Plate tectonics is a
scientific theory which describes the large scale
motions of Earth's lithosphere. The theory builds on the
older concepts of continental drift, developed during
the first decades of the 20th century by Alfred Wegener,
and seafloor spreading, developed in the 1960s.
The lithosphere is broken up into what are called
tectonic plates. In the case of Earth, there are
currently seven to eight major (depending on how they
are defined) and many minor plates. The lithospheric
plates ride on the asthenosphere. These plates move in
relation to one another at one of three types of plate
boundaries: convergent, or collisional boundaries;
divergent boundaries, also called spreading centers; and
transform boundaries. Earthquakes, volcanic activity,
mountain-building, and oceanic trench formation occur
along plate boundaries. The lateral relative movement of
the plates varies, though it is typically 0–100 mm
annually.
Tectonic plates are able to move
because the Earth's lithosphere has a higher strength
and lower density than the underlying asthenosphere.
Their movement is thought to be driven by the motion of
hot material in the mantle. Lateral density variations
in the mantle result in convection, which is transferred
into tectonic plate motion through some combination of
drag, downward suction at the subduction zones, and
variations in topography and density of the crust that
result in differences in gravitational forces. The
relative importance of each of these factors is unclear.
The location where two plates meet is called a plate
boundary, and plate boundaries are commonly associated
with geological events such as earthquakes and the
creation of topographic features such as mountains,
volcanoes, mid-ocean ridges, and oceanic trenches. The
majority of the world's active volcanoes occur along
plate boundaries, with the Pacific Plate's Ring of Fire
being most active and most widely known.
Four
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:
Transform 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.
Divergent boundaries
occur where two plates slide apart from each other.
Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active
zones of rifting (such as Africa's Great Rift Valley)
are both examples of divergent boundaries.
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.
Plate boundary zones
occur where the effects of the interactions are unclear
and the broad belt boundaries are not well defined.
Information
obtained from the Wikipedia web site. Wikipedia the free
encyclopedia.
