5.1 Mapping the Topography of the Ocean Floor; Underwater Volcanoes, Mountains and Valleys

The ocean is the largest feature on Earth, covering more than 70% of our planet’s surface. One of the main reasons that Wegener’s continental drift hypothesis was not widely accepted when first proposed was that so little was known about the ocean floor. Until the 20th century, investigators used weighted lines to measure water depth. In deep water these depth measurements, or soundings, took hours to perform and could be wildly inaccurate.

With the development of new marine tools following World War II, our knowledge of the diverse topography of the ocean floor grew rapidly. One of the most interesting discoveries was the global oceanic ridge system. This broad elevated landform which stands two to three kilometers above the adjacent deep ocean basins, is the longest topographic feature on Earth.

Today we know that oceanic ridges mark divergent plate margins, where new oceanic lithosphere originates. We also know that deep ocean trenches represent convergent plate boundaries, where oceanic lithosphere is subducted into the mantle. Because the process of plate tectonics is creating oceanic crust at mid-ocean ridges and consuming it at subduction zones, the oceanic crust is continually being renewed and recycled.

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If all water were drained from the ocean basins a great variety of features would be seen, including broad volcanic peaks, deep trenches, extensive plains, linear mountain chains, and large plateaus. In fact, the scenery would be nearly as diverse as that on the continents.

An understanding of seafloor features came with the development of techniques that measure the depth of the oceans. Bathymetry is the measurement of ocean depths and the charting of the shape or topography of the ocean floor.

The first understanding of the ocean floors’ varied topography did not unfold until the historic three-and-a-half-year voyage of the HMS Challenger. From December 1872 to May 1876, the Challenger expedition made the first, and perhaps still most comprehensive, study of the global ocean ever attempted by one agency. A trip of nearly 80,000 miles took the ship and its crew of scientists to every ocean except the Arctic. Throughout the voyage, they sampled a multitude of ocean properties, including water depth, which was accomplished by laboriously lowering a long weighted line overboard. Not many years later, the knowledge gained by the Challenger of the oceans’ great depth and varied topography was further expanded with the laying of transatlantic communication cables, especially in the North Atlantic Ocean. However, as long as a weighted line was the only way to measure ocean depths, knowledge of seafloor features remained extremely limited.

Today sound energy is used to measure water depths. The basic approach employs some type of SONAR, an acronym for SOund Navigation And Ranging. The first devices that used sound to measure water depth, called echo sounders, were developed early in the twentieth century. Echo sounders work by transmitting a sound wave, called a ping, into the water in order to produce an echo when it bounces off any object, such as a large marine organism or the ocean floor. A sensitive receiver intercepts the echo reflected from the bottom, and a clock precisely measures the travel time to fractions of a second. By knowing the speed of sound waves in water, about 4900 feet per second, and the time required for the energy pulse to reach the ocean floor and return, depths can be calculated. The depths determined from continuous monitoring of these Echoes are plotted so a profile of the ocean floor is obtained. By laboriously combining profiles from several adjacent traverses, a chart of the seafloor can be produced.

Following World War II, the U.S. Navy developed side-scan sonar to look for mines and other explosive devices. These torpedo-shaped instruments can be towed behind a ship where they send out a fan of sound extending to either side of the ship’s track. By combining swathes of side-scan sonar data, researchers produced the first photograph-like images of the seafloor. Although side-scan sonar provides valuable views of the seafloor, it does not provide bathymetric or water depth data.

This problem is not present in the high-resolution multi-beam instruments developed during the 1990s. These systems use hull-mounted sound sources that send out a fan of sound, then record reflections from the seafloor through a set of narrowly focused receivers and different angles. Thus, rather than obtaining the depth of a single point every few seconds, this technique makes it possible for a survey ship to map the features of the ocean floor along a strip tens of kilometers wide. When a ship uses multibeam sonar to make a map of a section of seafloor, it travels through the area in a regularly spaced back and forth pattern. Furthermore, these systems can collect bathymetric data of such high resolution that they can distinguish depths that differ by less than a meter.

Despite their greater efficiency and enhanced detail, research vessels equipped with multibeam sonar travel at a mere 6 to 12 miles per hour. It would take at least a hundred vessels outfitted with this equipment hundreds of years to map the entire seafloor. This explains why only about 5% of the seafloor had been mapped in detail, and why large areas of the seafloor have not yet been mapped with sonar at all.

Marine geologists are also interested in viewing the rock structure beneath the sediments that blanket much of the seafloor. This can be accomplished by making a seismic reflection profile. To construct such a profile, strong low frequency sounds are produced by explosions, depth charges, or air guns. These sound waves penetrate the seafloor and reflect off the contacts between rock layers and fault zones, just like sonar reflects off the bottom of the sea.

Another technological breakthrough that has led to an enhanced understanding of the sea floor involves measuring the shape of the surface of the global ocean from space. After compensating for waves, tides, currents, and atmospheric effects, it was discovered that the water’s surface is not perfectly flat. This is because gravity attracts water toward regions where massive seafloor features occur. Therefore, mountains and ridges produce elevated areas on the ocean surface and conversely, canyons and trenches cause slight depressions. Satellites equipped with radar altimeters are able to measure these subtle differences by bouncing microwaves off the sea surface. These devices can measure variations as small as 2 to 4 inches. Such data have added greatly to the knowledge of ocean floor topography. Cross-checked with traditional sonar depth measurements, the data are used to produce detailed ocean floor maps.