Oldest Rocks on Earth Found

Andrea Thompson, Senior Writer, LiveScience.com

Scientists have found the oldest known rocks on Earth.

They are 4.28 billion years old, making them 250 million

years more ancient than any previously discovered

rocks.

Earth formed about 4.6 billion years ago from a disk of

gas and dust circling the sun. Remnants of crust from

Earth's infancy are hard to come by because most of

that material has been recycled into Earth's interior

several times by the plate tectonics that continue to

shape our planet's surface.

In 2001, geologists found an expanse of bedrock,

known as the Nuvvuagittuq greenstone belt, exposed

on the eastern shore of Hudson Bay in northern

Quebec.

Suspecting that the rocks there could be from one of

the earliest periods of Earth's history, geologists took

samples to try and determine their age. They measured

tiny variations in the isotopes (or species of an element

that have different numbers of neutrons) of the rare

earth elements neodymium and samarium in the rocks

and determined that the samples were from 3.8 to 4.28

billion years old.

The oldest dates, which came from rocks that geologists

call "faux amphibolite," are thought to be ancient volcanic

deposits. They beat the previously oldest known rocks,

which are 4.03 billion years old and come from a

formation called the Acasta Gneiss in Canada's

Northwest Territories.

The only dates of crustal material older than the newly-

dated Nuvvuagittuq greenstone belt are from isolated

mineral grains called zircons that are highly resistant to

weathering and geologic processes. The oldest zircons,

from grains in Western Australia, are about 4.36 billion

years old.

The Nuvvuagittuq rocks are "the oldest whole rocks

found so far" though, said geologist Richard Carlson of

the Carnegie Institution, who analyzed the rocks with

Jonathan O'Neil, a Ph.D. student at McGill University in

Montreal. The team's findings are detailed in the Sept.

25 issue of the journal Science. Their work was

supported by the National Science and Engineering

Research Council of Canada, the U.S. National Science

Foundation and the Carnegie Institution of Washington.

Examining such ancient rocks "gives us an

unprecedented glimpse of the processes that formed the

early crust," Carlson said.

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