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2.7: Hotspots

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  • The Wilson Cycle provides a broad overview of the tectonic plate movement. To analyze plate movement more precisely, scientists study hotspots. First postulated by J. Tuzo Wilson in 1963, a hotspot is an area in the lithospheric plate where molten magma breaks through and creates a volcanic center, islands in the ocean and mountains on land [82]. As the plate moves across the hotspot, the volcano center becomes extinct because it is no longer over an active magma source. Instead, the magma emerges through another area in the plate to create a new active volcano. Over time, the combination of a moving plate and a stationary hotspot creates a chain of islands or mountains. The classic definition of hotspots states they do not move, although recent evidence suggests that there may be exceptions [83].

    The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.
    Figure \(\PageIndex{1}\): Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.

    Hotspots are the only types of volcanism not associated with subduction or rifting zones at plate boundaries; they seem totally disconnected from any plate tectonics processes, such as earthquakes. However, there are relationships between hotspots and plate tectonics. There are several hotspots, current and former, that are believed to have begun at the time of rifting. Also, scientists use the age of volcanic eruptions and shape of the chain to quantify the rate and direction of plate movements relative to the hotspot.

    Hotspots are scattered around the world.
    Figure \(\PageIndex{2}\): Map of world hotspots. Larger circles indicate more active hotspots.

    Scientists are divided over how magma is generated in hotspots. Some suggest that hotspots originate from super-heated material from as deep as the core that reaches the Earth’s crust as a mantle plume [84]. Others argue the molten material that feeds hotspots is sourced from the mantle [85]. Of course, it is difficult to collect data from these deep-Earth features due to the extremely high pressure and temperature [86].

    How hotspots are initiated is another highly debated subject. The prevailing mechanism has hotspots starting in divergent boundaries during supercontinent rifting [87]. Scientists have identified a number of current and past hotspots believed to have begun this way. Subducting slabs have also been named as causing mantle plumes and hot-spot volcanism [88]. Some geologists have suggested another geological process not involving plate tectonics may be involved, such as large space objects crashing into the earth [89]. Regardless of how they are formed, dozens are on the Earth. Some well-known examples include the Tahiti Islands, Afar Triangle, Easter Island, Iceland, Galapagos Islands, and the Samoan Islands. The United States is home to two of the largest and best-studied hotspots: Hawaii and Yellowstone.

    2.7.1: Hawaiian Hotspot

    There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.
    Figure \(\PageIndex{3}\): The Hawaii-Emperor seamount and island chain.

    The active volcanoes at the end of the represent one of the most active hotspot sites on earth. Scientific evidence indicates the Hawaiian hotspot is at least 80 million years old [90]. Geologists believe it is actually much older. However, any rocks with proof of this have been subducted under the ocean floor. The big island of Hawaii sits atop a large mantle plume that marks the active hotspot. The Kilauea volcano is the main vent for this hotspot and has been actively erupting since 1983.

    This enormous volcanic island chain, much of which is underwater, stretches across the Pacific for almost 6,000 km. The seamount chain’s most striking feature is a sharp 60-degree bend located at the midpoint, which marks a significant change in plate movement direction that occurred 50 million years ago. The change in direction has been more often linked to a plate reconfiguration [91], but also to other things like plume migration [83].

    The islands get older to the left.
    Figure \(\PageIndex{4}\):: Diagram of the Hawaiian hotspot and islands that it formed.

    In an attempt to map the Hawaiian mantle plume as far down as the lower mantle [92], scientists have used tomography, a type of three-dimensional seismic imaging. This information—along with other evidence gathered from rock ages, vegetation types, and island size—indicates the oldest islands in the chain are located the furthest away from the active hotspot.

    2.7.2: Yellowstone Hotspot

    Like the Hawaiian version, the Yellowstone hotspot is formed by magma rising through the lithosphere. What makes it different is this hotspot is located under a thick, continental plate. Hawaii sits on a thin oceanic plate, which is easily breached by magma coming to the surface. At Yellowstone, the thick continental plate presents a much more difficult barrier for magma to penetrate. When it does emerge, the eruptions are generally much more violent. Thankfully they are also less frequent.

    The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.
    Figure \(\PageIndex{5}\):: The track of the Yellowstone hotspot which shows the age of different eruptions millions of years ago.

    Over 15 million years of eruptions by this hotspot have carved a curved path across the western United States. It has been suggested the Yellowstone hotspot is connected to the much older Columbia River flood basalts [93] and even to 70 million-year-old volcanism found in the Yukon region of Canada [94].

    The eruptions trend eastward due to prevailing winds.
    Figure \(\PageIndex{6}\):: Several prominent ash beds found in North America, including three Yellowstone eruptions which are shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).

    The most recent major eruption of this hotspot created the Yellowstone Caldera and Lava Creek tuff formation approximately 631,000 years ago [95]. The eruption threw 1,000 cubic kilometers of ash and magma into the atmosphere, some of which was found as far away as Mississippi. Should the hotspot erupt again, scientists predict it will be another massive event. This would be a calamity reaching far beyond the western United States. These super volcanic eruptions fill the earth’s atmosphere with so much gas and ash, they block sunlight from reaching the earth. Not only would this drastically alter climates and environments around the globe, but it could also affect worldwide food production.