Where are volcanoes located? Why are they located in those places?
How are they Different?
Niomi
Hi Niomi,
Volcanoes are located in specific parts of the world, not all over the
place. The most common place to find volcanoes is near "subduction
zones". These are places where one of the Earth's oceanic plates is being
pushed under another plate, either another oceanic plate or a continental
plate. These subduction zones are found all around the Pacific Ocean and
consequently there are volcanoes all around the Pacific Ocean. What
happens is that the subducting plate gets pushed down at an angle.
Either it starts to melt when it gets down far enough where it is hot, or
it releases some fluids and gases that rise upwards and cause some of the
overlying mantle to melt. Either way, magma is generated by the
subduction process and this magma works its way to the surface to erupt
as a volcano.
Other places where volcanoes can be found are where two of the Earth's
plates are moving apart. When this happens magma from within the mantle
below rises up to erupt and fill in the space left behind as the plates
moved apart. This can happen under water, for example at mid-ocean
ridges, and in fact this is how new oceanic crust is formed. This
spreading apart can also happen on land, and indeed both the African and
Antarctic continents are slowly spreading apart. Magma is rising to fill
in the space and there are volcanoes along these regions.
The final place where you find volcanoes is over hotspots. We don't
really know what hotspots are or even how deep they are, but we do know
that they stay stationary over long periods of time while the plates move
around on the surface. We also know that they supply heat and magma
to the surface to form volcanoes. Since these don't depend on plate
boundaries (the way that subduction-zone volcanoes and spreading zone
volcanoes do) these hotspot volcanoes can be anywhere, even out in the
middle of a plate. The Hawaiian volcanoes were all formed by a hotspot
that has remained in one place while the Pacific plate has moved happily
by.
As for how are volcanoes different, here's the answer that I sent to
someone who may be your classmate:
Volcanoes are different in lots of ways, but almost all of those can be
blamed on one thing--the temperature of the magma that supplies the volcano.
If the magma is cool, then it has a high viscosity. This means that it
cannot flow easily. If you think about working in the kitchen you are
familiar with this relationship. There are lots of food items that are more
viscous (i.e. harder to stir) when they are cold but less viscous (i.e.
easier to stir) when they are hot. Magma works the same way.
Now, if the magma is viscous then it will not flow very easily.
Additionally, gas bubbles cannot rise through the magma and escape. You can
imagine that if you have lots of bubbles trying to escape the potential of
them building up explosive pressure is high. Sure enough, the volcanoes with
the cooler magma are the explosive ones. These volcanoes are called strato
volcanoes or sometimes composite volcanoes. Mt. St. Helens, Mt. Fuji, and
Mt. Pinatubo are examples of strato volcanoes. What then? The explosive
eruptions generate a high percentage of fine volcanic ash, and this is a
common feature of strato volcanoes. When actual lava does erupt it is very
viscous and usually forms lava domes or maybe short stubby lava flows. Lava
domes have steep sides, and as they get inter-layered with the ash, you
pretty soon end up with a cone-shaped steep-sided volcano.
However, when the
magma is hot, the gas can escape more easily. This means than when an
eruption does happen, either the gas has already escaped through pre-existing
cracks in the volcano or it just bubbles through the lava during the
eruption. A lava fountain at a Hawaiian volcano looks very spectacular but
it isn't really an explosion. The lava flows that are produced also have low
viscosities and they flow away from the vent very easily. They also are
pretty difficult to pile up into a steep mountain so the volcanoes formed
from hot magma have gradual slopes and are caled shield volcanoes.
There are numerous photos of both strato volcanoes and shield volcanoes in
VolcanoWorld. Take a look at some and you'll be able to get a good idea of
some of these things I'm talking about.
I hope this helped to answer your questions.
Sincerely,
Scott Rowland
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