1. Plate Tectonics
8th Science

2. Earth’s layers The Earth has 3 layers: core, mantle, crust.

3. 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.

4. 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.

5. Crust The 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

6. 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.

7. 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.

8. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

17. Try naming all the Plates
icearth/plate2.html

18. 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.

19. http://www. age-of-the-sage. org/tectonic_plates/volcanoes_earthquakes

20. Divergent 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.

22. 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; 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

23. http://bmsscience8209. edublogs

24. 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.

26. Resources Science, grade 8; McDougal-Little NC Edition; ©2005
Images: cited beneath