Plate boundaries are found at the edge of the lithospheric plates and are of three types, convergent, divergent and conservative. Wide zones of deformation are usually characteristic of plate boundaries because of the interaction between two plates. The three boundaries are characterized by their distinct motions.
The first sort of plate boundary is called a divergent boundary, or spreading center. At these boundaries, two plates move away from one another. As the two move apart, mid-ocean ridges are created as magma from the mantle upwells through a crack in the oceanic crust and cools. This, in turn, causes the growth of oceanic crust on either side of the vents. As the plates continue to move, and more crust is formed, the ocean basin expands and a ridge system is created. Divergent boundaries are responsible in part for driving the motion of the plates.
The mid-ocean ridges also play a very crucial role
in the development of plate tectonic theory, because of the unique quality
that minerals within the basalt possess. Basalt contains a fair amount
of magnetic minerals, which align to the Earth's magnetic field upon crystallization.
In the past, Earth's magnetic field has been known to change polarity,
causing a reversal in the magnetic field, which is preserved when the
crystals form. The alignment of these magnetic minerals can be used to
date the crust, since they can be correlated with ages of known magnetic
reversals in Earth's history. This plays a key role in the development
of plate tectonic theory because it was the first positive proof that
the plates were indeed moving and had been for most of geologic time.
By using the magnetic reversal information preserved in the minerals of
the mid-ocean ridge basalts, scientists were able to prove that the plates
were moving, and that new crust was being formed and old crust was being
destroyed in a continuous process that had been going on for most of Earth's
history. The oldest crust in the ocean dates back to the early Cretaceous,
100-65 million years ago, which is relatively recent in geologic time.
Subduction zones are the location of very strong earthquakes, which occur because the action of the down going slab interacts with the overriding slab. The "Ring of Fire" around the margins of the Pacific Ocean is due precisely to the subduction zones found around the edges of the Pacific plate. Subduction also is the cause of the volcanic activity in places like Japan: as the downgoing slab goes deeper beneath the overriding plate, it becomes hotter and hotter because of its proximity to the mantle. This causes the slab to melt and form magma, which moves upward through the crust and eventually forms volcanoes (island arcs) in oceanic crust or huge intrusive masses (plutons and batholiths) in continental crust. The Aleutian Islands are another example of a surface expression of subduction.
Sometimes, when there is a convergent boundary between two continental plates, subduction cannot occur. Since continental crust is more bouyant, or less dense, than oceanic crust, one plate does not easily override the other. Instead, the plates crumple as they plow into one another, and a very high mountain range is created. This is a special type of convergent boundary called a collisional boundary. The Himalayas in India are the result of two continental plates (the Indo-Australian and Eurasian plates) colliding head on.
Last modified on 9/8/98 by Maggi Glasscoe (firstname.lastname@example.org)