Divergent Plate Boundaries
Almost all the Earth’s new crust forms at divergent boundaries, but most
are not well known because they lie deep beneath the oceans. These are
zones where two plates move away from each other, allowing magma from
the mantle to rise up and solidify as new crust.
Convergent Plate Boundaries
Crust must be destroyed at about the same rate as it is being created,
as Harry Hess surmised. Such destruction (recycling) of crust takes
place along convergent boundaries where plates are moving toward each other,
and sometimes one plate sinks (is subducted) under another. The
location where sinking of a plate occurs is called a subduction zone.
The type of convergence – called by some a very slow "collision" – that
takes place between plates depends on the kind of lithosphere involved.
Convergence can occur between an oceanic and a largely continental plate,
or between two largely oceanic plates, or between two largely continental
Oceanic-Continental place convergence occurs when an oceanic plate
moves under (subducts) a continental plate. Oceanic-continental convergence
creates many of the Earth's active volcanoes, such as those in the
Andes and the Cascade Range in the Pacific Northwest.
As with oceanic-continental convergence, when two oceanic plates
converge, one is usually subducted under the other, and in the process
a trench is formed. Subduction processes in oceanic-oceanic plate
convergence also result in the formation of volcanoes. Over millions of
years, the erupted lava and volcanic debris pile up on the ocean floor
until a submarine volcano rises above sea level to form an island volcano.
Such volcanoes are typically strung out in chains called island arcs. As
the name implies, volcanic island arcs, which closely parallel the
trenches, are generally curved. The trenches are the key to understanding
how island arcs such as the Marianas and the Aleutian Islands have formed
and why they experience numerous strong earthquakes. Magmas that form
island arcs are produced by the partial melting of the descending plate
and/or the overlying oceanic lithosphere. The descending plate also provides
a source of stress as the two plates interact, leading to frequent moderate
to strong earthquakes.