Looking out at the vast, blue ocean, one might assume that its floor is as flat and featureless as its surface. But, in fact, the ocean floor has the same variety of topography that we see on land: mountains, hills, plains, basins and canyons. The ocean floor forms at the mid ocean ridges and moves off toward the oceanic trenches, where it is consumed back into the mantle; along the way, deposition, erosion, fault motion and volcanism mold its surface.
More than 30% of the ocean floor is covered by volcanic ridges; most of these are aligned along the spreading zones, where new ocean crust is forming and where the bordering tectonic plates are being pushed apart. In other areas, these ridges have developed over a volcanic hot spot and extend off in the direction of the plate motion; the Hawaiian Ridge is an excellent example of a regional volcanic ridge. The summit of most volcanic ridges are well below the surface of the ocean but some are tall enough to exceed sea level, forming clusters or chains of islands (Hawaii, Iceland, the Galapagos islands etc.).
In contrast, deep canyons (called trenches) form at subduction zones, where an oceanic plate is forced downward as it collides with another plate. Among the more famous oceanic trenches are the Philippine Trench (where the Philippine Plate is subducting beneath the Eurasian Plate), the Aleutian Trench (where the Pacific Plate is subducting beneath the North American Plate) and the Peruvian Trench (where the Nazca Plate is dipping below the South American Plate). The deep waters of these ocean canyons are among the least explored areas on Earth and likely harbor life forms that we cannot yet imagine.
