Physical Features of the Ocean Floor: Trenches, Vents, Mountains

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ocean floor map
Hydrography is the science that measures and describes the physical features of bodies of water and the land areas adjacent to those bodies of water, with an emphasis on the navigable portion of the Earth's surface. Hydrographic surveyors study these bodies of water to see what the "floor" looks like. In 1807, U.S. President Thomas Jefferson established the U.S. Coast Survey, the predecessor to NOAA, to conduct hydrographic surveys and create nautical charts of the young nation’s ports and waterways.[Source: NOAA]

Scientists and governments conducts hydrographic surveys to measure the depth and bottom configuration of water bodies. That data is used to update nautical charts and develop hydrographic models. This information is vital to navigating the ocean and our nation's waterways. Hydrographical surveys are also provide information for a number of purposes, including seafloor structural construction, laying pipelines and cables, dredging, anchoring and understanding fish habitats.

Hydrographers measure water depth, and search for shoals, rocks, & wrecks that could be hazards to navigation. They also collect information on water level & tides, currents, temperature and salinity, produce nautical charts, essential maps for safe marine navigation, and construct hydrographic models as baseline data for research and marine geospatial products and services. With only 5 percent of the ocean explored, many more scientific discoveries await to be made in this unknown frontier of our planet Earth.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium

Ocean Floor Physical Geography

volcanic eruption on a beach on Iwo Jima. Japan
Beneath the ocean surface is an underwater landscape as complex as anything you might find on land. The ocean has an average depth of 3.7 kilometers (2.3 mile) and the shape and depth of the seafloor is complex. Some features, like canyons and seamounts, might look familiar, while others, such as hydrothermal vents and methane seeps, are unique to the deep.

The seas cover the world’s deepest valleys, longest mountain ranges and most of the active volcanoes. Deep-sea trenches are the deepest points on the ocean. They are located at subduction zones often where continental and oceanic plate meet. More than 70 percent of the world’s oceans are over 1,000 meters deep. Fathoms used to be used to measure depth. One fathom equals about 6 feet or 1.822 meters.

The volume of the water in the ocean exceeds the volume of the ocean basins, and some water spills over on to the low lying areas of the continents. These shallow seas are the continental shelves. Some, such as the South China Sea, are more than 1100 kilometers wide. Most are relatively shallow, with typical depths of 50–100 meters. A few of the more important shelves are: the East China Sea, the Bering Sea, the North Sea, the Grand Banks, the Patagonian Shelf, the Arafura Sea and Gulf of Carpentaria, and the Siberian Shelf. The shallow seas help dissipate tides, they are often areas of high biological productivity, and they are usually included in the exclusive economic zone of adjacent countries.

Plate Tectonics and the Ocean

Bathymetry, the shape of the ocean floor, is largely a result of a process called plate tectonics. The outer rocky layer of the Earth includes about a dozen large sections called tectonic plates that are arranged like a spherical jig-saw puzzle floating on top of the Earth's hot flowing mantle. Convection currents in the molten mantle cause the plates to slowly move about the Earth a few centimeters each year. Many ocean floor features are a result of the interactions that occur at the edges of these plates. [Source: NOAA]

The shifting plates may collide (converge), move away (diverge) or slide past (transform) each other. As plates converge, one plate may move under the other causing earthquakes, forming volcanoes, or creating deep ocean trenches. Where plates diverge from each other, molten magma flows upward between the plates, forming mid-ocean ridges, underwater volcanoes, hydrothermal vents, and new ocean floor crust. Transform boundaries are faults that connect two areas where plates are converging or diverging. The edges of these continental boundaries usually form zig-zag patterns.

Robert Stewart wrote in the “Introduction to Physical Oceanography”: Earth’s rocky surface is divided into two types: oceanic, with a thin dense crust about 10 kilometers thick, and continental, with a thick light crust about 40 kilometers thick. The deep, lighter continental crust floats higher on the denser mantle than does the oceanic crust, and the mean height of the crust relative to sea level has two distinct values: continents have a mean elevation of 1100 meters, the ocean has a mean depth of -3400 meters. New crust is created at the mid-ocean ridges, and old crust is lost at trenches. The relative motion of crust, due to plate tectonics, produces the distinctive features of the sea floor including mid-ocean ridges, trenches, land arcs, and basins. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

Sea-Floor Features

The names of the sub-sea features have been defined by the International Hydrographic Organization. They include: 1) Basins, deep depressions of the sea floor of more or less circular or oval form; 2) Ridges, long, narrow elevations of the sea floor with steep sides and rough topography; 3) Sills, the low parts of the ridges separating ocean basins from one another or from the adjacent sea floor; 4) Canyons, relatively narrow, deep furrows with steep slopes. Canyons are common on shelves, often extending across the shelf and down the continental slope to deep water. [NOAA, Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

5) Continental Shelves are zones adjacent to a continent (or around an island) and extending from the low-water line to the depth, usually about 120 meters, where there is a marked or rather steep descent toward great depths. Continental slopes are the declivities seaward from the shelf edge into greater depth. The continental shelf may be narrow or nearly nonexistent in some places; in others, it extends for hundreds of kilometers. The waters along the continental shelf are usually productive, both from light and nutrients from upwelling and runoff.

6) Abyssal Plains are flat surfaces found in many deep ocean basins located between continental slopes and mid-ocean ridges. At depths of over 3000 meters (10,000 feet) and covering 70 percent of the ocean floor, abyssal plains are the largest habitat on earth and the smoothest surfaces on the planet. Sunlight does not penetrate to the sea floor, making these deep, dark ecosystems less productive than those along the continental shelf. But despite their name, these “plains” are not uniformly flat. They are interrupted by features like hills, valleys, and seamounts.

7) Mid-Ocean Ridge is an underwater mountain range, over 65,000 kilometers (40,3900 miles) long, rising to an average depth of 2,480 meters (8,000 feet) (See World’s Longest Mountain Range Below), Rising up from the abyssal plain, it embraces a system of underwater volcanoes that circle the globe longitudinally from north to south

8) Trenches are long, narrow, and deep depressions of the sea floor, with relatively steep sides. They usually form at the subduction zone of tectonic plates. The Mariana Trench is the deepest place in the ocean at 11,035 meters (36,201 feet). A few sea creatures such xenophyophores, amphipods and sea cucumbers, live in the Mariana Trench as well as undiscovered species. If a person were to try to swim there his body would be crushed by the enormous pressure and he would die in seconds. The 2nd deepest part of the ocean is the Tonga Trench. Situated south-western Pacific Ocean near Tonga, it is 10.9 kilometers (35,702 feet) deep. The five deepest parts of ethe main are oceans are: 1) The Mariana Trench of the Pacific Ocean, 2) the Puerto Rico Trench of the Atlantic Ocean, 3) the Diamantina Trench of the Indian Ocean, 4) the Molly Deep of the Arctic Ocean and 5) the South Sandwich Trench of the Southern Ocean.

Oceanic basin features

9) Seamounts are underwater mountains that stop short of breaking the surface of the sea and becoming islands. Many are volcanoes. By one count there are over 100,000 of them in the world’s oceans (See Below). 10) A guyot is a seamount with a flat top created by wave action when the seamount extended above sea level. As the seamount is carried by plate motion, it gradually sinks deeper below sea level. Depths are determined from echo sounder data collected from ship tracks (thin straight lines) supplemented with side-scan sonar data.

According to the “Introduction to Physical Oceanography”: Sub-sea features strongly influences the ocean circulation. Ridges separate deep waters of the ocean into distinct basins. Water deeper than the sill between two basins cannot move from one to the other. Tens of thousands of seamounts are scattered throughout the ocean basins. They interrupt ocean currents, and produce turbulence leading to vertical mixing in the ocean

World’s Highest Mountains and Greatest Relief Differences

By some reckoning Hawaii’s Mauna Kea is the world's tallest mountain. From top to bottom it is over 10,000 meters (32,280 feet) — 4,207.3 meter (13,803 feet) above sea level and around 6,000 meters (19,685 feet) below the sea. Chimborazo is 6.4 million meter (21 million feet) or 6,384 kilometers (3,967 miles) from the center of the earth.

Mauna Kea on the island of Hawaii stands 10,205 meters (33,480 feet) above the ocean floor and 4,205 meter (13,796 feet) above sea level. Mount Everest in 8,849 meters (29,030 feet) high. According to the Guinness Book of Records, Chimborazo, a 6,267-meter (20,560 foot) -high volcano in Ecuador, is 2,150 meters (7,054 feet) further from the center of the earth than Mt. Everest. It's distance from the earth's center is a result of the fact that Chimborazo is only 160 kilometers (98 miles) from the equator (the earth is slightly flat at the poles and wide at the equator). Chimborazo was thought to be the highest mountain in the world until the 1850s.

According to the Guiness Book of Records the highest mountain face underwater is Monte Pico, of Azores Islands of Portugal: With an altitude of 2,351 meters (7,711 feet) above sea surface and 6,098 meter (20,000 feet) below sea surface to the sea floor, Monte Pico in the Azores Islands (Portugal) is the highest underwater face mountain in the world. The 42 kilometer-long island formed by this volcano is one of the nine islands that conform the Azores and it is also considered to be the youngest. I guess here Guinness doesn’t count Mauna Kea as a mountains face.

Okeanos Seamount

How about the highest mountains completely submerged in the sea. Brian Kinsey, an IT professor, posted on Tallest mountain on earth — base to peak — is Mauna Kea in Hawaii... If you want the tallest that is completely underwater, that would be the Vema Seamount, which lies in the Atlantic off the coast of S. Africa. It’s over 15,000 feet [4,572 meters] high, and tops out just a few meters below the surface. From another perspective from the bottom of the 11,035-meter-deep (36,205-foot-deep) Marianas Trench to a 4000-meter-deep (13,123-foot-deep) hill next to the Marianas Trench a relief difference of over 7,035 meters (20,080 feet)

How about the greatest relief difference on Earth. The Tonga Trench is the second deepest oceanic trench in the world at around 10,800 meters below sea level. The island of Tonga is not far away. The highest mountains on Tonga is 1,030-meter-high (3,380-feet-high) Kao. From the bottom of the trench to the top of Kao is relief difference of 11,030 meters (36,188 feet). That is over one kilometers greater than the height of Mauna Kea from the ocean floor. The same reasoning can be applied to the West Coast of South America for two points further apart. The Peru–Chile Trench is located around 160 kilometers off the coast of Peru and Chile. It has a maximum depth of 8.06 kilometers. Aconcagua, the highest mountains in the America, is located on the Chile- Argentina about 250 kilometers away from the Peru-Chile Trench. Aconcagua is 6,960.8 meters (22,837 feet) high. The relief between it the deepest part of the Peru-Chile Trench is almost 15,000 meters (49,212 feet), about 10,000 higher than Mt. Everest.

World’s Longest Mountain Range

The longest mountain range on Earth is called the Mid-Ocean Ridge. Spanning 65,000 kilometers (40,390 miles) around the globe, it's truly a global marvel. About 90 percent of the mid-ocean ridge system is under the ocean. This system of mountains and valleys criss-crosses the globe, resembling the stitches in a baseball. It's formed by the movement of the Earth's tectonic plates.

As the great plates push apart, mountains and valleys form along the seafloor as magma rises up to fill the gaps. As the Earth's crust spreads, new ocean floor is created. This process literally renews the surface of our planet. [Source: NOAA]

If you look at a map of the world's volcanoes, you'll find that most of them form along the boundaries of this great system. In fact, the global mid-ocean ridge system forms the largest single volcanic feature on the Earth. The mid-ocean ridge consists of thousands of individual volcanoes or volcanic ridge segments which periodically erupt. The mid-ocean ridge is visible in this satellite imagery that captures bathymetric data

Atlantic Ocean crust

The 16,000-kilometer-long (10,000-mile-long) Mid-Atlantic Ridge (MAR) divides the Atlantic longitudinally into two halves. The longest mountain range in the world, the MAR rises two to three kilometers (1.2–1.9 miles) above the surrounding ocean floor. It s rift valley is the divergent boundary between the North American and Eurasian tectonic plates in the North Atlantic and the South American and African plates in the South Atlantic. The MAR produces basaltic volcanoes such as Eyjafjallajökull, Iceland, and produces pillow lava on the ocean floor. [Source: Wikipedia]

Earth’s Largest Waterfall — Is in the Ocean

The world’s largest waterfall is in the ocean beneath the Denmark Strait. Here, outhward-flowing frigid water from the Nordic Seas meets warmer water from the Irminger Sea. The cold, dense water quickly sinks below the warmer water and flows over the huge drop in the ocean floor, creating a downward flow estimated over 123 million cubic feet per second. [Source: NOAA]

Rivers flowing over Earth’s gorges create waterfalls that are natural wonders, drawing millions of visitors to their breathtaking beauty, grandeur, and power. But no waterfall is larger or more powerful than those that lie beneath the ocean, cascading over immense cataracts hidden from our view.

The Denmark Strait separates Iceland and Greenland. At the bottom of the strait are a series of cataracts that begin 610 meters (2,000 feet) under the strait’s surface and plunge to a depth of 3050 meters (10,000 feet) at the southern tip of Greenland—nearly a two-mile drop.

But how can there be waterfalls in the ocean? It’s because cold water is denser than warm water, and in the Denmark Strait, southward-flowing frigid water from the Nordic Seas meets warmer water from the Irminger Sea. The cold, dense water quickly sinks below the warmer water and flows over the huge drop in the ocean floor, creating a downward flow. Because it flows beneath the ocean surface, however, the massive turbulence of the Denmark Strait goes completely undetected without the aid of scientific instruments.

Warmer surface waters flow northward. These warmer waters gradually lose heat to the atmosphere and sink. Denser, cold water flows southward in a deep current along the sea floor over an undersea ridge in the Strait. The height of the Denmark Strait cataract is approximately 3,500 meters (11,500) feet. By comparison, the largest waterfall on land is 980 meters (3,212 feet).

“Leak” in the Bottom of the Ocean?

In 2023, scientists announced the discovery of a “leak” in sea floor off the coast of the Pacific Northwest. The “leak — technically a spring, known as Pythia’s Oasis — is likely venting water from beneath tectonic plates through the Cascadia Subduction Zone fault. Researchers worry the liquid it could increase the likelihood of a destructive earthquake as it is likely acting as a lubricant between the two plates colliding at the fault. [Source: Jackie Appel, Popular Mechanics, April 17, 2023]

Popular Mechanics reported: Technically, it’s a spring, because water is flowing in and not out. But in the ways that matter, it definitely is a leak. It’s a spring of almost-fresh water most welling up from under the ocean floor. It was accidentally discovered by then-grad-student-now-White-House-policy-advisor Brendan Philip—who spotted the bubbles that the spring carried to the surface—and a study on the vent was released by Philip and the rest of the research team from the University of Washington. “They explored in that direction and what they saw was not just methane bubbles, but water coming out of the seafloor like a firehose. That’s something that I’ve never seen, and to my knowledge has not been observed before,” Evan Solomon, a seafloor geologist and one of the authors on the paper, said.

The Cascadian Subduction Zone is a large strike-slip fault off the coast of the Pacific Northwest. That’s where two of the tectonic plates that make up the Earth’s crust meet up and slide alongside each other. And the reserve of water bubbling up from Pythia’s Oasis acts as lubrication between these two plates. “The megathrust fault zone is like an air hockey table,” Solomon said in a news release. “If the fluid pressure is high, it’s like the air is turned on, meaning there’s less friction and the two plates can slip. If the fluid pressure is lower, the two plates will lock – that’s when stress can build up.”

And therein lies the issue. If stress starts to build up, it eventually has to go somewhere. When the stress is too much and the system has to jerk into a new position, the jerk triggers an earthquake. Most likely, a big one. Scientists believe a release of stress in the Cascadia Subduction Zone could trigger a magnitude-9 earthquake that would affect many of those living in the Northwestern U.S. “Pythias Oasis provides a rare window into processes acting deep in the seafloor, and its chemistry suggests this fluid comes from near the plate boundary,” Deborah Kelley, an oceanographer and one of the authors on the study, said

Hydrothermal Vents

Hydrothermal vents — that spew out water hotter than 260 degrees C (500 degrees F) and emit minerals such as hydrogen sulfide — are located is places in the ocean, often at great depths, where there is volcanic activity. Water seeps down through cracks in the sea floor and is heated by magma and rushes back to the surface through vents. Different chemical reactions occur where the mineral-laden vent water meets the cold water of the sea. The build up of sulfide minerals produces chimneys. Minerals nourish bacteria, which in turn feeds tube worms and mollusks that feed forms of life further up the food chain. The first ones were discovered near the Galapagos islands at a depth of 2,800 meters (9,200 feet) in 1977.

A venting black smoker emits jets of particle-laden fluids. The particles are predominantly very fine-grained sulfide minerals formed when the hot hydrothermal fluids mix with near-freezing seawater. These minerals solidify as they cool, forming chimney-like structures. “Black smokers” are chimneys formed from deposits of iron sulfide, which is black. “White smokers” are chimneys formed from deposits of barium, calcium, and silicon, which are white. [Source: NOAA]

Underwater volcanoes at spreading ridges and convergent plate boundaries produce hot springs known as hydrothermal vents. Scientists that discovered hydrothermal vents in 1977, while exploring an oceanic spreading ridge near the Galapagos Islands, were amazed to the vents were surrounded by large numbers of organisms that had never been seen before. These biological communities depend upon chemical processes that result from the interaction of seawater and hot magma associated with underwater volcanoes.

Hydrothermal vents form at locations where seawater meets magma. They are the result of seawater percolating down through fissures in the ocean crust in the vicinity of spreading centers or subduction zones (places on Earth where two tectonic plates move away or towards one another). The cold seawater is heated by hot magma and reemerges to form the vents. Seawater in hydrothermal vents may reach temperatures of over 700° Fahrenheit. Hot seawater in hydrothermal vents does not boil because of the extreme pressure at the depths where the vents are formed.

Previously, sunlight was thought to be the energy source that supported the base of every food web on our planet. But organisms in these deep, dark, ecosystems have no access to sunlight, and instead metabolize hydrogen sulfide or other chemicals in a process called chemosynthesis. Since this discovery, sonar, submarines, satellites, and robots have helped find and explore this type of ecosystem and other deep ocean formations around the globe.

Mapping the Ocean Floor

Historically, the sea floor was mapped by ships that took depth “soundings” by dropping a lead line off of a ship until it reached the bottom and the line went slack. The term “soundings” comes from the Old French “sonder”offsite link (to plumb) or Old English “sund”offsite link for swimming, water, or sea. But coincidentally, this work is now usually done using sound! Today, ships equipped with multibeam echo sounders are the primary method of gathering this information, although hydrographers use many other technologies as well. [Source: NOAA]

By mapping out water depth, the shape of the seafloor and coastline, the location of possible obstructions and physical features of water bodies, hydrography helps to keep our maritime transportation system moving safely and efficiently. Multibeam echo sounder beams sweep the seafloor as the ship passes over the survey area. Multibeam echo sounder beams bounce off the seafloor and return to the ship where the depth is recorded. Hydrography is also used to identify shipwrecks and preserve our maritime heritage.

Very little of the ocean floor has been mapped directly. So how do we make maps of the global ocean floor? It turns out that satellites can “see” below the sea surface. With careful processing, small differences in sea surface heights and gravity can reveal detailed maps of the seafloor.

age of rocks on ocean floor

World’s Deepest Trenches

The deepest parts of the ocean trenches, up to 11 kilometers (seven miles) deep, are in the “Hadal zone”, named after the god of the underworld. This region is often referred to as an “abyss” Greek for “bottomless void” or “pit of hell”. The deepest trenches — the deepest of the deep on the ocean floor — are:

1) Mariana Trench is 11,034 meters (36,201 feet) deep. Located in the western Pacific Ocean, south of Guam, it contains the Earth’s deepest point — Challenger Deep — and was formed by the collision of converging tectonic plates. At the collision point, one plate descends into the Earth’s mantle, and a trough has formed where the plates converge. See Below. [Source: Marine Insight]

2) Tonga Trench lies around 10,882 meters (35,702 feet) under the surface of the sea. Located 2,500 kilometers from New Zealand, northeast to the island of Tonga, in the southwest Pacific Ocean, the Tonga trench was formed due to the subduction of the Pacific plate by the Tonga plate at the Kermadec Tonga Subduction Zone’s northern end, The deepest point in the Tonga trench, known as the Horizon Deep, is considered the second deepest point on Earth after the Challenger Deep and the deepest trench in the Southern Hemisphere. Some very powerful volcanos like the one that erupted catastrophically in 2022 are located nearby,

3) Philippine Trench is 10,540 meters (34,580feet) below sea level. Also known as Mindanao Trench, this submarine trench is located in the Philippine Sea and extends for 1,320 kilometers and averages 30 kilometers in width in the east of the Philippines. The third deepest point in the world, the Galathea Depth, is in the Philippine trench,. For a long time it was thought the Philippine Trench was Earth deepest places. According to scientists, the Philippine trench around 8 million years old by the collision of the Eurasian plate and the smaller Philippine plate. Other trenches in the Philippine Sea include East Luzon Trench, Manila Trench, Sulu Trench, Negros Trench, and Cotabato Trench.

4) Kuril- Kamchatka Trench is 10,500 meters (34,450 feet) below sea level. . Lying close to Kuril Island and off the coast of Kamchatka, this trench is north of China and east of Russia. Many vert active volcanoes in the region. The trench was formed by a subduction zone that is tens of millions of year old and aslo created the Kuril island and the Kamchatka volcanic arcs.

5) Kermadec Trench is 10,040 meters (32,940 feet) below the surface of the sea. Situated on the floor of the South Pacific Ocean, it extends for around 1,000 kilometers between the Louisville Seamount Chain and the Hikurangi Plateau and was formed by the subduction of the Pacific plate under the Indo-Australian Plate. The Tonga Trench in the north and the Kermadec Trench in the south creates the 2,000 kilometer-long, near-linear Kermadec-Tonga subduction system. Among the sea creature seen in the trench are giant 34-centimeter-long amphipod. The Nereus, an unmanned research submarine, imploded because of the high pressure in the Kermadec Trench at a depth of 9,990 meters while conducting explorations at the.

6) Izu-Ogasawara Trench has a maximum depth of 9,7800 meters (32,066 feet). Located in the western Pacific Ocean and also known as Izu-Bonin Trench, this deep trench stretches from Japan to the northern Mariana Trench and is an extension of the Japan Trench.

7) Japan Trench is 9,000 meters (29,527 feet) below sea level. Ppart of the Pacific Ring of Fire in the northern Pacific Ocean, it stretches from the Kuril Islands to the Bonin Islands and is extension of the Kuril-Kamchatka Trench to the north and the Izu-Ogasawara Trench to the south. The trench was formed due to the subduction of the oceanic Pacific plate beneath the continental Okhotsk Plate. Movement along the the subduction zone here has produced some very strong earthquakes and tsunamis.

8) Puerto Rico Trench is 8, 640 meters (28,346 feet) deep Located between the Caribbean Sea and the Atlantic Ocean, the Puerto Rico trench is the deepest point in the Atlantic Ocean. It is 800 kilometers long and tectonic activity here has generated tsunamis, volcanos and earthquake. Efforts to do a complete mapping of this trench have included expeditions by the French bathyscaphe Archimède in 1964, and a robotic vehicle in 2012.

9) South Sandwich Trench lies at a depth of about 8,420 meters (27,625 feet) The deepest trench in the Atlantic Ocean after Puerto Rico Trench, South Sandwich Trench, it stretched for over 956 kilometers. Located 100 kilometers east of the South Sandwich Islands in the southern Atlantic Ocean, this trench was formed by the subduction of the South American Plate’s southernmost portion beneath the small South Sandwich Plate.

10) Peru–Chile Trench is 8,060 meters (26,443 feet) below the surface of the sea. Also known as the Atacama Trench, it is located around 160 kilometers off the coast of Peru and Chile in the eastern Pacific Ocean. Its deepest point is called Richards Deep. The trench is 5,900 kilometers in length, making it the world’s longest trench, averages 64 kilometers in width and covers an area of about 590,000 square kilometers (227,8000 square miles). The Atacama Trench has been formed by subduction of the Nazca Plate under the South American Plate

Exploring the Mariana Trench — the Deepest Place on Earth

The Marianas Trench is the deepest point in the world's ocean. Running for 2,950 kilometers (1835 miles) on the eastern side of the Marianas islands, just south of Guam, it is a deep sea canyon with a maximum depth of 11,034 meters (36,201 feet). This is more than seven miles. Mount Everest is less than six miles high. The distance between the lowest point in the Marianas Trench and the highest on the Marianas islands is the greatest elevation change on earth.

The crescent-shaped Marianas Trench has an average width of 69 kilometers wide. Challenger Deep — its deepest point — is located several hundred kilometers southwest of Guam. At the bottom of the Marina Trench, the density of water is 4.96 percent higher than at sea level due to the high pressure. Even so expeditions have record a surprising number of creatures, including flatfish, large shrimp-like amphipods, crustaceans and of snailfish.

In 1960, Navy Lt. Don Walsh and Swiss oceanographer Jacques Piccard became the first people to descend to the deepest part of the ocean, the Challenger Deep in the Mariana Trench in a submersible called the Trieste. Designed by Piccard, the Trieste was designed like a hot air balloon, with a cylindrical top section composed of a float filled with gasoline and water to lift the vessel back to the surface after the dive. Attached to the bottom of the Trieste was a small, pressure-resistant sphere with enough room for just two people. “I think [the Trieste’s design was] pretty much a free balloon that would fly in the sky—except the balloon part was sausage-shaped rather than spherical because that’s an easier shape to tow,” Walsh told National Geographic

In March 2012, 2012, film director James Cameron reached the bottom of the Challenger Deep, the deepest part of the Mariana Trench in a submersible called the Deepsea Challenger. The maximum depth recorded during the record-setting dive was 10,908 meters (35,787 feet). The depth of the place where Deepsea Challenger touched down was 10,898 meter (35,756 feet).

Shortly after Cameron emerged from the craft he said, "My feeling was one of complete isolation from all of humanity...I felt like I literally, in the space of one day, had gone to another planet and come back. It's been a very surreal day." [Source: Seth Borenstein, Associated Press, NBC News, March 27, 2012]

Drilling into the Ocean Floor

Suzanne Oconnell of Wesleyan University wrote: It’s stunning but true that we know more about the surface of the moon than about the Earth’s ocean floor. Much of what we do know has come from scientific ocean drilling — the systematic collection of core samples from the deep seabed. This revolutionary process when the drilling vessel Glomar Challenger sailed into the Gulf of Mexico in August 1968 on the first expedition of the Deep Sea Drilling Project. [Source: Suzanne Oconnell, Professor of Earth & Environmental Sciences, Wesleyan University, Published: September 26, 2018]

I have participated in expeditions to locations including the far North Atlantic and Antarctica’s Weddell Sea. In my lab, my students and I work with core samples from these expeditions. Each of these cores, which are cylinders 31 feet (9.5 meters) long and 3 inches (7.5 centimeters) wide, is like a book whose information is waiting to be translated into words. Holding a newly opened core, filled with rocks and sediment from the Earth’s ocean floor, is like opening a rare treasure chest that records the passage of time in Earth’s history.

Over a half-century, scientific ocean drilling has proved the theory of plate tectonics, created the field of paleoceanography and redefined how we view life on Earth by revealing an enormous variety and volume of life in the deep marine biosphere. When scientific ocean drilling began in 1968, the theory of plate tectonics was a subject of active debate. One key idea was that new ocean crust was created at ridges in the seafloor, where oceanic plates moved away from each other and magma from earth’s interior welled up between them. According to this theory, crust should be new material at the crest of ocean ridges, and its age should increase with distance from the crest.

The only way to prove this was by analyzing sediment and rock cores. In the winter of 1968-1969, the Glomar Challenger drilled seven sites in the South Atlantic Ocean to the east and west of the Mid-Atlantic ridge. Both the igneous rocks of the ocean floor and overlying sediments aged in perfect agreement with the predictions, confirming that ocean crust was forming at the ridges and plate tectonics was correct.

The ocean record of Earth’s history is more continuous than geologic formations on land, where erosion and redeposition by wind, water and ice can disrupt the record. In most ocean locations sediment is laid down particle by particle, microfossil by microfossil, and remains in place, eventually succumbing to pressure and turning into rock. Plankton microfossils can be smaller than the width of a human hair but like larger plant and animal fossils, scientists can use these delicate structures of calcium and silicon to reconstruct past environments.

Thanks to scientific ocean drilling, we know that after an asteroid strike killed all non-avian dinosaurs 66 million years ago, new life colonized the crater rim within years, and within 30,000 years a full ecosystem was thriving. A few deep ocean organisms lived right through the meteorite impact. Ocean drilling has also shown that ten million years later, a massive discharge of carbon — probably from extensive volcanic activity and methane released from melting methane hydrates — caused an abrupt, intense warming event, or hyperthermal, called the Paleocene-Eocene Thermal Maximum (PETM). During this episode, even the Arctic reached over 73 degrees Fahrenheit. The resulting acidification of the ocean from the release of carbon into the atmosphere and ocean caused massive dissolution and change in the deep ocean ecosystem. Scientific ocean drilling has also shown that there are roughly as many cells in marine sediment as in the ocean or in soil. Expeditions have found life in sediments at depths over 8000 feet; in seabed deposits that are 86 million years old; and at temperatures above 140 degrees Fahrenheit.

Technology That Makes Ocean Floor Drilling Possible

Suzanne Oconnell of Wesleyan University wrote: Two key innovations made it possible for research ships to take core samples from precise locations in the deep oceans. The first, known as dynamic positioning, enables a 471-foot ship to stay fixed in place while drilling and recovering cores, one on top of the next, often in over 12,000 feet of water. [Source: Suzanne Oconnell, Professor of Earth & Environmental Sciences, Wesleyan University, Published: September 26, 2018]

Anchoring isn’t feasible at these depths. Instead, technicians drop a torpedo-shaped instrument called a transponder over the side. A device called a transducer, mounted on the ship’s hull, sends an acoustic signal to the transponder, which replies. Computers on board calculate the distance and angle of this communication. Thrusters on the ship’s hull maneuver the vessel to stay in exactly the same location, countering the forces of currents, wind and waves.

Another challenge arises when drill bits have to be replaced mid-operation. The ocean’s crust is composed of igneous rock that wears bits down long before the desired depth is reached. When this happens, the drill crew brings the entire drill pipe to the surface, mounts a new drill bit and returns to the same hole. This requires guiding the pipe into a funnel shaped re-entry cone, less than 15 feet wide, placed in the bottom of the ocean at the mouth of the drilling hole. The re-entry cone is welded together around the drill pipe, then lowered down the pipe to guide reinsertion before changing drill bits. The process, which was first accomplished in 1970, is like lowering a long strand of spaghetti into a quarter-inch-wide funnel at the deep end of an Olympic swimming pool.

This research is expensive, and technologically and intellectually intense. Today scientists from 23 nations are proposing and conducting research through the International Ocean Discovery Program, which uses scientific ocean drilling to recover data from seafloor sediments and rocks and to monitor environments under the ocean floor.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

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