News in Science

Geologists aim for mantle of the Earth

Diggin deep Scientists are planning to make a century-old geoscience's dream come true by drilling through the Earth's crust to retrieve a sample of mantle.

If successful, it would provide the first ever sample of mantle material, giving new insights into the origins and evolution of our planet.

Details were announced today in the journal Nature by co-chief project scientists; Dr Damon Teagle from the National Oceanography Centre in Southampton, England and Dr Benoît Ildefonse from the Montpellier University in France.

They say preliminary drilling will start next month, but could take 15 years to complete because reaching the mantle requires drilling through hard oceanic crust.

Three previous expeditions have already drilled down to more than 1.5 kilometres below the sea floor. The reseachers plan to drill down another 400 metres to retrieve the first ever gabbros (volcanic rock) from the lower crust, which will be the deepest types of rock ever extracted from beneath the sea floor.

Age old dream

The authors say the first attempts to drill to the mantle in 1961 failed, but did retrieve the first sea floor core samples.

It also developed new technologies including dynamic positioning, allowing ships to keep steady while drilling, techniques still used by the oil industry today.

Teagle and Ildefonse believe improved technology and a better understanding of geology mean reaching the mantle is now possible.

But they say deep drilling techniques capable of operating under 4 kilometres of water, in temperatures of 300 degrees Celsius, and ressures of 2 million hectopascals, still need development.

They're focusing on ocean drilling because the crust is only 6 to 8 kilometres thick, compared to 30 to 60 kilometres under the continents.

Three possible sites have been chosen off the coasts of Hawaii, Baja California and Costa Rica.

All are in the Pacific because its crust formed faster than other oceans, making a simpler, more uniform structure to drill through.

Most of the Earth is mantle

The 3000 kilometre thick mantle makes up 68 per cent of the Earth's mass, stretching from the crust to the outer core.

Teagle and Ildefonse say its composition varies, but is mainly composed of rocks called peridotites.

Dr Mark Tingay, a lecturer at the Australian School of Petroleum at the University of Adelaide, says understanding the composition and variability of the mantle will help scientists understand how the Earth formed and evolved.

"Almost all of Earth's oceanic crust came from the mantle," says Tingay.

"Some pieces, brought to the surface though mountain building, can be studied. Some were ejected from volcanoes, and sea-floor spreading delivered some to the ocean floor."

"But all have been chemically altered by processes bringing them to the surface, by exposure to sea water, air, degassing and weathering."

He says a few kilograms of fresh peridotite from beneath the crust would provide a wealth of new information about the Earth's geologic history.

World's deepest borehole

The former USSR's Kola Peninsula scientific Superdeep borehole reached a world record depth of 12,289 metres in 1989, but according to Tingay, that was through soft continental crust.

He says deep sea oil drilling also reaches depths of more than 10 kilometres, but only through soft shale and sedimentary layers."

"Drilling through hard basaltic ocean crust to the mantle will be a far tougher proposition."