History of plate tectonics
Plate tectonic theory had its beginnings in 1915 when Alfred Wegener proposed his theory of
"continental drift." Wegener proposed that the continents plowed through crust of ocean basins,
which would explain why the outlines of many coastlines (like South America and Africa) look like they
fit together like a puzzle. Wegener was not the first to notice this puzzle-like fit of the continents
(Magellan and other early explorers also noticed this on their maps), but he was one of the first to
realize that the Earth's surface has changed through time, and that continents that are separated now
may have been joined together at one point in the past.
Paleontologists had also found that there were fossils of similar species found on continents that are
now separated by great geographic distance. Paleoclimate studies, which concerns examining the climate in Earth's past,
revealed that glaciers covered large areas of the world which also are now separated by great geographic distances. These
observations seemed to indicate that the Earth's lithosphere had been moving over geologic time.
Searching for evidence to further develop his theory of continental drift, Wegener came across a old idea suggesting that a land
bridge had once connected Africa with Brazil. This proposed land bridge was an attempt to explain the well known observation
that the same fossilized plants and animals from the same time period were found in South America and Africa. The same was
true for fossils found in Europe and North America, and Madagascar and India. Many of these organisms could not have traveled
across the vast oceans that currently exist. Wegener's drift theory seemed more plausible than land bridges connecting all of the
continents. But that in itself was not enough to support his idea. Another observation favoring continental drift was the presence
of evidence for continental glaciation in the Pennsylvanian period. Striations (Scrapes and grooves) left by the scraping of glaciers
over the land surface indicated that Africa and South America had been close together at the time of this ancient ice age. The
same scraping patterns can be found along the coasts of South America and South Africa.
By 1922, Wegener had joined all of the present-day continents into one huge landmass. He used both fossil evidence, and eight
different geological indicators, including:
The coal fields shared by Britain, Belgium and the Appalachian Mountains of the USA,
The red sandstone band that passes through Norway, Britain, Greenland and Canada
The same fossilized plants and animals from the same time period were found in South America, Antarctica, and
Africa.
The same types of diamond fields of South Africa and Brazil.
The same scraping patterns from glaciers can be found along the coasts of South America and South Africa.
This continent he called Pangaea - from the Greek meaning "all land".
Mountains, he claimed, were the result of continents colliding and crumpling, and he cited the example of India colliding into
Asia to uplift the Himalayas. At this stage, he used his earlier measurements of the longitudinal change of Greenland to show
that the land was still moving. Unfortunately, this mistake (due to imprecise measurements) did nothing to convince the already
disbelieving skeptics, who disbelieved that continents could move at all, let alone at that speed.
Wegener's ideas were very controversial because he didn't have an explanation for why the continents moved, just that there
was observational evidence that they had. At the time, many geologists believed that the features of the Earth were the result of
the Earth going through cycles of heating and cooling, which causes expansion and contraction of the land masses. People who
believed this were called the anti-mobilists. The mobilists were in the opposite camp and supported Wegener's ideas, since many
of them had seen evidence for continental motion, especially in the Alps.
Although Wegener's "continental drift" theory was unaccepted, it was one of the first times that the idea of crustal movement
had been introduced to the scientific community; and it laid the groundwork for the development of modern plate tectonics. As
years passed, more and more evidence was uncovered to support the idea that the plates move constantly over geologic time.
Paleomagnetic studies, which examine the Earth's past magnetic field, showed that the magnetic north pole seemingly wandered
all over the globe. This meant that either the plates were moving, or else the north pole was. Since the north pole is essentially
fixed, except during periods of magnetic reversals, this piece of evidence strongly supports the idea of plate tectonics.
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Following World War II, even more evidence was uncovered which supports the theory of plate tectonics. In the 1960's a worldwide array of seismometers, (devices that measure vibrations in earth’s crust), were installed to monitor nuclear testing, and
these instruments revealed a startling geological phenomenon. It showed that earthquakes, volcanoes, and other active geologic
features for the most part aligned along distinct belts around the world, and those belts defined the edges of tectonic plates.
In addition, further paleomagnetic studies revealed a striped pattern of magnetic reversals in the crust of the ocean basins.
Basalt contains a fair amount of magnetic minerals called magnetite. When the lava from spreading centers in the oceans forms
and cools, these minerals align to the north pole. The Earth has undegone several magnetic reversals in the past, in which the
north and south poles are reversed for a period of time. When geologists and geophysicists discovered that the crust in the
ocean recorded these reversals, it was even more positive proof that the lithosphere had to be in motion, otherwise there would
be no "stripes" of normal and reversed polarity crust.
These were some of the final pieces of the puzzle that led to the development of modern plate tectonic theory. Since its
emergence in the 1960's, plate tectonic theory has gained wide-spread acceptance as the model of Earth processes.
QUESTIONS:
1) In which year did Wegener propose his idea of continental drift?
2) What previous theory was used to explain the same fossils found on Africa and South America?
3) What are striations?
4) What were three pieces of evidence that Wegener used to try and prove his theory?
5) What are two discoveries in the 1960’s that lead the scientific world to finally accept Wegener’s ideas?