الجديد فى عالم المعرفة

And life created continents...
24 March 2006
Exclusive from New Scientist Print Edition
Myles McLeod



A solid startLIFE on Earth may have driven the evolution of the planet itself. The idea is that ancient microbes provided the chemical energy to create the Earth's continents - a nod to the Gaia hypothesis, in which life helps create the conditions it needs to survive.

The theory would solve the puzzle of why the Earth's continental crust appeared when it did, and explain the presence of granite, a substance not found anywhere else in our solar system.

The Earth formed 4.6 billion years ago, coalescing as a homogeneous mass that in time separated into the discrete layers we know today: the core, mantle and crust, plus oceans and atmosphere. However, during the first 600 to 800 million years of Earth's existence there were no stable continents. The oldest vestiges of continental crust, which date from the Archaean aeon about 4 billion years ago, are in Acasta in north-west Canada.

These rocks are made from granite or a similar material, which is unique to Earth. It is created when basalt rock melts and reforms, becoming enriched in silica, aluminium and certain metals as it reacts with compounds in water. Granitic rocks are less dense than basalt, so they rise to the surface, forming a stable continental crust.

The ingredients for granite were there before the Acasta rocks, yet in all that time it didn't form. Now a team of geologists led by Minik Rosing of the Geological Museum and the Nordic Center for Earth Evolution at the University of Copenhagen, Denmark, says the appearance of photosynthetic life might have given this process the kick-start it needed.

Rosing's team was struck by how the appearance of the continental crust coincided with the rise of photosynthesis. The fossil record for the time is patchy at best, because microbes are small and fragile, but there is some geological evidence that photosynthesis might have arisen 3.8 billion years ago. Today, photosynthetic organisms, which convert solar energy into usable chemical energy, contribute three times as much energy to the Earth's overall geochemical energy cycle as geological activity driven by the Earth's interior.

The first photosynthetic life forms would have made solar energy available for chemical changes, cranking up the Earth's energy cycle and altering its geochemistry, Rosing's team believes (Palaeogeography, Palaeoclimatology, Palaeoecology, vol 232, p 99). "The energy capture from photosynthesis is used to keep oceans and atmosphere out of chemical equilibrium with the rock," says Rosing.

This tension enhances weathering cycles, causing more chemical breakdown in the crust compared with physical processes or the limited impact of more obscure organisms, such as microbes that thrive around thermal vents. Such breakdown of basalt produces smectite and illite clays, which in turn play a role in the creation of granite.

"The key point is that melting of basalt makes basalt again, while the melting of weathered basalt produces a small amount of granite," says Rosing. "Life might, in the end, be responsible for the presence of continents on Earth."

Other geologists agree that this is a novel and imaginative idea. But, they say, the evidence is still weak. For instance, the apparent rarity of granite may be due to other factors, says Martin Line of the University of Tasmania, Australia, such as the size of the Earth, the relative abundance of water on the Earth's surface, or simply because we have only sampled a few rocks on other solar bodies, such as the moon or other planets.
source
http://www.newscientist.com/article/mg18925444.200-and-...ated-continents.html