Hell on Earth? Scientists Debate Planet's Early Years

Richard A. Lovett
for National Geographic News

August 9, 2006

New evidence supports the theory that the young Earth was a hot and hellish place, scientists say.

Nobody doubts that Earth started out hot. But some researchers argue that it cooled rapidly after its birth, forming continents and oceans early in its history. That Earth, the researchers say, was a lot like the present, with only the occasional meteor strike to stir things up.

(Related story: "Meteorite Impact Reformulated Earth's Crust, Study Shows" [January 12, 2006].)

Before that theory became popular, however, scientists had imagined that frequent volcanic eruptions and earthquakes battered the early Earth, and asteroid impacts repeatedly vaporized its oceans.

This led researchers to call the earliest epoch in Earth's history the Hadean Period. Hades is the Greek word for hell.

Now, a new study of tiny crystals called zircons may throw a spanner in theories of a cool Earth.

The study appears in the August issue of the journal Geology.

Crystal Ball

One of the main problems with studying the Hadean Period is that no known rock formations have survived to tell us what Hadean conditions might have been like, says Laurence Coogan, a geologist at the University of Victoria, Canada, and one of the study's authors.

"The only material we have is zircons," Coogan said.

Smaller than a grain of pepper, zircons are made of zirconium silicate and other trace materials that crystallize out of cooling magma.

They are important to geologists because they are incredibly durable. They can survive the erosion of their parent rocks, burial in sediment, and recompaction into new rocks.

The oldest known zircons date back to 4.4 billion years ago, about 200 million years after Earth was formed. They lie in Australian rocks formed approximately 3 billion years ago from the erosional debris of prior rocks.

Geologists favoring the cold Earth theory argue that the chemical composition of these crystals shows that they formed from water-rich magma—similar to that which forms today's continents.

In other words, early Earth must have had climate conditions much like today's—and have been cool enough to allow continents to form.

But Coogan was suspicious. "To my mind, this is a large interpretation to make on the basis of a few crystals that aren't in their original rock," he said.

He and a Scottish colleague collected from seabeds samples of modern rocks formed by an entirely different process.

"We analyzed the composition of the zircons and found that it is basically the same as the Hadean ones," he said.

This shows that zircons don't have to form on continents.

But Coogan says more evidence is needed to conclusively support a theory of a hot early Earth.

"I certainly wouldn't say that we've closed the case [for a hot, nasty Hadean]," Coogan said.

Instead, the study shows that zircons don't provide good evidence for a cool Earth theory, he says.

Alternate Explanation

Norm Sleep, a geophysicist at Stanford University, disagrees.

The problem, he says, is that the zircons stuck around for more than a billion years. Rocks located in places other than continents disappear very quickly.

"Something sat around for 1.2 billion years or so before it got eroded. That's the key point," he said.

He offers an alternate explanation for the formation of Coogan's zircons. In today's oceans, he says, magma rises beneath mid-ocean ridges. Occasionally the roof of the magma chamber collapses, and water mixes with the magma.

That process can form a rock with zircons similar to those in continental crust, he says.

"[Coogan and his colleagues] are saying that the zircons from that kind of rock and the ones from the Hadean are about the same," he said. "That may be well and true," but it doesn't mean that Hadean zircons didn't come from continents.

For those reasons, Sleep doubts that early Earth was all that hellish. Asteroid impacts occurred, he says, but not frequently enough to turn the planet into a steamy slag heap.

"Unless you were very unlucky, you would not get killed by an asteroid," he said. "You would not have to sweep meteor dust off your driveway every night. Except right after an asteroid impact, it doesn't really correspond to what a good Baptist would think hell should be."

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