Download The Theory of Plate Tectonics

The Theory of Plate Tectonics
Tom and Lawrence
Contents
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How the theory came about
Evidence to support the theory
Types of plate boundary
Volcanoes
Earthquakes
How the theory came about
In the late 19th and 20th centuries, geologists believed
the plates were fixed except for varying degrees of
vertical movement. In 1912 German scientist, Alfred
Wegener put forward the idea that the large land
masses of Earth in fact fitted together. For example,
America’s coastline fits in to that of West Africa’s. From
then on Wegener publicly advocated his theory,
however there was not significant evidence to prove it
either way. It wasn’t until 33 years after his death that
the Royal Society for science in London adopted his
theory, the theory of continental drift.
Evidence for the theory.
Large land masses appear to fit together. Samples taken proved
India had once been in the Southern hemisphere as stated by
Wegener. Evidence for continental drift is now extensive. Similar
plant and animal fossils are found around different continent
shores, suggesting that they were once joined. The fossils of
Mesosaurus, a freshwater reptile rather like a small crocodile, found
both in Brazil and South Africa, are one example; another is the
discovery of fossils of the land reptile Lystrosaurus from rocks of the
same age from locations in South America, Africa, and Antarctica.
There is also living evidence — the same animals being found on
two continents. An example of this is a particular earthworm found
in South America and South Africa. An example of how the plates
once fitted together can be found on the next page.
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The various types of plate boundary
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Three types of plate boundaries exist, characterized by the way the plates move
relative to each other. They are associated with different types of surface
phenomena. The different types of plate boundaries are:
1. Passive boundaries occur where plates slide or, perhaps more accurately, grind
past each other along transform faults. The relative motion of the two plates is
either sinistral (left side toward the observer) or dextral (right side toward the
observer). The San Andreas Fault in California is an example of a transform
boundary exhibiting dextral motion.
2. Divergent boundaries occur where two plates slide apart from each other. Midocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's
Great Rift Valley) are both examples of divergent boundaries.
3. Convergent boundaries (or active margins) occur where two plates slide towards
each other commonly forming either a subduction zone (if one plate moves
underneath the other) or a continental collision (if the two plates contain
continental crust). Deep marine trenches are typically associated with subduction
zones. The subducting slab contains many hydrous minerals, which release their
water on heating; this water then causes the mantle to melt, producing volcanism.
Examples of this are the Andes mountain range in South America and the Japanese
island arc.
Volcanoes
Volcanoes are generally found where tectonic plates
are diverging or converging. A mid-oceanic ridge, for
example the Mid-Atlantic Ridge, has examples of
volcanoes caused by divergent tectonic plates pulling
apart; the Pacific Ring of Fire has examples of
volcanoes caused by convergent tectonic plates coming
together. Volcanoes are usually not created where two
tectonic plates slide past one another. Volcanoes can
also form where there is stretching and thinning of the
Earth's crust such as in the African Rift Valley, the Wells
Gray-Clearwater volcanic field and the Rio Grande Rift
in North America.
Earthquakes
An earthquake is the result of a sudden release of
energy in the Earth's crust that creates seismic
waves. Earthquakes are recorded with a
seismometer, also known as a seismograph. The
Richter magnitude of an earthquake is normally
reported, with magnitude 3 or lower earthquakes
being mostly unnoticeable and magnitude 7 causing
serious damage over large areas.