NOVA Online: Hot Lava
dry volcano 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 earth's interior.

NOVA: What makes that material different from the surrounding mantle material?

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 solid rock?

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.

grassy cliff

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 centers.

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 island.

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 that point.


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