Life Zones in the Devonian Seas
The ocean is divided into several important zones based mainly on depth and gradient. The nearest shore zone is the continental shelf, which is defined by the broad, flat, and relatively shallow area of the ocean known as the coastal plain. In times of Earth's history when sea levels were very high, the continental shelf included all areas of the continent that were submerged. During glaciations, water from the ocean basins becomes trapped in glaciers and ice caps. This process lowers sea level and exposes the continental shelf.
Although the modern day width of a continental shelf varies significantly; it extends approximately 70 kilometers (43 miles) into the sea. At this point there is a sudden steepening in the seafloor, known as the shelf break, which represents the proximal margin of the continental slope. This is the place where the coastal margin ends and the ocean basins begin. The gradient of this slope averages about 4 degrees globally, but may be drastically different depending on location.
At the base of the continental slope, the gradient begins to decrease. This mellowing in slope is known as the continental rise. It is in this region that the transition between the continental margin and the deep ocean takes place, and with it, a shift from the continental margin to the abyssal plain. The abyssal plain averages around 4500 – 6000 meters deep and is nearly flat lying, with a gradient of only a fraction of a degree, representing one of the deepest and flattest areas on the planet!