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World's Oldest Rocks Suggest Early Earth Was Habitable

John Roach
for National Geographic News
November 28, 2005
 
Astronauts in Earth's orbit today look down on a jigsaw puzzle of continents rising from vast oceans, the conditions ripe for supporting life.

Controversial new research suggests that the continents formed—and conditions for life arose—much earlier than was previously thought.

"If you'd come in a spacecraft 4.4 billion years ago, Earth would have looked a lot like it does today," said Mark Harrison, a geologist at the Australian National University in Canberra.

Harrison and his colleagues recently analyzed ancient minerals called zircons found deep in the Australian outback. They are the oldest known rocks on Earth.

Ancient metals in the zircons suggest that Earth had all the conditions necessary to make the planet habitable 4.4 billion years ago, within 200 million years of its formation, Harrison said.

But the research doesn't say life existed 4.4 billion years ago.

"Zircons don't have an opinion on that," Harrison said.

Harrison's team reported its findings last week in a paper posted on the Web site of the research journal Science.

Finding Questioned

The findings are in contrast to conventional theories that early Earth was either moonlike or dominated by oceans. Those theories hold that continental crust didn't form until 4 billion years ago, more than 500 million years after the planet formed.

Despite the findings of Harrison and his colleagues, several scientists still support the conventional theories.

Chris Fedo, a geologist at the University of Tennessee in Knoxville, asks: If the early Earth was full of continents, where are all the ancient rocks?

The zircons are the only known minerals older than four billion years.

"We know continental crust is incredibly hard to get rid of. It's a very durable object," Fedo said. "We have relics that go back up to four billion years, and yet why are there not whole strings of it earlier than that?"

Harrison admits his team's findings are controversial, but he says they paint a picture of an early Earth where active and rapid geological processes—called plate tectonics—would have recycled most of the earliest continental crust.

"This research is supporting a view put forth nearly 40 years ago, but seen as the lunatic fringe, that the Earth was essentially completely differentiated 4.5 billion years ago and has simply been in the process of recycling the surface ever since," he said.

Continental Zircons

To reach this conclusion Harrison and his colleagues examined the ratios of two rare elements—hafnium and lutetium—in zircon crystals from the Jack Hills in Western Australia.

One element is associated with the continental crust, and the other with the layer below the crust, called the mantle.

"Lutetium loves the mantle, and hafnium is a continent lover," Harrison said.

The signature in the zircons is that of a "continent lover," meaning that by nearly 4.4 billion years ago Earth must have had continental crust.

In addition, the zircons show signs that Earth's crust had remelted through plate tectonics and come back to the surface.

"Everything we're learning about the early Earth tells us in fact that almost immediately the Earth formed significant amounts of continental crust and there was plate boundary interaction going on pretty much like today," Harrison said.

"In fact the continents were recycling at a substantially greater rate early on than today," he said. "That makes more sense. You've got a lot more heat to drive the recycling."

Too Much Heat?

Balz Kamber, a geologist at Laurentian University in Sudbury, Ontario, Canada, said the radioactive heat 4.4 billion years ago would not have permitted continental crust to stabilize.

"The heat would have simply melted the continental crust," he said.

The zircons, if accurately dated, must represent a minority of the rocks constituting early Earth's surface, Kamber said.

Fedo, the University of Tennessee geologist, said the suggestion that only these Australian zircons survived the geological processes that created the original continental crust is a stretch.

Nevertheless, he added, the zircons demand an explanation.

"These are incredibly rare grains and the only things we have from this window of time on Earth," he said. "As much as we can study them, the better off we are. You've got to applaud any kind of attempt to continue to study these."

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