Marine geology
Marine geology or geological oceanography is the study of the history and structure of the ocean floor. It involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal zone. Marine geology has strong ties to geophysics and to physical oceanography.
Marine geological studies were of extreme importance in providing the critical evidence for sea floor spreading and plate tectonics in the years following World War II. The deep ocean floor is the last essentially unexplored frontier and detailed mapping in support of economic, natural disaster mitigation, and academic objectives.
History
The study of marine geology dates back to the late 1800s during the 4-year HMS Challenger expedition. HMS Challenger hosted nearly 250 people, including sailors, engineers, carpenters, marines, officers, and a 6-person team of scientists, led by Charles Wyville Thomson. The scientists' goal was to prove that there was life in the deepest parts of the ocean. Using a sounding rope, dropped over the edge of the ship, the team was able to capture ample amounts of data. Part of their discovery was that the deepest part of the ocean was not in the middle. These were some of the first records of the mid-ocean ridge system.Prior to World War II, marine geology grew as a scientific discipline. During the early 20th century, organizations such as the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution were created to support efforts in the field. With Scripps being located on the west coast of North America and WHOI on the east coast, the study of marine geology became much more accessible.
In the 1950s, marine geology had one of the most significant discoveries, the mid-ocean ridge system. After ships were equipped with sonar sensors, they travelled back and forth across the Atlantic Ocean collecting observations of the sea floor. In 1953, the cartographer Marie Tharp generated the first three-dimensional relief map of the ocean floor which proved there was an underwater mountain range in the middle of the Atlantic, along with the Mid-Atlantic Ridge. The survey data was large step towards many more discoveries about the geology of the sea.
In 1960, the American geophysicist Harry H. Hess hypothesized that the seafloor was spreading from the mid-ocean ridge system. With support from the maps of the sea floor, and the recently developed theory of plate tectonics and continental drift, Hess was able to prove that the Earth's mantle continuously released molten rock from the mid-ocean ridge and that the molten rock then solidified, causing the boundary between the two tectonic plates to diverge. A geomagnetic survey was conducted that supported this theory. The survey was composed of scientists using magnetometers to measure the magnetism of the basalt rock protruding from the mid-ocean ridge. They discovered that on either side of the ridge, symmetrical "strips" were found as the polarity of the planet would change over time. This proved that seafloor spreading existed. In later years, newer technology was able to date the rocks and identified that rocks closest to the ridge were younger than the rocks near the coasts of the Western and Eastern Hemispheres land.
At present, marine geology focuses on geological hazards, environmental conditions, habitats, natural resources, and energy and mining projects.
Methods
There are multiple methods for collecting data from the sea floor without physically dispatching humans or machines to the bottom of the ocean.Side-scan sonar
A common method of collecting imagery of the sea floor is side-scan sonar. Developed in the late 1960s, the purpose of the survey method is to use active sonar systems on the sea floor to detect and develop images of objects. The physical sensors of the sonar device are known as a transducer array and they are mounted onto the hull of a vessel which sends acoustic pulses that reflect off the seafloor and received by the sensors. The imaging can help determine the seafloors composition as harder objects generate a stronger reflectance and appear dark on the returned image. Softer materials such as sand and mud cannot reflect the arrays pulses as well so they appear lighter on the image. This information can be analyzed by specialist to determine outcrops of rock beneath the surface of the water.This method is less expensive than releasing a vehicle to take photographs of the sea floor, and requires less time. The side-scan sonar is useful for scientists as it is a quick and efficient way of collecting imagery of the sea floor, but it cannot measure other factors, such as depth. Therefore, other depth measuring sonar devices are typically accompanied with the side-scan sonar to generate a more detailed survey.
Multibeam bathymetry
Similarly to side-scan sonar, multibeam bathymetry uses a transducer array to send and receive sound waves in order to detect objects located on the sea floor. Unlike side-scan sonar, scientists are able to determine multiple types of measurements from the recordings and make hypothesis' on the data collected. By understanding the speed at which sound will travel in the water, scientists can calculate the two way travel time from the ship's sensor to the seafloor and back to the ship. These calculations will determine to depth of the sea floor in that area.Backscatter is another measurement used to determine the intensity of the sound that is returned to the sensor. This information can provide insight on the geological makeup and objects of the sea floor as well as objects located within the water column. Objects in the water column can include structures from shipwrecks, dense biology, and bubble plumes. The importance of objects in the water column to marine geology is identifying specific features as bubble plumes can indicate the presence of hydrothermal vents and cold seeps.
There are limitations to this technique. The distance between the sea floor and the sensor is related to the resolution of the map being created. The closer the sensor is the sea floor, the higher the resolution will be and the farther away the sensor is to the sea floor, the lower the resolution will be. Therefore, it is common for remotely operated vehicles and autonomous underwater vehicles to be equipped with the multibeam sensor or for the sensor to be towed by the ship itself. This ensures that the resolution of the collected data will be high enough for proper analysis.
Sub-bottom profiler
A sub-bottom profiler is another sonar system used in geophysical surveys of the sea floor to not only map depth, but also to map beneath the sea floor. Mounted to the hull of a ship, the system releases low-frequency pulses which penetrate the surface of the sea floor and are reflected by sediments in the sub-surface. Some sensors can reach over 1000 meters below the surface of the sea floor, giving hydrographers a detailed view of the marine geological environment.Many sub-bottom profilers can emit multiple frequencies of sound to record data on a multitude of sediments and objects on and below the sea floor. The returned data is collected by computers and with aid from hydrographers, can create cross-sections of the terrain below the sea floor. The resolution of the data also allows scientists to identify geological features such as volcanic ridges, underwater landslides, ancient river beds, and other features.
The benefit of the sub-bottom profiler is its capability to record information on the surface and below the seafloor. When accompanied with geophysical data from multibeam sonar and physical data from rock and core samples, the sub-bottom profiles delivers insights on the location and morphology of submarine landslide, identifies how oceanic gasses travel through the subsurface, discover artifacts from cultural heritages, understand sediment deposition, and more.
Marine magnetometry
Magnetometry is the process of measuring changes in the Earth's magnetic field. The outer layer of the Earth's core is liquid and mostly made up of magnetic iron and nickel. When the Earth turns on its axis, the metals release electrical currents which generate magnetic fields. These fields can then be measured to reveal geological subseafloor structures. This method is especially useful in marine exploration and geology as it can not only characterize geological features on the seafloor but can survey aircraft and ship wrecks deep under the sea.A magnetometer is the main piece of equipment deployed, which is typically towed behind a vessel or mounted to a AUV. It is able to measure the changes in fields of magnetism and corresponding geolocation to create maps. The magnetometer evaluates the magnetic presence generally every second, or one hertz, but can be calibrated to measure at different speeds depending on the study. The readings will be consistent until the device detects ferrous material. This could range from a ship's hull to ferrous basalt at the seafloor. The sudden change in magnetism can be analyzed on the magnetometer's display.
The benefit to a magnetometer compared to sonar devices is its ability to detect artifacts and geological features on top and underneath the seafloor. Because the magnetometer is a passive sensor, and does not emit waves, its exploration depth is unlimited. Although, in most studies, the resolution and certainty of the data collected is dependent on the distance from the device. The closer the device is to a ferrous object, the better the data collected.
Plate tectonics
Plate tectonics is a scientific theory developed in the 1960s that explains major land form events, such as mountain building, volcanoes, earthquakes, and mid-ocean ridge systems. The idea is that Earth's most outer layer, known as the lithosphere, that is made up of the crust and mantle is divided into extensive plates of rock. These plates sit on top of partially molten layer of rock known as the asthenosphere and move relative to each other due to convection between the asthenosphere and lithosphere. The speed at which the plates move ranges between 2 and 15 centimeters per year. Why this theory is so significant is the interaction between the tectonic plates explains many geological formations. In regards to marine geology, the movement of the plates explains seafloor spreading and mid-ocean ridge systems, subduction zones and trenches, volcanism and hydrothermal vents, and more.There are three major types of tectonic plate boundaries; divergent, convergent, and transform boundaries. Divergent plate boundaries are when two tectonic plates move away from each other, convergent plate boundaries are when two plates move towards each other, and transform plate boundaries are when two plates slide sideways past each other. Each boundary type is associated with different geological marine features. Divergent plates are the cause for mid-ocean ridge systems while convergent plates are responsible for subduction zones and the creation of deep ocean trenches. Transform boundaries cause earthquakes, displacement of rock, and crustal deformation.