Marine Geology and Geophysics

Marine Geology and Geophysics - California's central coast, just north of Pismo Beach
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 of sediment 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 underwater vehicles (AUVs) are unmanned robotic submarines equipped with cameras and instruments that operators control from a ship, much like a remote controlled car.

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.

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.