BACKGROUND INFORMATION
The term Plate Tectonics comes from two words of different origins. Plate is a geologic term
that refers to a “large, rigid slab of rock”. Tectonics is derived from a Greek word that means
“to build”. The theory of Plate Tectonics, which was not widely supported until the late 1960’s,
refers to the idea that the Earth’s outermost layer is broken into a number of slow moving
massive plates that interact with each other at their edges. The lithosphere, which is
composed of the crust and upper mantle and is approximately 100km thick, forms the Earth’s
tectonic plates. The layer below the lithosphere is the asthenosphere. It is an extremely hot,
partially melted, and plastic-like layer. The giant plates drift around on this layer, somewhat like
floats in a swimming pool. However, when one plate moves, its movement affects all the
others.
Plate movement is slow, one to two centimeters per year (less than one inch), yet the geologic
activity that occurs at the plate boundaries can be disastrous, often causing earthquakes,
volcanoes, hot springs and geysers. In the distant past, this geologic activity has given rise to
mountain ranges, volcanic islands, rift valleys and other major geologic formations. Most of the
plate boundaries are located on the ocean floor.
Scientists have designated three types of plate boundaries, which are determined by the
interaction occurring at adjacent plates; these boundaries are called divergent, convergent
and transform.
A divergent boundary is formed when two adjacent plates move apart. This causes molten
rock to erupt to the surface, creating new crust which is defined as sea floor spreading. The
discovery of sea-floor spreading, a theory proposed by Harry Hess in 1963, was the first
evidence to support Wegner’s’ theory of Continental Drift, which he proposed in 1910. As the
sea floor spreads, creating mid-ocean ridges, the direction of the Earth’s magnetic field is fixed
into rocks formed along these ridges. As the mid-ocean ridges continue to form new rocks
which move away from the ridges, the magnetic field alignment in the rocks reverses back and
forth in strips that run parallel to the ridges. These magnetic reversals showed that new rocks
are being formed at mid-ocean ridges which helped explain how crust could move. The
Continental Drift Theory had not been able to show this, but now evidence has been found. In
addition to causing mid-ocean ridges, such as the mid-Atlantic Ridge (bottom of the Atlantic
Ocean), plates moving apart can cause underwater volcanoes and earthquakes and form rifts
such as The Great Rift Valley in Eastern Africa.
A convergent boundary is formed when two plates move together. The type of crust the
plates are composed of determines the geologic activity at their interaction point. Two
continental crust plates, being equally dense, will buckle and be pushed upward, forming
mountain ranges such as the Himalayas (still occurring), the Alps and the Appalachians. In
contrast, the Rockies were formed by glaciers. Oceanic crust is denser than the thicker
continental crust. When the denser oceanic crust collides with the less dense continental crust,
the oceanic crust bends and slides under the continental crust and goes down into the mantle.
This interaction is called subduction, which results in earthquakes, volcanoes and rock
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deformation. Some volcanic mountain ranges, such as the Cascade Range in the Pacific
Northwest and the Andes Mountains in South America, were formed by this type of interaction.
A second variation of subduction occurs when two oceanic plates collide. The older, colder,
denser plate will bend and sink into the mantle. This forms a deep oceanic trench at the
boundary and at the surface gives rise to volcanic mountains or chains of volcanic islands such
as the Mariana Islands in the western Pacific. The Pacific Plate, which is the largest plate, has
an area along its edge referred to as the “Ring of Fire”. This area is made up of a line of major
trenches; when plates collide here there is tremendous friction and pressure, which causes
many volcanoes and frequent and severe earthquakes.
A transform boundary or transform fault margin is formed when two plates slide past each
other, usually moving in opposite directions. For example, one may be moving north while the
other moves south; however, they may also move in the same direction, but at different speeds.
A fault is a deep crack in the earth’s surface. At a transform boundary the plates catch and
grind but crust is not produced or destroyed here. The plates build up tension along the
boundary and then release it with a burst of energy and motion which causes an earthquake.
Most of these boundaries are located on the floor of the ocean. One notable exception is the
famous San Andreas Fault zone in California, where the Pacific Plate and the North American
Plate grind past each other at a rate of about five centimeters per year. This fault is
approximately 1,300Km long and more than 10Km wide in several places, passing through
approximately two-thirds of the state. Many earthquakes have occurred along this fault,
including those in San Francisco and Los Angeles.
The Earth’s structure can be divided into several layers: the core (inner and outer); the mantle
(upper and lower); and the crust. The center of the inner core, due to extremely high pressure,
is solid, dense and composed mostly of iron. The outer core is in the liquid state and is
composed mostly of iron. The mantle, the next layer out from the core, is the largest layer. It is
composed of silicon, oxygen, magnesium and iron and is divided into upper and lower divisions,
which are determined by different seismic wave speeds. The outermost layer, the crust, can
vary in thickness from more than 60km to less than 5km. It contains a higher percentage of
silicon and aluminum and a lower percentage of magnesium and iron than the mantle.
Generally, the crust is less dense than the mantle.
From learning as much as possible about the basic structure and makeup of the Earth,
scientists have gained tremendous knowledge about natural geological events. This knowledge
has enabled them to provide as much warning as possible prior to these events to attempt to
reduce their impact on society.
****The Plate Tectonics Word Search has thirteen words found in the above Background
Information. Circle these words and place a check mark by the words on the Plate Tectonics
Word Search Handout. Circle the same words in the above Background Information. Some words
may be either singular or plural on the Background Information or the Word Search, i.e. Plate or
Plates. There are five additional words on the Word Search NOT found on either of the
BACKGROUND INFORMATION sheets (Pangea: The Super Continent or Plate Tectonics: Slip,
Sliding Away). Make a note of these words and look up the definitions.
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