The Ocean Floor Geology
by Owen Borville
January 30, 2021
Learning, Geology, Science
The continental margin is the ocean floor that is between the shore line and the abyssal ocean floor, including the continental shelf, the continental slope and the continental rise. An active continental margin is the leading edge of a continental tectonic plate where an oceanic plate is being subducted underneath the continental plate. An active margin is characterized by earthquakes and volcanic activity. An example is the western continental margin of South America. A passive continental margin is a tectonically inactive continental margin characterized by a lack of earthquakes and volcanic activity. An example is the eastern continental margin of South America.
The continental shelf is the shallowest part of the continental margin and near the shoreline. The continental slope is the very steep or tall section of the continental margin connecting the continental shelf to the continental rise. The continental rise is the part of the continental margin where the ocean floor begins to rise in elevation and is adjacent to the continental slope.
Turbidites are sea bottom sediment deposits formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking, excessive sedimentation load, and storms. Turbidite sequences are deposited with the coarser sediment on the bottom and progressively finer grained sediment toward the top. These are also called Bouma sequences. Turbidity currents are density currents of sediment-rich turbulent water that produce turbidites.
Submarine canyons are deep V-shaped valleys located on continental shelves and slopes. These canyons are thought to be associated with past river valleys when sea level was lower. Submarine fans are fan-shaped deposits of sediment formed as a result of a direct connection to an adjacent submarine canyon or large river delta and thus are found immediately adjacent to continental areas. Submarine fans are supplied by turbidity currents. Submarine fans are similar to alluvial fans except in that a submarine fan is under water.
The abyssal plain is a flat region that lies beyond the continental rise on the ocean floor. This landscape is formed as layer upon layer of sediment settles on the ocean floor. This sediment is termed pelagic and consists of organic as well as inorganic matter. Pelagic sediment can be in the form of non-organic clay sized sediment or ooze, which is a combination of fine grained fragments of biologic material and fine grained sediment. Ooids are tiny concentric spherules of calcium carbonate that generally precipitate in warm, high-energy, and very shallow marine waters. When cemented, ooids form oolite or oolith, which is a limestone. Pisoliths (pisolites) are small round concretions greater than two millimeters in diameter and commonly consist of calcium carbonate.
Oceanic trenches are located just above a subduction zone and consist of two tectonic plates converging in the ocean and include the greatest depths in the oceans. Oceanic trenches are found mostly in the Pacific Ocean and a few are located in the Atlantic Ocean. An oceanic ridge is a rifting plate boundary along the ocean bottom where molten rock material from the mantle comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies. Aseismic ridges are long ridges or mountain ranges on the ocean floor that are not seismically active and are produced commonly by hot spot volcanism.
Seismic profiling is a method used to study the seafloor by using seismic waves. Seismic waves are emitted from the ocean surface, reflected from the seafloor, and return back to the surface where the wave travel times are recorded. The results of the seismic profiling allow geologists to calculate the ocean depths and create a view of the topography of the ocean floor including oceanic ridges and trenches.
A hydrothermal vent is a crack in the Earth’s surface where hot water is released and is usually located near a volcanic area on land, an ocean basin, or a hot spot. Submarine hydrothermal vents are associated with diverse biological communities and metallic ore deposits are precipitated when ocean water is heated and reacts with surrounding rock.
Other ocean floor features include seamounts, which are volcanoes scattered across the deep ocean floor that do not reach the sea level. Guyots are volcanoes that originally built up from the ocean floor above sea level. Over time, erosion by waves destroyed the tops of these volcanoes and resulted in a flattened peak or top. Due to the movement of the ocean floor away from oceanic ridges, the flattened guyots were eventually submerged to become undersea flat-topped peaks. Evidence that the tops of guyots were once at the surface is the presence of fossils such as coral reefs that only live in shallow water.
Deep-sea sediments are those found at ocean depths greater than 500 meters and cover roughly 67 percent of the Earth’s surface. These sediments consist of clays and oozes. Pelagic sediment exists on the deep ocean seafloor and can be in the form of non-organic clay sized sediment or ooze, which is a combination of fine grained fragments of biologic material and fine grained sediment. Oozes are pelagic deep-sea sediments containing biogenic skeletons, silt, and clay. Siliceous ooze is silica-rich deep sea sediment consisting of the skeletal remains of diatoms and radiolaria, which are microscopic single-cell organisms.
A reef commonly refers to an accumulation of dead skeletons of marine organisms in shallow tropical water. A reef can also refer to non-biological rock accumulations in shallow water. Coral reefs are very diverse marine organisms composed of calcium carbonate and are wave resistant structures. The growth of these structures is aided by algae, sponges, and other organisms, combined with a number of cementation processes. A fringing reef is a reef that is attached to the shore of an island or continent with no open water lagoon between the reef and the shore. Barrier reefs are reefs separated from the shoreline by a moderately deep body of water such as a lagoon. An atoll is an island in which coral reef has built up around the island edge while the island gradually sinks into the ocean over a period of time.
by Owen Borville
January 30, 2021
Learning, Geology, Science
The continental margin is the ocean floor that is between the shore line and the abyssal ocean floor, including the continental shelf, the continental slope and the continental rise. An active continental margin is the leading edge of a continental tectonic plate where an oceanic plate is being subducted underneath the continental plate. An active margin is characterized by earthquakes and volcanic activity. An example is the western continental margin of South America. A passive continental margin is a tectonically inactive continental margin characterized by a lack of earthquakes and volcanic activity. An example is the eastern continental margin of South America.
The continental shelf is the shallowest part of the continental margin and near the shoreline. The continental slope is the very steep or tall section of the continental margin connecting the continental shelf to the continental rise. The continental rise is the part of the continental margin where the ocean floor begins to rise in elevation and is adjacent to the continental slope.
Turbidites are sea bottom sediment deposits formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking, excessive sedimentation load, and storms. Turbidite sequences are deposited with the coarser sediment on the bottom and progressively finer grained sediment toward the top. These are also called Bouma sequences. Turbidity currents are density currents of sediment-rich turbulent water that produce turbidites.
Submarine canyons are deep V-shaped valleys located on continental shelves and slopes. These canyons are thought to be associated with past river valleys when sea level was lower. Submarine fans are fan-shaped deposits of sediment formed as a result of a direct connection to an adjacent submarine canyon or large river delta and thus are found immediately adjacent to continental areas. Submarine fans are supplied by turbidity currents. Submarine fans are similar to alluvial fans except in that a submarine fan is under water.
The abyssal plain is a flat region that lies beyond the continental rise on the ocean floor. This landscape is formed as layer upon layer of sediment settles on the ocean floor. This sediment is termed pelagic and consists of organic as well as inorganic matter. Pelagic sediment can be in the form of non-organic clay sized sediment or ooze, which is a combination of fine grained fragments of biologic material and fine grained sediment. Ooids are tiny concentric spherules of calcium carbonate that generally precipitate in warm, high-energy, and very shallow marine waters. When cemented, ooids form oolite or oolith, which is a limestone. Pisoliths (pisolites) are small round concretions greater than two millimeters in diameter and commonly consist of calcium carbonate.
Oceanic trenches are located just above a subduction zone and consist of two tectonic plates converging in the ocean and include the greatest depths in the oceans. Oceanic trenches are found mostly in the Pacific Ocean and a few are located in the Atlantic Ocean. An oceanic ridge is a rifting plate boundary along the ocean bottom where molten rock material from the mantle comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies. Aseismic ridges are long ridges or mountain ranges on the ocean floor that are not seismically active and are produced commonly by hot spot volcanism.
Seismic profiling is a method used to study the seafloor by using seismic waves. Seismic waves are emitted from the ocean surface, reflected from the seafloor, and return back to the surface where the wave travel times are recorded. The results of the seismic profiling allow geologists to calculate the ocean depths and create a view of the topography of the ocean floor including oceanic ridges and trenches.
A hydrothermal vent is a crack in the Earth’s surface where hot water is released and is usually located near a volcanic area on land, an ocean basin, or a hot spot. Submarine hydrothermal vents are associated with diverse biological communities and metallic ore deposits are precipitated when ocean water is heated and reacts with surrounding rock.
Other ocean floor features include seamounts, which are volcanoes scattered across the deep ocean floor that do not reach the sea level. Guyots are volcanoes that originally built up from the ocean floor above sea level. Over time, erosion by waves destroyed the tops of these volcanoes and resulted in a flattened peak or top. Due to the movement of the ocean floor away from oceanic ridges, the flattened guyots were eventually submerged to become undersea flat-topped peaks. Evidence that the tops of guyots were once at the surface is the presence of fossils such as coral reefs that only live in shallow water.
Deep-sea sediments are those found at ocean depths greater than 500 meters and cover roughly 67 percent of the Earth’s surface. These sediments consist of clays and oozes. Pelagic sediment exists on the deep ocean seafloor and can be in the form of non-organic clay sized sediment or ooze, which is a combination of fine grained fragments of biologic material and fine grained sediment. Oozes are pelagic deep-sea sediments containing biogenic skeletons, silt, and clay. Siliceous ooze is silica-rich deep sea sediment consisting of the skeletal remains of diatoms and radiolaria, which are microscopic single-cell organisms.
A reef commonly refers to an accumulation of dead skeletons of marine organisms in shallow tropical water. A reef can also refer to non-biological rock accumulations in shallow water. Coral reefs are very diverse marine organisms composed of calcium carbonate and are wave resistant structures. The growth of these structures is aided by algae, sponges, and other organisms, combined with a number of cementation processes. A fringing reef is a reef that is attached to the shore of an island or continent with no open water lagoon between the reef and the shore. Barrier reefs are reefs separated from the shoreline by a moderately deep body of water such as a lagoon. An atoll is an island in which coral reef has built up around the island edge while the island gradually sinks into the ocean over a period of time.
