By indexer
204 days ago

The origin of the oldest rocks

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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 many millions? 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.
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Borderline Very interesting, I really love Geology.
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Explorer2017 Informative, interesting. How does their age being measured? In trees, as I understand, the rings inside the tree signify the year the trees age.
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indexer @Explorer2017 Rocks are "aged" according to the proportion they contain of certain radioactive isotopes, especially those with very long half-lives. This allows very accurate measurements to be made.
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Explorer2017 @indexer I don't understand radioactive isotopes.
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indexer @Explorer2017 An isotope is - if you like - a variety of an element. The atomic number of an element is the total number of protons and neutrons in the core of an atom - hence the commonest form of carbon is carbon 12 with six protons and six neutrons, but you can also get carbon 13 (6 protons, 7 neutrons) and carbon 14 (6 protons, 8 neutrons). These varieties are what are known as isotopes.

Isotopes - especially "heavy" ones of atoms such as uranium and plutonium - tend to be unstable and break down as the nucleus divides, thus creating atoms of other elements and also emitting radiation. It is known how long it takes any particular radioactive isotope to split naturally, and for half the mass of any given quantity of that isotope to be lost - this is known as its half-life and it can vary from fractions of a second to billions of years.

Radioactive dating can be done by comparing the proportion of certain isotopes in a sample to that of the elements that result from its breakdown. The more of the latter in a sample, the older it must be, and knowing the half-life of the isotope in question allows very accurate measurements to be made.

Alternatively, you can compare the proportions of various isotopes of an element in a sample, given the knowledge of the expected proportions when the sample was "new". If the proportion has changed over time, the degree of change will indicate the age of the sample. This is how carbon 14 dating works in organic matter.
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Explorer2017 I don't understand it. It's too technical. @indexer
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indexer @Explorer2017 Sorry about that! However, I can assure you that there are very sound scientific methods that allow ancient rocks to be dated.
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Snezana nice
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