Kotzj
Kotzj
You've asked a question that nobody knows the answer to very well. We mainly know what a hotspot's effects on the surface are, but what a hotspot actually is or even where it is are pretty much unknown. We know that a hotspot causes hot basaltic magma to be erupted at a relatively small area on the Earth's surface, that this continues for 10's of millions of years, and that this location doesn't move. There are arguments about whether the hotspot actually supplies the magma or whether it only supplies the heat, and this heat partially melts the upper mantle to provide the magma.
The fact that hotspots don't appear to move, even when plate tectonics is shifting the earth's lithospheric plates around, argues for hotspots being deep. Some people even argue that they may be at the core-mantle boundary. We know from geochemistry that the magma does not come from that deep so if hotspots are indeed that far down then it is more likely that they are only supplying heat, and that the magma production occurs closer to the surface (around 100-150 km down).
This idea of non-moving hotspots and moving plates makes perfect conveyor belts for lines of volcanoes, and Hawai'i is the best and most-often studied example. Our hotspot is presently somewhere under the SE part of the big island (at the SE end of the Hawaiian chain). It is feeding 3 volcanoes--Mauna Loa, Kilauea, and Lo'ihi. Meanwhile, the Pacific plate, on which the volcanoes are built is drifting slowly to the NW. Eventually these 3 volcanoes will be carried so far off the hotspot that their plumbing systems can no longer be maintained, and new volcanoes will be built over the hotspot. This has been happening for at least the past 70 million years, evidenced by the chain of ever-older volcanoes that extends to the NW from the present hotspot location. For example, still on the big island, Hualalai, Mauna Kea, and Kohala are older (in that order), then on the island of Maui, the E. Maui and W. Maui volcanoes are older, then on to Moloka'i, and so on. Beyond Kaua'i, the volcanoes are only remnants, having both sunk and been eroded to bare nubbins. Beyond that are only atolls that have built on the completely sunken volcanoes, and beyond Kure and Midway, even the atolls have sunk below the ocean. Evidence of the earliest days of the hotspot, however, is provided by this chain of ever-older (to the NW) seamounts called the Emperor seamounts. The oldest is Meiji seamount at 60-70 million years old and not far from being subducted under Kamchatka.
A recent idea that is almost becoming a bandwagon is that hotspots are related to huge blobs of magma that rise from within the mantle and pool against the base of the lithosphere. You can imagine a large upside-down tear-drop shaped blob rising under the force of buoyancy through the mantle. It will have a big blobby head and a long trailing tail (sort of like the blobs in a lava lamp). The idea is that when the big head bumps into the base of the lithosphere it causes huge eruptions of basalt called flood basalts, and may even initiate the break-up of a continent to form a new spreading center and eventually a new ocean. Meanwhile the long trailing tail of the hotspot continues to rise, and long after the head part has had its effect, and 2 halves of the flood basalts have been moved away by plate tectonics, this tail is the hotspot. There a number of examples around the world where you can tie in a hotspot trace to a flood basalt. However, there are also a number of hotspots, particularly the Hawaiian hotspot, where you can't. The excuse is that perhaps the flood basalt associated with our hotspot has already been subducted under Asia (talk about sweeping the evidence under the carpet).
Well, anyway that may be more of an answer than you wanted, but it is a very fascinating subject, and one that many people are presently trying to unravel. Perhaps you'll join the effort someday!
Scott Rowland, University of Hawaii
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