Mantle plume

A mantle plume is an upwelling of abnormally hot rock within the Earth's (or another planet's) mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centers known as hotspots and probably also have caused flood basalts. It is a secondary way that Earth loses heat, much less important in this regard than is heat loss at plate margins (see Plate tectonics). Some scientists think that plate tectonics cools the mantle, and mantle plumes cool the core.

In 1971, geophysicist, W. Jason Morgan proposed the theory of mantle plumes. In this theory, convection in the mantle slowly transports heat from the core to the Earth's surface. Plumes of hotter-than-average material rise through the mantle till it reaches the Earth's crust where it causes a hotspot. The plumes originate at a thermal boundary layer at the core-mantle boundary. The only such layer known to exist in the deep mantle is the core-mantle boundary (D"), and thus Morgan-type plumes are generally assumed to rise from this layer. Due to the depth of such plumes, proving their existence is difficult and has led to some controversy over the theory.

Also, a "superplume" is the term for a larger-scale plume. It is usually defined as a plume that has a diameter of at least 1500-3000 km by the time the plume head spreads at the base of the lithosphere. A "superplume event" is "a short-lived mantle event (100 Ma) during which many superplumes as well as smaller plumes bombard the base of the lithosphere" (Condie et al. (2001)). It is believed that such an event may have occurred in the mid-Cretaceous.

Evidence for the theory

Mantle plumes are the generally accepted explanation for non-plate tectonic volcanism called 'hotspots'. The first observation to suggest this volcanism comes from deeply rooted mantle source comes from their stationary nature with respect to the 'hotspot reference frame' of plate motion (i.e. Morgan 1972 and Willson 1963). If a plume originates from Dˈˈ, the layer above the core-mantle boundary, as is commonly thought (i.e. see Labrosse, 2002), then as it rises through the lower mantle it will entrain primordial mantle material which can be observed at the surface. The most commonly used signature is the ³He/⁴He ratio, where an elevated value of ³He is thought to come from a primordial, non-degassed mantle (Anderson, 1989). Another way of identifying plumes is by measuring spatial variations in the time it takes seismic waves to travel through the earth. The relatively hot mantle plume will have a lower density and hence lower seismic velocity then its surroundings. Thus by observing regions where seismic waves take longer to arrive one can start to predict where the mantle is hot. By deploying a dense network of seismometers and a technique known as tomography scientists can construct 3-d images of seismic velocities to try and identify vertical plume like structures (ie see Yuan and Dueker 2005). Other indicators of plumes would be from the dynamic uplift of the surface (ie Burov 2005), an elevated heat flow, and a time progressive volcanic trend that mimics plate motion.

Density differences between a mantle plume and cooler material that surrounds it enable researchers to distinguish between the two. Seismic waves generated by large earthquakes are used to determine structure below the Earth’s surface. The waves slow down when they travel through low-density material.

By analyzing pressure pulses, or P-waves, a group of scientists at Princeton have identified 32 regions throughout the world where P-waves travel slower than average. They conclude that these areas are mantle plumes. The team used analysis of S-waves, another type of seismic wave generated by earthquakes, to determine that those plumes extend to the core-mantle boundary. (Montelli et al., 2004)

Computer modeling of the mantle plume theory shows that changes of temperature and chemical composition of rising plumes can lead to plumes of varying contours as opposed to the early conceptualization that plumes developed as a homogeneous mushroom shape (Farnetani & Samuel, 2005).

Clarification

In a 2004 paper, Don L. Anderson and James H. Natland wrote:

"Unfortunately, the terms hotspot and plume have become confused. In recent literature the terms are used interchangeably. A plume is a hypothetical mantle feature. A hotspot is a region of magmatism or elevation that has been deemed to be anomalous in some respect because of its volume or location. In the plume hypothesis, a hotspot is the surface manifestation of a plume, but the concepts are different; one is the presumed effect, and the other is the cause."

Mantle plume locations

Two of the most well known locations that fit the mantle plume theory are Hawaii and Iceland as both have volcanic activity.

The P-wave and S-wave images show other locations that fit the mantle plume model. Ascension Island and St. Helena appear to originate from the same plume. Similarly, volcanic activity in the Azores and Canary Islands branch from a single trunk.

South of Java and in the Coral Sea, the images show possible formation of future plumes that currently extend only halfway to the surface.

Ore deposit association with mantle plume activity

Nickel deposits
Copper deposits
Diamond deposits
Sulfide deposits
Gold deposits (to a lesser extent)

References

Anderson, Don L. & Natland, James H. (June 23, 2004). A brief history of the plume hypothesis and its competitors: Concept and controversy. Retrieved Sep. 15, 2004.
Courtillot, V., Davaille, A., Besse, J., Stock, J., 2003. Three distinct types of hotspots in the Earth's mantle. Earth and Planetary Science Letters 206, 295-308.
Farnetani, C.G., and H. Samuel. 2005. Beyond the thermal plume paradigm. Geophysical Research Letters 32(April 16):L07311. Abstract.
Ratajeski, K. (November 25, 2005). The Cretaceous Superplume

External links

MantlePlumes.org

Geology | Plate tectonics

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