The origin of the oldest rocks on Earth

The oldest rocks found so far on Planet Earth are at the Acasta River in Canada’s North-West Territories. Discovered about 30 years ago, these rocks are felsic granite, which means that they are rich in quartz and feldspar. 

How old are they? The estimate is that they were formed when Earth was only about 600 million years old, which puts their age at around 3.9 billion years. 

This type of rock, with its particular mix of elements, could only have formed at low pressure and at a temperature of 800 to 900 degrees Celsius during a period of temporary melting of Earth’s surface. But how could such a melting have occurred? 

The answer appears to be that the period of planetary formation was extremely violent for millions of years. For some 600-700 million years after the formation of planet Earth it was subject to almost constant bombardment by asteroids and meteorites, as was the Moon. The heating that this caused would have been sufficient to create the rocks found at the Acasta River. 

If that is the case, then why are similar rocks not found all over the world? This is because Earth – unlike the Moon – is subject to plate tectonics, which means that portions of the Earth’s crust are constantly being renewed and rocks as old as those in question are therefore extremely rare. 

It is thought that rocks of a similar age and type might be found in Siberia, although the discovery has yet to be made.

© John Welford