6.4: Seafloor Provinces

We now focus our attention on the main subject of this chapter, the seafloor. Because most of us grew up in classrooms with a globe or a world map hanging on a wall, the continents and their features are (hopefully) somewhat familiar. However, once we cross the water’s edge, our knowledge (understandably) diminishes considerably.

Traditionally, oceanographers have divided the ocean into three major seafloor provinces, regions of the seafloor bounded by recognizable features produced as a result of geologic processes (e.g., Lobeck 1939; Heezen et al. 1959). With the development of marine geomorphometry, oceanographers now define four major provinces (after Harris et al. 2014):

• The continental shelf province, the submerged flat portion of the continental margins
• The continental slope province, the steeply sloped part of the continental crust that extends from the shelf break to the abyss
• The abyssal province, from the foot of the continental slope to the start of the hadal regions
• The hadal province, the seafloor at depths below 19,685 feet (6,000 m)

The four major provinces encompass diverse seafloor features, “those parts of the ocean floor with measurable relief or delimited by relief” (e.g., Defense Mapping Agency 1981). Relief generally refers to the change in elevation from a high point to a low point, that is, how much something sticks up or goes down in comparison to its surroundings. A feature may have positive, negative, or no relief. A seamount looms above its surroundings like a mountain, positive relief. A submarine canyon cuts through the flatlands like Beggars Canyon on Tatooine (Lucas 1977), an example of negative relief. Relatively flat regions of the seafloor, the so-called abyssal plains, have little relief. As you read the descriptions below, take note of the seafloor features within each of the provinces. As noted by Harris et al. (2014), many seafloor features are unique to a particular province, but many can be found in more than one province.

The Continental Shelf Province

Though considered part of the seafloor, the continental shelf is underlain by continental crust. By definition the continental shelf begins at the low water line (defined as zero feet/meters and placed at the average height of the lowest tides) and ends where the slope of the seafloor increases sharply—the shelf break. The shelf and the slope encompass the submerged edges of the continents, what are known as the continental margins.

The continental shelf varies considerably in width. In Central California, the shelf narrows to less than a mile near the head of the Monterey Bay Submarine Canyon. By contrast, the Siberian Shelf in the Arctic Ocean spans 930 miles (1,500 km), the widest continental shelf in the world ocean.

Of course, the width of a particular continental shelf also depends on local sea level, or the height of the ocean relative to some point on land. During icehouse conditions (i.e., ice ages), when glaciers form on land, sea level may drop hundreds of feet below their present-day positions. Conversely, during hothouse conditions, sea level may rise as glaciers melt and seawater expands. This means that the position of the shoreline and the extent of the world’s continental shelves have changed throughout geologic time.

Just off Long Beach, California, for example, you can find an ancient sandy beach, a paleoshoreline, at a depth of 200 feet (61 m). The sand was deposited there some 22,000 years ago during the peak of the last glaciation. The rise and fall of sea levels shape our modern coastlines too. As sea level rise caused by global warming threatens to submerge low-lying coastal areas, scientists are taking great interest in paleoshorelines for what they can teach us about coastlines under different sea levels.

Continental shelves may host complex geological features, as evidenced by the coast of Southern California. This continental shelf region, known as the Southern California Continental Borderland, or simply California Borderland, extends almost 200 miles (322 km) from the coastline to the Patton Escarpment, a wedge of sediments at the seaward edge of the shelf. The California Borderland includes dozens of basins (officially, shelf-perched basins), submarine canyons, and submarine fans, features typically associated with slope and abyssal regions. The archipelago of California’s Channel Islands, first colonized by the Chumash and Tongva Native Americans, also rests within the California Borderland. These first settlers made extensive use of the rich oil deposits within the borderland deposited on beaches as asphaltum, a tar-like substance, used to waterproof boats and utensils (e.g., Brown et al. 2014). The California Borderland currently represents the largest repository of oil and gas on the West Coast of the United States (Zabanbark 2008). It also creates a mosaic of habitats important for diverse marine organisms and fisheries.

The Continental Slope Province

Beyond the continental shelf where the shelf break begins, we come to the continental slope. You may think of the continental slope as the edge of the continents, like the sides of a cake (except with a slope).

While continental slopes make up only 5.42 percent of the world ocean, they include one of its most fascinating features, the V-shaped submarine canyons. Submarine canyons offer complex and diverse habitats for benthic (i.e., bottom-dwelling) and pelagic (i.e., drifting or swimming in the water column) organisms. They also link the coastal and deep ocean, providing a conduit for sediments, biologically important nutrients, and, unfortunately, litter and pollutants. Submarine canyons occur in all ocean basins; a total of 9,477 of them have been identified in the entire world ocean (Harris et al. 2014).

Submarine canyons generally cut across the continental shelf perpendicular to the shoreline. The heads of these shelf-incised canyons are often very near the shore—within swimming distance in some places. Redondo Canyon in Redondo Beach, California (offshore from Veteran’s Park south of the Redondo Beach Pier), offers spectacular scuba diving, especially at night in December through March, when the market squid come up from the depths to mate.

The Monterey Bay Submarine Canyon, off California’s central coast, begins a half mile from the beach at Moss Landing and extends nearly 100 miles (161 km) out to sea, reaching depths of more than 2 miles (3.2 km). If you’ve ever seen Arizona’s Grand Canyon, you have some idea of the size of the Monterey Bay Submarine Canyon. Unfortunately, because it is underwater in complete darkness, its awe-inspiring majesty can only be imagined from illustrations or scale models.

Most submarine canyons formed when rising sea levels at the end of an ice age buried the coastal portion of a river valley, causing the now-submerged valley to become part of the seafloor. As rivers moved sediments downstream and into the ocean, those sediments piled up in the submerged river valleys. At some point the sediment accumulations became unstable and began to flow downhill in a water-sand-mud mixture called a turbidity current. Just like the action of a river carves a valley or canyon on land, turbidity currents carve submarine canyons.

Turbidity current modification of submarine canyons continues today as rivers deliver sediments to the heads of the canyons. At least half of the submarine canyons in the California Borderland continue to generate turbidity flows in this way (Normack et al. 2009). Because the downslope motion of sand tends to occur only occasionally and in great bursts, it creates quite a spectacle. Imagine an underwater landslide, and you get the idea. Tectonic events, such as an earthquake or volcano, may trigger underwater landslides. The devastating tsunami that hit Indonesia in December 2018 was caused by a massive landslide following the volcanic eruption of Anak Krakatoa in the Sunda Strait. More than 400 people lost their lives.

As mapped by Harris et al. (2014), polar submarine canyons are the largest, twice the average size of nonpolar submarine canyons. The Bering–Bristol–Pribylov Canyon complex—located on the continental shelf slope of the Bering Sea—measures 12,873 square miles (33,340 km²). It likely formed as a result of glacial runoff from rivers in Alaska and Siberia (Scholl et al. 1970).

A number of other processes, including faults, subterranean seepage, tectonic uplift, and subsidence, may also contribute to the formation and modification of submarine canyons. Oceanographers speculate that one or more of these processes may be responsible for headless canyons, a class of submarine canyons confined to continental slopes. Their heads may have been buried by sedimentation, or the canyon may have formed as a result of seepage of fluids along the slope that caused slumping of sediments. If you see the ghost of a headless canyon galloping on horseback down your street, fear not. They’re only dangerous underwater.

The turbidity currents that carve submarine canyons create their own sedimentary deposits—turbidites—easily recognized by the pattern of sediment sizes that occur within them. Because heavier sediments settle faster, turbidites display a sequence of coarse-to-fine sediments with each event. Think of upside-down pecan pie: coarse nuts, nutty goo, fine-grained crust. Multiple turbidity flows create a deposit with alternating layers of coarse-to-fine sediments. Stack a series of upside-down pecan pie pieces and you have an edible physical model of a turbidite deposit. You can observe turbidites for yourself in places like Dana Point Harbor in Southern California or Point Lobos in Central California. Or you just go to your local bakery and grab a pie.

Turbidity currents can be very destructive. On November 18, 1929, a large earthquake (7.2 Mw) on the Grand Banks of Newfoundland (the location for the film Perfect Storm; Petersen 2000) triggered a turbidity current and broke the transatlantic cable, a telecommunications cable laid across the North Atlantic Ocean. The event disrupted communications between America and Europe for nearly 10 months. It also generated a tsunami that killed 28 people (Ruffman and Hann 2006).

Like continental shelves, continental slopes may exhibit add-on features. Off the southeastern coast of the United States lies the Blake Plateau, a massive limestone platform between the shelf and deeper water. This carbonate platform was built and transformed by millions of years of biological, sedimentary, and geological activities (e.g., Sheridan and Enos 1979). Like the California Borderland, the Blake Plateau offers diverse habitats for marine organisms (e.g., Ross 2007; Hourigan et al. 2017).

The Abyssal Province

Beyond the continental shelf and slope, we encounter the largest province of the seafloor, the abyssal province. This region of the seafloor extends from the base of the continental slope to the edge of oceanic trenches (discussed below). The abyssal province includes many familiar seafloor features, such as basins, seamounts, oceanic ridges, and abyssal plains and hills. We also find less familiar features, such as troughs, bridges, sills, and plateaus. Much remains to be learned about the abyssal ocean. Once thought to be featureless and practically lifeless, the abyssal regions have revealed greater complexity and more abundant life as new technology brings this region into better focus.

Many features of the abyssal regions interact with the shelf and slope, especially near the continental margins. For example, sediments that flow down submarine canyons often spread out when they reach the abyssal seafloor, forming what are called abyssal fans. They’re the underwater counterpart of alluvial fans, where eroded sediments spread out in the bird-tail shape of a hand fan at the base of canyons in mountains.

Delivery of sediments to the seafloor by submarine canyons contributes to the formation of the continental rise, a gently sloping region of sediments at the base of the continental slope, now considered part of the abyssal province. These deposits—some of the thickest sediment deposits found in the ocean—make up 8.24 percent of the global seafloor. In the Southern Ocean, the continental rise completely encircles the Antarctic continent (Harris et al. 2014).

Continental-rise sediments also arrive as a result of transport and deposition by contour currents, so called because they tend to flow along certain depth contours on the bottom. Oceanographers refer to contour current–generated sediment deposits as contourites to differentiate them from turbidites.

Beyond the continental margins, we encounter the ocean basins, formally defined as “seafloor depressions with closed bathymetric contours,” that is, depth contours that form a closed curve. Harris et al. (2014) identify at least 29 major ocean basins larger than 309 square miles (800 km²). The deepest occur in the northwestern Pacific. Of all the world ocean features, basins cover the largest percentage of the seafloor, some 43.8 percent, nearly a third of Earth’s surface (31.1%).

Ocean basins may be further subdivided into flat regions, the abyssal plains (with a relief less than 984 ft or 300 m above the seafloor); hilly regions, abyssal hills (984–3,281 ft, 300–1,000 m above the seafloor); and mountainous regions, the abyssal mountains (greater than 3,281 ft, or 1,000 m above the seafloor), which include features such as seamounts, guyots, and similar elevated areas. They also feature the granddaddy of all oceanic features—oceanic ridges. Defined as an elongated narrow region of varying complexity with height exceeding 3,281 feet (1,000 m) and a length-to-width ratio greater than two, oceanic ridges appear much like the seams on a baseball, with fractures that look like stitches and a central valley where the seams come together. Oceanic ridges form a network of mountains that traverse the ocean basins for more than 47,000 miles (75,000 km; Harris 2020).

Abyssal plains make up 27.9 percent of the seafloor. They represent relatively flat regions where the slow buildup of sediments has smoothed out the seafloor. By way of comparison to a land feature, Colorado’s Grand Dunes National Park, home of the tallest sand dunes in the United States (some as tall as 700 ft or 213 m), would qualify as an abyssal plain if it was underwater. Abyssal hills, covering 41.3 percent of the seafloor, resemble the Basin and Range Province of the southwestern United States. Abyssal hills (and the basin and range) were created by a pulling apart of Earth’s crust, a geologic process called extensional tectonics. Together, abyssal plains and hills make up 69.2 percent of the seafloor, making them the most common feature on Earth (49.1%).

Within the category of abyssal mountains, we find another feature that bears resemblance to one we see on land. Oceanic seamounts resemble terrestrial volcanoes. The International Hydrographic Organization (IHO) defines a seamount as “a distinct generally equidimensional elevation greater than 3,281 feet (1,000 m) above the surrounding relief as measured from the deepest isobath that surrounds most of the feature” (IHO 2019). Think underwater stratovolcano—tall (more than 32,808 ft or 10,000 m) and cone-shaped, like Mount Shasta in Northern California, Popocatépetl near Mexico City, Mount Fuji in Japan, or Mount Etna in Italy (e.g., Lutgens et al. 2018).

The exact number of seamounts in the world ocean varies with research methods and definitions and ranges from tens of thousands (Etnoyer et al. 2010; Yesson et al. 2011; Harris et al. 2014) to over 100,000 (Wessell et al. 2010). Restricting their definition to conical forms, Harris et al. (2014) identified 9,951 seamounts worldwide, with the majority of those (6,895, or 69%) occurring in the Pacific Ocean. Some scientists view height restrictions on the definition of a seamount to be an arbitrary distinction. Seamounts on the order of 328 feet (100 m) in height exhibit many of the same characteristics of larger seamounts, so some authors are content to relax the height requirement.

Oceanographers increasingly recognize the importance of seamounts as marine habitats (e.g., Etnoyer et al. 2010). Some marine scientists declare them the most important habitats in the world ocean. Seamounts support dense populations of marine organisms in what otherwise might be considered oceanic deserts. Seamounts act as a kind of underwater gathering spot for many pelagic species, including tuna, billfish, sharks, and marine mammals. Here they find food, mates, and cleaning stations—places where cleaner organisms (small shrimps and fishes) remove parasites.

The importance of seamounts as hot spots of biodiversity has led to efforts to protect them. In 2006 President George W. Bush established in the uninhabited islands to the northwest of Hawaii the Papahānaumokuākea (pronounced pa-pa-ha-no-mo-ku-ah-kay-uh) Marine National Monument, part of the 3,600-mile-long (5,800 km) Hawaiian–Emperor seamount chain. Expansion of the monument by President Barack H. Obama in 2016 now makes it one of the largest protected marine areas in the world, covering 582,578 square miles (1,508,870 km²)—almost as large as the state of California. In the same year, President Obama also created the Northeast Canyons and Seamounts Marine National Monument, making it “the first and only national marine monument in the Atlantic Ocean” (NOAA Fisheries 2022).

Guyots represent the flat-topped cousins of seamounts. They may form on the top of a sinking island or seamount through a buildup of reef materials (i.e., carbonates), erosion, or both (e.g., Buchs et al. 2018). Other than their buzz cut, guyots exhibit all the characteristics of seamounts. Harris et al. (2014) mapped 283 guyots in their study. The largest—the 48-million-year-old Koko Guyot, named after the 58th emperor of Japan—can be found in the Hawaiian–Emperor seamount chain in the North Pacific Ocean.

Oceanic plateaus represent “flat or nearly flat elevated regions of the seafloor that drop off abruptly on one or more sides” (IHO 2019). They are thought to be formed from the breakup of supercontinents, from tectonic uplift, or from massive volcanic eruptions of lava on the seafloor. This latter process mirrors the formation of continental flood basalts on land, such as the Columbia Plateau that covers parts of Washington and Oregon. Harris et al. (2014) identified 184 oceanic plateaus covering about 5.11 percent of the seafloor.

The most extensive plateaus occur in the South Pacific and Indian Oceans. The Challenger Plateau, located west of New Zealand, is one of the largest. It’s thought to represent a fragment of submerged continental crust—part of a complex that once belonged to an ancient continent known as Zealandia. Some geologists argue that Zealandia fits the definition of a continent and deserves designation as Earth’s seventh geological continent (Mortimer et al. 2017).

One interesting area of research involves the effects of the formation of oceanic plateaus and continental flood basalts on Earth’s atmosphere. The massive eruptions that formed these features likely released tremendous amounts of carbon dioxide into the atmosphere. The resultant atmospheric warming and ocean acidification are implicated in mass extinction events in Earth’s geologic past.

Smaller elevated features, including banks, domes, knolls, sedimentary ridges, and terraces, also populate the basins of the world ocean. This is a subject that could fill volumes, yet we have other magnificent features to visit. Readers interested in exploring less-well-known features of the seafloor may refer to the Undersea Feature Names Gazetteer, maintained by the General Bathymetric Chart of the Oceans (GEBCO; see https://www.ngdc.noaa.gov/gazetteer).

The Hadal Province

We now visit the hadal province, the seafloor deeper than 19,685 feet (6,000 m). Much of the hadal province consists of oceanic trenches—narrow, V-shaped depressions with steep walls. Ocean trenches rank as the deepest seafloor features in the world ocean, often descending more than two miles deeper than the surrounding seafloor. Not all trenches plunge to hadal depths, however, so we find trenches in both the abyssal and hadal provinces. Because oceanic trenches originate from movements of Earth’s crust—plate tectonics—we’ll cover them in greater detail in our next chapter.

The depths of the deepest oceanic trenches exceed the height of the highest places on land. The absolute deepest place in the world ocean, the Challenger Deep—located in the Mariana Trench off the coast of Guam—plunges to 6.788 miles deep (35,876 ± 20 ft, or 10,935 ± 6 m; Greenaway et al. 2021). Compare that to Mount Everest at 5.498 miles high (29,028.87 ft or 8,848 m). If you were Captain Planet (Pyle and Turner 1990–1996) and could uproot Mount Everest and place it on the bottom of the Challenger Deep, the top of the mountain would still be 1.29 miles (2.07 km) below the surface of the ocean. Of course, if you could do that, you probably wouldn’t be reading this book; you’d be out doing superhero things, like stopping volcanoes and diverting raging rivers.

Harris et al. (2014) identified 56 trenches, a mere 0.95 percent of the seafloor. Most trenches occur in the Pacific Ocean, where they form a circle spanning the western coastlines of South, Central, and North America, the Aleutian Islands of Alaska and Russia, Japan, the Philippines, New Guinea, the Solomon Islands, New Caledonia, American Samoa, Tonga, and New Zealand. The Atlantic and Indian Oceans contain only a few short trenches, the Puerto Rico Trench and the Java Trench, respectively, among the most notable.

Despite their reputation as lifeless wastelands, ocean trenches host an extraordinary diversity of life with representatives from nearly every major group of marine organisms (except photosynthetic ones). Though often located far from land in waters with very low productivity, many receive particles of organic matter from shallower depths. Some are close enough to continents that supplements of plant and seaweed debris sink into their confines. Unfortunately, they also receive human debris. A plastic bag found at the bottom of the Mariana Trench made headlines in 2018. A 30-year study of plastic pollution in the abyss revealed that single-use plastics make up 92 percent of all plastics found at hadal depths (Chiba 2018).