December 23, 2010
|Travertine forming at a hotspring, Yellowstone hotspot, Fall 2005.|
1. A place where you can obtain internet in order to write your geology blog.
2. A thermal anomaly within Earth’s mantle, generally consisting of a hot, rising plume of mantle material that generates volcanism- or increased volcanism- on Earth’s surface.
Hotspots are aptly named- they are spots of the Earth that are hot. They are spots because they are limited in area- no more than a few hundred kilometers in diameter at the largest. They are hot because their temperature is hotter (generally by 100-200 degrees C) than the ambient, surrounding mantle.
Although they produce volcanism in Earth’s crust, hotspots are really features of Earth’s mantle. Hotspots are likely- at least in the case of the largest ones such as Hawaii and Iceland- plumes of hot, rising material that originate in the lower mantle, perhaps even as deep as the core-mantle boundary. Hotspots are almost stationary features in the mantle. There is evidence that hotspots can drift extremely slowly in the mantle, but hotspots are essentially stationary relative to the faster-moving tectonic plates.
As a tectonic plate moves over a mantle hotspot, a chain of volcanoes is produced. The most famous of these chains is the Hawaiian Islands & Emperor Seamounts in the Pacific Ocean. These islands and seamounts are age progressive. The youngest island, Hawaii, is volcanically active and is where the crust is currently located over the hotspot. As the crust moves, another island will eventually form- in fact, one is forming underwater now, and it is named Loihi! Behind the main island of Hawaii, there are other, older islands that are not currently volcanically active. Beyond the islands, there is a long chain of underwater seamounts- these seamounts used to be subaerial islands but because of subsidence and erosion they are now underwater hotspot remnants.
|The Hawaiian-Emperor hotspot trail. Image from Wikipedia Commons.|
|The Hawaiian Hotspot. Image from Tasa Graphics.|
Hotspots occur all over the Earth- they can produce volcanism both on oceanic crust and on continental crust. Famous hotspots include Hawaii, Iceland, Yellowstone, Afar, Reunion, Ninetyeast Ridge-Kerguelen, Galapagos, and the Azores. One interesting aspect of many hotspots is that they produce volcanism in the middle of tectonic plates. Volcanism generally occurs at plate boundaries, not in the middle of plates such as in the case of Hawaii. A quick look at the location of tectonic plates and the location of worldwide volcanism (see maps below) shows that this is true- hotspots are, indeed, places of anomalous volcanism.
Some hotspots do occur at plate boundaries. Iceland is the most well-known example of this and is located along the divergent plate boundary of the Mid-Atlantic Ridge. Plate boundaries (at least divergent and convergent/subducting ones) generally produce volcanism. When a hotspot is located along a plate boundary, more volcanism is produced. In the case of Iceland, so much volcanism is produced that the mid-ocean ridge is actually exposed subaerially. Iceland is the only place in the world where you can go and walk along an active mid-ocean ridge. You can actually walk the ocean floor (well, sort of… technically it is a subaerial island) in Iceland!
|Tectonic Plates. Image from USGS, taken from Wikipedia Commons.|
|Global Map of Volcanoes. From the Global Volcanism Program website.|
You can explore locations of volcanoes on the Global Volcanism Program website here.
Hotspots are capable of generating enormous amounts of lava and volcanism. When hotspot plumes first form, they are thought to produce large flood basalts. For instance, the Deccan Flood basalts (associated with the Reunion hotspot), the Kerguelen flood basalts (associated with the Heard Island & Ninetyeast Ridge hotspot), and the Columbia River flood basalts (associated with the current Yellowstone hotspot) are all gigantic volumes of basaltic lava that is thought to have been produced when a hotspot plume “head” first reached the Earth’s surface. After this initial outpouring of basalt, the hotspot plume “tail” is thought to then produce a more steady, smaller amount of lava that creates a chain of volcanoes.
|Plume head and tail. Image from Tasa Graphics.|
Even plume “tails” can generate enormous amounts of volcanism. For instance, Mauna Kea on the island of Hawaii is only about 1 million years old but is actually (measured from the seafloor) the tallest mountain on Earth.
So, we’ve established that hotspots are spots of hot that produce volcanism. But why does being hot produce volcanism? This may sound like a simple question but, most of the time, lava- or melted rock- is not produced through heating. Think about it- the crust and upper mantle (where melts are produced) is actually colder than the lower Earth. So as mantle material rises, it generally becomes colder- not warmer. The wonderful diagram below (from Wikipedia Commons) explains how melts are produced in the Earth. The geotherm is the rate at which the temperature changes with depth in the Earth. The solidus is the line below which the mantle is solid. Above this line, the mantle starts to melt. When the geotherm crosses the solidus, melts are produced.
In the normal case, the solidus and the geotherm do not cross and no melting (and thus no volcanism) is produced. When plate diverge, mantle material rises and decompresses- the mantle melts because it encounters a lower pressure. When plate converge and subduction occurs, the subducting plate releases volatiles (such as water and carbon dioxide) and these volatiles lower the solidus temperature and the mantle melts. At hotspots, the geotherm is higher (by about 100-200 degrees C) and melting is able to occur.
|Excellent diagram showing the three ways that melts are produced on Earth. Click on image for a larger view. From Wikipedia Commons here.|
Hotspots were first postulated in 1963 by J. Tuzo Wilson. Wilson (or J.T. as I like to call him) proposed that chains of volcanic islands could be produced as tectonic plates moved across a deep thermal anomaly within the Earth. Since J.T., hundreds (thousands?) of geologists have studied hotspots and tried to understand them. We have come a long way in understanding hotspots, but there is still debate and still much that is unknown.
One thing we know is that the simple Hawaiian model does not work everywhere. Not all anomalous (i.e. not at a plate boundary) volcanoes are produced because of a thermal anomaly. Some may be produced because of compositional anomalies in the mantle- pieces of mantle that are easier to melt than the “normal” mantle. Even Hawaii is somewhat anomalous- there is no clear flood basalt associated with Hawaii. So, what happened to the plume head? Were flood basalts produced? If so, where did they go?
Clearly, we still have much to learn about our mantle and about the nature of hotspot volcanism. There is even a minority group of scientists who believe that mantle plumes do not exist at all. There is a great website called Do Mantle Plumes Exist? where scientists on both sides of the plume debate engage in conversation. I think that the plume deniers take things a little too far- it’s clear that the well-known hotspots such as Iceland, Hawaii, and Yellowstone are produced by long-lived, deep, mantle plumes. Geophysics has even allowed us to image these amazing plumes. The plume deniers do have some excellent points, however. Not all “hotspot” or “anomalous” volcanism can be explained by mantle plumes. I think that it is important to listen to these plume deniers and their criticisms. Because good science is about discussion and refinement of ideas based on evidence.
|Image of the Iceland plume. Taken from here.|