Far below the ocean's surface, volcanic mountain chains are growing in mid ocean zones in which plates pull apart. Elsewhere, deep trenches descend in subduction zones in which plates collide and one dives below the other.
In 1977, scientists employed submersible vehicles to discover the seabed and found vents gushing dark plumes of superhot, mineral-rich water. These black smokers, are due to volcanic activity at mid ocean ridges. Water getting into cracks in the crust is warmed by magma and combined with mineral sulphides, then belched forth in dark clouds.
Marie Tharp, together with colleague Bruce Heezen, made the initial detailed map of the ocean floor making use of sonar readings. In the late forties, Tharp found a rift valley running down the centre of the Mid-Atlantic Ridge and came to realize that a chain of mid ocean ridges circles the globe.
The base of a body of water is referred to as the benthic zone, no matter how deep it occurs. In coastal waters the ocean floor sits upon the continental shelf and is typically less than 200m deep. Most of the ocean floor lies upon the sea crust and is between 4000m to 6000m deep.
Scientists who research the benthic zone can come from several disciplines and each will be interested in various aspects. The reasons for study are just as diverse but can include:
Resources: Locations and quantities of minerals, oil, gases, and food supplies.
Understanding: Exploration of areas, studying about ecosystems and habitats.
Geohazards: Landslides including the one that caused the 2004 Tsunami.
The ocean floor has the identical general character as the land areas of the world: plains, mountains, canyons, channels, exposed rocks and sediment-covered areas. The insufficient weathering and erosion in many areas, however, permits geological processes to be seen more clearly on the seafloor than on land. Undisturbed sediments, for instance, contain a historical record of past climates and the state of the ocean, which has allowed geologists to locate a close relation between past climates and the variation of the length of the Earth from the Sun (the Milankovich effect).
Because electromagnetic radiation cannot penetrate any significant range into the sea, the oceanographer utilizes explosives, acoustic signals and earthquakes, as well as magnetic fields and gravity, to probe the seafloor and the structure under.