| Spouts Beneath the Plates | |
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When the theory of plate tectonics came together in the 1960s, it did what all scientific theories do—explained a great many things and at the same instant highlighted many new strange things. It explained a host of mysteries such as the movements of continents, the form and behavior of the ocean floor, the birth and death of mountain chains, the locations of volcanoes and earthquakes, the distribution of ancient life, even the rise and fall of sea level over geologic time. By clarifying crustal behavior, plate tectonics made the Earth's crust clear in almost a literal sense, letting us focus on the strange things beneath.
Hotspots were obvious to the first plate-tectonic theorists. When they sketched the crustal plates on the globe and saw the world's volcanoes fall into line along the plate edges, the exceptional volcanoes suddenly made sense in a new way.
The Hawaiian volcanoes are the textbook case. At one end of this island chain is Hawaii itself, with three active volcanoes. Away from this end, the islands get older and smaller, and beyond the last island marches a long train of undersea mountains, each one older still. With plate tectonics, the island chain was seen to be the product of a single "hot spot" fixed in the mantle, punching one volcano after another through the crust as it moves overhead. This illustration from VolcanoWorld shows the track of the Hawaiian hotspot and some others in the Pacific.
It didn't take long to find many dozens of hotspots around the world, in a wide range of sizes.
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Most of these are in the ocean, more than you would expect. Only one is definitely inside a continent, the one that underlies Yellowstone National Park. (Here's a more detailed look at that hotspot track.)
Exactly what hotspots are and what they mean is a hot subject. Some of them formed at definite times in Earth history. Some of them are dead. Some of them can be traced as far back as the seafloor record takes us, about 150 million years, and presumably beyond. They stay where they are despite all the movements of the crustal plates—at least they seem to move only very slowly.
Everyone agrees that hotspots arise in the mantle, but explanations vary. The majority opinion is a bottom-up scenario, that hotspots boil upward from a deep-seated source of heat, the way plumes of smoke rise from a fire, and why not? The latest thinking is that the Earth's center may be practically as hot as the Sun's surface. However, until recently, seismic studies (discussed in Part 4) didn't show even vague images of mantle plumes deeper than the upper mantle, a few hundred kilometers down.
For a long time, therefore, others have found room for a top-down explanation of hotspots based on plate tectonics. In this view, openings between plates at the surface, by lowering the pressure beneath, cause mantle rock to begin melting and rise as hotspots. Even after the plates move away, the hotspots continue to churn. The irrepressible Don L. Anderson of Caltech is among these people, and in a 2001 Science article "Topside Tectonics" (read the PDF) he lays out the no-plumes hypothesis in tantalizing if sketchy form.
The argument is not over, and both sides may be right. Nevertheless a consensus is growing that while many hotspots are fairly shallow, some are true plumes with their roots at the very bottom of the mantle, almost 3000 kilometers down. A Web search for "mantle plume" will bring you a vast list of research on hotspots.
But we still don't know very well what kind of rock is down there. Hotspots are not like the kimberlites and lamproites in Part 2, which deliver clean chunks of the upper mantle to the surface like an oil gusher. A mantle plume is more like the hot-water inlet in a hot tub—the hot water jet is quickly diluted into a gentle warm current. In the same fashion, plumes deliver magmas to the surface that are mixed with every part of the mantle along the way. The resulting geochemical evidence is complex, a tangle of many threads, and deep-Earth experts are having the time of their lives following them.
More college-level geology class material is going on the Web every day. Try the Yellowstone hotspot module on the Network Montana Program. You'll learn about two things at once—geology and current Web-based education.
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