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On Petrology

The most important Australian cities are situated in the eastern part of the country. The spectators, who watch the game of cricket sitting inside beautiful stadia in Sydney, Melbourne, Canberra or Brisbane, are all from the eastern part of Australia. Western Australia is like a desert. The natural beauty of this land, one of the oldest in the world, is unique and breath-taking. However, the tourists usually visit the eastern side of Australia. In the early 21st Century, noted US Geologist John Valley and his colleagues visited the Jack Hills Desert in Western Australia, in search of ancient rocks. They wanted to explore the past by examining the ancient stratified rocks. The US geologist reportedly discovered the oldest rock in the world, as small as sand, and as old as 4.4 billion years, there.

Jack Hills in Western Australia

Jack Hills has microscopic zircon crystals embedded in small rock-like particles. Zircon crystals, found in nature, behave like a time machine. As this element of nature is not easily destroyed, analysis of its composition reveals when this stone was formed. Valley and his co-researchers expected that they would be able to get an idea about the temperature and other atmospheric conditions of that period of time by analysing the ancient zircon crystals. However, an experienced geologist, like Valley, was stunned by his own discovery. He realised that zircon crystals available in the Jack Hills region were around 4.3 billion years old.


Valley was well aware of the fact that it would be possible to determine whether water reacted with granite rocks at the time of their formation by analysing the ratio of Oxygen-18 and Oxygen-16 isotopes in zircon crystals. Granite rocks, found on the surface of the Earth, usually have more Oxygen-18 isotopes, as they come in touch with water. This is exactly what was found in the ancient zircon crystals of Jack Hills. A 4.4 billion years old granite rock melted down 4.3 billion years ago, and came in contact with cold water, resulting in the formation of ancient zircon crystals. Later, ancient zircons were found in Pilbara and other regions of Western Australia. Researchers came to the same conclusion after analysing those zircon crystals.

A false-colour microscope image of a 4-billion-year-old zircon (Image credit: John Valley)

The Earth is estimated to be 4.54 billion years old, plus or minus about 50 million years. After analysing the zircon crystals in Western Australia, researchers have come to the conclusion that water flooded the Earth within 15-25 billion years of its birth. This study, published in Nature in 2001, claimed that the Earth’s climate and the existence of liquid water had made it habitable around 4.4 billion years ago. It may be noted that the Hadean is a geologic eon of Earth history, preceding the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the International Commission on Stratigraphy (ICS), four billion years ago. For the ancient Greeks, Hades is basically a polite term for hellish. Earlier, scientists used to think that water had first come into existence in the Archean era 3.6 billion years ago. Later, they realised that water was there in the Hadean era, and the connection of life with water is very strong.

Geological time scale

In 2006, Professor Mark Harrison invented the titanium thermometer, and measured the density of titanium in zircon with it. From the density of titanium, it is possible to know about the climate in which the zircons were crystallised. They scanned 40,000 zircons in four years to find that hundreds of crystals were 4.2 billion years old or more. Valley and his co-researchers were also able to measure the temperature at which rocks were formed by analysing the density of titanium in at least 50 zircon crystals, which were 4-4.35 billion years old. At that time, the crystals were created at a temperature of 690 degree Celsius, and not 800-1200 degree Celsius, as predicted earlier. Researchers are of the opinion that it is not possible to melt rocks at such a low temperature without sufficient water.

At that period of time, there were water, sea, as well as atmosphere, on the Earth. Water on planet Earth had come into existence long before the last heavy bombardment (meteor shower) 4.1-3.8 billion years ago. In the beginning, innumerable small planets and meteors used to fall on the Earth. Those meteors, comets, or asteroids contained water, resulting in the formation of oceans and atmosphere of the Earth. Although most of the world’s land mass is covered by the oceans today, the total mass of oceans is a very small fraction (only 0.023%) of the mass of the entire Earth. However, the amount of water stored inside the mantle and core of the Earth has not been fully measured, as yet. According to scientists, the mass of this water is several times heavier than the mass of the oceans… yet, it is insignificant compared to the total mass of the Earth.

A timeline of the history of our planet places the formation of the Jack Hills zircon and a ‘cool early Earth‘ at 4.4 billion years (Image credit: Andree Valley)

The Earth was not like hell in the Hadean eon. Now, the question is whether this name would be changed. Of course, one can rewrite Dante‘s concept of Inferno, as well.

Courtesy: The Hindu (May 10, 2018)

1) Wilde, S. A.; et al. (2001). “Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago”, Nature, 409 (6817): pp.175-178. doi:10.1038/35051550.
3) “ANU – Research School of Earth Sciences – ANU College of Science – Harrison”. Archived from the original on 2006-06-21. Retrieved 2009-08-20

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