43 Hot Spots

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. They may be on, near to, or far from tectonic plate boundaries.

Currently, there are two hypotheses that attempt to explain their origins. One suggests that they are due to hot mantle plumes that rise as thermal diapirs from the core-mantle boundary. An alternative hypothesis postulates that it is not high temperature that causes the volcanism, but lithospheric extension that permits the passive rising of melt from shallow depths. This hypothesis considers the term “hotspot” to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well known examples include Hawaii and Yellowstone.



The origins of the concept of hotspots lie in the work of J. Tuzo Wilson, who postulated in 1963 that the Hawaiian Islands result from the slow movement of a tectonic plate across a hot region beneath the surface.

It was later postulated that hotspots are fed by narrow streams of hot mantle rising from the Earth’score-mantle boundary in a structure called a mantle plume.



Oblique view of the principal Hawaiian Islands and (the still submarie) Loihi Volcano. Inset gives a closer view of three of the five volcanos that form the Island of Hawaii (historical laval flow run down the southern coast and from Mauna Loa down the middle of the island). The longest duration historical eruption on Kilauea's east-rift zone at Pu'u O'o, which began in January 1983, continues unabated (as of spring 2015). The oldest volcanos are in the northwest and get younger as the chain travels southeast. This is due to the motion of the pacific plate as it drags the plume head. From north to south the volcanos (which have become islands) are Ni'ihau, Kaua'i, O'ahu, Moloka'i, Maui, Hawai'i (which includes Mauna Loa, Kilauea, and Lo'ihi).
Figure 3. The Hawaiian Islands are a beautiful example of a hotspot chain.
Kilauea volcano lies above the Hawaiian hotspot. Mauna Loa volcano is older than Kilauea and is still erupting, but at a lower rate. The islands get progressively older to the northwest because they are further from the hotspot. Loihi, the youngest volcano, is still below the sea surface. Notice on the left side seamount chains (former volcanoes, currently under the sea level.

Whether or not such mantle plumes exist is currently the subject of a major controversy in Earth science. Estimates for the number of hotspots postulated to be fed by mantle plumes has ranged from about 20 to several thousands, over the years, with most geologists considering a few tens to exist. Hawaii, Réunion, Yellowstone, Galápagos, and Iceland are some of the currently most active volcanic regions to which the hypothesis is applied.

Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, in which water is trapped under the overriding plate. Where hotspots occur in continental regions, basaltic magma rises through the continental crust, which melts to form rhyolites. These rhyolites can form violent eruptions. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history. However, when the rhyolite is completely erupted, it may be followed by eruptions of basaltic magma rising through the same lithospheric fissures (cracks in the lithosphere). An example of this activity is the Ilgachuz Range in British Columbia, which was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.

The hotspot hypothesis is now closely linked to the mantle plume hypothesis.

Hotspot volcanic chains

The joint mantle plume/hotspot hypothesis envisages the feeder structures to be fixed relative to one another, with the continents and seafloor drifting overhead. The hypothesis thus predicts that time-progressive chains of volcanoes are developed on the surface. Examples are Yellowstone (Figure 5), which lies at the end of a chain of extinct calderas, which become progressively older to the west. Another example is the Hawaiian archipelago, where islands become progressively older and more deeply eroded to the northwest.

15 million years ago, the hotspot was along the Nevada/Oregon border. 14 million years ago, it was at the corner of Orgeon, Nevada, and Idaho. 12 million years ago, it was almost completely in Idaho, crossing just a bit into Nevada. 11 million years ago, It was fully in Idaho, and it had moved east above Utah, rather than Nevada. 10 million years ago, it was just above pocatello, further north and east from its previous position. 4 to 6 million years ago, it had moved further northeast. 0.6 to 2 million years ago, it had traveled further northeast, crossing into Wyoming, though about one third of the hotspot remains in Idaho.
Figure 5. Volcanic activity above the Yellowstone hotspot on the North American Plate can be traced from 15 million years ago to its present location.


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Geology 101 for Lehman College (CUNY) Copyright © by Yuri Gorokhovich and Lumen Learning is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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