News in Science

Missing crust feeds new theory on Earth's lumps

exposed mantle Scientists studying a bulge on the Earth's surface where the crust is missing have found the exposed mantle contains more magnesium than usual making it lighter.

The discovery by Dr Henry Dick from the Woods Hole Oceanographic Institute in Massachusetts and Dr Huaiyang Zhou from Tongji University in Shanghai, suggests the Earth's mantle is not as uniform in composition as previously believed.

The finding is based on a study of the Marion Rise, a bulge in the Earth along the Southwest Indian Ridge, associated with the break-up of the Gondwana supercontinent into Australia, Antarctica and Africa, 180 million years ago.

The ridge is an oceanic spreading point between Africa and the Antarctic, where the Indian, Southern and Atlantic oceans converge.

Bulges have previously been blamed on hot spots, where deep plumes of hot mantle rock well up through convection, swelling the crust. If the magma melts through the crust, it erupts onto the sea floor and builds up into islands such as Hawaii and Iceland.

Chemistry rather than temperature

Writing in the Journal Nature, Dick and Zhou say the exposed mantle and lack of crust along the Marion Rise indicate a different mechanism was in place to form this bulge.

"It was always thought these rises were supported by hot mantle plumes," says Dick.

"But several of them, such as the Southwestern Indian Ridge off Marion Island seem too large to be supported by a tiny hot spot, and the Marion Island hot spot is absolutely itty-bitty compared to Hawaii or Iceland."

Dick and Zhou examined the chemical composition of the mantle rock along the bulge and found it contained more magnesium than usual.

This made the mantle lighter causing it to rise, forming a bulge on the surface.

"Up until recently we thought the earth's mantle, at least for several hundred kilometres beneath the surface, was very uniform in composition," says Dick.

"What we're finding now is that there are large variations."

Supercontinent breakup

The researchers say the changes in mantle composition are due to past events in Earth's history.

"The event which gave rise to this area of very light mantle under the Marion Rise was the break-up of the supercontinent Gondwana," says Dick.

"At that time 180 million years ago, there were enormous eruptions known as the Karoo volcanic event, that came out of the Earth and covered all southern Africa, a large percentage of Antarctica, and a chunk of Australia."

As Australia went east, Antarctica went south and Africa went north, the Southwest Indian Ridge formed in between.

The mantle substrate, which the ridge overlies, is the residue of the mantle melting event.

"We're now studying the chemical composition of mantle rocks in different sections of the Marion Rise to confirm they're lighter than rocks further away," says Dick.