Monday, 30 May 2016

Grain of Sand

It is no wonder that geologists swarm to the Jack Hills, Western Australia -- for it is the site of the oldest rocks on Earth.

There are larger old rocks in Canada, but they are 500,000,000 years younger than the Jack Hills rocks. There are older meteorites in Antarctica, but they were not formed on Earth. The rocks at the base of the Grand Canyon -- thought by many to be the oldest rocks on Earth -- are a whopping 2,650 million years younger than the rocks of Jack Hills.

Being about 4.4 billion years old, some rare Jack Hills sand grains came from the first rocks Earth ever had. However, the old rocks of the Hills are just that: a few very rare sand grains, called zircons, which are deeply embedded in sedimentary (made of sand) and very metamorphic ('changed') rocks. How did these tiny grains appear here in the first place, and how might they have survived the wear of time? Why are they so rare in the first place? Let's go back in time to find the answer.

The Crust Solidifies

4.4 billion-years-old Earth is not a place you would want to live. A human, dropped on the prehistoric planet's surface, would be fried by nuclear radiation, burned by lava, choked to death by poisonous gases, and crushed by meteorites within the first five minutes. One good thing about early Earth: Oceans. Scientists have found out that the Jack Hills zircons were created in water; water which could have come either from meteorites or the planet itself. In any case, Earth had oceans, but still wasn't cooled enough for the igneous rocks to turn into anything else, whether sediment or metamorphic rock. Also, there was virtually no oxygen. This meant, for now, that the early rocks were safe from change. That is, until . . .

Life Begins

Life is currently thought to have originated around 4.0 or 3.9 billion years ago. The oldest evidence for life comes, like the earliest evidence for water, from those same Jack Hills sand grains. Life did not have much effect on early rocks until about 3.4 billion years ago, when the Earth had cooled down significantly and erosion had begun creating the first sedimentary rocks. Around this time, moss-like cyanobacteria began making the first stromatolites (crazily, the only colony of stromatolites left is within sight of the oldest rocks). Cyanobacteria use photosynthesis, a complicated process which turns carbon dioxide, water, and sunlight into sugar and oxygen. The latter was released on a massive scale into the atmosphere. This would not have been that bad, but there was a lot of iron in the volcanic rocks. The oxygen and iron combined to form rust, and immediately the age-old rocks from Earth's creation began to fall apart. Jack Hills zircons, not being made of iron, had survived for the time being. However, an important factor was now coming into play . . . .


There were trillions of trillions of zircons on Earth when it was first created. Corrosion and heat did not change their numbers very much. However, around three billion years ago, the zircon crystals began to break apart, due to a process known as metamictization.

Early Earth was very, very radioactive. Rare elements today, like actinium, used to be very common four billion years ago. Uranium-238, the most common radioactive element, has a half-life of about 4.5 billion years. This means that a 200-atom sample of uranium from the creation of Earth would have about 100 atoms now (the other atoms would have turned into something like radium). A by-product of radioactivity is radiation, in this case in the form of alpha particles. You may expect a zircon to have had about a million atoms of uranium in it 4.4 billion years ago. By now, the decaying uranium would have released at least 500,000 alpha particles -- more because what uranium decays into, decays into something else. 500,000 alpha particles are more than enough to destroy the crystal.

A few, very rare zircons would have survived long enough to endure the next test.

Plate Tectonics

In 2013, scientists were shocked to recognize the remains of a massive continental plate, lodged deep within the Earth underneath North America. This plate was called the 'Farallon Plate', and was later discovered to have been shoved underneath the crust by the Pacific and North American plates.

As shocking as it may be, it is not uncommon for a continental plate to slide underneath the crust, never to come back again. It has happened throughout the history of the Earth since plate tectonics began, around 4 billion years ago. Every 300 million years or so, the Earth's crust is recycled. Our zircons could hold out under the immense pressures of the Mantle for a while. Eventually, however, even the strongest crystal on Earth could not survive.

What saved our zircons is exactly what destroyed all the old rocks: Erosion.

The zircons, swept by the wind, would have spread across the world, minimizing the chance of all being destroyed. They would become part of normal sandstone rocks, which would erode and the zircons would have been released again. Nevertheless, the oldest zircons became rarer and rarer. Finally, around 600 million years ago, a group of zircons became embedded in some sandstone rocks. Eventually, as most of their neighbors were slowly destroyed, the last zircons got buried under heaps of volcanic rock. The surrounding land went through cycles of burial and erosion, but the zircons were protected by the volcanic rock. After being warped by pressure, the sandstone containing the zircons slowly, but surely, was uncovered. It was now part of the western Australian plate.

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