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Moon Formed Volcanoes Early, Rock Study Shows

Mason Inman
for National Geographic News
December 5, 2007
 
Magma-spewing volcanoes developed on the moon soon after its formation, according to a new study of a moon rock that fell to Earth.

The findings will help researchers understand how planets develop in their early stages, the study authors say.

An international team of researchers examined a 30-pound (13.5-kilogram) moon rock found in 1999 in the Kalahari Desert in the southern African country of Botswana.

"It seems like it came upon Earth relatively recently, unlike many lunar meteorites," said team member Mahesh Anand of Open University in Milton Keynes, England.

The meteorite landed on Earth about 200 to 300 years ago, Anand said. It was probably blasted free from the moon when an asteroid hit the lunar surface.

The age of the rock suggested that volcanoes had already begun erupting just 150 million years after the moon's creation.

The new study appears this week in the journal Nature.

Rocky History

Most researchers agree the moon formed around 4.5 billion years ago, when a Mars-size planet slammed into early Earth with a glancing blow.

The bulk of that other planet's matter stuck to Earth, but some of the matter shot out and then congealed to form the moon.

Earlier studies—based on rocks collected from the moon's surface during the Apollo missions between 1969 and 1972—suggested that the bulk of the moon's volcanic activity occurred after 3.9 billion years ago.

"Apollo-era research suggested that it took a long time for the moon's volcanoes to erupt," Anand said. (Read more about moon exploration.)

But the Kalahari meteorite is made of basalt, a common rock that forms from cooling magma that is a telltale sign of volcanic activity.

The researchers examined small grains of two phosphate-containing minerals—apatite and merrillite—inside the rock.

Once magma solidifies, it becomes closed like a time capsule, and its radioactive uranium begins to split into other elements. Measuring the ratios of these elements reveals the rock's age.

This showed that the Kalahari meteorite is made of magma that solidified 4.35 billion years ago, soon after the moon formed.

So "volcanism on the moon had started much earlier than we had thought," Anand said.

"This does not definitively pin down the age of the moon," Anand added. "But it does tell you that the moon must have formed and cooled down by 4.35 billion years ago."

Scientists believe volcanism on the moon ended about three billion years ago.

Planet Formation Clues

The new research also sheds rare light on the earliest stages of planet formation. That information has largely been lost on Earth, where plate tectonics continually recycle the crust, sucking continents into the planet's interior and melting the rocks.

Because of this, even tiny bits of rock from early Earth are extremely scarce.

But since the moon is much smaller, it cooled quickly and never developed plate tectonics. Many of its ancient rocks are still on or near the surface.

"The beauty of the moon is that it preserves almost the entire history of the solar system," Anand said.

Mars should also have very ancient rocks on or near its surface, Anand said, but there has never been a return mission from Mars that brought back rocks.

And of about 40 Mars meteorites found, only one is more than four billion years old.

"Snapshot" of Early Earth

Irene Antonenko is a geologist with Geosoft, a geology software company based in Toronto, Canada, who was not involved with the new study.

She said the work is important because "we are looking at the moon for a snapshot of what the early Earth was like."

Other studies had also suggested that the moon was volcanically active in its very early days, Antonenko said. But these were based on more indirect evidence—such as counting the impact craters on the surface in rocks from different eras.

The new study shows "one of the earliest physical samples" of volcanic activity on the moon, she added.

The Apollo rocks had "been pounded ... over billions of years," but the Kalahari meteorite is a large rock, she said.

"That's one of the really neat things about it," she said. "It's something that hasn't been battered to hell."

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