Shedding light on deep-sea thermal vents

Complex plumbing: The vents are found thousands of metres below the sea surface along mid-ocean ridges, which are geologically active underwater mountain ranges. They host a variety of weird creatures that get their energy from metabolising sulphur in the water. Credit: Wikimedia

PARIS: Marine seismologists have overturned an existing theory about the internal workings of deep-sea hydrothermal vents.

In a study published today in the British journal Nature, experts studying a site in the East Pacific detail new insights into how the unique plumbing system of the vents work.

Mountains in the deep sea

Thirty years ago, scientists exploring the ocean depths came across jets of hot water, spewing from the sea floor, that hauled up flecks of gold and other minerals from Earth’s interior and nurtured weird, resilient life forms – these are known as hydrothermal vents.

The jets are found thousands of metres below the surface on the mid-ocean ridges – geologically active ‘mountain ranges’ – formed from mighty tectonic plates that push into each other and form spines along the ocean floor.

Until now, the main hypothesis about hydrothermal vents has been that gigantic pressure forces seawater through large faults along the flanks of the ridge. The water, the theory goes, is then heated by coming into proximity with volcanic rock before re-emerging at the middle of the ridges, where the vents are usually clustered.

A different picture

But in the first detailed investigation into vent circulation, a team led by Maya Tolstoy of Columbia University’s Earth Observatory in New York City has come up with a different picture.

They placed seismometers over a four-square-kilometre area of the East Pacific Rise, about 800 km southwest of Acapulco, which has been under study for the past 15 years. The sensors monitored tiny earthquakes that happen 2.5 km below the surface. Around 7,000 of these brief, shallow quakes were recorded in 2003 and 2004 alone.

The tremors also built up an image of how the water circulates, because the quakes were intriguingly clustered around where the cold water entered the rock.

The map drawn by Tolstoy’s team shows a downflow ‘pipe’ that descends about 700 m into the ridge, then fans out for about 200 m. The water then plunges down another 600 metres until it arrives just above a bulge of magma. There, the water is heated and disgorged along the ridge through a dozen vents about two kilometres north of the entrance pipe.

Physical stresses

Tolstoy’s team contends that what appear to be tiny quakes are caused by the physical stress of cold water passing through hot rocks.

And, contrary to the prevailing hypothesis, they believe that the water travels not through large faults but through systems of minute fissures, and at a much higher rate of turnover than previously thought.

The research adds critical knowledge about seafloor currents and the nutrient flows that feed them. It also furthers understanding about the mechanics of heat transfer from Earth’s crust to the seafloor.

Hydrothermal vents are sometimes called black smokers for the bilious clouds of material that emits from their chimneys.