essay on volcanic Island.
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Volcanic islands occur in ocean basins (such as the Hawaiian Islands) or on or near ocean ridges (e.g., St. Paul Rocks and Ascension Island in the Atlantic Ocean) (Figure 1.3). They are large volcanoes erupted on the seafloor whose tops have emerged above sea level. Volcanic islands and seamounts (submerged volcanoes) range in tectonic stability from intermediate or unstable in areas where volcanism is active (like Hawaii and Reunion) to stable in areas of extinct volcanism (such as Easter Island). Volcanic islands range in size from < 1 to about 104 km2. Guyots are flat-topped seamounts produced by erosion at sea level followed by submersion, probably due to sinking of the seafloor. Coral reefs grow on some guyots as they sink, producing atolls. Some of the large volcanic islands may have developed over mantle plumes, which are the magma sources.
It is remarkable that the strength decay is exponential, as for a Maxwell solid, only within interval of a few My; after that, strength remains unchanged or slowly increases with time following the common thermal age dependence. The subsidence data suggest that the characteristic relaxation times in the lithosphere are on the order of several My. Compared to the Maxwell relaxation times in the asthenosphere (10–100 years).