Seamounts
Introduction
The landscapes found deep beneath the ocean are as dramatic as any found onshore. Jagged peaks loom high above the seabed, and rolling volcanic highlands give way to the muddy plains of the abyss. Among the most imposing seafloor features are ancient, mostly extinguished, volcanoes. Some are jagged or flat-topped cones, while others rise in a series of long, serrated ridges. Many stand alone, but most are gathered in long meandering chains.
Undersea volcanoes are classified by their height. Those that are large enough to break the sea surface are known as volcanic islands. Smaller forms that hug the seabed are called hills and knolls. Volcanoes that are taller than 1,000 meters, but still hidden beneath the waves, are called seamounts.
Tens of thousands of seamounts are scattered throughout the deep-sea, in every ocean on Earth. The patterns and placement of seamounts help illuminate the geologic forces at work beneath the sea. Many seamounts mark the location of hot spots – narrow, persistent plumes of magma that rise from the earth’s interior and burn through the plates above. Others grow near the edges of the planet’s tectonic plates, where slabs of the crust slam together or tear apart.
The origin of some seamounts is enigmatic. Perhaps forming when the interior of a plate gives way under geologic stresses, these peaks may one day provide new insights into tectonic behavior. Regardless of size, shape, and origin, all seamounts share the ability to profoundly influence the environment around them. They are biologic storehouses, home to diverse and abundant animal communities. Life flocks to seamounts because they provide a bonanza of food and shelter unmatched elsewhere in the open seas.
The source of this bounty lies in currents. The complex topography of seamounts deflects deep-sea circulation. Water flows turn and accelerate as they pass over and around these mountains. This has two important results. First, large areas on the flanks and tops of the seamounts are swept clear of the muck and the mud that cover the rest of the deep sea floor. The bare rock exposed by currents is a rare and valuable habitat for cementing and encrusting organisms. Second, the swirling currents carry with them bits of organic debris and small floating animals. Eddies that curl around the ridges and rocks concentrate nutrients and allow life to flourish.
In consequence, many seamounts are zones of unusually high productivity, and extremely diverse and intricate communities. On the seamount itself, clusters of corals, sponges, crinoids, and anemones dot the ground. Crabs and snails and other invertebrates scurry about, finding food and shelter within these deep-sea gardens. In turn, larger animals – fish, sharks, jellies, and octopi – forage the waters just above, and use the complex topography as a place to lay eggs and nourish their young. Even the leviathans of the deep – the great whales – may be lured by the teeming life around seamounts. Scientists believe that sperm whales congregate around some seamounts to hunt giant squid.
The influence of seamounts can extend far above their summits. The currents that are deflected off the deep seafloor by these mountains inject cool, nutrient-rich water into the upper ocean. This stimulates the growth of plankton, which attracts many species of open-ocean fish. These schools draw in flocks of sea birds, dolphins, turtles, and other predators.
Seamounts are cloaked by depth and darkness, and the study of their biology is still in its infancy. However, it appears that seamount communities harbor a mix of endemic and cosmopolitan species, and that they play important roles in the evolution and distribution of marine organisms. Endemic species – those found nowhere else on earth – make up a large proportion of seamount faunas. For less mobile organisms, seamounts may act as incubators for evolution, allowing new species to develop atop their splendid isolation.
For species that reproduce with floating larvae, the powerful currents that sweep across seamounts provide a free ride to the next volcanic outcrop. Over time, species may use seamounts like stepping stones to cross from one side of an ocean to the other.
Global Impact
There may be 100,000 or more seamounts scattered throughout the world’s oceans. Only a few hundred have even begun to be explored. These mysterious, little known peaks stand as both a challenge and an opportunity to scientists. For geologists, questions about their formation are a reminder that there is still much to learn about the complex workings of plate tectonics. For biologists, their rich and unique faunas offer hope of fresh insights into evolutionary patterns. For medical researchers, seamount life holds out the promise of new drugs and therapies.
Unfortunately, the vibrant communities that have thrived for eons on these watery slopes are under siege at the same time they are under study. The fish that gather around seamounts to spawn and to feed include many commercially harvested species. As traditional fisheries fail, seamount stocks are being targeted. But many of them are slow to grow and late to mature, and once over fished, their populations need years to recover. Several seamount fisheries have already been depleted, and others are in danger.
Fishing can also disrupt the bottom-living communities on seamounts. When trawlers drag their nets across the mountainsides, corals and sponges are uprooted and destroyed. These creatures, which grow slowly over decades and even hundreds of years, are the backbone of bottom faunas. Their destruction means the death of the entire community.
Such threats have given scientific studies of seamount habitats an added sense of urgency. Research institutions from around the globe are currently working together to devise strategies to preserve and protect these fragile ecosystems and their inhabitants.