NOVA: What creates a volcano on an island?
RHODES: Well it's melting within the mantle. That's producing the magma which
is then erupting initially on the sea floor and then gradually building up a
volcanic structure which then leads to an island.
NOVA: Do we know really what is the hot spot?
RHODES: The hot spot idea is a theory. What we do know is that there are certain
places on the earth's surface, Hawaii's probably the best example, where we've
had volcanic activity for very long periods of time. Say in the case of Hawaii
as much as 70 million years. These hot spot locations or centers of volcanic
activity appear to have remained largely stationary, even though the earth's
plates are sort of slowly moving around a few centimeters a year. And so
although there have been a lot of ideas floating around, the one that
scientists are latching onto at the moment is the idea that these hot spots are
the surface expression of a deep mantle plume. In other words a material
that's welling up from deep within the mantle and remains essentially
stationary with time. Now, in addition to that, when you look at the
topography of the sea floor around the islands of Hawaii, they're sort of bowed
upwards, and this is perhaps best explained as a sort of welling up of buoyant
mantle material immediately beneath the Hawaiian islands. Another line of
evidence was that when one looks at the composition of the lavas that are
erupted on Hawaii, they're quite different from lavas that have been erupted,
say, elsewhere on the ocean floor. And the, these chemical characteristics
have led us to believe that the source of these lavas is much deeper than the
lavas that have formed along mid-ocean ridges and so the inevitable conclusion
that one comes to is that the source of the Hawaiian islands is deep within the
NOVA: What makes that material different from the surrounding mantle
RHODES: It's basically hotter and more buoyant than the surrounding material.
One idea is that it's hotter and more buoyant because it contains a larger
proportion of radioactive elements, such as potassium, uranium and thorium.
And these elements generate the heat which makes this particular part of the
mantle somewhat more buoyant than the surrounding mantle.
NOVA: And this one kind of solid rock can move up through another kind of
RHODES: Well it's the same idea as, for instance, the hot air balloon. When
you look at a hot air balloon it's got air inside it and it's moving upwards
through the atmosphere. The only difference is the the air inside the balloon
is hotter and therefore more buoyant and therefore the balloon rises. It's the
same sort of idea with the mantle plume.
NOVA: But at a very, very slow pace.
RHODES: An extremely slow pace. In the order of centimeters per year. The
idea of mantle plumes really originated in the 1970's. It did so because in
the late 1960's the plate tectonics hypothesis explained a lot of the features
of the earth but it failed to explain why one had these centers of volcanic
activity that had been active for long, long periods of time. And so, as a
consequence, the idea of mantle plumes were developed to explain these volcanic
NOVA: How long has the big island been under construction?
MR: Oh, less than a million years....On the island of Hawaii itself we've got
four very large volcanos that have grown within the last one million years from
the ocean floor up to heights of about 30 thousand feet. On the other hand, if
one were to look at the Rocky Mountains in the past millions years, they
wouldn't look too much different a million years ago than they do today....I
can't think of a faster way of creating land than through volcanic activity.
NOVA: Explain the processes that happen when a volcano rises up to form a new
RHODES: You're going to have the sea water flashing into steam, ash being
blasted into the atmosphere and there will be a constant battle going on
between the volcano which is trying to establish itself as an island and the
sea which is persistently eroding it away. My guess is that perhaps within a
few years an island would become established and then continue to grow from
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