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The Zircons

Zircons can tell us about the very earliest days of the planet.

They are tiny crystals of zirconium silicate. Zircons are found in most granites. As fresh granite cools, any zirconium in the melt takes up silicate and crystallises out as zircons. Zircons are useful because they are particularly tough. They can survive temperatures of up to 1600°C. And they can survive high pressures without melting or undergoing metamorphosis. Nothing else can survive such punishment. Even diamond. This photo is by Lois Brynes

Rocks usually contain a certain amount of uranium. So do zircons. This enables their formation to be dated (more). An undamaged zircon can be dated to within 1%. So we’re talking about an accuracy of ±40 million years. The oldest zircon found so far, comes from the Jack Hills in western Australia and dates to 4.4 thousand million years. That’s within a hundred million years or so of the birth of the planet.

The Earth’s first crust

Zircon specialists can deduce a huge amount by a detailed analysis of their crystals. The oldest zircon contains quartz. Now quartz is the sparkly bit in granite. You don’t get it in basalt. You don’t see basalt much, except in places like Iceland or the Devil’s Causeway. It’s a ‘primitive’ rock and lives almost entirely in the Earth’s interior.

By contrast, granite is an ‘evolved’ rock, and most of the Earth’s continental crust is made of it. Granite is basalt that has been worked and reworked in volcanoes and such. In the process it has lost a lot of its heaviest components. You may think that granite is heavy, but if so you have never tried to lift a lump of basalt.

Granite will ‘float’ on molten basalt. It is a sign of permanent crust. So it looks as though the first permanent crust was formed within the first hundred million years. If ever there was a ‘wow’ statement, this is it.

(To satisfy the scientists, we should point out that the silica doesn’t prove that our zircon formed in granite. It seems that there are other possible explanations. On the other hand, there is other evidence for a very early continental crust. For my money, the evidence favours early crust, particularly if you read on …)

But that’s not all. The scientists are able to conclude that there was plenty of water around when this earliest zircon was formed. They have suggested that entire oceans of water probably existed at that time. This is amazing, and flies in the face of everything we thought we knew.

However it doesn’t mean that there was always water from then on. A serious bombardment of space chunks, over the next few hundred million years, is very probable. And it could easily have boiled all the water away.