January 4: The oldest rocks on earth

by Richard I, Gibson

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The oldest dated rocks on Earth are from about 3.8 billion years ago -- but as with a lot of the Earth's story from that long ago, there's some controversy and question about the details.

Zircon

Transcript: Once the near continuous bombardment and possible re-melting of earth by meteors was over, about 3.9 billion years ago, solid rocks began to form, and some of them survive to this day.

First, we need to define what we mean by “oldest rocks.” Pretty much it refers to the time when rocks solidified from molten magma. That’s the time when our measuring stick, radioactive decay of elements in a solid rock, starts the clock ticking in a way that we can determine.

The honor of identifying the oldest rock makes this a hot pursuit, and it’s a moving target. Errors and improvements in age-dating techniques mean that many reported “oldest” rocks are usually at least somewhat controversial and subject to a lot of scientific scrutiny. That’s a good thing, but it makes it hard to say flat-out “the oldest rocks on earth are….”

Some of the oldest dated rocks have been found near Yellowknife on the Great Slave Lake in northwestern Canada. They were heated and deformed at least 3,840,000,000 years ago, and because they are metamorphic rocks (that means they have been changed from their original state by heat and pressure), there must have been older rocks that were altered. Some individual zircons – microscopic crystals that can survive temperatures that would melt most of the rest of a rock – have given age dates for those Canadian rocks as old as 4 billion 30 million years. And some zircons from similar rocks in Western Australia have been dated at 4.4 billion years, just 200 million years after the earth was assembled. Update: confirmation.

Those zircon studies may indicate that there were continental masses on earth rather earlier than is generally accepted, or those zircons might have crystallized down in the earth’s hot mantle but did not solidify into solid rocks for hundreds of millions of years.

This is an ongoing area of exploration and analysis. I think it’s safe to say that the oldest surviving rocks on earth are about 3.8 billion years old, but solid rocks could have been around before that time. In fact there almost certainly were solid rocks, which were either melted by the meteoric bombardment that ended about 3.9 billion years ago, or maybe they are still out there, and just haven’t been found yet.

This is a work in progress! Please bear with us as we figure out how to handle the audio and discussions! Thanks to Robert Edwards for his contribution to the discussions.

The zircon image is by Chd from Wikipedia under the Creative Commons license.