gathered data in
1968 by drilling ocean floor
 Found younger rock are located at the mid-oceanic
ridges and older rock found farther away
 Mid-oceanic ridges have molten rock, heat, exotic
life-forms
Evidence for Spreading
 Magnetic clues: when molten rock cools, it cools
with magnetic attractions to the north and south
poles
 Found that magnetic north has changed several
times in earth’s history and the magnetic poles are
preserved in the rock at mid-oceanic ridges
 On each side of the mid-ocean ridges portrays these
magnetic changes
 Magnetometer detects the magnetic field reversals
on both side of ridges indicating that the ridges are
moving apart when new rock is formed
Seafloor spreading and Magnetic reversals
Theory of Plate Tectonics
 Plate Movement: plate tectonics combine the
knowledge of continental drift and seafloor
spreading in the Theory of Plate Tectonics
 Earths
crust and upper mantle are broke into
plates
 These
plates ride on top a plastic-like layer of the
mantle much like a raft on water
Composition of Earth’s Plates
 Lithosphere is the outer layer of earth’s crust
 Made of less dense material than plastic-like layer
below it (asthenosphere)
Plate Boundaries
 When plates move they interact with each other in
many ways
 They can move toward, pull away or slide past each
other.
 When plates interact, the result of their movement is
seen at the plate boundaries
Plate Boundaries
 Plates Move Apart: Divergent boundaries are
areas where plates move away from each other
 Mid-Atlantic Ridge is divergent boundary between
North American Plate and the Eurasian and African
Plate
 Great Rift Valley in eastern Africa is result of
continental plate being pulled apart
Plate Boundaries
 Plates Move together: Convergent
boundaries are areas where plates move together
 As crust is added in one area, crust disappears in
other areas when seafloor cools, becomes denser and
sinks (convergent boundaries)
 When oceanic crusts converge with less dense
continental crust, the denser oceanic plate sinks
under the continental plate in subduction zones
 Deep-sea trenches are associated with convergent
boundaries

http://www.geocraft.com/WVFossils/subduction.html
Plate Boundaries
 As ocean plates sink under continental plates,
they warm and melt and magma forms. This less dense
magma rises to surfaces and form volcanoes

Cascade Mt ranges are result of convergent boundaries and
subduction zones
 A subduction zone can form from two oceanic plates
that collide where the older, denser plate sinks (Mariana
Islands are chain of volcanic islands )
 When 2 continental plates collide, usually they
crumple up forming mountain ranges (earthquakes
common, but no volcanoes) (Himalayan Mts)
Plate Boundaries
 When plates slide past each other:
transform boundaries are result where two
plates slide past each other
 They move in opposite direction and usually
earthquakes are associated with transform
boundaries
 San Andreas fault is an example
Causes of Plate Tectonics
 Question about what was force that would cause
plates to move were finally answered by convection
currents
 Convection currents in earth’s crust are currents
caused by warm less dense magma rising
 Some magma reaches the surface at ridges, some is
moved away from rising magma and drags the plates
with them. They cool and settle back to earths center
where it moves and is heated up and cycle repeats
Causes of Plate Tectonics
Causes of Plate Tectonics
Features Caused by Plate Tectonics
 Interactions of plates build mountains, create ocean
basins and cause volcanoes

Earthquakes are associated with these interactions
 Tension forces result in earth crust being pulled
apart and form normal faults
 Causes large blocks of crust to break, tilt, or slide
down and large fault-block mountains form
Rift valleys and mid-oceanic ridges are associated
with divergent boundaries where tension forces pull
crust apart and normal faults occur
Tension forces and Normal faults
Rock layers above the fault move down when
compared with rock layers below the fault
Compression Forces cause Reverse Faults
 Where plates meet, compression force cause
massive folding and faulting into mountain ranges
(Himalaya) by reverse faulting
 Rock layers above the fault surface is moved up
relative to rock layers below the fault
Reverse and normal faulting
 Reverse fault:
 Compression forces
 Normal fault:
 Tension forces
Strike-slip Faults
 Caused at transform boundaries where 2 plates move
past each other (usually in opposite directions, but
can be where both plates are moving in same
direction but at different speeds)
 These are called strike-slip faults
New technology and Plate tectonics
 With GPS and modern technology we have been able
to record crustal movement on plates as low as 1 cm
per year
 Pacific plate is moving at rate of 2.5 cm per year
average