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While the Wilson Cycle can give a general overview of plate motions in the past, another process can give more precise, but mainly recent, plate movement. A hot spot (map) is an area of rising magma, causing a series of volcanic centers which form volcanic islands in the ocean or craters/mountains on land. There is not a plate tectonic process, like subduction or rifting, that causes this volcanic activity; it seems as if disconnected to plate tectonics processes. Also first postulated by J. Tuzo Wilson, in 1963, hot spots are places that have a continual source of magma with no earthquakes, besides those associated with volcanism. The classic idea is that hot spots do not move, though some evidence has been suggested that the hot spots do move as well. Even though hotspots and plate tectonics seem independent, there are some relationships between them, and they have two components: Firstly, there are several hot spots currently and several others in the past that are believed to have begun at the time of rifting. Secondly, as plate tectonics moves the plates around, the assumed stationary nature of hot spots creates a track of volcanism that can measure past plate movement. By using the age of the eruptions from hot spots and the direction of the chain of events, one can identify a specific rate and direction of movement of a plate over the time the hot spot was active.

A diagram showing the Hawaii hotspot and the inferred underlying mantle plume in cross-section.

Hot spots are still very mysterious in their exact mechanism of magma generation. The main camps on hotspot mechanics are opposed. Some claim deep sources of heat, from as deep as the core, bring heat up to the surface in a structure called a mantle plume. Some have argued that not all hot spots are sourced from deep within the planet, and are sourced from shallower parts of the mantle. Others have mentioned how difficult it has been to image these deep features. The idea of how hot spots start is also controversial. Usually, divergent boundaries are tabbed as the start, especially during supercontinent break up, though some question whether extensional or tectonic forces alone can explain the volcanism. Subducting slabs have also been named as a cause for hotspot volcanism. Even impacts of objects from space have been used to explain plumes. However they are formed, there are dozens found throughout the Earth. Famous examples include the Tahiti, Afar Triangle, Easter Island, Iceland, the Galapagos Islands, and Samoa. The United States has two of the largest and best-studied examples: Hawai’i and Yellowstone.

Hawaiian Hot Spot

The big island of Hawai’i (map) is the active end of the Hawaiian-Emperor seamount chain, which stretches across the Pacific for almost 6000 km. The evidence for this hot spot goes back at least 80 million years, and presumably, the hot spot was around before then, but rocks older than that in the Pacific Plate had already subducted. The most striking feature of the chain is a significant bend that occurs about halfway through the chain that occurred about 50 million years ago. The change in direction has been more often linked to a plate reconfiguration, but also to other things like plume migration. While it is often assumed that mantle plumes do not move, much like the plumes themselves, this idea is under dispute by some scientists.

3D seismic imaging, called tomography, has mapped the Hawaiian mantle plume at depths including the lower mantle. Within the Hawaiian Islands, there is clear evidence of the age of volcanism decreasing, including island size, rock age, and even vegetation. Hawai’i is one of the most active hotspots on Earth. Kilauea, the main active vent of the hot spot eruption, has continually erupted since 1983.


Yellowstone Supervolcano

The Yellowstone Hot Spot (map) is formed from rising magma, much like Hawai’i. The big difference is Hawai’i sits on a thin oceanic plate, which makes the magma easily come to the surface. Yellowstone, however, is on a continental plate. The thickness of the plate causes the generally much more violent and less frequent eruptions that have carved a curved path in the western United States for over 15 million years (see figure). Some have speculated an even earlier start to the hotspot, tying it to the Columbia River flood basalts and even 70 million-year-old volcanism in Canada’s Yukon.

The most recent significant eruption formed the current caldera and the Lava Creek Tuff. This eruption threw into the atmosphere about 1000 cubic kilometers of magma erupted 631,000 years ago. Ash from the eruption has been found as far away as Mississippi. The next eruption, when it occurs, should be of similar size, causing a massive calamity to not only the western United States, but also the world. These so-called “supervolcanic” eruptions have the potential for volcanic winters lasting years. With so much gas and ash filling the atmosphere, sunlight is blocked and unable to reach Earth’s surface as well as usual, which could drastically alter global environments and send worldwide food production into a tailspin.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-04)
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