What is a hotspot and how do you know it's there?
Most volcanic eruptions occur near the boundaries of tectonic plates, but there are some exceptions. In the interior of some tectonic plates, magma has been erupting from a relatively fixed spot below the plate for millions of years. As the plate continuously moves across that spot, a trail of progressively older volcanic deposits is left at the surface. The Hawaiian Islands are a good example of this. The island of Hawai‘i currently sits above the active hotspot, while a chain of older (and no longer active) island volcanoes extend to the northwest, in the direction of plate movement. A few hotspots (like the one in Iceland) have also been found at diverging plate boundaries.
Scientists don’t fully understand how and why hotspots occur, and there is vigorous scientific debate about their origins. A frequently-used hypothesis suggests that hotspots form over exceptionally hot regions in the mantle, which is the hot, flowing layer of the Earth beneath the crust. Mantle rock in those extra-hot regions is more buoyant than the surrounding rocks, so it rises through the mantle and crust to erupt at the surface.
Hotspots and their trails on the earth’s surface do not develop suddenly (within the span of a human lifetime, for example). Scientists are only able to identify hotspots because of their relatively fixed locations beneath the tectonic plates, which produce tracks of surface volcanism spanning millions of years.
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Map of the Hawaiian Islands.
Hotspots
This is a map of the earth. The thick dashed line shows the ridge formed by the Hawaiian hot spot during the last 80 million years. The arrows show the direction and rate of motion of selected plates relative to the hot spots. The plate boundaries are shown by the lines of medium weight, and the "Pacific ring of fire" plate boundaries by heavier lines. The dots mark the
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Track of Yellowstone hotspot
Shaded relief map showing the path of the Yellowstone hotspot. Yellow and orange ovals outline past caldera eruptions during the time periods indicated (orange calderas are the most recent). The calderas progress from oldest in northern Nevada to youngest in Yellowstone National Park as the North American plate passed over the relatively stationary hotspot. Black lines
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