Transcript GTPlate Tectonics, Continental Drift and Seafloor Spreading

Continental Drift, Plate
Tectonics, and Seafloor
Spreading
Physical Geography 110
Where it all began...
Animations
http://www.suu.edu/faculty/colber
g/Hazards/PlateTectonics/18_Pang
aea.html
http://education.sdsc.edu/optipute
r/flash/pangea_4.htm
http://www.wwnorton.com/college
/geo/egeo/flash/2_1.swf
Video – 10 minutes
• http://www.youtube.com/watch?v=lsZq1pNufNg
Theory of Continental Drift
• First mentioned in 1596, by Abraham Ortelius
• Fully developed in 1912 by Alfred Wegener
• The theory proposed the all of the earth’s
continents were once a single super continent
called “Pangea”.
• Pangea existed 200-300 million years ago.
Wegener’s Evidence
• 1. The way that the continents seem to fit like
the pieces of a puzzle (rock formations).
Wegener’s Evidence
• 2. Glacial deposits from similar places in places
that were no longer cold.
• Striations from glaciers also showed that these
places used to be in colder places (near poles).
Wegener’s Evidence
• 3. The same fossils found in continents that
were across the ocean from each other.
Mesosaurus
• Mesosaurus was a 1 metre long reptile whose fossils
have been found in areas of South America and
Africa that were once believed to be connected.
Lystrosaurus
• Therapsid the size of a pig
• Fossils found in Antarctica, India, and South
Africa
Continental Drift
• Many believed Wegener’s theory was just that,
merely a theory.
• Many said there was no mechanism to explain
how the continents could have moved apart.
• Until.....
Theory of Plate Tectonics
• Wegener’s work combined with new
knowledge from other scientists.
• Arthur Holmes believed there may be plate
boundaries under the sea.
• Holmes also suggested convection currents
within the mantle as the driving force for plate
tectonics.
Theory of Plate Tectonics
• Plate tectonics is the study of how these plates
move and effect the earth.
• The lithosphere (upper layer) is split into
plates that are floating on top of the
asthenosphere.
– Convection current set plates in motion
• Plates – 100km thick, have a defined
boundary, move independently.
Plate Boundaries
• Notice the location of plate boundaries in the
next slide.
• Commonly associated with land forms such as
mountains, volcanoes, faults, and trenches.
• Also associated with events such as
earthquakes.
• Creates land forms (mountains etc. ), and
distributes resources (geological/biological).
Plate Boundaries
Plate Movements
• Plate movements are responsible for 3
processes.
1. Folding – bending and twisting of rocks
commonly found where plates come together.
2. Faulting – rocks moving past each other.
3. Vulcanism – movement of magma above or
below the earth’s surface.
The Theory of Plate Tectonics
• This theory states that the Earth’s crust is
broken into many pieces called plates. These
plates and a portion of the upper mantle
beneath them, called the lithosphere, move
around on top of the rest of the mantle, called
the asthenosphere. Plates are thought to move
around in response to convection currents in
the mantle of the Earth.
Why do Plates move –
Convection Currents
• Magma in the Earth’s surface becomes hot in
hot spots.
• Hot magma is less dense therefore it rises.
• As it rises it cools and spreads out.
• As it spreads it drags the plates with it.
• The magma cools and moves back down.
• These are convection currents.
Convection Currents
Three types of Plate Boundaries
1. Convergent – where two plates meet each
other and crash/crunch against each other.
Two types are subduction zone (under the
ocean) and collision zone.
2. Divergent – where plates split and move
apart.
3. Transform – plates slide along or past each
other due to shear force.
Convergent Plate boundary
Convergent Plate boundary
Divergent Plate Boundary
Divergent Plate Boundary
Transform Plate Boundary
San Andreas Fault
Seafloor Spreading
• Proposed by Harry Hess
• This theory states that the Atlantic Ocean is
spreading/growing from the mid-ocean ridge.
• The mid-ocean ridge is 10,000km long and
3.2km high above the seafloor.
• Some scientists believe the Atlantic Ocean will
one day grow to be larger than the Pacific.
• Cuts directly through Iceland.
Mid-Ocean Ridge
Evidence of Seafloor Spreading
• Oceanic rocks are youngest near the mid ocean
ridge.
• Continental rocks are 4 billion (4,000,000,000)
years old while the oldest oceanic rocks are
only 180,000,000 years old.
• Parallel bands of rocks on either side of ridge
that had matching but alternating magnetic
poles.
Mountain Ranges Below the Sea
Seafloor Spreading with Bill Nye
• http://www.youtube.com/watch?v=6CsTTmvX6mc
Mid-Ocean Ridges:
http://www.youtube.com/watch?v=FQsbT9j3P44&feature=related
Aliant Learning Seafloor Spreading
http://learning.aliant.net/