Download 8. Earth`s Moving Plates

8. Earth's Moving Plates
Many of the important discoveries
about the structure of the earth were made
by scientists who found evidence in the
oceanic crust for regular patterns of crustal
movement. As we will see, these movements are thought to be related to convection currents in the mantle.
An example of a convection current in
liquids is shown in Fig. 8-1. In the beaker,
hot water rises at the point where heat is
applied, goes to the surface, then spreads
out and cools. Cooler liquid sinks to the
bottom at places where there is less heat.
The rising up, spreading out, and sinking of
gas, liquid, or molten material produces
convection currents.
Fig. 8-1.
liquid
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Convection currents in a beaker of
Cracks in the Earth's Crust
The solid crust acts as a heat insulator
for the hot interior of the earth. Below the
crust, in the mantle, is the molten material
called magma. Tremendous heat and pressure within the earth cause the hot magma
to flow in convection currents. Periodically
it rises to the surface, cracks the crust,
erupts as lava, steam, or ash, and releases
heat. The lava cools to form new rock on
the oceanic or continental crust.
A worldwide system of great cracks in
the oceanic and continental crust has been
mapped. These cracks divide the crust into
rigid plates, huge sections of the earth that
move relative to each other. See Fig. 8-2.
Movements of the Earth's Plates
There are four major kinds of movements of the plates in the earth's crust:
1. subduction, when one plate plunges beneath another
2. collision, when two continental plates
are shoved together
3. seafloor spreading, when two plates are
pushed apart
4. transform faulting (also called transverse fracturing), when two plates slide
past each other
Subduction is the downward movement of an oceanic plate into the mantle.
Locate the Nazca and South American
plates (Fig. 8-2), then follow the process of
subduction shown in Fig. 8-3. The Nazca
Plate plunges under the South American
Plate at a rate of 2 to 3 em per year. As the
• • 1 Rift valley system
~ Ridge
WI.WJ. Subduction zones
A Principal vo lcanoes
.~
J:J
ANTARCTIC PLATE
f;j
Fig. B-2. The earth's plate system
pl
e
UNIT 1. EARTH AND OCEAN BASINS
Fig. 8-3. Subduction of the Nazca Plate below the South American Plate forming composite
volcanoes
e
oceanic crust enters the mantle, pressure
breaks the crustal rock, heat from friction
melts it, and a pool of magma develops.
This thick magma, called andesite lava,
consists of a mixture of basalt from oceanic
crust and granite from continental crust.
Forced by tremendous pressure, it eventually flows along weaker crustal channels
toward the surface. The magma periodically breaks through the crust to form great,
violently explosive composite volcanoes,
steep-sided, cone-shaped mountains like
those in the Andes at the margin of the
South American Plate.
Ocean trenches form at the regions
where one plate moves downward beneath
another. These trenches are deep (up to 10.8
km), narrow (about 100 krn), and long
(from 800 to 5,900 km) , with very steep
sides. Places where subduction occurs are
also sites of deep earthquakes caused by
rocks slipping over other rocks deep in the
mantle.
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Subduction pulls the seafloor steadily
downward , as if it were a giant conveyor
belt. Continental collision occurs if two
plates carrying continents collide and the
subduction is interrupted. Because continental crusts are composed of low-density
material, they do not sink. So when the
continents collide, the crust moves upward ,
and the crustal material folds, buckles, and
breaks. Many of the great mountain ranges
were formed by the collision of continents.
Fig. 8-4 illustrates the continuous process of plate movement over very long periods of time. (A) shows how a subduction
zone forms when oceanic crust slides under
continental crust. (B) shows how the collision of two continental crusts interrupts the
subduction process and forms a new mountain chain. (C) shows how the oceanic crust
continues sliding under the continental
crust, forming a new subduction zone and a
new submarine trench, and how two continental crusts begin to fuse.
A
subduction zone
(ocean trench)
c
ocean
new subduction zone
(ocean trench)
ocean
Fig. 8-4. Subduction and continental collision
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UNIT 1. EARTH AND OCEAN BASINS
Seafloor Spreading and Transform
Faults
While one boundary of an oceanic plate
is being pushed down into the mantle by
subduction, new material is coming up at
the opposite boundary by a process called
seafloor spreading. See Fig. 8-5. Great
cracks develop in the ocean floor where hot
magma rises to the crustal surface and
forms midocean ridges or rises. As the
lava cools, it forms new seafloor features
such as rift valleys, seamounts (volcanic
peaks that extend more than I km above the
seafloor) , and abyssal hills (volcanic peaks
that extend less than I km above the
seafloor). The outpouring of material continues as the oceanic plates pull apart.
Crustal movement related to this spreading
causes frequent shallow earthquakes with
distinct seismic wave patterns. In fact,
earthquake data were used to help map active oceanic ridge systems.
Today we know that midocean ridges
and rises are the largest continuous features
on the earth. They are tens of thousands of
kilometers long, running through and connecting most of the ocean basins. Oceanographic data reveal that seafloor spreading
is slowly widening the Atlantic Ocean, the
Red Sea, and the Gulf of California. Similar spreading on land is also causing a large
rift valley in Africa that may eventually
split that continent.
As we might expect, spreading does not
occur evenly along a ridge. Perpendicular
breaks or fracture zones, called transform
faults, occur when the sections of the plates
slip by each other and displace segments of
the midocean ridges. See Fig. 8-5. Great
tension can build up before slippage occurs
and causes shallow earthquakes. People
living near the San Andreas Fault, a transform fault in California, regularly experience such quakes.
' I rift valley
abyssaI hII s
'"
Fig. 8-5. Seafloor spreading and the formation of transform faults
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Topic 8. Earth's Moving Plates
ACTIVITY
Interpret information about the earth's
moving plates from the map in Fig. 8-2.
MATERIALS
• pencil
• crayons or colored pencils (red, green,
yellow, orange)
• world map
5. Follow the areas of seafloor spreading
around the map. What do you notice
about the relationship between them
and how far around the earth they
extend?
6. Why are the lines representing the
midocean ridges jagged rather 'than
smooth? What type of movement explains this irregularity?
PROCEDURE
1. On the map in Fig. 8-2, put a star on the
plate where you are now.
2. Locate ridges (the areas of seafloor
spreading) and color them green.
7. Locate the areas of seafloor spreading
at the boundaries of the Pacific Plate.
In what direction does spreading seem
to be driving the plate?
8. Mount St. Helens, a volcano in Wash3. Locate areas of subduction and color
them red.
4. Locate the areas where composite volcanoes are occurring and color them yellow.
5. Find Iceland and the region where the
Mid-Atlantic Ridge comes to the surface
and pushes the Eurasian and North
American plates apart. In this area volcanic activity and earthquakes happen at
the surface. Color this region orange.
ington state, erupted violently a few
years ago. What might account for the
eruption of this mountain? How was
the volcano probably formed?
9. Locate the island arcs that form the
Aleutian Islands, Japan, and the Philippines. What processes might have
formed them? Explain.
10. In what direction does the Indo-Australian Plate seem to be moving? Describe what is happening at the northern boundary of this plate.
QUESTIONS
1. How many plates are there?
2. Which plate are you on?
3. Which plates, if any, carry no continents?
4. What kinds of structures are found at the
boundaries of plates?
11. Inspect Fig. 8-2 to find other locations
where continents have collided. What
mountain ranges were formed?
12. San Francisco is approximately 10,000
km from Tokyo. The Japan Trench is
gobbling up crust at the rate of 3 em a
year. How many years will it take until
San Francisco collides with Tokyo?
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UNIT 1. EARTH AND OCEAN BASINS
13. Scientists use the term plate tectonics
to refer to the theory that major structural features on the earth's surface
form as crustal plates move. How
could convection currents of materials
within the earth be related to the theory
of plate tectonics?
Hot Spots
Some volcanoes form over stationary
hot spots in the middle of plates. See Fig.
8-6. Magma flowing from these volcanoes
is called basalt Java. It is high in iron and
magnesium. This lava flows like hot, thick
syrup, gradually forming shield volcanoes
shaped like domes with gently sloping
sides. These volcanoes are much less explosive than the composite volcanoes
formed at regions of subduction. Some
shield volcanoes begin on the ocean floor
and, with repeated eruptions, grow slowly
until they reach the surface of the water and
Fig. 8-6. Formation of volcanic islands
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form islands. Peaks of some of these islands reach 3.6 km above sea level in regions where the ocean is 7 kID deep, making them the largest mountains on earth as
measured from their bases on the seafloor
to their summits. Almost all ofthe mid-Pacific and mid-Atlantic islands formed in
this way.
Over time, as the plate moves or twists,
a volcano that was over the hot spot moves
away, ceases to erupt, and becomes extinct.
New islands in the island chain (archipelago) form as other parts of the oceanic crust
move over the hot spot. See Fig . 8-6.
Erosion (wearing away) and subsidence
(sinking of the earth 's crust) eventually
cause older islands to sink below sea level.
Atolls, groups of islands that ring a shallow
lagoon, form when coral reefs grow on top
of submerged volcanoes. Found even
deeper are guyots, which are extinct, flattopped, eroded volcanoes 1,800 to 3,000 m
below the ocean surface.