Origin of the oceans' largest plateau.

Origin of the oceans' largest plateau

A 2,300-kilometer-long mystery known as the Kerguelen plateau stretches across the southern Indian Ocean near Antarctica. This structure, the world's largest submerged plateau, has long invited speculation and debate concerning its origins. But scientists who spent March and April drilling into the plateau retrieved hard evidence to help them reconstruct its history for the last 100 million years.

Because of the plateau's large size, some geoscientists thought it might be a fragment of Antarctica that splintered off when the Indian Ocean began to form. However, when the crew on Leg 120 of the Ocean Drilling Program bored into the basement rock of the submerged plateau, they found no evidence of continental material, says staff scientist Amanda Palmer of Texas A&M University in College Station. Instead, the basement turned out to be basalt, the typical oceanic crust formed from molten mantle rock that rises to the surface. In chemical composition, the Kerguelen plateau basalts are a rare intermediate between two types of basalt: those that flow out of midocean ridges and those that erupt out of hotspot locations such as Hawaii. It remains unclear what kind of volcanic activity formed the plateau, says Palmer.

While more than a kilometer's depth of water now covers the plateau in all but three locations, the researchers believe these basement basalts erupted when the area was either above water or very close to the surface about 97 million years ago. Soil and vegetation probably covered parts of the plateau because there are claystones, siltstones and small pieces of wood in the sediments blanketing the basalts.

The plateau then apparently began sinking slowly into the ocean. Limestones containing fossils of urchin-like creatures and other animals testify that the plateau was once at a shallow depth. Over the last 60 million to 70 million years, however, this area has subsided more rapidly. At some time, the ocean over the plateau began to cool as a continent-circling ocean current started to seal off the Antarctic climate, sending it into its current deep-freeze state. It appears that icebergs from a glacial cap on Antarctica first reached the Kerguelen plateau during the early Oligocene period, which began 37 million years ago.