Plate tectonics and volcanism are very closely related, and all due to the
fact that the Earth still contains a lot of heat which is trying to escape
to the surface. The current idea is that there are great huge convection
currents within the Earth's mantle (like when you cook soup - the hot soup
rises while the cool soup sinks, then the hot soup cools off and sinks
meanwhile the cool soup gets heated up and rises, and around and around
and around. If you imagine huge volumes of plastic (sort of between solid
and liquid) rock doing the same kind of convecting (very slowly) then you
can get a picture of what is going on down under the surface of the earth.
Where a couple of these convection cells rise next to each other, hot
mantle material is brought near the surface where it melts to form magma
which rises to erupt at spreading centers. Most of these are at mid-ocean
ridges. The convection cells move laterally after rising and this helps
to drag the plate along. Where two cells bump into each other after
flowing laterally they have to dive back into the mantle. The plates that
are being dragged along by these cells will collide with each other. The
most common thing that happens is that one of the plates is oceanic (and
therefore relatively dense), and it will dive under
the other to form what is called a subduction zone. As this plate dives
into the mantle it heats up and all the water that it collected while
being the ocean floor is boiled off. This water vapor rises up towards
the surface and has the effect of lowering the melting temperature of the
upper mantle rocks that it rises through. This causes melting, the
generation of magma, and thus volcanoes are very strongly correlted with
subduction zones. There are subduction zones adjacent to all the lines of
volcanoes that comprise the pacific ring of fire.
Plate tectonics seems to have been happening for at least the past 3
billion or so years of the Earth's history, with the continental
fragments of crust sometimes moving apart from each other (as oceanic
crust is created to fill in the space), and at other times with the
continental pieces of crust colliding. At at least one time all the
oceanic fragments mashed together to form one super continent, and
geologists have named this "Pangea".
I've only given you a quick answer to what is a complex question. If you
want more information, just about any introductory geology book will
contain a good explanation as well as diagrams.
Scott Rowland, University of Hawaii
Tom Wynne
Other Categories
Other Questions
To VolcanoWorld