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Locations of Volcanoes We currently think that hot spots
are caused by plumes that rise from the lower mantle. Remember that not all of the
Earth's internal heat is transported by mantle convection, but that part is
transported by plumes of very hot rock. Experimental and computer models suggest that when
a plume reaches the lithosphere, the plume flattens out against the bottom of the
lithosphere, heats it, and causes it to bulge and fracture. Decompression melting converts
some of the plume material to lava that rises through cracks to the surface to form a huge
flood basalt volcano. Later, as the plate continues to move over the hot spot, the
"head" of the plume is sheared off, leaving the narrower hot "tail."
Lava rising up the tail and through the plate over a long period of time leaves a trail of
shield volcanoes leading away from the huge flood basalt deposit. Since the lavas
come from the mantle, the plumes are basaltic in composition and low in dissolved gases.
Plumes come in all different sizes and can occur anywhere.All of this should sound familiar. Virtually all of these model predictions match features seen in hotspot volcanism, which is why the plume model is the leading hypothesis for hotspot volcanoes.
(Now all you have to do is figure out the probability of that actually happening in your lifetime!)
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An interesting consequence of
hotspot volcanism is that a hotspot volcano can occur anywhere, even in your own backyard! In fact, a volcano did occur recently in one farmer's backyard.
In 1943 Dionisio
Pulido found a crack spewing out red hot rocks and lava in his cornfield. Within a year
the crack grew into a volcano--Paricutin--that covered his field, his farm, his local
village, and everything else within five miles. Paricutin is still active in southwestern
Mexico. Of course, a glance at the map will show that Paricutin is in a zone of many
active volcanoes caused by plate subduction under Mexico; therefore, its location was not
exactly a complete surprise. On the other hand, it is estimated that hotspot volcanism
will affect every part of the Earth's surface on an average of every 500 million to 800
million years. So even if you live in Lafayette, Indiana, you might wake up some morning
to find a brand new hotspot volcano in your own backyard! Photo:
Courtesy of NGDC/NOAA.