The oldest known rocks on Earth are 4.03 billion years old from the Acasta Gneiss in Canada. Other old rocks have been found in Greenland and Australia. Zircons from the Jack Hills in Australia are the oldest rocks at 4.4 billion years old and suggest the oceans may be older than previously thought. Only 7% of the world’s continental rock is over 2.5 billion years old. The Earth’s surface had a higher heat flux in ancient times, leading to rapid recycling of crustal material. Further research is needed to uncover more about the earliest era of the planet’s history.
As of 2008, the oldest known rocks on Earth are 4.03 billion years old, from the Acasta Gneiss in the Northwest Territories of Canada. These rocks have been found in the Canadian Shield, a very ancient section of the earth’s crust. The rock was originally part of a very ancient mountain range, the core of which has been exposed for billions of years due to glacial activity. The rock was formed during the Hadean period, the oldest geological division of time on Earth, beginning with planet formation 4.57 billion years ago and continuing until the dawn of the Archean 3.8 billion years ago. Many of the oldest rocks in the world are from this era.
Other extremely old rocks have been found in western Greenland and Western Australia. These are at most about 3.8 billion years old, and their age is used as a dividing marker between the Hadean and Archean eons. The continental crust as a whole is quite ancient, with ages in the order of billions of years. This is very different from oceanic crust, which is continually recycled through subduction zones and has an average age of only 100 million years.
Even older than the Acasta gneiss are individual zircons from the Jack Hills in Australia, dated to 4.4 billion years ago. These are the oldest rocks, forming just 130 million years after the formation of the Earth itself, and represent when the crust started to cool. The Earth was initially in a completely molten state and only cooled once the energy from its contraction dissipated – these zircons represent that first cooling. One controversial aspect of Jack Hill’s findings is that zircons appear to have formed in the presence of liquid water, which was not previously thought to exist on the surface of the Earth until about 3.8 billion years ago. So it turns out that the oceans could be much older than we suspected, although the analysis here is controversial.
Less than 7% of the world’s continental rock is thought to be more than 2.5 billion years old. Although the continents are relatively stable, they regenerate over time due to large-scale volcanism and erosion. Heat flux at the Earth’s surface in very ancient times was about three times that of today, leading to rapid recycling of crustal material. More research will be needed to uncover more older rocks and use them as clues to the conditions that existed in the earliest era of our planet’s history.
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