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Plate Tectonics
8th Science
Earth’s layers

The Earth has 3 layers: core, mantle, crust.
http://geology.com/articles/mohorovicic-discontinuity.shtml
Core
The core is the densest region.
 It is made up of the inner core, a ball of
hot, solid metals, and the outer core, a
layer of liquid metals surrounding the
inner core.

http://web.ics.purdue.edu/~braile/edumod/threedearth/threedearth_files/image017.jpg
Mantle
The mantle is the thickest layer, measuring
about 1700 miles (2900 km).
 It is made of hot rock less dense than the
core.
 The top part of the mantle is cool and
rigid.
 Just under that, the rock is hot and soft,
like a thick paste.

Crust
The crust is a thick layer of cool rock.
 There are two basic types of crust:
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◦ Continental crust: all continents and some
major islands
◦ Oceanic crust: all ocean floors
http://www.enchantedlearning.com/ogifs/outerlayersearth.GIF
Lithosphere
The crust and the very top of the mantle
together form the lithosphere.
 The word litho- means “stone” or “rock”
in Greek.
 This is the most rigid of all layers.
 It sits atop the asthenosphere.
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Asthenosphere
This layer is hotter, softer rock in the
upper mantle.
 The word asthenés means “weak” in
Greek.
 This layer is not weak, though, but moves
slowly like hot tar.

http://www.radford.edu/jtso/Geol
ogyofVirginia/Photos/Tectonics/As
thenoLitho.jpg
Tectonic Plates
The lithosphere is broken into many large
and small slabs of rocks, called tectonic
plates.
 These plates fit together like a puzzle.
 They include both the continental crust
and the oceanic crust, which means parts
of the plates (or in some cases the entire
plate) can lie underwater.

http://2.bp.blogspot.com/-BxljyV06U2Y/TcFRtTvJjoI/AAAAAAAAALE/0soH2qBqzk8/s1600/tectonic%2Bplate%2Bboundaries.jpg
Continental Drift
The discovery of tectonic plates led to the
discovery of continental drift.
 In 1912, Wegener first proposed this idea based
on several things:

◦ Fossils of the ancient reptile Mesosaurus had been
discovered both in South America and Africa.
◦ Scientists have discovered tropical plant fossils in
Greenland (which is near the Arctic circle).
◦ The types of rocks that make up the continents are
probably the best evidence that the continents have
moved. The type of rock found in Brazil matched the
type of rock found in western Africa; the limestone
found in Appalachian Mountains is exactly like the
limestone found in Scotland.
Pangaea
Pangaea is the name given to the super
continent.
 The word means “all lands” in Greek.
 Wegener believed that all the continents
were at one time joined together
approximately 200 million years ago.

http://images.yourdictionary.com/images/3980.16.pangaea.jpg
More evidence
In the 1950s, scientists began to map the ocean floor
and discovered huge underwater mountain ranges,
called mid-ocean ridges.
 The mid-ocean ridges appear in every ocean, circling
the Earth.
 These ridges form along cracks in the crust; molten
rock rises through these cracks, cools, and forms new
oceanic crust. The old crust is pulled away to make
room for new material, which means the sea floor
slowly spreads apart.
 Ocean trenches, which are like deep canyons in the
oceans, are actually sites where the oceanic crust is
sinking into the asthenosphere. This is why new crust
forms but the Earth doesn’t actually get any bigger.

How do the plates move?
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The tectonic plates rest on the asthenosphere,
which is soft, hot rock that moves slowly.
Rock in this layer (and in the mantle) move by
convection, which is the energy transfer by the
movement of a material.
The hot, soft rock of the asthenosphere rises to
the surface, then cools and sinks, where it then
heats back up and rises again.
The sinking-rising motion is called a convection
current, a motion that transfers heat energy in a
material.
This process takes a long time, the rock only
moves a few centimeters a year.
Two other possible movements
Slab pull happens when gravity pulls the
edge of a cool, dense plate into the
asthenosphere; because the plates are
rigid, the entire plate is dragged along.
 Ridge push happens when material from a
mid-ocean ridge slides downhill from the
ridge; the material pushes the rest of the
plate.

Theory of Plate Tectonics
The theory says the Earth’s lithosphere is
made up of huge plates that move over
the surface of the Earth.
 There are 19 major tectonic plates:
African, the Antarctic, the Australian, the
Indian, the Eurasian, the Nazca, the North
and South American, and the Pacific
plates.

http://www.enchantedlearning.com/egifs/Earthsplates.GIF
Try naming all the Plates

http://www.learner.org/interactives/dynam
icearth/plate2.html
How plates move

These plates can move in 3 ways:
◦ A divergent boundary is when the plates
move apart.
◦ A convergent boundary is when the plates
push together.
◦ A transform boundary is when the plates
scrape past each other.

The plate movements cause great changes
in the Earth’s crust, such as earthquakes,
volcanoes, and mountain ranges.
http://www.age-of-the-sage.org/tectonic_plates/volcanoes_earthquakes.gif
Divergent Boundaries
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When plates move apart, the sea floor spreads.
The mid-ocean ridges are the sites of divergent boundaries.
As the ridges widen, a gap is formed, called a rift valley.
The rift valley is where the molten material rises to build
new crust.
The world’s longest ridge is the Mid-Atlantic Ridge and runs
the entire length of the Atlantic Ocean.
◦ The North and South American plates are moving away from the
African and Eurasian plates.
◦ It extends from Iceland in the north to Antarctica in the south,
running approximately 6214 miles (11,000 km).
◦ The rift valley is approximately 15 miles wide (24 km) and 6
miles (9 km) deep.
http://www.gweaver.net/techhigh/projects/period1_2/Yellowstone/Images/Divergent%20Boundary.jpg
Convergent Boundaries
When one plate sinks beneath another it is called
subduction.
 There are three types of convergent boundaries:

◦ Continental-continental subduction: two plates carrying
continental crust push together; neither plate sinks, so the
land crumbles and folds, forming mountains
◦ Oceanic-oceanic subduction: two oceanic plates collide,
the older plate sinks beneath the younger; the older crust
gets pushed into the asthenosphere where it melts; deepocean trenches and island arcs (chains of volcanic islands)
are formed
◦ Oceanic-continental subduction: oceanic crust sinks under
continental crust; deep ocean trenches and coastal
mountains can be formed
http://bmsscience8209.edublogs.org/files/2010/10/convergent-COLLISIONS-205lo60.jpg
Transform
This is where two plates slide past each
other in opposite directions.
 The edges will scrape and grind against
each other.
 This occurs mostly on the sea floor near
mid-ocean ridges.
 The San Andreas Fault in California is an
example of a transform boundary.

http://earth.rice.edu/mtpe/geo/geosphere/topics/plate_tectonics/saf_final.jpg
Resources
Science, grade 8; McDougal-Little NC
Edition; ©2005
 Images: cited beneath
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