Download Sample Question Answer (300 Words)

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Sample Question Answer (300 Words)
Que.
‘‘Plate tectonic has evolved as a grand framework for understanding
endogenic forces in long sequence of time.’ Elaborate’’
Ans. Plate tectonics provides us with a grand framework for understanding the
extensive crustal rearrangement that apparently has taken place during the
relatively recent history of Earth. Wegener’s Pangaea is now generally accepted
as having existed. There is substantial evidence to indicate that, 450 million
years ago, five continents existed; these continents sutured to form Pangaea. For
the next 200 million years or so, Earth had but a single major continent and
single world ocean. About 250 million years, Pangaea began to break up, first
into two massive pieces – Laurasia in the Northern Hemisphere and
Gondwanaland in the Southern Hemisphere – and then into a number of smaller
bits. One of the great triumphs of the theory of plate tectonics is that it explains
topographic patterns. It can account for the formation of many mountains,
midocean ridges, oceanic trenches, island area, and the associated earthquake
and volcanic zones. Where these features appear, there are usually plates either
colliding or separating. Despite the correlation of mountain building with plate
convergence, the contemporary pattern of plates does not explain all mountain
belts. Many of the major mountain ranges of North America and Europe are in
the middle of plates rather than in boundary zones. The genesis of such midplate
ranges is not fully understood but presumably is related to changing tectonic
conditions in the past. There is convincing evidence that during some past eras
there were fewer plates than there are today and during other eras there were
more plates than there are today. In addition, the sizes and shapes of past plates
different from the current sizes and shapes. For example, there was no Atlantic
Ocean 250 million years ago, and yet today it is a major feature of the planet,
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widening at a long-term average rate of about 2 inches (5 centimeters) per year.
Seafloor spreading is proceeding even faster in parts of the Pacific.
Indeed, some geophysicists have postulated that oceans are being created and
removed by crustal rearrangement on about a 100-million-year cycle.
Modifications to the Original Theory: With each passing year, we learn more
about plate tectonics. Two examples of recently acquired knowledge are
accreted terranes and mantle plumes.
Accreted Terranes: A terrane is a small-to-medium mass of lithosphere carried
a long distance by a drifting plate that eventually converges with another plate.
The terrace is too buoyant to be subducted in the collision and instead is fused
(“accreted”) to the other plate, often being fragmented in the process. Terranes
are distinctive geologically because their lithologic complement (types of rock) is
generally quite different from that of the plate to which they are accreted. It is
generally believed that every continent has grown outward by the accumulation
of accreted terranes on one or more of its margins. North America is a prominent
example. Most of Alaska and much of western Canada and the western United
States consist of a mosaic of several dozen accreted terranes, some of which
have been traced to origins south of the equator.
Mantle Plumes: There are many places on Earth where magma from deep in the
mantle comes either to or almost to the surface at locations that are not
anywhere near a plate boundary; these leaky spots in the interior of a plate are
referred to as mantle plumes or hot spots. They are believed to be relatively
stationary over long periods of time (tens to hundred of millions of years). We do
not yet know the cause of these plumes, but more than 100 of them have thus far
been identified. As the magma rises through the crust, it creates volcanoes
and/or hydrothermal (hot water) features on the crust surface. The plate
containing the hot spot is on the move, however, and so the volcanoes or other
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plume features appear to drift away from the plume generating a “hot spot train.”
From a surface perspective, the hot spot appears to migrate in the direction
opposite the direction of plate movement, the volcanoes produced by hot spots
because younger in the opposite direction from that which the plate is moving.
Thus these traits can be used to determine absolute plate motions.
One striking result of mantle plume is the Hawaiian Islands, where the ancient
volcanic remnants of Midway Island are now 1600 miles (2500 kilometers)
northwest of the presently active volcanoes on the big island of Hawaii, although
both developed over the same mantle plume, separated in time by several million
years. The volcanoes of the Hawaiian chain are progressively younger from west
to east, as the Pacific plate drifts northwestward and while new volcanoes are
produced on an “assembly line” moving over one persistent hot spot.
Mantle plumes help explain both topographic development and the rate and
direction of plate movement. Because the plumes are effectively fixed in position,
the “traits” they produce indicate absolute plate motions. Despite the intellectual
and scientific enrichment we have received from plate tectonics theory, we still
have many unanswered questions.