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Continental Drift
Johnson Space Center Collection/NASAJohnson
Space Center Collection/NASA
Over long periods of time, the continents drift into new configurations, as the plates
carrying them move across Earth’s surface. The plates—and thus the continents—
move at a rate of about 0–5 inches (0–13 centimeters) per year. North America, for
example, is currently moving northwest at a little over an inch (three centimeters)
per year. On the other hand, Antarctica is hardly moving.
About 270 million years ago, nearly all the land on Earthformed one enormous
continent called Pangea. This “supercontinent” began to break apart about 200
million years ago during the early Jurassic period, fracturing into two large pieces
that drifted away from each other. These continents were Laurasia in the north and
Gondwana in the south. Laurasia consisted of the land that now makes up North
America, Europe, and Asia (except India and Arabia). Gondwana included what are
now South America, Africa, Australia, Antarctica, India, and Arabia. Later, Laurasia
and Gondwana fractured into pieces. Gondwana began to break apart about 180
million years ago, and Laurasia about 66 million years ago. The fragments of these
continents drifted over time to form the present continents. Evidence suggests that
this process has probably occurred several times during Earth’s long history, as
continents came together to form supercontinents, which later broke apart.
The idea that the continents move about the surface of the planet is an old one. It
was formulated initially to explain the striking parallels of the shape of the Atlantic
coast of Africa with that of South America. The shape of these continents suggests
that they would fit together well. However, the close geometric fit of the continental
margins was not universally accepted as evidence that they were once joined together
and that the Atlantic Ocean was once closed.
Major Tectonic
Plates, Encyclopedia Britannica, Inc
In the early 20th century the German meteorologist Alfred L. Wegener and others
developed a comprehensive theory of continental drift. In 1912 Wegener proposed
that all the land was once joined into the supercontinent Pangea, which broke up
over time into the present continents. In 1937 the South African geologist Alexander
Du Toit modified Wegener’s hypothesis by suggesting that Pangea first fractured into
Laurasia and Gondwana.
In addition to the geometrical fit of the continents, there is much evidence
supporting the idea that the continents were once united. The main lines of evidence
are based on the study of ancientclimates and the matching of fossils, rocks, and
geological structures on land on opposite sides of oceans. A belt of ancient rocks
along the Brazilian coast, for example, matches one in West Africa. This matching
suggests that these pieces of land were once joined together. As the continents drift
into new positions, their climates change. There is evidence that Antarctica, southern
South America, southern Africa, India, and Australia were extensively covered by
glaciers from 380 to 250 million years ago. The pattern of the ancient glaciers
suggests that these landmasses once formed a single large continent near the south
polar region.
At first, continental drift theory met with strong opposition. Many scientists,
especially those in North America, rejected the theory. They opposed it partly
because it could not adequately explain what causes the continents to move. They
were also unwilling to scrap their existing theories that involved unmoving
landmasses.
Starting in the 1950s, new data about the magnetic patterns of rocks provided strong
support for continental drift. When certain rocks form, their particles are aligned
according to the direction of Earth’s magnetic poles. This magnetization helps
scientists determine how the rocks were positioned when they first formed. Evidence
from such rocks suggests that the continents were indeed once joined together.
Scientists discovered major features of the seafloor in the early 1960s. A new theory
to explain these features proposed that the seafloor spreads at oceanic ridges. Molten
rock material from the mantle wells up at the ridges and pushes aside the existing
oceanic crust, thereby causing the seafloor to spread.
By the late 1960s the satisfactory parts of the theories of continental drift and
seafloor spreading were combined to form the theory of plate tectonics. According to
this revolutionary theory (as mentioned above), Earth’s outer shell is divided into
rigid plates that carry the continents and the ocean basins. Oceanic crust descending
at subduction zones pulls the plates along. Seafloor spreading pushes plates apart.
Many scientists think that the motion of the plates may be driven by the circulation
of heated material in the mantle. Plate tectonics has very successfully explained
many phenomena. After decades of controversy, the vast majority of scientists came
to accept the theory of continental drift as explained by plate tectonics.
See table: The continents compared
Modern technology has provided tools that can actually measure the slow relative
motion of the continents. Extensive networks of radio telescopes, the Global
Positioning System (GPS) of satellites, and a technique known as satellite laser
ranging (SLR) have all successfully been used to detect continental drift.
Charles W. Finkl, Jr.
The continents compared
region
area (sq mi)
area (sq km)
population, 2008
Africa
11,678,801
30,247,722
955,761,100
Antarctica
5,500,000
14,200,000
—
Asia
12,239,721
31,700,654
4,018,522,000
Europe
8,896,305
23,041,330
735,213,700
North America
9,418,467
24,393,718
526,827,700
Australia & Oceania
3,287,718
8,515,146
35,120,640
South America
6,882,027
17,824,370
378,448,500
World
57,903,039
149,922,940
6,649,893,640
region
literacy (%) (male)
literacy (%) (female)
life expectancy (male), 2005
Africa
69.6
52.0
51.8
Antarctica
—
—
—
Asia
82.5
65.2
67.2
Europe
99.4
98.5
71.0
North America
89.2
87.1
71.9
Australia & Oceania
94.7
91.9
74.5
South America
89.7
88.6
68.9
World
84.0
70.8
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