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Wednesday, November 3, 1999 Published at 12:25 GMT


The history of rock

Dr Goodge has collected tonnes of Antarctic rock

By BBC News Online Science Editor Dr David Whitehouse

A single rock collected in Antarctica has revealed some of the major geological events of the past three billion years.

The geologist who found it says: "It has been a witness to some of our planet's most turbulent geological history."

But Dr John Goodge does not remember picking the rock up. "I remember the particular outcrop from where it came but I do not remember finding that particular specimen," he says.

Barely explored mountains

Dr Goodge, a geologist at the Southern Methodist University in Texas, has been to the wilderness that is the Transantarctic Mountains many times. The range pokes out of the ice from the East Antarctic shield, one of the oldest rock formations on Earth. Barely explored, it has a rich and largely untold geological story.

[ image: The layered gneiss records as turbulent geological history]
The layered gneiss records as turbulent geological history
And one sample, called 85-20H, tells some of that story. It is the story of a region of the Earth and its wanderings across the globe over billions of years before it crawled to the pole and became entombed in ice.

"After each expedition to Antarctica, I come back with a few thousand pounds of rocks that need studying and classifying," he told BBC News Online. Then he got the chance to use one of the most sophisticated analytical machines in the world.

The probe is based at the Australian National University in Canberra and is able to date individual parts of a rock specimen so that its history can be determined.

"When we looked at 85-20H, I got a surprise. In this rock, some of the major events of the past three billion years are recorded," he says. "What is unique about this sample is that we have evidence for four events separated over two and a half billion years of time."

Subterranean forge

The rock is a sample of gneiss, containing many zircon crystals that can be dated.

[ image: The tiny zircon crystals grow incrementally over billions of years]
The tiny zircon crystals grow incrementally over billions of years
"The ion probe is a very sensitive instrument," he says. "It focuses a tiny beam of ions on particular parts of the sample. The material that is given off is collected and analysed. From its chemical composition, the age of individual grains in the sample can be determined."

The detailed findings of this research are reported in the November issue of the journal Geology. And it is quite a story.

Three billion years ago, there were a series of widespread and violent events which formed much of the mass of rock that in the distant future was to become Antarctica.

"This particular sample was probably formed well below the surface, in the middle of the crust when tongues of molten rock infiltrated existing rock formations," Dr Goodge says.

"We can see evidence that the rock formed about three billion years ago in a major crustal movement. Looking at the grains in the sample, we can also see evidence for the rock having been altered about 2.9 billion years ago."

Large supercontinent

"After that, things settled down for a while until about 1.7 billion years ago when the major land masses of that time began coming together to build a large supercontinent.

"Finally at about 530 million years ago, we see the final set of changes in the rock. It was heated and cooked during the widespread events that created the superstructure for the Transantarctic Mountains as we know them today."

Dr Goodge and colleagues believe that it was altered near a geologically active region on the edge of Gondwanaland - an enormous supercontinent that broke up to form the pattern of the continents we see today.

He is soon to return to the Transantarctic Mountains. "We're off in just a few days and this time I will have a helicopter for a while that will help enormously in our exploration.

"It is a pretty remote place to work," he says. "The mountains are about half way between the base on near the coast and the base at the South Pole. We fly in on C130 Hercules aircraft that are fitted with skis and they land right on the ice.

"It is pretty neat - ancient rocks that poke out of the ice that tell us what the world was like billions of years ago."

[ image: Rocky bluffs in Antarctica overlook the sea ice]
Rocky bluffs in Antarctica overlook the sea ice

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