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Hot Spot Volcanism
As this slide from the esteemed Professor Karen Harpp's
Powerpoint presentation on Plate tectonics demonstrates,
hot spot volcanism occurs in the middle of a plate,
rather than at its edges (where most volcanic activity
can be found).
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a mantle plume releases magma towards the surface,
plates slowly move. Over time this leads to
the creation of a series of volcanic formations.
Also note that older formations are subject to
destructive forces of nature such as erosion.
This means formations underwater may once have been
islands.4 |
The hot spot theory claims that
sometimes in the center of continental plates mantle
plumes exist in which magma from the asthenosphere/lithosphere
boundary escapes to the surface.4
As the material from the boundary
rises, it undergoes decompression melting.
This occurs when materials at depth are hot enough under
normal (i.e. surface) pressure conditions to be liquid.
The pressure of the depth, however, causes the material
to remain solid. As it rises this pressure
decreases (there is less material pushing down on it),
and it melts, despite remaining approximately the same
temperature. The melt is now a lower density than
the material around it, and continues on to breach the
surface.4
When the lava is released at the
surface it is cooled by the ocean waters almost
immediately and begins to pile up around the vent. In the case of island hot spots (such as
Easter Island), this build up is eventually great enough
to breach the water level. The most telling
feature of a hot spot is that it will create a string of
islands and/or sea mounts, which are essentially
'underwater islands' (raised geologic formations beneath
the surface of the water).5
If a chain of islands and sea
mounts is the product of hot spot volcanism they should
be arranged in order from oldest to youngest. Why
is this? This reflects the fact that plates on the
surface of the earth move
while mantle plumes are stationary. As a
plate slowly moves over a hot spot it will form a sea
mount or island. When the plate has moved enough,
that formation will stop growing/showing volcanic
activity, and a new one will begin forming right next to
it. Then the same process will repeat, with the
result being a set of volcanic formations roughly in a
line from oldest to youngest.4
As E.
Bonatti et al. wrote in 1977, such a chain
phenomenon exists with Easter Island, located over the
Nazca Plate citing it as a "prominent east-west feature"
extending from Easter Island almost to the South
American continent.11,
pg. 2458
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Theories as late as the 1930s that Easter Island was
once a great archipelago empire demonstrate the
mystery surrounding the island into modern times, as
well as the advances that have been made in
volcanological science in the past century.7 |
The existence of such a pattern
around Easter Island initially led to some wild
speculation about the island and its inhabitants.
For instance, New York Times articles from as late as
the 1930s state that, "Today's theory is that Easter
Island was at one time the necropolis of a lost empire,
probably of a vast archipelago, which has sunk to the
bottom of the Pacific."7
Such theories are based upon observations of
seamounts in the vicinity around Easter Island, referred
to in the article as "submarine islands."7
The hot spot theory proves to be significantly
more consistent with archaeological and geologic
evidence about the island.
Easter Island's largely basaltic
geologic makeup(6, pg.
14) is also consistent with the hot spot
theory. This basaltic composition is related to
the fact that the process of decompression melting
allows magmas to erupt without losing much heat, and
magmas therefore are subject to minimal crystallization.
The resulting lava flows are thus relatively hot and low
in silicates, indicators of low viscosity.
Eruptions are therefore likely effusive, which
corresponds to the fact that lavas flows (rather than
explosions) have tied the three volcanoes of Easter
Island together.8, pg
137
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