Plate Tectonics – The Moving Crust
Name: ______________________________________
Date: ______________________
The surface of the earth (the crust) is not one solid continuous covering (like an egg shell).
Instead, it is broken into many smaller areas known as PLATES. It is more like a hard
boiled egg that has been dropped and the shell have cracked in many places. The map
below shows the major plates of the earth's surface.
The name of the plate where you live is called "The __________________________ Plate".
All of these plates are constantly moving very slowly over the surface of the earth. The
movement of the plates causes some plates to be coming toward each other, some to be
moving away from each other and others to be sliding past each other. These different types
of motion cause different effects on the earth: mountain building, volcano formation, new
land formation, and earthquakes.
The movement of the plates is caused by the flowing of the
heated magma in the mantle. How? As the magma closer to
the core becomes hot, it rises toward the crust and the cooler
magma closer to the surface of the earth falls toward the
core. This up and down, circular motion is called a
convection current. Convection currents force the plates to
be pushed along the surface in various directions.
What happens where the plates meet?
Types of Plate Boundaries
Name: ______________________________________
Date: ______________________
The plates of the earth are always moving, very slowly, but moving none-the-less. The
edges of a plate where two or more plates meet are called boundaries. What happens at the
boundaries depends on whether the plates are coming toward each other, moving away
from one another or sliding along each other.
There are 3 main types:
1) Divergent boundaries
- Plates moving apart
2) Convergent boundaries
- Plates moving toward each other
3) Transform boundaries
- Plates grinding along, past each other
- First proposed by J. Tuzo Wilson (Canadian)
Divergent Boundaries
Diverging Boundaries are where plates are moving away from each other.
New crust is forming when molten magma from deep down in the mantle oozes out of the
cracks where the plates are moving apart.
There are 2 main types of divergent boundaries:
1)
Two Oceanic Crusts Moving Apart
This happens on the ocean floor where plates are diverging
(moving apart). If enough lava bubbles up, underwater
volcanoes and ridges of land can be formed.
One of the best examples of a divergent boundary is the
mid-Atlantic ridge. It is spreading at a rate of 2.5 cm/year
(or 25 km in a million year).
This explains why the island country of Iceland, which sits
on top of the mid-Atlantic ridge, is growing larger every
year.
2)
Two Continental Crusts Moving Apart
This happens where a plate boundary exists on
land. The changes seen on land happen in a
number of steps:
 a valley-like rift develops first. This rift
valley is a dropped zone where the plates are
pulling apart.
 As the dropped zone widens and thins,
valleys and volcanoes form.
 Early in the rift formation, streams and rivers
flow into the low valleys and long, narrow
lakes can be created
 Eventually, the crust may become thin enough that a piece of the continent breaks
off, forming a new and separate land mass. At this point, water from the ocean
will rush in, forming a new sea or ocean basin in the rift zone.
 An excellent example of this is the Great Rift Valley of Central Africa.
Convergent Boundaries
Converging Boundaries are where plates are moving toward each other. When these slow
moving plates push toward each other, we feel the pressure in the form of earthquakes. As
the plates are squeezed together, one plate (the older, denser plate) gets pushed down under
the other and the other plate crumples and gets shoved up into the air – forming a mountain
or a volcano. There are three main types of converging boundaries:
1)
Ocean-Continent
 Oceanic crust tends to be denser and thinner than
continental crust.
 The oceanic crust gets pushed under the
continental crust. This is called “subduction”.
 This forms a subduction zone.
 The sinking crust creates a deep oceanic trench, or
valley, at the edge of the continent.
 The crust continues to be forced deeper into the earth, where high heat and
pressure melt the crust & begin to push it up through the continental crust,
forming a volcano.
 When this magma finds its way to the surface through a vent in the crust, the
volcano erupts.
An example of this is the band of active volcanoes that encircle the Pacific Ocean
(like the Rocky Mountains), often referred to as the Ring of Fire.
2)
Oceanic-Ocean Plates Collide
 When two oceanic plates collide, the older plate is
forced below the younger plate.
 This creates another subduction zones.
 Instead on forming volcanoes on the land, it creates a chain of volcanic islands
known as island arcs.
 The deepest trenches on the planet also occur this
Examples include the Mariana Islands in the western Pacific Ocean and the Aleutian
Islands, off the coast of Alaska.
3)
Continent-Continent
When two continental crusts bump into each other,
neither is more dense than the other so no subduction
occurs. The edges simply crumple and push high into the
air forming very high MOUNTAINS. This is how the
Himalayan mountains in Asia were formed.
Its best known peaks, Mount Everest and K2, are among
several mountains that measure over 8,000 meters high at their summits. Since the
Indian Plate is continuing in its northward movement into Asia, the Himalayas
continue to grow higher each year by small amounts (5 to 20 mm or 1 inch per year).
Colliding Plates – Tsunami Makers
 The collision and subduction of oceanic plates creates large, powerful earthquakes.
 Earthquakes generated in a subduction zone can also give rise to tsunamis.
 A tsunami is a huge ocean wave caused by a sudden shift on the ocean floor.
If the wave reaches land, it can cause incredible destruction, like the Asian Tsunami, which
killed more than 200,000 people in 11 countries across the Indian Ocean region on
December 26, 2004.
Transform Boundaries
Transform Boundaries are where plates are moving along beside
each other in opposite directions.
This does not occur in a smooth, gliding motion, though. Instead, the rocks of the two plates
grind against each other and rip and tear.
This produces shaking motions in the crust known as
EARTHQUAKES.
This is the type of motion we see along the west coast of
California in the US. The North American plate is moving
in a southerly direction while the Pacific plate is sliding up
toward the north. This explains why there are so many
earthquakes in the San Francisco area.