Sea Floor Geology


Geological oceanography is a field that studies the ocean floor’s history and structure; that is, studying the solid rocks and basins where the oceans lie. It relies on other fields such as paleontology, sedimentology, geochemistry, geophysics, etc.

You would wonder why oceanographers study the ocean floor. They do so to comprehend and predict climate changes and their effect on the oceans and shores. Moreover, studying ocean floor helps preserve the planet and its equilibrium, and discover resources as petroleum and minerals.

At the start of the 20th century, oceanographers faced problems with sea floor mapping. The use of sonar echo sounding in naval warfare in World War I and World War II aroused civilian scientists’ curiosity; they started using this cheaper method to study the sea floor.

Sonar echo sounding sends sound waves to the sea floor awaiting their echo sounding. Since sound travels at a fixed speed in water, the time it takes to travel through water and echo back to the ship provides the distance to the sea floor; therefore, the faster the sound echoes back, the shallower the water is.

Columbia University geological oceanographers Bruce Heezen and Marie Tharp were the first to create a three-dimensional physiographic map of the world’s ocean basins, printed by the National Geographic Society in 1956. This bathymetric map led to the discovery of chains of underwater volcanoes and deep ocean trenches surrounding Earth, resulting in a revolution in geological oceanography and plate tectonics theory in the following two decades.

Oceanographers create a bathymetric profile of an ocean basin; for example, one of the Pacific Ocean, which highlights the sea floor features:

  • Continental Shelf: It is the continents’ shoal submerged margins. Some are extremely wide like North America and South America East Coasts, while others are extremely narrow as their West Coasts. Across geological time, the shorelines retreat or advance according to the growth or shrinkage of the ice of the two Poles and the global rise and fall of sea levels.
  • Continental Slope and Rise: The Slope is the sudden transition from the continental shelf to the ocean depths. It is interrupted by large canyons, which move turbidity currents; the rise is the accumulation of sediments at the slope’s bottom.
  • Mid-Ocean Ridge: an uninterrupted chain of low, uniform volcanoes, which stretches along the ocean basins. It is a vast elevation with a tiny valley at its center, such as the Mid-Atlantic Ridge between Africa and South America. Gentle volcanic eruptions pour molten lava into the ridge axis valley which when it cools down, becomes the new sea floor.
  • Ocean Trenches: bottomless, curved underwater valleys present on the edges of the ocean basins; they are the oceans’ deepest locations. At this point, a process known as subduction occurs, where the travelling sea floor is recycled inside the Earth’s interior at these trenches.
  • Abyssal Plains: They are enormous, flat terrains of deep ocean floor. Sometimes small groups of sharp-topped ridges, known as abyssal hills, cut this almost smooth sea floor. On inspecting the abyssal hills, scientists found out that they are the tops of slant rock blocks under a layer of deep ocean sediment.

The solid rock on the sea floor is unlike that present on the continental rock. The former is thinner, deeper, and dark-colored, and contains more magnesium and iron than that of the Earth’s surface. Mid-ocean ridge volcanoes deposit basalt, which is an integral part of the ocean floor. The sediment that covers the abyssal plains on the ocean floor, known as pelagic ooze, formed by the constant slow amassment of silica-and-calcium that are abundant in microscopic animals and plant remnants, which fall until they reach the ocean floor. The latter contains unpolished, light-colored rocks such as granite.

The borders separating ocean rocks from continental ones are under large sediment fans, which create continental margins. Continental margins, like continental slopes and shelves are thick sediment layers, which glaciers and rivers move from the continental interior.

This highly interesting field of study keeps progressing mainly due to increased interest in the effect of sea floor geology on the environment and the discovery of new resources. This inspires many people to pursue a career in this field due to its importance in today’s world.


*The original article was published in SCIplanet, Earth Sciences (Spring 2017) issue.

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