نطق دوم

Lecture 2 - Formation of some special volcanos

Narrator: Listen to part of a lecture in an Earth Science class. The class has been discussing volcanoes.

Professor: Okay. We know the Earth’s surface, the crust, is made up of tectonic plates, and these huge slabs of rocky crust are slowly sliding over or under or past each other, and we said that most of the world’s volcanoes occur at the boundaries of these tectonic plates where you have hot molten rock squeezing up through gaps between the plates.

But some volcanoes occur not at the edges, but in the middle of a continental or oceanic plate. The Hawaiian Islands, for example, are thousands of kilometers away from any plate boundary, and yet you have vast amounts of magma, molten rock or lava, flowing up through the earth’s crust, which means, of course, that volcanic activity there can’t be explained simply by plate tectonics. So, how do we explain these volcanic anomalies, these exceptions to the general rule?

Well, back in 1963, a geophysicist by the name of Wilson came up with a hot-spot theory to explain how this particular type of volcanic activity can occur, and can go on for maybe tens or even hundreds of millions of years. Wilson’s theory was that: hot spots exist below tectonic plates, and they’re the cause of these volcanoes. But what causes the hot spots?

Hmm, well, the most popular theory that’s been proposed is the plume hypothesis. According to this hypothesis, plumes, uh, basically columns of extremely hot magma. These plumes well up from deep inside the planet’s interior, maybe even as deep as its core, and rise all the way up to melt through the Earth’s crust.

Imagine a burning candle, and imagine moving a sheet of heavy paper slowly over the flame of the candle. You’re gonna get a series of burned spots in the paper, well, that’s just like what’s happening with the Hawaiian Islands, but instead of a sheet of paper, you’ve got a tectonic plate, and it’s moving over this plume of intensely hot magma, and rather than a series of burned spots in the paper. You’re getting a chain of volcanic islands where the hot plume melts through the crust under the Pacific Ocean at one point after another with active volcanoes on the younger islands that are now just above the plume, and the other islands, well, the farther away from the plume they are now, the older they are, and the longer ago their volcanoes went dormant or extinct.

Incidentally, volcanic islands may seem small, but the island known as the Big Island Hawaii is one of the tallest topographic features on the planet, more than five kilometers from the sea floor to the ocean surface, and almost that much again, up to its highest peak. That’s nearly ten kilometers from ocean floor to the highest point on the island, which makes it taller even than Mount Everest. So, you can imagine the huge amounts of magma, or lava, that’ve flowed up to form even just this one island, much less the whole chain of islands.

Now, the Plume Hypothesis provides a pretty elegant explanation for a volcanic anomaly, like the Hawaiian Islands. But, while it’s hypothetically attractive, there’s very little direct evidence to support the theory, because so far, no one’s been able to actually observe what’s happening that far beneath the Earth’s crust.

Some studies have been done, seismographic, geochemical, where the data’s consistent with the model, but they aren’t definitive proof. Even the model supporters are uncomfortable claiming that it explains every volcanic anomaly.

And like any popular theory, I suppose, it has some determined critics. These critics have put forth a number of alternative theories, all unproven so far. But one well-regarded theory is the crack hypothesis, which assumes that hot spots are created when a piece of the crust gets stretched thinner and thinner and the resulting stress causes small cracks to open up at weak spots in the crust, and it’s through these cracks that magma pushes up to form volcanoes.

Proponents of the crack hypothesis consider this a widespread phenomenon and believe that magma’s not coming up from deep within the Earth’s interior, but rather from just beneath the surface crust.

This hypothesis is attractive, because it fits with what we already know about plate tectonics and it fits what we know about some secondary smaller hot spots, but how well does it explain the Hawaiian Islands? Could a series of random cracks produce that same particular string of Islands that’s sequenced so neatly from old to young? You know, it worries me when a theory depends on coincidence to produce results.