Geology of the Ocean Floor

Geology is the study of the solid Earth and its history. Marine geology is the study of the solid rock and basins that contain the oceans. The rocks and sediments (particles of sand, gravel, and silt) that lie beneath the oceans contain a record book of Earth's past. Topographic features (the physical features of the surface of Earth) and geologic processes in the ocean basins hold the keys to plate tectonics, a fundamental theory of geology that explains the movement of the continents and seafloor over time. (A plate is a rigid layer of Earth's crust and tectonics is the large scale movements of the crust.) Only when scientists began to successfully probe the secrets of the seafloor in the mid-twentieth century did they begin to understand the complex workings of the solid Earth.

If marine geology is the study of the ocean soup bowl, then oceanography is the study of the broth, and marine biology is the study of the vegetables and meat in the broth. These three branches of ocean science are closely linked. The mountains and valleys of the seafloor, together with the continental margins (edges), act to guide ocean currents (a steady flow of water in a prevailing direction) that in turn regulate global climate. Moving water shapes the seafloor by eroding (wearing away) and depositing sediments (particles of gravel, sand, and silt). The seafloor provides shelter and nutrients (food) for marine (ocean) plants and animals, and living organisms play a role in shaping the seafloor. They burrow (dig holes and tunnels), build shelters, and consume nutrients during their lifetimes, and their remains form layers of sediment on the seafloor. Practical knowledge of seafloor topography (called bathymetry) and marine geology is essential for human navigators, coastal and marine engineers, naval tacticians, as well as petroleum (oil and gas) and mineral prospectors.

Depth and shape of the seafloor

Bathymetry beyond shallow coastal waters was a complete mystery until the middle of the 1800s. Until then, navigators used relatively short ropes and chains to make water depth measurements, called soundings, and to construct charts of shallow coastal waters where seafloor topography is a shipping hazard. By the 1860s, however, advances in science had raised a number of intriguing questions about the nature of the deep ocean floor. Royal Society of London naturalists aboard the ship Challenger used newly developed steel cables to take more than 500 soundings and to dredge 133 rock and sediment samples from the deep ocean during their expedition from 1872 to 1876.

The Challenger scientists discovered that the oceans are very deep. They took their deepest sounding in an ocean trench near the Mariana Islands in the western Pacific Ocean. Today, it is known that Earth's lowest point, the Challenger Deep in the Mariana Trench, is 36,201 feet (11,033 meters) below sea level. In comparison, Earth's highest point, the peak of Mt. Everest in the Himalayan Mountains, is a mere 29,035 feet (8,850 meters) above sea level! The average water depth in the main oceans is 12,200 feet (3,729 meters), deeper than the highest points in 38 of the 50 states of the union. Dredge samples from the Challenger expedition showed that ocean rocks and sediments are fundamentally different from those found on land.

Plate Tectonics

Earth's rocky outer shell, the lithosphere, is broken into rigid pieces, or plates, that move over time. This fundamental theory of geology is called plate tectonics. It explains the jigsaw puzzle fit of continents across ocean basins as well as patterns of mountain ranges, earthquakes, volcanoes, and different types of rocks on Earth's surface. Plate tectonics also explains the observation that Earth's land masses have formed different patterns over geologic history, and have even been joined at times. Some plates, like the North American Plate, are composed of both continental and oceanic crust. Others, like the oceanic Pacific Plate, contain mostly one type of crust.

In 1912, German meteorologist Alfred Wegener (1880–1930) suggested that Earth's continents were joined about two hundred million years ago and have since drifted apart. Other scientists doubted him because he had no explanation for how the continents moved. (This was unfortunate because Wegener's theory of continental drift was correct.) The complete theory of plate tectonics was finally developed in the 1960s and 1970s by marine geologists studying new images and rock samples from the deep ocean floor.

Lithospheric plates move by processes that occur at their boundaries. There are three types of plate boundaries: divergent (plates move away from each other), convergent (plates move toward each other), and transform (plates move horizontally by each other).

Plates move about an inch (a few centimeters) per year, about the speed that fingernails grow. Places where oceanic and continental crust are connected and do not move relative to each other, like the east coasts in North America, are called passive margins. Passive margins are not plate tectonic boundaries. (The Mid-Atlantic Ridge in the central Atlantic Ocean is the eastern boundary of the North American Plate.)

In spite of the tantalizing clues turned up by nineteenth century British scientists, a full picture of the ocean basins did not come into focus until the late 1950s. In the early twentieth century, the spirit of scientific inquiry during the Challenger era was replaced by more practical reasons to map the seafloor—naval warfare during the first and second world wars. A new technique, called sonar echosounding, replaced expensive, relatively inaccurate wire soundings. An echosounder works by bouncing a sound wave off the seafloor. Sound travels at a constant velocity (speed) in water, so the time it takes for the sound to travel through the water and echo back to the ship gives the distance to the seafloor. The faster the sound returns, the shallower the water. American ships carrying troops and supplies to Europe and Asia carried echosounders that recorded the water depths along their routes.

Civilian scientists were intrigued by what they saw on the wartime bathymetric profiles, and they set out to survey the seafloor using the new, accurate, inexpensive echosounders. The first complete maps of the Pacific, Atlantic, Indian, and Arctic Ocean basins were compiled by Columbia University marine geologists Bruce Heezen and Marie Tharp and published by the National Geographic Society in the mid-1950s. The features that were clearly visible on these bathymetric maps, globe-encircling chains of underwater volcanoes and deep ocean trenches, led to a revolution in marine geology and the theory of plate tectonics during the 1960s and 1970s.

Seafloor features

A bathymetric profile (cross-section) of a major ocean basin like the Pacific Ocean shows the typical features of the seafloor: continental shelf, continental slope and rise, mid-oceanic ridge, ocean trench, and abyssal plain.

• Continental shelf: Continental shelves are the relatively shallow, submerged margins of the continents. Some shelves, like the east coasts of North and South America, are very wide. Others, like the west coasts of North andSouth America, are very narrow. Over geologic time, the shorelines on continental shelves retreat and advance as the ice in the North and South Poles grow and shrink and global sea-level rises and falls.
• Continental slope and rise: The continental slope is the steep transition from the continental shelf to the floor of the abyssal (deep) ocean. The slope is cut by huge canyons that carry underwater landslides, called turbidite flows, downslope at speeds of up to 40 miles (64 kilometers) per hour. The continental rise is the deposit of sediments at the base of the continental slope.
• Mid-oceanic ridge: The most striking feature of Heezen and Tharp's bathymetric map was the mid-oceanic ridge system, a continuous chain of low, symmetrical volcanoes that extends through all the ocean basins. A mid-oceanic ridge, like the Mid-Atlantic Ridge between South America and Africa, is a broad uplift with a small valley at its axis (center). Mild volcanic eruptions fill the ridge axis valley with molten lava that cools to become new seafloor. The ocean basins on either side of a mid-oceanic ridge are symmetrical mirror images that are moving away from the ridge axis over time.
• Ocean trenches: Trenches are deep, arc-shaped submarine valleys along the edges of the ocean basins. They are the deepest parts of Earth's oceans. Scientists now know that the moving seafloor is recycled into Earth's interior at trenches, a process called subduction. Chains of large volcanoes, called arc volcanoes, form on the outer edges of trenches. The Andes Mountains of South America and the islands of Japan are examples of arc volcanoes. Friction between rocks during subduction also causes very large earthquakes. The geologically active subduction zones that surround the Pacific Ocean are called the "ring of fire."
• Abyssal plains: The abyssal plains are vast, flat areas of the deep-ocean floor. In some places, small repeating sets of sharp-peaked ridges, called abyssal hills, interrupt the nearly featureless abyssal seafloor. Cross-sections through the seafloor show that abyssal hills are the tips of tilted blocks of rock beneath a blanket of deep-ocean sediment.

Ocean rocks and sediments

The solid rock, called the basement, that acts as the floor of the deep ocean is different from that of the continents. Earth's rocky outer crust comes in two varieties, continental and oceanic, that have very different properties and compositions. Ocean crust is denser, thinner, darker-colored, and contains more of the chemical elements iron and magnesium than continental crust. The basement of the ocean basins is mostly made of black, volcanic rock called basalt. Mid-oceanic ridge volcanoes produce basalt. The centers of the continents are composed mainly of coarse-grained, light-colored rocks like granite. The blanket of sediment that covers the floors of the abyssal plains is called pelagic ooze. Oozes form by the slow, steady accumulation of silica- and calcium-rich remains of microscopic animals and plants that sink to the deep seafloor.

Tsunamis

Tsunamis are ocean waves caused by disturbances on the seafloor. Underwater earthquakes, volcanic eruptions, landslides, and man-made explosions create these very large waves that travel great distances across ocean basins. Tsunamis are particularly common in the Pacific Ocean where large seafloor earthquakes and volcanic eruptions occur regularly around the "ring of fire." The largest earthquake of the twentieth century occurred in 1960 in the Peru-Chile Trench offshore of South America. It triggered a tsunami that traveled throughout the Pacific Ocean. The huge waves caused widespread destruction and killed 231 people when they washed ashore at the Hawaiian Islands, the west coast of the United States, Japan, and the Philippines.

The word tsunami means "harbor wave" in Japanese. In the open ocean, tsunami waves are very broad, but not very tall. When they approach land, the waves get shorter and much taller; they appear to spring up from the ocean near coastlines. Tsunamis are sometimes mistakenly called tidal waves because an approaching tsunami can resemble a rapidly falling and then rising tide. (Tides are daily sea level rises and falls.) Tsunami waves can be as tall as a six-story building and are very unpredictable; they are not generated by wind like most ocean waves, and can arrive from distance sources during calm weather conditions.

The boundary between deep ocean rock and continental land rock lies beneath massive fans of sediment that form the continental margins. Continental margins, including the continental shelf and slope, are composed of thick stacks of layered sediment that rivers and glaciers have carried from the continental interior. Some shelves, called carbonate shelves, are composed of the calcium carbonate shells and skeletons of organisms like corals and mollusks. Along many continental margins, continental sediments gradually give way to pelagic oozes at the toe of the continental rise.

Laurie Duncan, Ph.D.

For More Information

Books

Byatt, Andrew, et al. Blue Planet. London: DK Publishing, 2002.

Earle, Sylvia. Atlas of the Ocean: The Deep Frontier. Washington, DC: National Geographic, 2001.

Kennett, James. Marine Geology. Upper Saddle River, NJ: Prentice-Hall, 1981.

Websites

"Marine Geology: Research Beneath the Sea." U.S. Geological Survey (USGS).http://walrus.wr.usgs.gov/pubinfo/margeol.html (accessed on August 12, 2004).

"Pacific Tsunami Museum." Pacific Tsunamis, Hilo, Hawaii.http://www.tsunami.org (accessed on August 12, 2004).

"This Dynamic Earth: The Story of Plate Tectonics." U.S. Geological Survey (USGS).http://pubs.usgs.gov/publications/text/dynamic.html (accessed on August 12, 2004).

"Tsunami Research Program." U.S. Department of Commerce National Oceanographic and Atmospheric Administration (NOAA).http://www.pmel.noaa.gov/tsunami (accessed on August 12, 2004).