Relationships cut two ways. If the Gaian view of the Earth is correct, we must entertain the notion, preposterous though it sounds, that Gaia not only regulates the Earth’s temperature and atmosphere, but that it affects the movements of the Earth’s tectonic plates. That Gaia extends beyond the biosphere to include the Earth’s crust.
So far, the Earth is the only planet known to support life. It’s also unique, in that its crust is composed of thin plates, jigsaw pieces that drift over the mantle. At the mid-Atlantic (a spreading zone) they separate, to allow lava from below to create new crust. Elsewhere as in the Alaska trench or the Mariana trench, the Earth’s crust is consumed. A tectonic plate sinks into the Earth’s mantle where it is melted, a process called subduction.
Take a look at the plate tectonic movie made by the BBC.
Plane tectonics is necessary for life on Earth to develop and flourish because it regulates the amount of carbon in the atmosphere. At spreading centres, carbon dioxide gasses out from the Earth’s interior. Weathering, erosion and life processes lock up carbon dioxide in carbonate rocks, deposited on the oceanic floor. Subduction takes those rocks down into the mantle.
Neither Venus nor Mars — a much smaller planet than Earth, have tectonic plates. The crust on Venus is continually renewed by periodic melting. Mars is geologically dead in the sense that it hasn’t seen volcanic activity or significant mountain building for over a billion years. What makes Venus, Earth and Mars so different?
The recent discovery of many Earth-like planets orbiting distant stars brought the question into focus. Geologic models reported by Diana Valencia and Richard O’Connell (2007) show that for planets more massive than our Earth, plate tectonics is inevitable. Those Super-Earths have stronger mantle convection (upwelling of molten rock and its sinking down to the core). This increases the stress on the planet’s crust and makes it fragment into tectonic plates. In a small planet like Mars, mantle convection is so low that we wouldn’t expect the crust to fragment. But what about Venus, a planet only slightly smaller than our Earth? Valencia and O’Connell’s work suggest that a critical mass is necessary for a planet’s crust to fragment — a mass curiously similar to the Earth’s mass. On the Earth, plate tectonics almost didn’t happen. A factor, other than plain geology may have nudged our planet’s crust during its early history and initiated plate tectonics. Something called life.
When the Earth’s crust first cooled, it probably fragmented into several pieces. According to work by Kent Condie (2008), plate movement didn’t begin until around 3.8BY ago.
To initiate plate tectonics a horizontal force was necessary. It’s generally accepted that the force that drives plate tectonics today is the weight of the sinking slab. The cold oceanic crust that sinks back into the mantle in a subduction zone, drags the entire plate with it, and causes tectonic plates to spread apart elsewhere. What kick-started started that process? The best answer we have is water. Our early oceans.
Water can jump-start plate tectonics in two ways.It acts as a lubricant, reduces the friction between the descending plate and the overlying crust. Water also reacts with oceanic basalt, weakens it and makes it more pliable. Such crust when loaded with an extra layer of sediments, or carbonates becomes massive enough to sink into the Earth’s mantle. Without the presence of the early oceans, plate tectonics would have probably never begun.
And so the chain of events leads back to Gaia. Venus, the Earth and Mars all had early oceans — water that originated in the cosmic material from which the planets first formed, but only the Earth has retained its oceans. It wasn’t easy. The early atmosphere had very little oxygen. Water reacts with basalt and splits into hydrogen and oxygen. Hydrogen, the lightest element, tends to escape into space. Also, the sun’s increased ultraviolet radiation (remember there was no ozone layer in those days), tends to split water into its constituents. In those circumstances the early oceans would have evaporated into space. That they didn’t may be thanks to the ocean’s burgeoning bacteria population, some of which produce water as a byproduct. Stephan Harding and Lynn Margulis make that case in their recent essay, Water Gaia. One bacteria (Prokaryotes) flourish close to subsea volcanic vents (near spreading centres) where they feed on hydrogen sulphide and carbon dioxide to produce methane and water. Cyanobacteria, which populated the early oceans and were the first organisms to use photosynthesis, began to produce large amounts of oxygen. This oxygen recombined with free hydrogen to re-create water.
Today, do living processes still affect plate tectonics? I suspect that further research will confirm that relationship. That the biosphere not only kick-started plate tectonics 3.5 BY ago, but in ways not yet understood, it maintains a role in regulating it.