What is the relationship between volcanoes, earthquakes, and plate-tectonics?
Plate tectonics is the over-lying theory presently used by most Earth
Scientists to describe motion within the outer-most layer of the solid
Earth (also know as the lithosphere). Individual plates of varying size
move about the surface of the Earth at varying speeds. Where
plate pull apart, slide by each other or collide, there is tectonic
activity manifested as earthquakes. The great majority of seismicity
on the planet occurs at plate boundaries, although intra-plate
seismicity can occur as well when stresses build up in the plate.
For instance the Mew Madrid Fault zone of the mid-western USA is an
example of a intra-plate seismic belt.
(for more info on this subject see
http://www.soest.hawaii.edu/GG/ASK/NewMadrid.html) In general,
the deepest plate boundary earthquakes are at plate collision
(or subduction) zones, and the shallowest are at divergent margins.
Volcanism is associated with two of the plate boundary types: divergent
and convergent margins. The former manifest themselves as long volcanic
rifts mostly in the ocean basins (ocean ridges) whereas the latter
typically make individual volcanoes on the plate that "wins out" in
the collision process (i.e., does not subduct). Where two plates
containing continental crust at their margins collide, there is little
or no volcanism (such as at the Himalaya). Occasionally, plate
boundaries where plates are mostly sliding by each other can
experience small amounts of volcanism as well if there is a component
of extension across this boundary.
Volcanism can also occur at intraplate volcanoes. These volcanoes
are believed to have sources deeper down in the Earth's mantle that
remain in a relatively fixed location relative to the always migrating
plate boundaries. Mauna Loa and Kilauea in Hawaii are the classic
examples of intraplate
volcanoes. Such volcanoes can also be seismically active, particularly
when volcanic structures are built up rapidly. The crust must respond
to the extra load and relieves this stress through tectonic
activity. There are a number of other postings about hotspots and
Hawaii at this web site if you are interested (you can search from
the site home page).
Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822