Geological Makeup of Marine Environments
The Continental Shelf
Most continents extend far beyond the point where ocean meets land. Their extended perimeter is referred to as the continental shelf. The ocean becomes very deep at the steep slope called the shelf break where the continental shelf ends and the abyssal plain, (or flat ocean floor) begins. Most shelves have a width of approximately 80 km and can measure anywhere between 30 m to 600 m in depth.
The largest shelf, the Siberian shelf in the Arctic Ocean, stretches 1,500 km wide. Another large shelf is found in the South China Sea called the Sundra shelf. This shelf joins Borneo, Sumatra, and Java to the Asian mainland. The North Sea and the Persian Gulf also contain shelves. Some geographical areas do not have a continental shelf. This common where the leading edge of moving oceanic plates is found beneath the continental crust. There are no shelves off the coast of Chile or the west coast of Sumatra.
Continental shelves are places of great biodiversity and marine life due to the relative abundance of sunlight available in their shallow waters. In contrast, the abyssal plain has been described as a biotic desert. Shelves eventually become a source of fossil fuels if oxygen poor conditions in sedimentary deposits continue over long periods of time.
The shelf is the most familiar and well-understood area of the ocean floor to humans due to its great diversity of life. Fish species that inhabit continental shelves are quickly becoming overexploited because they are more easily accessible than deep water species. Continental shelves are also valuable for oil and gas exploration. Consequently, marine nations claimed sovereign rights to their continental shelves in the Convention on the Continental Shelf created by the UN’s International Law Commission in 1958.
The law was partially overridden by the 1982 United Nations Convention on the Law of the Sea.
The Mid-Atlantic Ridge
The Mid-Atlantic ridge runs from Iceland to Antarctic and is the longest underwater mountain range on Earth. The ridge was formed by an oceanic rift separating the North American Plate from the Eurasian Plate in the North Atlantic Ocean. In the South Atlantic, the Mid-Atlantic ridge separates the South American Plate from the African Plate. The Mid-Atlantic ridge sits atop of the highest point of the mid-Atlantic rise, a bulge in the ocean floor where upward convective forces in the asthenosphere push up the oceanic crust and lithosphere. The discovery of the Mid-Atlantic ridge in the 1950s by Bruce Heezen led to the theory of seafloor spreading and the acceptance of Wegener’s theory of continental drift. The Mid-Atlantic ridge runs along plates that become increasingly more separated according to plate tectonics, a theory developed to explain continental drift.
There is constant movement in the ocean floor due to tectonic plates shifting, submerging deeper, or, as in the case of the Mid-Atlantic ridge, moving away from each other. In the area left behind, new crust is created when magma pushes up from the mantle. The rate of spreading is about 2.5 cm (~1 inch) per year or 25 km in one million years. Although this rate is relatively slow to humans, in terms of geologic time the plates have moved thousands of kilometers. A good example of seafloor spreading is the Atlantic Ocean, which has transformed from a small inlet between Europe, Africa, and the Americas into the enormous ocean it is today (» read more).
The Theory of Plate Tectonics states that plates make up the outer layer of the Earth and have slowly moved long distances throughout the history. This theory explains how the continents once fit together in a single continent called Pangaea. The movement of continents explains how animals became separated onto different continents, and it explains how mountains, volcanoes, and ocean trenches were formed, and why earthquakes occur. The underlying theory behind plate tectonics is that the force of gravity is stronger on a heavy, cooled ocean floor than it is on a hotter and lighter floor. The boundaries at which plate tectonics occur are: convergent boundaries, divergent boundaries, collisional boundaries, and transform boundaries. Crust is formed at divergent boundaries and consumed at convergent boundaries. Crustal plates collide at collisional boundaries and slide against each other at transitional boundaries.
The Theory of Plate Tectonics came about in the 1960s to explain seafloor spreading and continental drift. Around 1915 a scientist named Alfred Wegener published the first edition of “The Origin of Continents and Oceans” in which he proposed that the shapes of the east coast of South America and the west coast of Africa indicate that may have been attached at some point in history.
In 1962, American geologist Harry Hess suggested that, instead of continents moving through the ocean crust, an entire ocean basin and its connected continent actually moved together as a plate. Once the Theory of Plate Tectonics was accepted, a multitude of questions were explained and a scientific revolution occurred in geophysics and geology.
Other scientific phenomena were explained by plate tectonics as well, such as how the collisions of converging plates had enough force to lift the sea floor into thinner atmospheres.
The Science of Plate Tectonics
Tectonic plates are solid bodies of rock floating on top of the asthenosphere, an area that is partially molten. The plates themselves are what make up the lithosphere, which is the Earth’s crust and the solid portion of the upper mantle.
The lithosphere is cooler, heavier, and more rigid than the asthenosphere and is made up of seven large plates including: the African Plate, the Antarctic Plate, the Indo-Australian Plate, the Eurasian Plate, the North American Plate, the South American Plate, and the Pacific Plate.
Continental crust and oceanic crust behave differently due to their varying composition, so scientists named two types of lithosphere: the continental lithosphere and the oceanic lithosphere. Oceanic lithospheres are denser than continental lithospheres due to the high mafic mineral content in the ocean.
The Details of Plate Boundaries
As mentioned above, plates move in four different ways: convergent, divergent, collisional, and transform boundaries. As these moving plates meet along their boundaries, earthquakes are caused and volcanoes, mountains, and oceanic trenches are formed.
Convergent boundaries occur where two plates slide towards each other and form a subduction zone, where plates slide underneath each other or an orogenic belt (a.k.a. collisional boundary) where plates simply collide and compress. When a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is usually pushed underneath, forming a subduction zone where the ocean floor looks like an oceanic trench on the ocean side and a mountain on the continental side. A subduction zone is found on the western coast of South America where the oceanic Nazca Plate is in the process of subduction beneath the continental South American Plate. The continental spine of South America is dense with volcanoes. These volcanoes are formed by the transfer and heating (by friction) of organic material from the bottom, a process that releases many dissolved gases that can erupt to the surface.
Another example is the Cascade mountain range in North America, which extends north from California’s Sierra Nevada range. Volcanoes such as these are known for long periods of quiet and episodic eruptions starting with the expulsion of explosive gas containing fine particles of glassy volcanic ash and spongy cinders. A rebuilding of the pressure with hot magma follows this phase. The Pacific Ocean is completely surrounded by volcanoes; hence, it is called The Pacific Ring of Fire. Crumpling of both plates or compression of one plate occurs when two continental plates collide and one overrides the other. For example, the Himalayas were formed when an Indian subcontinental plate was thrust under part of the Eurasian plate. In Japan, it is common to see two oceanic plates converging to form an island arc as one plate is subducted under the other plate.
Divergent boundaries occur where two plates slide apart. The space created here is filled up with crust newly brought up from molten magma below. The East African Great Rift Valley is an example of a rift formed by a divergent boundary. Divergent boundaries most likely form at hotspots where convective cells bring large quantities of molten material from the asthenosphere up and there is enough kinetic energy to break through the lithosphere. The Mid-Atlantic Ridge is thought to have come from a hotspot that widens a few centimeters every century. Hotspots may be a future source of energy as they are thought to be an abundant source of hydrogen. Countries like Iceland are actively researching geothermal energy as a source for the world’s first hydrogen economy.
Fracture zones or fault zones in the oceanic ridge system are created by divergent boundaries due to a non-uniform rate of spreading. These fracture zones result in many submarine earthquakes and appear on a map as patterns divided by lines perpendicular to the ridge line. The patterns are caused by a conveyor belt motion away from the center of divergence.
A major piece of evidence supporting the theory of sea-floor spreading was found at the mid-ocean ridges when airborne geomagnetic surveys revealed an odd pattern of symmetrical polar magnetic reversals on opposite sides of the center of each ridge. These reversals corresponded directly with the Earth’s polar reversals and the theory that the sea floor moves in huge plates was confirmed. Additional evidence was supplied by measuring the ages of rocks in each band. With all the evidence combined, it was obvious that huge plates moved away from each other and collided. With the new information it was even possible to create a detailed map of the rate of spreading.
Transform boundaries are formed when plates grind past each other along transform faults. Huge plates grinding past each other cause immense friction and the effects of the stress build-up are highly visible. With transform boundaries, stress builds up in both plates until they reach the slipping point where the built-up potential energy is released in the form of a motion along the fault line. In many cases this energy becomes an earthquake. The San Andreas Fault is an example of a transform boundary where the movement of the Pacific and North American plates builds potential energy released in the form of earthquakes.
The Movement of Oceanic Plates
Tectonic plates are able to move because they float on the relatively fluid asthenosphere. The source of energy for the movement of tectonic plates is thought to be the loss or dissipation of heat from the mantle of the Earth. Dissipation of heat from the mantle is converted into two forces, the force of friction and the force of gravity.
The transmission of convection currents in the mantle through the asthenosphere is driven by the force of friction occurring between the asthenosphere and the lithosphere known as mantle drag. Trench suction occurs when local convention currents pull plates at subduction zones and ocean trenches downward with the force of friction.
Gravity is another force in the movement of tectonic plates. There are several different types of plate motion caused by gravity including ridge-push plate motion and slab-pull plate motion. With ridge-push plate motion plates at oceanic ridges are higher in elevation and are prone to sliding down due to the force of gravity. The name is actually not representative of what is actually happening, since there is no pushing going on but rather the sliding down of plates. The underlying cause of motion is upwelling from the convection occurring in the mantle, and this is what triggers the sliding of plates. Slab-pull plate motion is the other gravitational force and is the result of cold, dense plates sinking into the mantle at places where there is a trench.
The idea that convection, or the circulation of liquid or gas, occurs within the mantle is supported by strong scientific evidence. Scientists are almost certain that the upwelling of materials particularly around the mid-ocean ridges is caused by convection. It is still not clear what forces move tectonic plates. At first, it was thought that the force of friction between the asthenosphere and the lithosphere was key and that plates sit on top of huge convection cells like conveyor belts. As more data was gathered it became clear that the force of friction was not significant enough to drive the motion of tectonic plates alone. Now it is thought that slab-pull is the strongest force and trench suction follows closely in significance.
The Movement of Plates
Over long periods of time, geologic in scale (approx. every 500 million years), supercontinents form and break up. For example, the supercontinent known as Rodinia formed approximately a billion years ago and was the starting material for all the current Earth’s continents. About 750 million years ago, this supercontinent broke up into eight different continents which then formed two supercontinents called Laurasia (North America, Europe and Siberia/Asia) and Gondwana (China, India, Africa, South America, and Antarctica) about 350 million years ago which then reassembled about 275 million years ago to form what is known as the supercontinent Pangaea. Tectonic forces then began breaking Pangaea apart which continues today.
The Earth is not the only planet where plate tectonics can occur. It is thought from observations made in 1999 of the magnetic fields of Mars by the Global Surveyor spacecraft that plate tectonics may have once been at work on Mars.
Hydrothermal vents are fractures (cracks) in Earth’s surface where geothermally-heated water pulses through. Usually, a hydrothermal vent is found where hot magma is close to the surface crust as in volcanically active locations. The Earth has quite a few geothermal vents due to its geologically active nature and the vast amount of water present at the surface. Hot springs, fumaroles, and geysers are all examples of geothermal vents on land. The Yellowstone National Park in the United States is home to a spectacular display of hydrothermal vents.
Hydrothermal vents in the ocean floor are called submarine hydrothermal vents or black smokers and were discovered in 1977 around the Galapagos Islands by the National Oceanic and Atmospheric Administration using a small submersible called Alvin. They are usually hundreds of meters wide and are formed when water heated geothermally to extreme temperatures (up to 400° C) rises through the ocean floor. The water never boils because of the extreme pressure it is under at that depth. They contain many dissolved minerals like sulfides that usually crystallize into a chimney-shaped structure. The black color comes from the precipitation of minerals when the cold ocean water and the superheated water collide. These black smokers exist in the Pacific and Atlantic Oceans usually at about 2,100 m in depth.
The areas around black smokers contain entire complex communities of organisms who survive using chemicals dissolved in the fluids of the vents. These waters are too deep for sunlight to penetrate, so the vent communities rely on chemosynthesis instead of photosynthesis where heat, methane, and sulfur compounds are converted into energy. At the base of the food chain are chemosynthetic microorganisms that support entire communities of giant tubeworms, clams, and shrimp. Other planets that may have hydrothermal vents are Mars and Europa.
Fascinating new species are always being discovered in and around black smokers. The Pompeii worm (Alvinella pompejana) is an animal living exclusively in hydrothermal vents of the Pacific Ocean. This worm was discovered in the early 1980s by French researchers and is known now to be the “hottest animal on Earth” and an example of an extremophile (or an animal that can live in these extreme environments). In 2001, the scaly-foot gastrod (Crysomallon squamiferum), was discovered during an expedition to the Indian Ocean’s Kairei hydrothermal vent field. This animal utilizes pyrite and greigite, iron sulfide compounds, to build it’s hardened body parts instead of the usual calcium carbonate. The armor plating is most likely used to defend the animal from predatory snails with huge teeth in the same area. It is thought that iron sulfide is stabilized by the extreme pressure of 2,500 m ocean water allowing the compound to be used by organisms.
The Lost City
In December 2000, a series of hydrothermal vents were discovered during a National Science Foundation expedition to the mid-Atlantic ocean that are very different from the black smoker hydrothermal vents found in the 1970s. A subsequent expedition in 2003 used Alvin to explore these vents and the details of data recovered were published in March 2005. What is being called The Lost City is a series of vents located on the seafloor mountain Atlantis Massif, where hydrogen rich fluids and methane are produced by reactions between seawater and the upper mantle peridotite. The fluid produced here is highly basic (pH 9-11) with temperatures that range from 40° to 90° C. The Lost City consists of about 30 chimneys composed of calcium carbonate and standing 30-60 m tall in addition to many smaller chimneys.
The vents found at the Lost City release methane and hydrogen into the ocean water and in contrast to black smokers, they do not release a lot of carbon dioxide, hydrogen sulfide or metals. The Lost City is also much older than black smoker vents, an observation made possible using strontium, carbon and oxygen isotopic data and radiocarbon ages to document 30,000 years of hydrothermal activity. Consequently, organisms living in Lost City vents are completely different from those living near black smokers; hence, Lost City vents do not contain a lot of chemosynthetic microorganisms found near volcanically driven black smokers. The animals that do live near Lost City vents consist of small invertebrates with carbonate structures like snails, bivalves, polychaetes, amphipods and ostracods. Microorganisms living here include: Methanosarcina-like archaea and bacteria related to the Firmicutes. In addition to the variety of scientifically interesting chemosynthetic organisms supported by black smokers, the Lost City provides scientists with a model of an ecosystem driven by abiotic methane and hydrogen. A large portion of scientific research into whether life exists on other planets, relies on the study of extremophiles like these here on Earth.
A cold seep or cold vent is a vent that is not superheated but hydrogen sulfide, methane and hydrocarbon-rich fluid still seep out into the surrounding water. The reactions between seawater and methane create carbonate rock formations and reefs over time. Cold seeps do not work in short, unpredictable bursts like many hydrothermal vents but are instead slow and dependable.
Light independent organisms exist in entire communities in and around cold seeps. The organisms that live in cold seeps usually live longer than those at hydrothermal vents due to the relative stability of resources. The longest-lived noncolonial invertebrate is a cold seep tubeworm that lives from 170-250 years. Most of the organisms living in cold seeps are reliant on a symbiotic (each benefits from the other) relationship with chemoautotrophic bacteria, organisms that process sulfides and methane through chemosynthesis into chemical energy. The chemoautotrophic bacteria are classified into archaea and eubacteria and power the lives of larger organisms like vesicomyid clams and vestimentiferan tubeworms. In exchange, the clams and tubeworms provide a safe haven for the bacteria and also a source of food. The entire structure and formation of the reefs and rock formations could be dependent on bacterial reactions. The strategy of some bacteria in cold seeps is to create mats, covering the ocean floor. One example is the beggiatoal bacterial mat in Blake Ridge, off the coast of South Carolina.
Dr. Charles Paull is credited with the discovery of cold seeps in 1984 at a depth of 3,200 m in the Gulf of Mexico, although the deepest cold seep found is located in the Sea of Japan at a depth of 5,000-6,500 m. Since then, additional cold seeps have been found in the Gulf of Mexico, the Sea of Japan, and in waters off the Alaska’s coast.