The attraction of renewable energy sources is that nature continuously replenishes them. As fuels, they may not be infinitely renewable or completely environmentally benign, but they all have one thing in common: a smaller carbon footprint than fossil fuels. Examples that come to mind include power-generating geothermal energy systems that use deep wells to extract hot brine (
POWER, December 2007,
“Raft River Geothermal Project”) or naturally produced steam (Calpine’s Geysers Project in Northern California). Both are good examples of renewable, yet finite renewable energy sources.
Not all geothermal projects have a finite life, at least not as we normally characterize finite. Southern California inventor Bruce C. Marshall (www.marshallsystem.com) laments the lack of attention paid to a huge untapped renewable energy source—hydrothermal vents on the ocean floor. Magma from deep within the earth is constantly pushing its way to the surface, forcing the planet’s tectonic plates apart. As two plates separate, the rock below them partially melts to produce magma that rises and fills the gap and produces further seafloor spreading (Figure 1).
1. What lies beneath. Ridges are formed as tectonic plates pull away from each other, forming hydrothermal vents along the boundary. Source: U.S. Geological Survey
Drilling down
A tectonic plate is a massive section of earth’s crust (lithosphere) that “rides” on the asthenosphere, a hot, semiplastic layer. Tectonic plates move independently, sometimes colliding, sometimes sliding against each other. The earth’s surface has 10 to 12 major tectonic plates and many smaller minor ones. These plates, each about 60 miles thick, move relative to one another about 1 inch a year.
Hydrothermal vents are natural geysers of magma-superheated water found atop mid-ocean ridges across the planet at an average depth of 7,500 feet (Figure 2). Islands such as Iceland, Bermuda, and the Azores are products of tectonic plate movement along the Mid-Atlantic Ridge.
2. Where to expect earthquakes and active volcanoes. The red dots indicate known hydrothermal and geothermal locations. This is only a partial map, as there are tens of thousands of kilometers of mid-ocean ridges that have never been explored. Source: U.S. Geological Survey
Tectonic plate movement opens fissures on the seafloor into which seawater is forced down by the high pressure of the water column above it. Magma superheats the seawater to a temperature as high as 750F, creating a geyser (an underwater version of Old Faithful) with a velocity of 3 to 15 feet per second (fps). Because the pressures at depth are up to 3,200 psi, the hot fluid stream doesn’t flash into steam. But the cold surrounding seawater causes a cocktail of metals and minerals to precipitate out of the fluid and rain down on the seabed. Precipitates include iron, gold, silver, copper, and zinc, to name a few elements (Figure 3).
3. Thank you for smoking. Plumes from sites on the ridge crest are often caused by the intrusion of a magma dike during an episode of seafloor spreading. The hot fluids rise and are diluted by the surrounding seawater. Courtesy: U.S. Geological Survey
If you’re thinking that 7,500 ft is beyond human reach, think again. Oil and gas platforms in the Gulf of Mexico are located in water that’s 1 mile (5,280 ft) deep, and they support drills that go down another 7 miles or more. Mapping and tapping volcanic vents may not any more technically challenging than drilling for oil. It will, however, probably be much harder to capture hydrothermal energy and economically convert it to electrical energy.
With the cost of all kinds of energy rising, the appeal of undersea geothermal energy could grow to match its ubiquity and large scale. The sizes of the vents that have been surveyed are truly amazing. For example, the Juan de Fuca Ridge 200 miles off the coast of Seattle has an active vent field over 500 feet wide and 1,000 feet long. Within that field are more than 15 vents up to 90 feet in diameter that continuously eject water at 700F at a rate of 9 fps to 15 fps. Each of the vents theoretically offers 40 GW equivalent of thermal energy.
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