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.
4.
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.
9.
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.
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
18.
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.
20.
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.
22.
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.
24.
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.
26.
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.
