Saturday, March 8, 2014

Terra Firma

Dating of the oldest rocks on Earth sheds light on how the planet cooled to a life-supportable solid crust from a nascent sphere of molten magma. A cathodoluminescence image of a 400-mm zircon crystal found in a remote part of Western Australia known as Jack Hills is dated at 4.4 billion years old.

A zircon crystal estimated to be 4.4 billion years old
John Valley, University of Wisconsin
Penetrating so many secrets, we cease to believe in the unknowable. But there it sits, nevertheless, calmly licking its chops.
H. L. Mencken
Genesis Rock from the moon.
To date, Jack Hills zircon crystals are the oldest crystals discovered in-situ on the face of the Earth.

The Genesis Rock, collected by Apollo 15 astronauts during their moon landing, has been dated at 4.46 billion years. Although it originated on the Moon, it is one of the oldest known rocks.

Scientists hypothesize that the Earth began orbiting the sun about 4.6 billion years ago. As a molten protoplanet, the Earth needed a cooling period before a rocky crust was formed that was cool enough to support carbon-based organisms. It cooled from its molten state to solid earth. Terra firma is Latin phrase meaning solid earth. Terra translates to earth. Firma translates to solid.
"You're on Earth. There's no cure for that."
Samuel Beckett
The dating of the Jack Hills crystal suggests that by its 200-millionth birthday, the Earth had a rocky crust.

Chemical hints in the Isua sediments found on the southwestern coast of Greenland suggest life existed as early as 3.8 billion years ago.
"What are men to rocks and mountains?"
Jane Austen, Pride and Prejudice