Mantle plumes and hotspots
Not all volcanic activity can be related to present day active plate margins. The Pacific Ocean shows 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 that in 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 are also much younger than the ocean crust that they are built on.
These islands are all shield volcanoes built up of basaltic lavas. The basalt that makes up these islands 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 hot spots within the mantle.
The hot spots create localised plumes of hot, rising mantle material. As the plume rises towards the 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 the oceanic crust and erupts onto the ocean floor to build up an active volcanic island. As the plate carries on moving over the plume, the original island is carried away from the magma source and becomes extinct. The plate acts as a conveyor belt so as the old island is carried away, a new volcanic island is formed in its place above the hot spot. This process builds up a chain of islands, with the age of each increasing with distance away from the currently active island. As the old islands are carried away from the hot spot, they subside (sink down into the crust) and are eroded by the sea, so that many of the older islands are now under the sea surface and form guyots.
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 99mm per 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 volcanic chains preserves evidence of a change in the direction of plate motion about 45 million years ago.
How do mantle plumes form?
Some geologists consider that mantle plumes are generated in the lower levels of the asthenosphere by the decay of concentrations of radioactive isotopes. However, many believe that they are generated at much deeper levels in the lower mantle, perhaps even at the boundary of 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 persist for periods of approximately 200 million years), whereas patterns of mantle convection currents seem to be a lot more erratic. In excess of 70 hot spots have been identified on the Earth’s surface, and they can occur under both continental and oceanic crust. Where hot spots underlie continental crust, the crust becomes domed and extensive volcanic activity can occur. The huge volumes of flood basalts erupted in the Deccan Traps of India during the Tertiary period are thought 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 million years. It is thought that the associated climatic changes may brought about the mass extinctions seen at the Cretaceous-Tertiary boundary.
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