Geography Reference
In-Depth Information
people of California are intimately aware of this action because it causes the earthquakes they feel (and
fear) as California is dragged north by the plate beneath the Pacific Ocean.
The second type of boundary is called
diverging
, for the spots where plates are moving away from each
other in opposite directions. This is most common under the ocean, as plates carrying the sea floor head in
opposite directions. As this happens, magma, the molten material from beneath the earth's crust, wells up
and forces itself between the two plates, building massive, submerged ocean mountain ridges. The longest
mountain range on earth is the mid-Atlantic ridge that marks the plate boundary between the North Amer-
ican and Eurasian plates, which are heading away from each other. Sometimes this eruption moves above
the surface. In 1963 such an immense upheaval occurred near Iceland, where a new island, Surtsey, was
born as 10.5 billion feet of molten lava rose out of the sea. But divergent boundaries also exist under land
masses; the best example is Africa's Great Rift Valley, where the continent is literally going to be torn apart
as the plates go their separate ways. Don't worry. This will take a while and at the present rate, none of us
or our descendants will be around to see it.
Finally, there are
convergent
boundaries marking plates meeting head-on. Usually, one of the plates at
these boundaries is pushed under the other in a process called
subduction.
The material in the plate getting
pushed down is then melted by the intense heat of the mantle, the chief way in which the earth's materi-
als get recycled. If the plates that converge are sea floor plates, one result of the subduction process is a
deep oceanic trench, such as the Mariana Trench located in the Pacific, south of Japan, which at more than
35,000 feet (11,022 meters) deep is the deepest spot in the world's oceans. If one of the plates converging
does carry land, the result may be a “crumpling” that forms a long mountain chain with a nearby ocean
trench. The best example of this action is the Andes Range in South America. And if both plates in the
collision carry land, the result is even more spectacular, as the two land-bearing plates are welded together.
The Urals, a 1,500-mile-long chain in Russia, marks where Europe and Asia collided and were joined. But
probably the most extraordinary collision occurred when the Indian subcontinent rammed into and then
joined the rest of Asia.
Where Are the Tallest Mountains?
The collision of India and Asia created the world's tallest mountain ranges, which include Mount Everest,
the world's tallest peak. Plate tectonics at work is evident in the fact that Everest is still growing at the
rather healthy rate of a centimeter—.394 inch—every year, a veritable growth spurt by geologic standar-
ds. Although its precise height is disputed, Everest rises to some 29,108 feet (8,872 meters), or about 5.5
miles high, the tallest in the greatest chain of mountains on earth, the Himalayas (Sanskrit for “abode of
snow”). A series of three parallel mountain ridges, the Himalayas run for about 1,500 miles, extending
from northwest Pakistan and across Kashmir, northern India, Tibet, Nepal, Sikkim, and Bhutan to the bor-
ders of China in the east. The three great rivers of India—the Ganges, the Brahmaputra, and the Indus—all
begin in the Himalayas.
Ironically, Everest was named by British mapmakers for a man who may have never seen the mountain.
From 1830 to 1843, George Everest was the leader of the British team charged with mapping all of India,
a task begun in the eighteenth century and continuing well into the nineteenth, and part of the basis for
Rudyard Kipling's
Kim
, the first espionage novel. The India survey was an incredible achievement that
required extraordinary bravery and sacrifice in the face of the enormous physical difficulties faced by the
surveyors. When the survey reached the Himalayas, Englishmen, along with any other foreigners, could
go no farther than the Tibetan border, by order of the Chinese emperor. Instead, the British recruited local
men who were trained in surveying skills and equipped with measuring chains disguised as prayer beads.
