During WWII, a large number of studies were conducted in order to accurately map the seafloor. This was done primarily because submarines needed accurate data in order to navigate safely, and to locate potential hiding spots for Allied and enemy submarines. These studies, which continued after the war, led to a number of astonishing discoveries including ocean ridges and ocean trenches.

• Ocean Ridges: Studies showed a large oceanic mountain chain with a narrow valley down its center extending the length of the entire Atlantic Ocean. This Mid-Atlantic Ridge also connected to other ranges of mid-oceanic ridges that extended completely around the entire planet.


Did You Know?
The Mariana Trench vs. Mt. Everest
The Mariana Trench in the Pacific Ocean has a depth far greater than the height of the world’s tallest mountain, Mt. Everest. Mt. Everest has a height of 8,848 meters (29,000 feet), while the Mariana Trench has a depth of 10,971 meters (36,000 feet). This means that the world’s deepest trench exceeds the world’s highest mountain by a distance of over 2 kilometers (1.25 miles)!
• Ocean Trenches: Deep sea trenches thousands of kilometers (thousands of miles) long and many kilometers (miles) deep were also found. These trenches were often found at the edges of continents or along a chain of islands. The majority of ocean trenches seemed to be located in the Pacific Ocean.

Samples of sediment and rock collected from the seafloor also revealed a number of interesting facts. The sediment layer on the seafloor was much thinner than what scientists had expected. Also, none of the sediment samples were more than 180 million years old. This was extremely young when compared to the age of continental crust which is measured in billions of years.

The lack of old oceanic crust was explained in 1962 by a theory developed by American geologist Harry Hess (1906–1969). Hess’ theory was known as seafloor spreading. This theory states that hot magma from the Earth’s mantle rises up through the mid-oceanic ridges. This magma cools and flows sideways forming new seafloor. Hot magma continues to rise from the mid-oceanic ridge, constantly producing new oceanic crust. The older crust becomes denser as it slowly cools and sinks, gradually building up the mid-oceanic ridges. Eventually, this newly formed ocean crust is carried away from the mid-oceanic ridge by the spreading seafloor. This ocean crust is then recycled millions of years later when it returns to the mantle by descending into the deep ocean trenches. Persuasive evidence to support seafloor spreading was quick to follow Hess’ hypothesis.