Plate TectonicsPlate Tectonics8th Science

Earth’s layersEarth’s layersThe Earth has 3 layers: core,

mantle, crust.

http://geology.com/articles/mohorovicic-discontinuity.shtml

CoreCoreThe 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

MantleMantleThe 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.

CrustCrustThe crust is a thick layer of cool

rock.There are two basic types of

crust:◦Continental crust: all continents and

some major islands◦Oceanic crust: all ocean floors

http://www.enchantedlearning.com/ogifs/outerlayersearth.GIF

LithosphereLithosphereThe 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.

AsthenosphereAsthenosphereThis 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/GeologyofVirginia/Photos/Tectonics/AsthenoLitho.jpg

Tectonic PlatesTectonic PlatesThe 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 DriftContinental DriftThe 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.

PangaeaPangaeaPangaea 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 evidenceMore 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?How do the plates move?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 Two other possible movementsmovementsSlab 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 TectonicsTheory of Plate TectonicsThe 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 PlatesTry naming all the Plateshttp://www.learner.org/interactiv

es/dynamicearth/plate2.html

How plates moveHow plates moveThese 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 BoundariesDivergent Boundaries 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 BoundariesConvergent BoundariesWhen 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; deep-ocean 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

TransformTransformThis 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

ResourcesResourcesScience, grade 8; McDougal-Little

NC Edition; ©2005Images: cited beneath