Why Hawaii's islands get older toward the northwest
In 1963 J. Tuzo Wilson proposed that Hawaii formed as a tectonic plate slid slowly over a hot patch beneath it, leaving a trail of volcanoes behind. The idea explains why Hawaiian islands grow older and more eroded to the northwest, and why Yellowstone sits at the young end of a caldera chain.
Most volcanoes cluster where plates pull apart or collide. Hotspots are the exceptions, volcanic regions that sit wherever they like with respect to plate boundaries, such as Hawaii, Iceland and Yellowstone. Because the heat source stays roughly put while the plate drifts, a single hotspot can build a whole line of volcanoes, each one carried away from the source and going extinct as a newer one rises behind it.
Two explanations compete. The favoured one says hotspots are fed by mantle plumes, columns of unusually hot rock rising from the boundary between core and mantle. The rival plate theory argues the mantle below is not especially hot at all; the crust above is simply thin or weak, letting melt rise passively from shallow depths as the plate stretches. Whether plumes exist has been a major controversy, but seismic images now broadly support the idea. Guesses at how many plume-fed hotspots exist run from about 20 to several thousand, and most geologists settle on a few dozen. The plumes imaged so far are tilted and irregular rather than neat vertical pipes, and only Yellowstone has been consistently traced from the deep mantle to the surface.
Hotspot lava is usually basalt, as on Hawaii and Tahiti, so eruptions tend to be gentler than at subduction zones, where water dragged down with a sinking plate fuels explosive island arcs like the Aleutians. Under continents it is different: rising basalt melts the crust into rhyolite, which can erupt violently. The Yellowstone Caldera was made by some of the most powerful explosions in Earth's history.
Geologists hoped hotspot trails would work as fixed markers for plate motion, but chains such as the Galápagos are not neatly time-ordered, and Hawaii and Iceland appear to drift relative to each other. In 2020 Wei and colleagues used seismic tomography to find an ocean plateau about 100 million years old, thought to come from the plume head that began the Hawaii-Emperor chain, now sunk 800 kilometres beneath eastern Siberia.
Source: Hotspot (geology)
