Scattered around the globe are more than one hundred regions of volcanic
activity known as hot spots. Unlike most volcanoes, hot spots are rarely found
along the boundaries of the continental and oceanic plates that comprise the
Earth’s crust; most hot spots lie deep in the interior of plates and are anchored
deep in the layers of the Earth’s surface. Hot spots are also distinguished
from other volcanoes by their lavas, which contain greater amounts of alkali
metals than do those from volcanoes at plate margins.
In some cases, plates moving past hot spots have left trails of extinct volcanoes
in much the same way that wind passing over a chimney carries off puffs of
smoke. It appears that the Hawaiian Islands were created in such a manner by a
single source of lava, welling up from a hot spot, over which the Pacific Ocean
plate passed on a course roughly from the east toward the northwest, carrying
off a line of volcanoes of increasing age. Two other Pacific island chains—the
Austral Ridge and the Tuamotu Ridge—parallel the configuration of the Hawaiian
chain; they are also aligned from the east toward the northwest, with the most
recent volcanic activity near their eastern terminuses.
That the Pacific plate and the other plates are moving is now beyond
dispute; the relative motion of the plates has been reconstructed in detail.
However, the relative motion of the plates with respect to the Earth’s interior
cannot be determined easily. Hot spots provide the measuring instruments for
resolving the question of whether two continental plates are moving in opposite
directions or whether one is stationary and the other is drifting away from it.
The most compelling evidence that a continental plate is stationary is that, at
some hot spots, lavas of several ages are superposed instead of being spread
out in chronological sequence. Of course, reconstruction of plate motion from
the tracks of hot-spot volcanoes assumes that hot spots are immobile, or nearly
so. Several studies support such an assumption, including one that has shown
that prominent hot spots throughout the world seem not to have moved during the
past ten million years.
Beyond acting as frames of reference, hot spots apparently influence the
geophysical processes that propel the plates across the globe. When a
continental plate comes to rest over a hot spot, material welling up from
deeper layers forms a broad dome that, as it grows, develops deep fissures. In
some instances, the continental plate may rupture entirely along some of the
fissures so that the hot spot initiates the formation of a new ocean. Thus,
just as earlier theories have explained the mobility of the continental plates,
so hot-spot activity may suggest a theory to explain their mutability.
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