When we think of Earth we think of the land, the mountains, the trees and many other things that exist on land. In reality, when we think of Earth, we should think of the world ocean. The world ocean covers three-fourths of the Earth s surface. The major goals of studying the ocean, known as oceanography, are to understand the geologic and chemical processes involved in the evolution and alteration of the ocean and its basin. Also, the interaction between the ocean and the atmosphere is studied so there will be a greater knowledge of the climate and its variations.
The world ocean covers 71 percent of the Earth s surface, or about 361 million sq. km. The average depth of the ocean is 5000 m and its total volume is about 1,347,000,000 cu km. The world ocean is divided into four oceans: the Atlantic Ocean, the Pacific Ocean, the Indian Ocean and the Arctic Ocean. From the shorelines of the continents, a submerged portion of the continent, called the continental shelf, extends into the ocean at an average distance of 75 km. The shelf then drastically drops off at a depth of about 200 m to a much steeper zone known as the continental slope. The continental slope then extends into the ocean at a depth of about 3500 m. At the base of the continental slope is the continental rise. The continental rise is a gradually sloping zone of sediment that is considered part of the ocean bottom. The continental rise is formed by sediment that moves along the slope and is abruptly stopped due to the flat ocean floor.
After the continental slope, the ocean extends into the abyssal plains. The abyssal plains average between 4500 m and 6000 m deep. They are formed by fine particles of sediment that slowly drift down the slope and over millions of years, build up to create thick sedimentary deposits.
After the abyssal plains, the ocean floor extends into the mid-ocean ridge. The mid-ocean ridge is an extensive mountain chain with inner troughs that are heavily intersected with crack, called fracture zones. The mid-ocean ridge plays a key role in plate tectonics because from these troughs is where the molten rock up wells from the Earth s mantle and spreads on both sides of the ridge. This is constantly adding new material to the Earth s crust.
This leads into plate tectonics in the Earth s world ocean. The Earth is comprised of about a dozen large plates that make up the lithosphere. These plates are always moving across the surface horizontally. The boundaries of these plates make up many of the Earth s features, like volcanoes, mountains and the ocean basins. The plates are moving apart, currently at a rate of 1 to 10 cm per year and are being forced into adjacent plates. About 200 million years ago, all of the continents were connected to form a super continent called Pangaea. From the Mid-Atlantic Ridge, the continents, which rest on the plates and were once joined, have moved away from one another into the placement we have today. In the Pacific Ocean, plates are also moving apart from the East Pacific Rise, but the bordering plates are overlapping them and forcing them under at the edges. At these places, along almost the entire rim of the Pacific, deep trenches are formed as crust is subducted and returned to the mantle. The Pacific Trenches commonly reach depths of more than 7 km. The deepest known point is in the Mariana Trench east of the Philippines and it is about 11 km deep. Trench areas, or subduction zones, are characterized by volcanic activity and seismic activity, due to the motions and stresses of the Earth s plates.
The topography and structure of the ocean floor are studied through the use of satellite mapping, which measures the level of the ocean s surface to estimate the shape of the ocean floor; sonar, which measures the depth of the oceans; and seismic techniques, which measures the thickness of sediments of the ocean floor. Depth measurements are made by sonar from ships that travel slowly, so only a small fraction of the ocean s floor has been mapped from depth measurements. Even using the latest sonar techniques, it would take about 125 years to map the ocean floor with depth measurements.
The ocean floor is covered by an average of 0.5 km of sediment, but the thickness varies up to about 7 km in the Argentine Basin in the South Atlantic. Some regions, particularly in the central parts of the mid-ocean ridge where new crust is formed, have little, if any, sediment on them. The sediments are studied by dredging and by deep-sea exploration projects such as the Ocean Drilling Program, which obtains core samples of seafloor sediment from all the world s oceans.
The sediments are found to consist of rock particles and organic remains; the make-up of the sediment depends on depth, distance from continents, and local variants such as submarine volcanoes or high biological productivity. Clay minerals, which are formed by the weathering of rocks found on the continents and carried out to sea by rivers and wind, are usually abundant in the deep sea. Thick deposits of this material are often found near mouths of rivers and on the continental shelves. Also accumulating as sediment in the deep ocean are the calcium carbonate shells of small organisms such as foraminifera and the siliceous shells of marine protozoans.
The ocean that surrounds us influences much of the world we live in today. When we think of the Earth, we tend to only think about the land we live on and have a tendency to forget about the oceans that are not as visible to us. The oceans are what bring life to this planet. Much is to be learned about this planet we live on, and the place to learn it is the ocean. From studying oceanography we can learn what our plant use to look like, why we have earthquakes and volcanic eruptions, where mountains and deep-sea trenches come from, and also why such islands as Hawaii and Iceland form. There is so much to be learned about our oceans and we have just reached the tip of the iceberg.