Have you ever wondered how the ocean would look without all that water? How it would look if it were just dry land? Well, if you’re standing on the shore watching the sandy beach disappear beneath the waves, you may think that it is probably like a big sandy desert. It’s not. The ocean floor is as varied and irregular as the land we can see. It has mountains and plains and valleys and ridges and volcanoes and just about any other land feature you could name.
Surrounding nearly all continents is a shallow extension of that landmass known as the continental shelf. This shelf is relatively shallow, tens of meters deep compared to the thousands of meters deep in the open ocean, and extends outward to the continental slope where the deep ocean truly begins.
Sediment from the erosion of land surfaces, washed into the sea by rivers and waves, nourishes microscopic plants and animals. Larger animals then feed upon them. These larger animals include the great schools of fish, such as tuna, menhaden, cod and mackerel, which we catch for food.
The continental shelf regions also contain the highest amount of benthic life (plants and animals that live on the ocean floor).
The continental slope connects the continental shelf and the oceanic crust. It begins at the continental shelf break, or where the bottom sharply drops off into a steep slope. It usually begins at 430 feet (130 meters) depth and can be up to 20 km wide. The continental slope, which is still considered part of the continent, together with the continental shelf is called the continental margin.
Submarine canyons cut through many of the continental margins. Some of these have been carved by turbidity currents, which are bottom currents that carry lots of sediment.
Past the continental slope, we find the continental rise. As currents flow along the continental shelf and down the continental slope, they pick up and carry sediments along and deposit them just below the continental slope. These sediments accumulate (gather) to form the large, gentle slope of the continental rise.
The deep ocean basin, which is about 2.5 to 3.5 miles deep, covers 30 % of Earth’s surface and has features, such as abyssal plains, deep-sea trenches and seamounts.
The abyssal plain is the flat, deep ocean floor. It is almost featureless because a thick layer of sediment covers the hills and valleys of the ocean floor below it.
Deep-sea trenches are the deepest parts of the ocean. The deepest one, the Marianas Trench in the South Pacific Ocean, is more than 35,000 feet (10,668 meters), or almost 6.6 miles (10.6 kilometers) deep. A Navy-owned submarine, the Trieste, still holds the record for diving to the bottom of the deepest part of the Marianas Trench, the Challenger Deep, on January 23,1960.
The mid-ocean ridge is two chains of mountains separated by a large depression (or rift valley) that form at a spreading center (or where two plates are drifting apart). The mountain ranges can have peaks as high as 12,000 feet (2,500 meters meters) and some even reach above the ocean’s surface. Iceland, along the mid-Atlantic Ridge, is an example of this.
In the rift valley, which can be 15 to 30 miles (24 to 48 kilometers) wide, new oceanic crust is being made, which means lots of seismic activity is happening. Hydrothermal ventswere discovered in rift valleys.
The plates are spreading at a rate of 2.5 centimeters a year. This means that every thousand years or so the plates spread and grow about 25 meters.
Most seamounts began life as volcanoes formed over hot spots in the ocean floor. After the crust moves off the hot spot, the volcanic activity stops.
Seamounts are usually 25 miles (40 kilometers) in diameter and can be 10,000 to 15,000 feet (3000 to 4500 meters) tall. In fact, some are so tall that their peaks pierce the ocean surface forming a volcanic island or, if there are more than one seamount, a volcanic island chain (think of the Hawaiian Islands).
Seamounts whose peaks have eroded and become a flat surface are called guyots.
Coral reefs sometimes grow around seamounts that rise above the ocean waters. As the seamount sinks or its peak erodes, the seamount will disappear beneath the water leaving the coral ring. This is called an atoll.