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Marine Geology and Geophysics

Marine Geology and Geophysics

Marine geology and geophysics are scientific fields that are concerned with solving the mysteries of the seafloor and Earth's interior. Marine geologists, like all geologists, seek to understand the processes and history of the solid Earth, but their techniques differ from geologists who work on land because they study geologic (Earth's) features that are underwater. The oceans cover more than 70% of Earth, and water obscures a wealth of information about the rocks and sediments (particles of rock, sand, and other material) in the ocean basins. Marine geologists rely mainly on physical techniques to uncover the features and processes of the seafloor.

Geophysicists are scientists who study the physical properties of the solid Earth, and often work closely with marine geologists. Geophysicists use experiments and observations to determine how Earth materials such as rock, magma (molten rock), sediments, air, and water affect physical phenomena such as sound, heat, light, magnetic fields (a field of magnetic force), and earthquake tremors (seismic waves). Marine geologists and geophysicists make images and maps of the seafloor, along with maps ofsediment and rock layers below the seafloor. They also use instruments to measure changes in Earth's gravity (the attraction between two masses), magnetic field, and the pattern of heat flow arising from deep in the Earth that help to explain geologic features of the ocean basins.

Marine geology and geophysics involve many different fields of science. Many marine geoscientists (a group including both marine geologists and marine geophysicists) have backgrounds in such diverse academic fields as physics, chemistry, oceanography, engineering, and paleontology (study of biological life in the fossil record). Most marine geologists are familiar with the theories and techniques of geophysics, and most geophysicists understand the geological significance of the processes and features they are working to clarify. Marine geology is also closely linked to the sciences of oceanography and marine biology. Oceanographers study the physical and chemical properties of the water in oceans and marine biologists study the living organisms in oceans. In order to completely understand the cycles, structures and processes of the oceans, scientists from many fields must collaborate.

Why study the seafloor?

The ocean basins hold keys to understanding the two most important theories of geological science: plate tectonics and the sedimentary record of geologic history. Marine geologists and geophysicists were the first to discover the globe-encircling chain of volcanic mountains, called the mid-ocean ridge system, where new ocean floor is created.

Using their observations of the seafloor, these scientists developed the theory of plate tectonics, the idea that Earth's outer shell (lithosphere) is made of rigid pieces (plates) that move relative to one another over time. Plate tectonic theory explains the worldwide distribution of mountain ranges, ocean trenches (deep, arc-shaped valleys along the edges of the ocean basins), volcanoes, rock types, and earthquakes. By studying plate tectonics, scientists can better understand and predict geologic actions of today, such as volcanic activity and earthquakes. Scientists also know from studying plate tectonics that the moving seafloor is recycled into Earth's interior at trenches, a process called subduction. Like the theories of evolution (change over time) in biology and relativity in physics, plate tectonics is a unifying theory that has general significance to all of science. Marine geologists and geophysicists also study layered sedimentary rocks (strata) on the seafloor that hold clues to the chemical, biological, and geographic history of the oceans.

The ocean basins hold a vast wealth of economically important minerals, such as manganese and nickel, and hydrocarbons (oil and natural gas). Petroleum (oil and gas) and mining companies hire marine geologists and geophysicists to find offshore sources of petroleum. They rely heavily on marine scientific techniques to locate petroleum reservoirs and mineral deposits.

Studying the seafloor

Marine geology and geophysics use a number of technologies uniquely adapted for ocean exploration. Many of the methods used are geophysical because they allow a "hands off" approach to seafloor observation. In other words, geophysical technologies allow marine geoscientists to "see" through water, rock, and sediment. (Techniques that involve observing or measuring the properties of land, sea, and seafloor surface from a distance are generally termed remote sensing.)

Like all geologists, marine geologists collect rock and sediment samples. They use dredges, which are metal buckets or claws that are lowered from a ship and dragged along the sea floor, and coring (drilling) devices to bring materials up from the bottom of the sea. Scientists then examine the materials' physical, chemical, and biological properties. Seafloor samples are, however, difficult and very expensive to obtain, especially in very deep water. Marine geologists usually collect them from a few critical locations within a study area and then use geophysical images to generate a big picture of the study area. Sediments and deep rock samples are collected using shipboard drills that bring back cores (metal tubes) that are filled with several meters of sample. By using samples together with seafloor maps and profiles (cross-sections) through the rock and sediment layers below the seafloor, marine geologists construct three-dimensional representations of their study areas.

Although most features that interest marine geologists, such as submarine (underwater) volcanoes, massive sand dunes, and deep trenches are too large to observe from the seafloor, direct observations by divers, submersibles, and remotely-operated vehicles (ROVs) can be useful in some cases. Geologists use waterproof cameras and other instruments carried by divers, lowered on cables from ships, or attached to remotely operated watercraft to capture details of the seafloor environments. Submersibles are small submarines that are capable of carrying passengers to the deep seafloor. ROVs and autonomous under-water vehicles (AUVs) are unmanned robotic submarines equipped with cameras and instruments that operators control from a ship, much like a remote controlled car.

Deep Ocean Drilling

Deep ocean drilling allows scientists to recover cores of ocean sediments and underlying oceanic crust for mapping the ocean floor. The core is brought back to the surface where scientists analyze the sediments' history and composition. Deep ocean drilling can be expensive and in 1964, several U.S. institutions interested in studying the sea floor pooled their resources and formed an organization called Joint Oceanographic Institution for Deep Earth Sampling (JOIDES). JOIDES directed the first Deep Sea Drilling Project (DSDP) that used a ship named the Glomar Challenger. The Glomar Challenger was a customized ship that had powerful thrusters that kept the ship centered over the top of the ocean floor target. The Challenger could lower the drill through up to 20,013 feet (6100 meters) of ocean water and drill up to 2,500 feet (760 meters) of sediment once it hit the sea floor.

Over 15 years the DSDP drilled more than 600 core holes during 96 legs (voyages) worldwide. Seismic and magnetic surveys of the ocean floor were made while the ship was in motion. When the cores were brought up they were analyzed in ship-board laboratories. Data from the DSDP project proved that Earth's ocean basins are relatively young when compared to the continents and contributed to an understanding of sea floor spreading and plate tectonics.

In 1984 a group of 21 nations formed the Ocean Drilling Program (ODP). ODP used the drillship JOIDES' Resolution for drilling in poorly sampled areas, especially along the margins of continents and ocean trenches. The ODP also drilled holes in which instruments were lowered to the seafloor, providing a global network to study earthquake movements (seismic waves) on the ocean floor. The JOIDES Resolution drilled 650 holes over 110 legs.

In October 1993, ODP became the Integrated Ocean Drilling Project (IODP). IODP has two ships, the Chikyu, built by Japan, and an upgraded Resolution. Chikyu will drill in areas where plates converge, slide beneath one another, and produce earthquakes. Resolution will concentrate on recovering sediment cores worldwide to help scientists study climate.

Marine geologists rely on sonar (short for "sound navigation and ranging"), which is the use of underwater sound waves. Sound travels at a constant velocity (speed) in water, so the time it takes for the sound wave to travel through the water and echo back to the ship illustrates variations in the seafloor. Sonar is used to measure bathymetry, the topography or layout of the sea floor. A "chirp" is transmitted from a ship hull and travels until it reaches the sea floor and bounces back to a receiver on the ship where the travel time is recorded. To determine the distance from sea level to the ocean bottom, scientists multiply the time it takes for the sound wave to travel to the ocean floor and back by the rate (speed) at which the sound wave travels in water.

Submersibles, ROVs, and AUVs

Submersibles, remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs) are motorized crafts that are designed to withstand the pressure of the deep ocean. Submersibles carry passengers, usually a pilot and two scientific observers, while ROVs and AUVs are remotely operated. These crafts were originally built of steel, but now are built of light materials such as titanium. Although these type of deep-diving craft are expensive to build and can be dangerous to people riding in the submersibles, they offer scientists a unique look at the deep oceans. Several of the important capabilities of these crafts include cameras that record underwater conditions in real-time; instruments that record temperature, pressure, and chemistry; and robotic arms that can retrieve specimens.

The first submersible, named Alvin, was built in 1964 and is still operated by Woods Whole Oceanographic Institute (WHOI). Alvin was the first deep-sea submersible that could carry passengers. Lowered from a ship platform, Alvin can dive to a depth of 14,764 feet (4,500 meters) and remain under water for 10 hours. In case of emergency, Alvin's life support system can sustain three people for 72 hours. Alvin is 23 feet (7 meters) long and can travel 6 miles (9.6 kilometers) from the ship platform with maximum speed of 2 knots. Alvin is responsible for important observations of hydrothermal vents (geysers on the ocean floor) near the Galapagos Islands, and helping to find the wreckage of the Titanic, a passenger ship that sank in 1912 and resulted in the deaths of over 1,500 people.

ROVs are small crafts that carry video cameras deep and record or transmit live footage back to a screen on a ship. ROVs like Jason, also operated by WHOI, do not carry passengers, but are driven like a remote control car. As images are transmitted from the ROV back to the ship, the ROV operator can steer the ROV in the direction he or she wants.

AUVs are used for longer-term projects. While submersibles and ROVs are good for intensive short-term studies, AUVs can remain in one location for up to a year. An AUV operator can program a computer inside the AUV to sit on the sea floor for a predetermined time and can "wake up" to perform surveys then return to sleep mode until the next scheduled survey. AUVs can record changes that occur in one location over time and are often used in between ROV or submersible visits in one location.

Scientists can also map ocean floor bathymetry using satellite (vehicles in orbit around Earth) instruments. The ocean surface is not completely flat, but mimics the sea floor by bulging upward and downward. Satellite observations reveal detailed patterns of mid-ocean ridges and trenches and underwater volcanoes, thus confirming plate tectonics. Magnetometers, towed behind a ship, measure small changes in Earth's magnetic field. Sensitive shipboard gravimeters record subtle changes in Earth's field of gravity (the attraction between the Earth and another body).

Marine geoscientists also use seismology (earthquake waves) to make an image of the seafloor. A ship tows several air guns that make an underwater explosion using compressed air. The shock waves from the explosion are the same type as waves made in an earthquake. These waves penetrate layers of rock underlying the surface of the ocean and bounce back to hydrophones (receivers). The waves travel at different speeds depending on the type of rock.

Laurie Duncan, Ph.D., andMarcy Davis, M.S.

For More Information

Books

Fowler, C. M. R. The Solid Earth, An Introduction to Global Geophysics. Cambridge, UK: Cambridge University Press, 1990.

Kennett, James. Marine Geology. Washington, DC: Prentice-Hall, 1981.

Websites

"Exploring with Satellite Altimeter Data." Satellite Geodesy.http://topex.ucsd.edu/marine_grav/explore_grav.html (accessed on August 26, 2004).

"Integrated Ocean Drilling Program." IODP Website.http://www.oceandrilling.org (accessed on August 26, 2004).

"Ocean Explorer, Technology." National Oceanographic and Atmospheric Administration.http://oceanexplorer.noaa.gov/technology/subs/subs.html (accessed on August 26, 2004).

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