Download 8.1 Earth has several layers

8. 2 Continents change
position over time
Learning Goals
• Students will:
-explain how the continental drift
hypothesis was developed.
-explain evidence for plate movement
that scientists have gathered from
observations of the sea floor.
-describe how scientists developed the
theory of plate tectonics.
3-Minute Warm-Up
• Draw a diagram showing and labeling
Earth’s four layers. Identify the
lithosphere and asthenosphere on your
diagram.
Continents join together and
split apart
• Mapmakers noticed that eastern coast of S.
America and western coast of Africa fit
together
• Alfred Wegner
– Hypothesized that Earth’s continents were
once joined in a single mass
– They gradually moved apart
– People didn’t believe him at first
• continental drift—the hypothesis that Earth’s
continents move
Evidence of Continental Drift
• Fossils (pg. 271)
– Fossils found in S. America and Africa and
no where else
• Climate
– Ancient climate zones show tropical plants on
Greenland, which is now mostly ice. Rocks with
deep ice scratches can be found in South
Africa, which is now a warm climate.
• Geology
– Rocks found in today’s continents
matched rocks are found in different
areas and different continents
Pangaea and Continental Drift
• Supercontinent of all continents together
was called Pangaea—“all lands”
• Pangaea—a supercontinent that included
all of the landmasses on Earth.
• Wegener could not explain how it moved,
so people rejected his hypothesis
Plate Tectonics
• The theory of plate tectonics explains how
plates and their continents move
• Wegner’s idea was pushed aside, until
scientists discovered that tectonic plates
move
Evidence from the Sea Floor
• mid-ocean ridges—a long line of
sea-floor mountains where new
ocean crust is formed by volcanic
activity. (underwater mountain
ranges)
• Sea-floor spreading
– Ridges form along cracks in the
crust
– Molten rock rises from these
cracks
– As the sea floor spreads (pulls
apart) the old crust is pulled
away, making room for new
material
•
•
http://emvc.geol.ucsb.edu/3_downloads/M1GTect/eSoAtlCutGlobeMovi
es/SoAtlantic_CutGlobe.mov
http://emvc.geol.ucsb.edu/3_downloads/M3RegTect/bNoCentAtlantic/N
oCent_Atlantic.mov
• Age of the sea floor
– Scientists discovered that
the rocks that were
further from the ridge
were older, so this was
evidence of sea-floor
spreading
•
http://emvc.geol.ucsb.edu/3_downloads/M3RegTect/aSoAtlanti
c/SoAtlanticY.mov
• Ocean trenches
– Because the sea floor was spreading and
increasing, scientists wondering why the
Earth did not grow
– Reason it did not grow is because there
are places where denser oceanic crust is
sinking into the asthenosphere
•
•
http://emvc.geol.ucsb.edu/3_downloads/M3RegTect/dNoPacific/NoPPacific.mov
http://emvc.geol.ucsb.edu/3_downloads/M1GTect/eSoAtlCutGlobeMovies/SoAtlantic_
CutGlobe.mov
Causes of Plate Movement
• Tectonic plates rest on the asthenosphere
(layer of soft, hot rock)
• Convection currents within Earth helps to
move the plates
• convection—the transfer of heat by the
movement of a material
• convection current—a motion that transfers
heat energy to a material
• Moves very slowly, a few centimeters a year
• Push and pull motion
Putting the theory together
• theory of plate tectonics—the theory that states that
Earth’s lithosphere is made up of huge plates that
move over the surface of Earth.
• Scientists combine their knowledge of Earth’s plates,
sea floor spreading and the asthenosphere to
create the theory of plate tectonics
• Plates can move apart, push together, or scrape
past each other
• Plate movements cause great change—
earthquake, volcanoes, and mountains appear
where tectonic plates meet
Earth’s tectonic plates. Arrows indicate the direction of movement.
• http://emvc.geol.ucsb.edu/3_downlo
ads/M1GTect/aPangaeaMovies/Pang
aea.mov
Questions
• What evidence did Alfred Wegner use
to hypothesize his theory of continental
drift?
• What does Pangaea mean?
• What is evidence that the sea floor is
spreading?
• Why doesn’t the Earth grow because
of sea floor spreading?
• In what ways do plates move?