In geologic terms, a plate is a large, rigid slab of solid rock.
The word tectonics comes from the Greek root "to build."
Putting these two words together, we get the term plate tectonics,
which refers to how the Earth's surface is built of plates. The theory
of plate tectonics states that the Earth's outermost layer is fragmented
into a dozen or more large and small plates that are moving relative to
one another as they ride atop hotter, more mobile material. Before the advent
of plate tectonics, however, some people already believed that the present-day
continents were the fragmented pieces of preexisting larger landmasses ("supercontinents").
The diagrams below show the break-up of the supercontinent Pangaea
(meaning "all lands" in Greek), which figured prominently in the
theory of continental drift -- the forerunner to the theory of plate
tectonics.
According to the continental drift theory, the supercontinent
Pangaea began to break up about 225-200 million years ago, eventually
fragmenting into the continents as we know them today.
Plate tectonics is a relatively new scientific concept, introduced some
30 years ago, but it has revolutionized our understanding of the dynamic
planet upon which we live. The theory has unified the study of the Earth
by drawing together many branches of the earth sciences, from paleontology
(the study of fossils) to seismology (the study of earthquakes).
It has provided explanations to questions that scientists had speculated
upon for centuries -- such as why earthquakes and volcanic eruptions occur
in very specific areas around the world, and how and why great mountain
ranges like the Alps and Himalayas formed.
Why is the Earth so restless? What causes the ground to shake violently,
volcanoes to erupt with explosive force, and great mountain ranges to rise
to incredible heights? Scientists, philosophers, and theologians have wrestled
with questions such as these for centuries. Until the 1700s, most Europeans
thought that a Biblical Flood played a major role in shaping the Earth's
surface. This way of thinking was known as "catastrophism," and
geology (the study of the Earth) was based on the belief that all
earthly changes were sudden and caused by a series of catastrophes. However,
by the mid-19th century, catastrophism gave way to "uniformitarianism,"
a new way of thinking centered around the "Uniformitarian Principle"
proposed in 1785 by James Hutton, a Scottish geologist. This principle is
commonly stated as follows: The present is the key to the past. Those
holding this viewpoint assume that the geologic forces and processes --
gradual as well as catastrophic -- acting on the Earth today are the same
as those that have acted in the geologic past.
The belief that continents have not always been fixed in their present positions
was suspected long before the 20th century; this notion was first suggested
as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus
Geographicus. Ortelius suggested that the Americas were "torn away
from Europe and Africa . . . by earthquakes and floods" and went on
to say: "The vestiges of the rupture reveal themselves, if someone
brings forward a map of the world and considers carefully the coasts of
the three [continents]." Ortelius' idea surfaced again in the 19th
century. However, it was not until 1912 that the idea of moving continents
was seriously considered as a full-blown scientific theory -- called Continental
Drift -- introduced in two articles published by a 32-year-old German
meteorologist named Alfred Lothar Wegener. He contended that, around 200
million years ago, the supercontinent Pangaea began to split apart. Alexander
Du Toit, Professor of Geology at Johannesburg University and one of Wegener's
staunchest supporters, proposed that Pangaea first broke into two large
continental landmasses, Laurasia in the northern hemisphere and Gondwanaland
in the southern hemisphere. Laurasia and Gondwanaland then continued to
break apart into the various smaller continents that exist today.
In 1858, geographer Antonio Snider-Pellegrini made these
two maps showing his version of how the American and African continents
may once have fit together, then later separated. Left: The formerly joined
continents before (avant) their separation. Right: The continents after
(aprés) the separation. (Reproductions of the original maps courtesy
of University of California, Berkeley.)
Wegener's theory was based in part on what appeared to him to be the remarkable
fit of the South American and African continents, first noted by Abraham
Ortelius three centuries earlier. Wegener was also intrigued by the occurrences
of unusual geologic structures and of plant and animal fossils found on
the matching coastlines of South America and Africa, which are now widely
separated by the Atlantic Ocean. He reasoned that it was physically impossible
for most of these organisms to have swum or have been transported across
the vast oceans. To him, the presence of identical fossil species along
the coastal parts of Africa and South America was the most compelling evidence
that the two continents were once joined.
In Wegener's mind, the drifting of continents after the break-up of Pangaea
explained not only the matching fossil occurrences but also the evidence
of dramatic climate changes on some continents. For example, the discovery
of fossils of tropical plants (in the form of coal deposits) in Antarctica
led to the conclusion that this frozen land previously must have been situated
closer to the equator, in a more temperate climate where lush, swampy vegetation
could grow. Other mismatches of geology and climate included distinctive
fossil ferns (Glossopteris) discovered in now-polar regions, and
the occurrence of glacial deposits in present-day arid Africa, such as the
Vaal River valley of South Africa.
The theory of continental drift would become the spark that ignited
a new way of viewing the Earth. But at the time Wegener introduced his theory,
the scientific community firmly believed the continents and oceans to be
permanent features on the Earth's surface. Not surprisingly, his proposal
was not well received, even though it seemed to agree with the scientific
information available at the time. A fatal weakness in Wegener's theory
was that it could not satisfactorily answer the most fundamental question
raised by his critics: What kind of forces could be strong enough to move
such large masses of solid rock over such great distances? Wegener suggested
that the continents simply plowed through the ocean floor, but Harold Jeffreys,
a noted English geophysicist, argued correctly that it was physically impossible
for a large mass of solid rock to plow through the ocean floor without breaking
up.
Undaunted by rejection, Wegener devoted the rest of his life to doggedly
pursuing additional evidence to defend his theory. He froze to death in
1930 during an expedition crossing the Greenland ice cap, but the controversy
he spawned raged on. However, after his death, new evidence from ocean floor
exploration and other studies rekindled interest in Wegener's theory, ultimately
leading to the development of the theory of plate tectonics.
Plate tectonics has proven to be as important to the earth sciences as the
discovery of the structure of the atom was to physics and chemistry and
the theory of evolution was to the life sciences. Even though the theory
of plate tectonics is now widely accepted by the scientific community, aspects
of the theory are still being debated today. Ironically, one of the chief
outstanding questions is the one Wegener failed to resolve: What is the
nature of the forces propelling the plates? Scientists also debate how plate
tectonics may have operated (if at all) earlier in the Earth's history and
whether similar processes operate, or have ever operated, on other planets
in our solar system.
URL: http://pubs.usgs.gov/publications/text/historical.html
Last updated: 05.05.99
Contact: jmwatson@usgs.gov
Tectonic plates [65 k]
Rejoined continents
[48 k]
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