Wednesday, September 30, 2009

Samoa & Sumatra

Over the past 24 hours, powerful earthquakes have occurred along the west coasts of Samoa and Sumatra. A secondary tsunami has caused significant damage in Samoa but it is too early to know if the Sumatran quake will result in a tsunami across the Indian Ocean.

The Sumatran quake occurred south of the epicenter of the devastating earthquake and tsunami that shocked the planet earlier this decade but, like its many predecessors, it surely developed along the west Sumatran subduction trench, where the Australian Plate is dipping below the Eurasian Plate. In such regions, the overlying plate is pulled downward by friction with the subducting plate; this building pressure is intermittently released, producing an earthquake and, by lifting the overlying ocean water, often triggering a tsunami.

The Samoan quake is less easy to explain. Not lying along a plate margin, the Samoan Islands have long been characterized as volcanic hotspots, produced by the movement of the Pacific plate over a mantle plume; in this respect they would be similar to the Hawaiian Ridge and other mid-plate island chains. However, geologists who have studied the age of volcanic rocks on the Samoan Islands and other regional archipelagos, have not found the age progression that typifies a hot spot chain. Some believe that these islands have developed above a fracture in the Pacific plate, which may have resulted from stress within the ocean crust; such a fracture would permit mantle intrusion, leading to volcanism and island formation. Yesterday's earthquake seems to lend support for that theory.