Continental Drift
How many continents are
there?
Were the continents always
located in the same position?
Drift animations
http://www.classzone.co
m/books/earth_science/t
erc/content/visualizatio
ns/es0806/es0806page0
1.cfm?chapter_no=08
http://www.wwnorton.com/co
llege/geo/egeo/flash/2_1.swf
A continental puzzle
• In 1912 a German meteorologist named
Alfred Wegener noticed that the continents
fit together like puzzle pieces.
• He proposed that the continents were
joined together in the past, in a large land
mass called Pangaea.
http://www.sci.csuhayward.edu/~lstrayer/geol2101/2101_Ch19_03.pdf
• Over time, the continents drifted apart –
Wegener named his theory “Continental
Drift”.
He supported his theory with four
pieces of evidence.
1. Puzzle like fit of continent edges
http://maps.google.com
2. Matching fossils of plants and animals on
once connected land areas.
Fossils of Glossopteris are found in Permian
rocks of South Africa, India, Australia, South
America, and Antarctica
3. Rock similarities and ages
Mountains in South America and Antarctica are
believed to have formed as part of the same
mountain chain.
4. Climate evidence (Glacial evidence in
Africa, South America, Australia, and
India and tropical plant fossils in Arctic
areas)
Fossils found in Antarctic soil indicate that the now
frigid continent was once lush with trees and
ferns, and home to dinosaurs, amphibians, and
later, marsupials.
Wegener’s theory made sense, but no one
wanted to accept it until they knew HOW the
continents moved.
• In the 1960’s, a Princeton University
scientist named Harry Hess, discovered
how the continents drifted.
Continental Drift Whiplash
Seafloor Spreading
• Magma in the mantel rises and pushes the
plates apart, forming new oceanic crust.
http://education.sdsc.edu/optiputer/flash/seafloorspread.htm
Seafloor Spreading causes
Continental Drift
Supporting evidence of seafloor
spreading
• 1. Magnetic iron particles record the time
of the rock formation.
http://www.wwnorton.com/college/geo/egeo/flash/2_3.swf
• 2. Rocks farther away from the opening
age at the same rate on both sides.
http://www.wwnorton.com/college/geo/egeo/flash/2_5.swf
A map of the ocean floor provides
even more evidence
http://maps.google.com/
Iceland shows seafloor spreading
above the water, which makes it
easier to study
Plate Tectonic Theory
• Theory of Plate Tectonics -Earth’s crust
is broken into plates which float and
move.
Earth’s crust made of many plates
is similar to the panels on the
outside of a soccer ball.
There are about 13 plates covering
Earth’s surface
Plate Boundaries (edges)
When the plates move, their
boundaries, or edges, can scrape
and collide.
http://www.wwnorton.com/college/geo/egeo/flash/2_6.swf
Convergent Boundary
• Plates move toward each other
Convergent Boundary
• When two continental plates move into
each other, the plates combine and form
mountains. (India into Asia)
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1105/es1105page01.cfm?chapter_no=visualization
Convergent Boundary
• When an oceanic plate runs into a
continental plate, the heavier oceanic
plate subducts (sinks) back into the
mantle.
• Volcanic mountains are created along
this edge.
Oceanic plate into continental
• Example: Pacific plate (oceanic) subducts
(sinks) under Japan (continental).
http://maps.google.com/
Divergent Boundary
• Plates move apart
When both diverging plates are both
oceanic, it is called seafloor
spreading
(Mid-Atlantic Ridge)
When both diverging plates are
continental it is called rift valley
formation (Africa)
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0072402466/30425/19_21.swf::Fig.%2019.21%
20-%20Evolution%20of%20a%20Divergent%20Plate%20Boundary
Transform Boundary
• Plates slide past each other
San Andreas Fault
San Andreas Fault, CA
• Each time the plates slide past each other,
an earthquake occurs
Find the three boundaries
Asthenosphere
• A plastic-like layer found below the
lithosphere.
• The rigid oceanic and continental plates of
the lithosphere sit on top
The Asthenosphere is heated by
the hot Outer Core
Convection Current
crust
mantle
core
• Hot material rises, cooler material
sinks, creating a current, called a
Convection Current
When the asthenosphere moves, it carries
the lithospheric plates (divergent,
convergent, and transform motions)
Convection currents cause plate motion
http://www.absorblearning.com/media/attachment.action?quick=12p&att=2775
Every time these plates move we
get earthquakes, volcanic
eruptions, and possibly tsunamis