And life created continents...
24
March 2006
Exclusive from New Scientist Print Edition
Myles McLeod
LIFE 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.
From issue 2544 of New Scientist magazine, 24 March 2006, page 12
http://www.newscientist.com/article.ns?hbxmail=nl&id=mg18925444.200
