By 1965, investigations led to the proposal that Earth's
surface was broken into seven large plates and several
smaller plates. It was further suggested that Line these
plates are rigid, and that their boundaries are
Line 5 marked by earthquakes and Volcanic activity. In recent years,
satellite pictures have documented the existence of
plate boundaries. An especially visible example is the
San Andreas Fault in California.
Plates interact with one another at their boundaries
10 by moving toward, away, or alongside each other.
Faults are examples of boundaries where two plates
slide horizontally past each other. Mid-ocean ridges
mark boundaries where plates are forced apart as new
ocean floor is being created between them. Mountains,
15 volcanic-island arcs, and ocean trenches occur at
the boundaries where plates are colliding, causing
one plate to slide beneath the other. The network of
crustal plates and the geologic activity caused by their
movement is referred to as plate tectonics.
20 The original continental-drift theory suggested
that continents plowed through the ocean floor
like ships. Plate tectonics, on the other hand, holds
that continents are carried along together with
the surrounding seabed in huge plates-much like rafts
25 frozen into the ice on a flowing stream.
There are several major plates. The North American
plate comprises North America and the Western half of
the North Atlantic seafloor. The South American plate
includes South America east to the Mid-Atlantic Ridge.
30 The African plate contains Africa and its surrounding
seafloor. The Antarctic plate has Antarctica and
surrounding seafloor. The Eurasian plate includes
Europe, Asia, and nearby seafloor. The Pacific plate
underlies the Pacific Ocean. Recently, an international
35 team of geologists and other researchers analyzing
seafloor measurements discovered that what was
classified as the Indo - Australian plate may actually be
two separate plates-one With the Indian subcontinent
and the adjacent seabed, and the other with Australia
40 and surrounding Waters.
middle mantle, and an outer crust.) These plumes
of molten rock, often called hot spots, rise and erupt
through the crust of a moving plate
Most of the isolated mid-plate Volcanoes, such as
50 those of Hawaii and Yellowstone, lie at one end of a
line of extinct volcanoes that grow steadily older with
distance from the active center. Hawaii's Mauna Loa
is at the extreme southeastern end of the rest of
the Hawaiian island chain. The Volcanoes in this chain
55 become steadily older and less active to the northeast.
Likewise, Yellowstone's hot springs and geysers are
at the eastern end of a line of extinct volcanoes that
extend into Idaho. Such a line of volcanoes suggests
that the crust of Earth is passing over a hot spot, or hot
60 spots, in the deeper mantle. As the crustal plate
moves, the hot spot "punches" up a line of volcanic and hotspring activity.
Indeed, the motions of the Pacific plate are
compatible With the direction of the Hawaiian chain
65 and the ages of its volcanic islands. Plate motion has
slowly moved the volcanic islands away from the hot
spot that created them. In other Words, the Hawaiian
island chain traces the motion of the Pacific plate.
As molten rock flows up along the mid-ocean
70 ridges to create new seafloor, the lava flows more
abundantly in certain spots, producing Volcanic
islands. Scientists believe that these places of abundant
lava flow may be hot spots that occur between two
separating plates. TWO such plates underlie the large,
75 highly volcanic island of Iceland, which straddles
the Mid-Atlantic Ridge. On one side is the North American plate;
on the other, the Eurasian plate. Similarly,
ridges extend from the active volcanic island
of Tristan da Cunha westward to South America and
80 eastward to Africa. Some geologists propose that,
although such hot spots do not actually move plates,
they may mark Weak points in the mantle, which
in turn help determine the lines along which plates
fracture and separate.
85 Hotspots of volcanic activity often occur at
the junction where plates collide. Examples are
the Volcanic Azores, which arise where the North
American, Eurasian, and African plates meet.
Another-Macquarie Island, south of New Zealand
90 marks the meeting point for the Pacific, Antarctic, and
Indo-Australian plates. These hot spots may be fueled
in part by plate collision.
