This document discusses plate tectonics and the evidence that supports the theory. It describes how Alfred Wegener first proposed continental drift in 1912 and how the theory was rejected until the 1950s. The discovery of mid-ocean ridges provided evidence that the ocean floors were spreading and pushing the continents apart. Rocks and fossils found on separated continents matched and indicated they were once joined together in a supercontinent called Pangaea. The theory of plate tectonics emerged, linking continental drift and seafloor spreading to explain how the Earth's crust has evolved over time through the movement of tectonic plates.

1. Plate Tectonics

2. Earth’s Drifting Continents
• Theory of Continental drift- the theory,
proposed by Alfred Wegener, that the
continents were once joined together and
have since drifted apart.
– Alfred Wegener concept in 1912.
– He thought some of the continents coasts look like
they could have fit together at one time.
– Discredited by many scientists until the 1950’s.
– Went against many geological basic principles.

3. • Pangaea- means all Earth; is the name of the
super continent from millions of years ago.
– Fossil records showed that the same kind of
animals and plant remains were separated by the
Atlantic Ocean.
– Evidence from Rocks- when you line South
America and Africa up together there is rock
formations that line up and are the same age as
each other.

5. • Rock deposits left from glaciers.
– Many of the same rock deposits have been found
in South America, Africa, India, Australia, and
Antarctica.
– Ancient glacial deposits have been found in areas
with very warm climates.

6. Earth’s Spreading Floor
• During the 1950’s, new mapping techniques
discovered midocean ridges.
– Midocean Ridges- an undersea mountain chain
where new ocean floor is produced.
• This gave the final piece of evidence for the
Theory of Continental Drift to be expected.

7. • Midocean Ridges form the worlds longest
mountain chain (80,000 km).
– Great deal of volcanic activity occurs in this area.
– When the ocean floor moves apart, lava wells up
and hardens causing new oceanic crust.

8. • Ocean-floor spreading- the process in which
old ocean floor is pushed away from a
midocean ridge by the formation of new
ocean floor.
– Helped explain Continental Drift.
– When the ocean floor moves, it takes the
continent with it.
• Transform fault- a fault that runs across a
midocean ridge.

10. • New deep-sea drilling also provides evidence
to support the idea of ocean-floor spreading.
• Rock samples from the ocean floor indicate
that rocks next to a midocean ridge are
younger than rocks farther away.
• Magnetic stripes in ocean-floor rocks further
convinced scientists of ocean-floor spreading.

11. • Trenches- deep V-shaped valley that lie along
the bottom of the ocean.
– They are found near some continents or near
strings of islands.
• Subduction- process in which crust plunges
back into the Earth.
– The older oceanic crust is pushed under the
continental crust.

13. Homework
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• Page 64 1-3

14. Earth’s Moving Plates
• Theory of Plate Tectonics- links together the
ideas of continental drift and ocean-floor
spreading, explains how the Earth has evolved
over time. It helps to explain the formation,
movements, collisions, and destruction of the
Earth’s crust.
• Provides a framework for understanding
mountains, volcanoes, earthquakes, and other
landforms and processes of the physical Earth.

15. Plate Tectonics
• Gives scientists insight into how and why life
on earth has evolved.
• Helps people to understand the past and to
predict the future.

16. Lithospheric Plates
• Lithosphere- the topmost solid part of the
Earth.
– Made up of a number of plates.
– Plates contain a thin layer of crust above a thick
layer of relatively cool, rigid mantle rock.
– Plates usually contain both oceanic and
continental crust.

17. 7 Major Lithospheric Plates
• Pacific Plate- covers 1/5 of the Earth’s surface.
• North American
• South American
• Eurasian
• African
• Indo-Australian
• Antarctic

19. Plate Boundaries
Three Main Types of Boundaries
• Divergent Boundaries- a plate boundary at
which plates move apart.
– This type occurs at the midocean ridges.
– Also called constructive boundaries.

21. • Convergent Boundaries- a plate boundary at
which plates come together.
– This plate boundary has trenches.
– They are also called destructive boundaries.
– Cause tremendous pressure and friction.
– Causes severe earthquakes.
– When the plate material melts in the Earth’s
mantle, some of it surges upward to produce
volcanoes.

23. • Strike-Slip Boundaries- two plates grind
together and slip past each other horizontally.
– No new plate material is made, and no plate
material is destroyed.
– It is also called the conservative boundary.
– Earthquakes often occur along these boundaries.

25. Plate Motion
• Convection Currents- a movement of material
caused by differences in temperatures.
– convection currents move air in the atmosphere
and water in the ocean. And they may move the
plates of the lithosphere as well.
– A rising and sinking cycle repeats over and over.

27. • Subduction- the process in which crust
plunges back into the interior of the Earth.
– The denser plate is subducted, and the other plate
edge floats over it.
– When two oceanic plates collide, the older
oceanic plate is subducted under the younger
plate.
– When two continental plates collide that have the
same density, neither plate is subducted, and they
push upward to form a mountain.

31. Homework
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