Monday, 31 August 2015

On The Origin Of The Continents

The Pilbara region of Australia is underlain by a craton
When the Earth formed it was a molten planet bombarded by meteorites. Gradually the dense elements in the magma sank to the centre to form the core, leaving behind a scum of aluminium and silicate.

This cooled and solidified to form the first crust, a thin and highly unstable skin on top of a roiling ocean of liquid rock. When the temperature dropped below 100 degrees Celsius, quadrillion tonnes of water vapour, which had sat in the atmosphere for tens of millions of years, condensed and fell in the greatest storm the planet has ever experienced.

The erosive power was such that any elevated crustal sections were scoured flat. The Earth was all but sunk beneath the waves, save for the clusters of basalt islands produced by submarine volcanoes. Yet where continents came from is more of a mystery. They are assemblages of rock formations from different times in the Earth's history. Younger formations, however, are clustered around central cores of ancient sections of crust known as cratons.

Cratons are immensely stable composed of resistant crystalline rocks with roots which extend deep into the mantle. As such they have largely survived destruction by the tectonic forces which shape the Earth's landmasses. All the cratons are over two billion years old; the oldest extend back to over four billion years. Yet they have played a crucial role in Earth's history by acting as nucleation points for larger landmasses. Understanding the origin of cratons is vital in understanding the origin of the continents.

Several theories exist, but Vicky L. Hansen from the University of Minnesota puts forward a new angle which comes from the examination of the geology of Venus. In contrast to Earth, Venus does not have a hydrological cycle and so its early geological features have not been destroyed by erosion. Venus also failed to develop plate tectonics, but its early geological history would have been similar to that of the Earth prior to the origin of plate tectonics and so its geological history gives us insight into the conditions and processes which resulted in the formation of the cratons on Earth. Indeed the lack of tectonic processes is a blessing as they may have destroyed the early geological features.
A comparison between Venus and Earth's tectonic histories

Hansen's theory suggests large meteorites pierced the early thin crust, causing massive partial melting in the ductile mantle. The melt escaped upward, forming the cratonic crust whilst strong, dry melt residue formed cratonic roots, serving as uniquely buoyant 'life preservers' during future plate-tectonic recycling. This study shows how useful other planets can be in interpreting Earth's early geological history. The planet's rocks have been destroyed and reconstituted countless times. Other planets have their geological histories frozen in their youth, giving us a glimpse into a past which cannot be obtained on the Earth.