Subject: Plate Tectonic, Volcanoes and Earthquakes

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 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

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