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Godzilla Megamullion offers glimpse into Earth's mantle

The Yomiuri Shimbun/ANN | Monday, Jan 20, 2014

JAPAN - An enormous dome-shaped rock formation - the largest known protrusion of the Earth's mantle-has been discovered on the ocean floor about 700 kilometers southeast of Okinotorishima island, Japan's southernmost point. The discovery comes as a result of investigations by the Japan Coast Guard, the Japan Agency for Marine-Earth Science and Technology and others.

The formation is expected to help clarify the makeup of the Earth's interior, by serving as a window into the Earth's mantle, which usually cannot be directly observed. It is also drawing attention as a new source of ocean floor resources, as the Earth's mantle includes minerals from which useful metals and gems can be recovered.

Okinotorishima island is under the jurisdiction of Ogasawara, Tokyo.

The newly discovered rock formation is about three times the area of Tokyo, spanning 125 kilometers by 55 kilometers and stretching about four vertical kilometers from its deepest to highest point.

Because its great size evokes a monster on the ocean floor, the find has been dubbed the Godzilla Megamullion after Japan's most internationally famous special-effects movie. Megamullion refers to a giant cylindrical rock.

The oceanic crust is generally composed of basalt, the product of magma that has been cooled and solidified. However, the Godzilla Megamullion is composed of materials including olivine, a silicate of magnesium and iron, which is a component of the Earth's mantle.

The Earth is composed of layers, in descending order, the crust, mantle and core. Of the Earth's 6,400 kilometer radius, the crust ranges from several kilometers to 30 kilometers thick. Below that is the Earth's mantle, which reaches to a depth of about 2,900 kilometers. The mantle's uppermost section is hard rock, and together with the crust, makes up plates that cause earthquakes.

One primary mineral in the mantle is olivine, which is the same as the gemstone peridot.

The Godzilla Megamullion is believed to be the result of ocean floor movement that occurred about 5 million to 10 million years ago, which caused a part of the Earth's crust to tear, as if it were dragged, allowing the interior of the Earth to push through.

"It's like the Earth's crust was peeled away," said JCG senior researcher Yasuhiko Ohara.

A fault that appears to be a remnant of the shift has also been confirmed.

In addition to the adoption of the name Godzilla Megamullion by academic papers, 14 hills on the formation have been confirmed and been called the object's head, neck and tail.

Similar protrusions have been discovered off the coast of Chile, in waters near Japan and elsewhere. The Godzilla Megamullion is a particularly large protrusion, but the reason for its size is unknown.

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