The Atlantic Ocean and its marginal seas cover approximately 20% of Earth’s surface, extending from the eastern shores of North and South America to the western coasts of Europe, the Strait of Gibraltar and Africa. To the north is the Arctic Ocean and, some 60 degrees south of the Equator, the Atlantic ends in the Southern Ocean. Although it has always been the most heavily traveled of Earth’s oceans, we knew very little about the Atlantic's floor until the 20th century, when improved exploration technology revealed many interesting facts.
The continental shelf
On average the Atlantic is 2.07 miles (3.34 kilometer) deep, but near the continents the sea floor is relatively shallow, tens to a few hundreds of feet/meters below the surface. This is because most continents extend quite a distance out beyond the coast line. On this continental shelf, sunlight easily penetrates all the way down, warming the water and providing energy for photosynthesis. Most of the Atlantic marine life is found on the continental shelf, and these regions are centers of commercial and recreational fishing. Petroleum and gas deposits are often found here, too.
During the last Ice Age, parts of the continental shelves all around the Atlantic were above sea level. Glaciers left their tracks over this exposed ocean floor. Long-lost rivers carved deep canyons in it, and land sediments were deposited there. Today, these features as well as fossils and relics from early humans may be found offshore on the continental shelf today, underneath shipwrecks and other marine debris from more recent times.
Eventually the shelf ends and a vast continental slope marks the transition of the ocean floor down to great depths. According to naval oceanographers, this abyssal plain appears overall very flat because a thick layer of sediment covers its hills and valleys. The flatness is relieved in places by seamounts and guyots, as well as a number of local rises (such as that around Bermuda), plateaus and ridges, and a few deep trenches and canyons.
The wreck of the "Titanic" sits on the abyssal plain about 2½ miles/4 kilometers below the surface, but the deepest part of the Atlantic sea floor is twice that depth: 5.35 miles (8.6 km), in the Puerto Rico trench.
Commercial fishing is impossible in the deep Atlantic because the ocean bottom is too far down. Lack of sunlight, high pressures and very cold water temperatures limit opportunities for life down there. Even if fish were as abundant there as they are on the continental shelf, where the sea floor is much closer to the surface, we would lack the technology to harvest them.
However, the abyss may still be economically important. Slow, steady sedimentation onto the sea floor is a good way to form some ore deposits. Manganese, an important metal alloy used in making steel, has been found there in nodules and crusts. Explorers are curious about the possibility that a lode of rare earth elements, like the one Japan found in 2011 at the bottom of the Pacific, might also exist in the Atlantic Ocean.
The Mid-Atlantic Ridge
A range of volcanic mountains with a central rift valley rises at the center of the Atlantic Ocean basin. Here, basaltic material from the Earth’s mantle has been welling up, forming new sea floor, for millions of years, ever since this ocean first opened up. The submarine ridge peaks are generally some 1½ miles/2.5 kilometers below sea level, but in the North Atlantic the ridge rises above water and is associated with the island of Iceland.
The western part of the Mid-Atlantic Ridge (including western Iceland) is part of the North American tectonic plate; the eastern side (including eastern Iceland) is part of the Eurasian plate. The physics of such movement on a sphere like Earth requires the Mid-Atlantic Ridge to form in linear segments, each with its own fracture zones and transform faults so the sea floor always moves away from the ridge.
Life abounds at the Mid-Atlantic Ridge, where hydrothermal vents provide the necessary ingredients for an extensive marine ecosystem based on chemicals in the water instead of photosynthesis. Circulation of sea water through these vents also concentrates valuable minerals like gold, copper and nickel, raising interest in and concerns about deep sea mining at the ridge.
Scholars had wondered for centuries about the apparently close fit between South America’s eastern coast and the west coast of Africa, but no one could adequately explain it until the 20th century when, according to the United States Geological Survey, proof was found on the floor of the Atlantic Ocean for a radical geological theory, first called continental drift and now known as plate tectonics.
It was very controversial at first, but supporters pointed out some irresistible facts. Of primary importance, of course, was the discovery of the Mid-Atlantic Ridge. Scientists soon also learned that the youngest ocean floor is found at the ridge. The age of the sea floor increases with distance from the ridge, both to the east and the west. Too, studies showed that the sea floor contains a record of Earth’s periodic reversals of its magnetic poles. There are identical “magnetic stripes” on either side of the Mid-Atlantic Ridge that must have formed together, even though they are now hundreds to thousands of miles apart.
Unlike the Pacific’s “Ring of Fire,” where many deep subduction trenches line the ocean's edges, the floor of the Atlantic Ocean is fairly quiet except at the central ridge, where sea floor spreading causes some earthquakes, and in the Caribbean, a very active tectonic region. There are some troughs and trenches down there, but most are not tectonically active. Two exceptions are the South Sandwich Trench in the South Atlantic, where subduction is occurring, and the deep Puerto Rico trench, where the North American and Caribbean plates are sliding past each other.
No one knows all the facts about the Atlantic Ocean yet. In 1929, a magnitude 7.2 earthquake south of Newfoundland triggered a submarine landslide that caused a tsunami. No one is sure what caused such a large earthquake at a passive margin. Another unexplained mystery is the “hole” in the sea floor 1118 miles/1800 km east of Barbados, where the basalt crust is missing, leaving a frozen part of the Earth’s mantle exposed. With the help of new and better technological tools, we will expand our knowledge of the Atlantic Ocean many times over in coming years.