The ground beneath our feet is understood to be made up of a rigid layer (called the lithosphere) that rests on a mass of molten, slow-flowing rock. Apparently, this rigid outer layer averages some 60 miles [100 km] in depth and makes up only about 0.6 percent of the volume of the planet. The outermost part of it, the crust, is uneven, thicker beneath the continents and as thin as 3.5 miles [6 km] beneath the mid-ocean ridge system.
Furthermore, this solid outer shell is not one piece, like the shell of an undamaged egg. Instead, it appears to be broken into a number of large, rigid plates and many smaller ones, all of which are called tectonic plates. These make up the continents and ocean basins. The plates move in relation to one another. Where they draw apart, they thin out and form the rifts of the mid-ocean ridges. Worldwide, plates move at an average of about one inch [3 cm] per year.
According to the plate tectonic theory, as the plates diverge along the ridge system, they allow hot rock from the mantle, the region below the crust, to rise. The hot material forms new oceanic crust along the rift zone, but this does not result in the plates’ fusing together. Instead, they continue to part, which makes the rift system resemble a massive wound that never heals.
While a plate has new layers added to it at the mid-ocean ridges, its other extremity slowly slips beneath its neighbor and descends into the hot mantle below. There it becomes assimilated into the mantle. The region where a plate descends is called a subduction zone. Subduction zones contain some of the world’s deepest trenches. The Mariana Trench off Guam in the Pacific Ocean, for instance, is over 36,000 feet [11,000 m] deep. Were Mount Everest, the highest terrestrial mountain, placed in this trench, its summit would still be 7,000 feet [2,000 m] below sea level.
An Oasis—Of Toxins
Because of its highly unstable and volcanic nature, the globe-encircling mid-ocean ridge system is riddled with lava flows and hydrothermal vents. The vents spew out a toxic, superheated concoction of water and dissolved minerals from inside the earth. Yet, amazingly, this inhospitable realm, which is also under pressures hundreds of times greater than those at sea level, does not repel life but, rather, attracts it—and in abundance! The hundreds of species living there include bacteria, giant clams—perhaps a foot in length—and, strangest of all, thickets of crimson-plumed tube worms anchored firmly to the seafloor and standing up to six feet [1.8 m] tall.
When brought to the surface, vent creatures smell like rotten eggs! The stench comes, not from decay, but from hydrogen sulfide—an offensive-smelling and highly poisonous chemical that is abundant in hydrothermal vents. Vent water is also highly acidic and contains many metals, including copper, magnesium, iron, and zinc. But instead of barely coping in this environment—which has been compared to a toxic-waste site—tube worms and other creatures thrive! How? In order to understand, let us take a closer look at the tube worm.
A Living Enigma
When biologists examined the tube worms, they found the animals to be a living enigma. They had no mouth and no digestive system. The question arose, How did they eat and assimilate food? Then came a startling discovery: The worms had red blood—not a bloodlike fluid but actual blood rich in hemoglobin—circulating through their body and featherlike plume.
The mysteries deepened when biologists opened up the flaccid sac of the tube worm’s body. Its tissues contained a bacterial culture composed of some 285 billion [10 billion]bacteria per ounce [gram] of tissue! In 1980 a biology student theorized that the tube worm lives by means of symbiosis—an arrangement where two species cooperate for mutual benefit. Research confirmed her hypothesis by showing that the tube worm, as host, feeds the bacteria, and the bacteria feed the worm.
Like gills, the plumes of the tube worm gather the ingredients, such as oxygen and carbon, that the bacteria need to manufacture food. The plumes do not wave directly in the searing vent water—that would be suicide—but in the region close to where near-freezing seawater and vent water mix. Of course, this food-manufacturing process requires energy. On the earth’s surface—and in the upper part of the ocean—sunlight energizes food production by causing vegetation to grow. But sunlight comes nowhere near the abyssal home of the tube worm.
Energy From the Belly of the Earth
Ingeniously, the Creator has arranged for the belly of the earth to provide the necessary energy via the hydrothermal vents and that obnoxious-smelling compound hydrogen sulfide. As the “sunlight” of the vent community, hydrogen sulfide provides the energy that the bacteria need to have to go about their food-manufacturing business. Meanwhile, the bacteria are the “plants” of the vent community because they are at the base of the vent food chain.
In order to bind all the chemicals needed by the bacteria, tube worm blood is composed of hemoglobin molecules that are 30 times larger than hemoglobin molecules in humans. The blood transports these chemicals to the hungry bacteria, and the bacteria, in turn, manufacture food for the tube worm.
Vent Life—A Zoo of Organisms
Indeed, no vent creature ought to go hungry, for bacteria blanket practically everything—at times up to inches thick.Even in the warm turbulence above the vents, bacteria sometimes congregate in great blizzards, forming, in effect, a living soup. Like tube worms, some animals enjoy a symbiotic relationship with the bacteria, while others graze directly on these microorganisms. Indeed, vent communities are so productive and energetic that they have been compared to salt marshes, tropical rain forests, and shallow-water coral reefs.
In fact, some 300 new species have already been identified near the vents. These include giant white clams and mussels (pigment is superfluous in a world of eternal night), octopuses, and voracious white crabs that relish the delicate plumes of tube worms. For protection, the worms have a snappy reflex that promptly retracts the plume into the safety of the tube.
Other vent creatures include sea spiders, snails, dancing shrimps, limpets, copepods, eellike fish that slither about on the bacteria- and sulfur-laden surfaces, smaller species of tube worms, and other worms. The latter include spaghetti worms and Pompeii worms. Appropriately named, spaghetti worms resemble handfuls of white spaghetti draped over rocks. What makes the Pompeii worm unique is its ability to tolerate temperatures of up to 176 degrees Fahrenheit [80°C.]! Of course, vent bacteria, which coat the Pompeii worm, are also able to withstand high temperatures.