Missing crust feeds new theory about clods of earth ›News in Science (ABC Science)

Marion Island sits over a scar in the earth’s crust where the mantle is exposed (Source: NASA)

exposed coat Scientists studying a bulge on the earth’s surface that is lacking the crust have found that the exposed mantle contains more magnesium than usual, making it lighter.

The discovery of Dr. Henry Dick of the Woods Hole Oceanographic Institute in Massachusetts and Dr. Huaiyang Zhou from Tongji University in Shanghai suggests that the composition of the Earth’s mantle is not as uniform as previously thought.

The finding is based on a study of Marion Rise, a bulge in the earth along the Southwest Indian Ridge that was linked to the collapse of the Gondwana supercontinent in Australia, Antarctica and Africa 180 million years ago.

The ridge is an oceanic dispersal point between Africa and Antarctica, where the Indian, southern and Atlantic oceans converge.

So far, hotspots have been blamed for bulges in which deep, hot mantle clouds rise well through convection and swell the crust. When the magma melts through the crust, it breaks out on the ocean floor and builds up into islands like Hawaii and Iceland.

Chemistry instead of temperature

Dick and Zhou write in the journal Nature that the exposed mantle and lack of crust along the Marion Rise suggest that another mechanism was present to form this bulge.

“It was always thought that hot mantle clouds aided these climbs,” says Dick.

“But some of them, like the Southwestern Indian Ridge off Marion Island, seem too big to be supported by a tiny hotspot, and Marion Island’s hotspot is absolutely tiny compared to Hawaii or Iceland.”

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

This made the coat lighter, causing it to lift and form a bulge on the surface.

“Until recently, we thought that the Earth’s mantle, at least a few hundred kilometers below the surface, had a very uniform composition,” says Dick.

“What we are now finding is that there are big differences.”

Supercontinent separation

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

“The event that led to this very light-cloaked area under the Marion Rise was the breakup 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 of southern Africa, a large percentage of Antarctica and part of Australia.”

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

The clad substrate over which the ridge lies is the residue of the clad melt event.

“We are now studying the chemical composition of mantle rocks in different sections of the Marion Rise to confirm that they are lighter than rocks further away,” says Dick.

Keywords: Earth sciences, geology

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