I accept that it is unlikely, but is it at least theoretically possible that a fault in the Earth鈥檚 crust on an ocean floor could drain an enormous quantity of seawater? What would be the effect, not only on life on Earth, but on the planet鈥檚 structural integrity if sea levels underwent a colossal drop? Of course it鈥檚 hypothetical, but indulge me.
鈥� This is theoretically possible only in the sense that nothing is ever absolutely impossible.
Your hypothesis requires the improbable assumption of an enormous void, or a series of interconnected voids, under the ocean floor, containing nothing more than a gas. If the overlying rock were breached by a fault, any gas would either bubble to the surface or be compressed to a much smaller volume by the weight of the water above, lowering sea level as water drains in.
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The effect on the planet would depend on the circumstances. If the void were close to the surface, where the surrounding rock is cool, the planet鈥檚 structural integrity might be improved. A void filled with an incompressible liquid would be more stable than one containing only a compressible gas.
鈥淲e might be less concerned with the drop in sea level than with the resulting鈥�
If the void were a little deeper, the resulting geyser might be a spectacular tourist attraction for anyone left to see it. Still deeper and the intense heat would suddenly generate huge volumes of steam and could blow away large portions of the planet, possibly all of it. After all, we are considering the improbable.
The lowered sea level alone would cause dramatic climate changes. If enough water were drained away to significantly reduce the surface area of the oceans, the resulting decrease in evaporation and rainfall would be catastrophic. Life as we know it would have to retreat to the few remaining niches, which would probably not support our species and certainly not our civilisation. For us, evolution would have to start again. Eventually intelligent life might emerge, this time perhaps with better things to do than ponder such bizarre scenarios.
Peter Bauer, Tuckerton, New Jersey, US
鈥� The oceans fall and rise by hundreds of metres every time an ice age starts and ends. The process takes centuries or millennia, and each time life replaces life.
Beneath the sea some tectonic plates continuously slide under others, taking many cubic kilometres of seawater with them. But it takes more water than that to affect ocean depths noticeably, and anyway that water mostly re-emerges in volcanic eruptions. This explains why volcanoes near subduction zones emit large quantities of water.
As for sea-floor chasms opening catastrophically, this is highly improbable. For a chasm to open, something must make room. That would require either an abrupt swelling of the Earth鈥檚 interior, or a sudden piling up of sea-floor plates. Either process would require such unthinkable energies that sea level would be the least of our worries. In practice, sea floors continually split open at mid-ocean ridges, and magma from beneath immediately congeals and fills the cracks.
Jon Richfield, Somerset West, South Africa
鈥� I once attended a lecture by the famous Colonel Brian D. Shaw, a British army explosives expert, in which he demonstrated the explosive power of 1 millilitre of water when it was instantaneously vaporised. He did this by suspending sealed ampoules of water above Bunsen burner flames.
After about 15 minutes, during which he continued his lecture, there were almighty explosions which toppled his safety screens and made the audience jump out of their seats.
He pointed out that these were the results from only a tiny volume of water, and that when Krakatoa erupted it involved the instantaneous vaporisation of 2 cubic kilometres of water. Two-thirds of the island disappeared. I imagine therefore that if more water than this fell through the Earth鈥檚 crust into the molten interior, we might be less concerned with the drop in sea level than with the resulting explosion.
Kenny Walker, Dunblane, Perthshire, UK