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Mantle plumes and hotspots

Not all volcanic activity can be related to present day active plate margins. The Pacific Oceanshows a number of sub-parallel chains of volcanic islands which run diagonally across the Pacific

plate (see map below). Isotopic dating of the lavas making up the volcanic islands shows thatin all cases, the age of the islands increases from SE to NW. The youngest islands in the chains

are all volcanically active, but as the islands increase in age they become extinct. The islands arealso much younger than the ocean crust that they are built on.

Above: Map showing the volcanic islands in the Pacific. Click to enlarge.

These islands are all shield volcanoes built up of basaltic lavas. The basalt that makes up theseislands contain significantly more potassium and sodium than the basalts which make up the

ocean crust. This suggests that while the mantle forms the source for both these rocks suites,there must be a different mechanism involved in their formation.

The formation of these volcanic islands is related to the occurrence of long-lived, stationary hotspots within the mantle.

Above: map showing distribution of mantle hotspots. Click to enlarge.

The hot spots create localised plumes of hot, rising mantle material. As the plume rises towardsthe base of the lithosphere, the reduction in pressure allows partial melting of the mantle

material within the plume to form basaltic magma. The magma melts its way through theoceanic crust and erupts onto the ocean floor to build up an active volcanic island. As the platecarries on moving over the plume, the original island is carried away from the magma source andbecomes extinct. The plate acts as a conveyor belt so as the old island is carried away, a newvolcanic island is formed in its place above the hot spot. This process builds up a chain of islands,

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with the age of each increasing with distance away from the currently active island. As the oldislands are carried away from the hot spot, they subside (sink down into the crust) and areeroded by the sea, so that many of the older islands are now under the sea surface and formguyots.

Above: sequences in the formation of a volcanic island above a mantle plume.

Dating of islands in the Pacific chains gave an average plate movement of approximately 99mmper year over the last 90 million years, although there is no evidence to suggest that this rate of

plate motion has been constant over time. Changes in the alignment of some of the volcanicchains preserves evidence of a change in the direction of plate motion about 45 million years ago.

H ow d o m a n t l e p lu m e s f o r m ?

Some geologists consider that mantle plumes are generated in the lower levels of theasthenosphere by the decay of concentrations of radioactive isotopes. However, many believethat they are generated at much deeper levels in the lower mantle, perhaps even at the boundaryof the mantle with the outer core. One of the reasons for this is that the plumes seem to have

remained in the same position over very long periods of geological time (they appear to persistfor periods of approximately 200 million years), whereas patterns of mantle convection currentsseem to be a lot more erratic. In excess of 70 hot spots have been identified on the Earths

surface, and they can occur under both continental and oceanic crust. Where hot spots underliecontinental crust, the crust becomes domed and extensive volcanic activity can occur. The hugevolumes of flood basalts erupted in the Deccan Traps of India during the Tertiary period arethought to be caused by the eruption of a mantle plume head at the surface. During the Deccan

Traps event, in excess of 50,000 km2 of lavas were erupted over the period of only a few millionyears. It is thought that the associated climatic changes may brought about the mass extinctionsseen at the Cretaceous-Tertiary boundary.