USGS/Cascades Volcano Observatory, Vancouver, Washington
What is a tectonic plate?
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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.
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How do these massive slabs of solid rock float despite their tremendous
weight? The answer lies in the composition of the rocks. Continental crust
is composed of granitic rocks which are made up of relatively lightweight
minerals such as quartz and feldspar. By contrast, oceanic crust is composed
of basaltic rocks, which are much denser and heavier. The variations in
plate thickness are nature's way of partly compensating for the imbalance
in the weight and density of the two types of crust. Because continental
rocks are much lighter, the crust under the continents is much thicker
(as much as 100 km) whereas the crust under the oceans is generally only
about 5 km thick. Like icebergs, only the tips of which are visible above
water, continents have deep "roots" to support their elevations.
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Plate Tectonic Theory
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Before the 19th century, people thought the ocean floor was flat and featureless.
In reality, the ocean floor has rugged mountain ranges. Though hidden underwater,
the global mid-ocean ridge system is the most prominent topographic feature
on the surface of our planet. In 1961, scientists began to theorize that
mid-ocean ridges mark structurally weak zones where the ocean floor was
being ripped in two. New magma from deep within the Earth rises easily
through these weak zones and eventually erupts along the crest of the ridges
to create new oceanic crust. This process, called seafloor spreading, has
built the mid-ocean ridges.
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How can new crust be continuously added along the oceanic ridges without
increasing the size of the Earth? Henry Hess reasoned that the ocean basins
were perpetually being "recycled," with the creation of new crust and the
destruction of old oceanic lithosphere occurring simultaneously. He suggested
that new oceanic crust continuously spreads away from the ridges in a conveyor
belt-like motion. Many millions of years later, the oceanic crust eventually
descends into the oceanic trenches -- very deep, narrow canyons along the
rim of the Pacific Ocean basin. This means the Atlantic Ocean is expanding
while the Pacific Ocean is shrinking. As old oceanic crust was consumed
in the trenches, new magma rose and erupted along the spreading ridges
to form new crust. Thus, Hess' ideas explained why the Earth does not get
bigger, why newer rocks are found near mid-ocean ridges, and why oceanic
rocks are much younger than continental rocks.
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Further, improvements in seismology led to the knowledge that earthquakes
tend to be concentrated in certain areas, most notably along the oceanic
trenches and spreading ridges. By the late 1920s, several prominent earthquake
zones were identified parallel to the trenches that typically were inclined
40-60° from the horizontal and extended several hundred kilometers
into the Earth. But what was the significance of the connection between
earthquakes and oceanic trenches and ridges? This connection helps to confirmthe
seafloor-spreading hypothesis by pin-pointing the zones where oceanic crust
is being generated (along the ridges) and the zones where oceanic lithosphere
sinks back into the mantle (beneath the trenches).
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Scientists now have a fairly good understanding of how the plates move
and how such movements relate to earthquake and volcano activity. Most
movement occurs along narrow zones between plates where the results of
plate-tectonic forces are most evident.
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From: Tilling, 1985, Volcanoes: USGS General Interest Publication
URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>
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03/02/01, Lyn Topinka