The
Plates
The outer part of the earth is broken into rigid plates approximately 62 miles
thick. These outer plates are called lithosphere and include rocks of the earth's
crust and upper mantle. Below the rigid lithosphere is the asthenosphere, a zone
around the earth that is approximately 90 miles thick and behaves like a plastic
because of high temperature and pressure. The lithosphere plates move over the
plastic asthenosphere at a rate of an inch or more per year. Eight large plates
and a few dozen smaller plates make up the outer shell of the earth.
The internal heat of the earth is the most likely cause of plate movement;
this heat is probably generated by the decay of radioactive minerals.
The entire surface of the earth is moving, and each plate is moving
in a different direction than any other. We now believe that plate movement
is responsible for the highest parts of the continents and the deepest trenches
in the oceans. Such movements also cause catastrophic events like earthquakes,
volcanoes and tsunamis.
Plate Boundaries
Plate boundaries are of three types: a diverging plate boundary is a boundary
between plates that are moving apart; a converging plate boundary is one where
plates are moving towards each other; and a transform plate boundary is one
at which two plates move past each other.
Diverging
Boundaries
Diverging boundaries occur where plates are moving apart. Most of these boundaries
coincide with the crests of the submarine mid-oceanic ridges. These ridges form
by ascending hot mantle material pushing the lithosphere upward. When heat rises,
molten rock moves upward and the expansion from the heart and pressure causes
the ridge plate to bow upward and break apart at the spreading centers. Tension
cracks form parallel to the ridge crest and molten rock from magma chambers
in the mantle is intruded through the fractures. Magma errupts into submarine
volcanoes and some of it solidifies in the fissure. New crust forms in rifts
at the spreading centers. As new magma is extruded, it accretes to both sides
of the plates as they are pushed or pulled apart. As the plates continue to
pull apart, new tension fractures form and fill with magma. This cycle repeats
itself again and again.
Transform Boundaries
The transform boundary occurs where two plates slide past one another. The San
Andreas Fault is one of the best known land exposures of a transform boundary.
Converging Boundary
A converging boundary where plates move toward each other is responsible for
the origin of most of Idaho's igneous rock as well as most of the major structural
features of the state. Where one plate is covered by oceanic crust and the other
by continental crust, the less dense continental plate will override the denser
oceanic plate. The older the oceanic plate, the colder and more dense it is.
Where two plates collide, the dense plate is subducted below the younger and
less dense plate margin. At this boundary, a subduetion zone forms where the
oceanic plate descends into the mantle beneath an overriding plate. As the oceanic
plate descends deeper into the earth it is heated progressively hotter. Also
the friction caused by the two plates grinding past each other leads to greater
temperatures.
At the subduction zones, submarine trenches form, representing the deepest
parts of the ocean basins. Earthquakes continuously occur at the plate margins
where the overriding plate is grinding and abrading the subducted laver. The
subducted plate causes earthquakes all along its downward path as it slowly
moves into the earth's mantle. By measuring the depth and position of the earthquakes,
geologists are able to determine almost exactly the position and orientation
of the subducting plate.
Age of Ocean Basins
The youngest rocks are found at the spreading centers, and become progressively
older in both directions away from the spreading centers. The oldest rocks in
the ocean basins are approximately 150 million years old as compared to the
oldest rocks on the continents of 3.8 billion years.
Physical Properties
of Oceanic Rocks
Rocks forming the ocean basins are dark, iron-rich and have a higher specific
gravity than those forming the continents. Rocks of the continents tend to be
low in density, light colored and rich in silica and aluminum. Because of the
lower density of continental rocks, they are floating on the denser oceanic-type
rocks.
Generation
of Magma, Volcanoes and Batholiths
When the plate reaches a certain depth, the heat and pressure melts the lighter
minerals within it. This light molten rock or magma coalesces at depth and floats
upward through the more dense rock towards the surface of the earth. Where these
globs of molten rock break through the oceanic crust, they form chains of volcanic
islands. The Aleutian Islands are a well-known example.
The portion of the magma that manages to break through the surface forms volcanoes
like Mount St. Helens and is classified as volcanic or extrusive rock. The portion
that does not break through the earth's surface, but instead solidifies within
the earth's crust, is classified as intrusive igneous or plutonic rock.
Where an oceanic plate is subducted below a continent, the rising globs of
magma melt and absorb portions of the silica-rich, low specific gravity continental
rocks. Where magma manages to break through the continental crust, the extrusive
products are much more siliceous than their oceanic counterparts.
Geologists have found that rocks intruded through continental rocks have strontium
isotope values much different than those intruded through oceanic rocks. Consequently,
we are able to determine, on the basis of strontium isotope ratios, whether
or not a particular intrusion passed through continental rocks.
Rocks in the vicinity of a subduction zone are drastically changed by the intense
heat and pressure. If these rocks do not melt, they become metamorphic rocks.
Also rocks in the continental crust in the path of a rising magma chamber are
metamorphosed by the heat and pressure exerted by the upward-moving molten rock.
Mantle Plumes
Mantle plumes are believed to form where convection currents in the earth's mantle
cause narrow columns of hot mantle rock to rise and spread radially outward. One
of the most convincing theories for the origin of the Snake River Plain proposes
that a hot mantle plume tracked across the plain from west to east and was the
source for most of the volcanism in the Snake River Plain. This hot mantle plume
is now thought to underlie the caldera at Yellowstone Park. Many of the volcanic
islands of the Pacific Ocean may have originated from a mantle plume. The best
known examples are the Hawaiian Islands.