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Chapter 7: The Seafloor

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When you stand in the shallow water on the foreshore of a beach, you stand on the outermost edge of an expanse of Earth that covers 71 percent of our planet’s surface. Simply put, you are standing on the floor of the ocean, the seafloor. 

If you were to keep walking—outfitted, of course, in a deep-sea atmospheric dive suit (ADS) with plenty of rations for you and some fishy friends—you would come across some of the most extraordinary regions of our planet: grand canyons, majestic mountain ranges, and massive dunes that rival those of Arrakis, the desert planet in Frank Herbert’s best-selling 1965 sci-fi novel Dune. The seafloor boasts its own Yellowstone-like geysers and mud pots and bizarre pools of methane that look like underwater lakes. It’s a world largely unseen and unknown.

But technology is changing our ability to “see” the seafloor. Thanks to new tools for mapping the seafloor (Chapter 5) and telepresence technology that streams pictures and video to any internet-connected device (Chapter 4), you can explore these incredible habitats in the comfort of your home. In the 21st century, the seafloor has become visible to us in ways we could only dream of a few decades ago. Submersibles have provided access to the very deepest parts of the ocean (Chapter 3). Satellites and their ability to very precisely map the sea bottom from bumps and wiggles in the sea surface have brought the major seafloor features into view (Chapter 4). Remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) have expanded our ability to see (via cameras) and map (via sonar devices) the seafloor (Chapter 4). Multibeam and side-scan sonar allow us to visualize the three-dimensional topography of the ocean bottom in unprecedented detail (Chapter 5). So while it’s popular to say that we know more about the surface of the Moon or Mars than we know about the bottom of the ocean, that perception ignores the great advances that have been made in recent decades (e.g., Hillier et al. 2008; Harris and Macmillan-Lawler 2018; Jamieson 2023).

In this chapter, we survey the major features of the seafloor. In particular, we emphasize the significance of these features to geological, chemical, physical, and biological processes in the world ocean. If you’ve ever traveled anywhere on land, you know that a little advance knowledge of the places you intend to visit can make the experience more enjoyable. So, too, can knowledge of the seafloor open your eyes to a geological wonderland beneath the waves. So grab the controls of your virtual ADS and let’s dive in, ocean explorer.

7.1 Planet Earth vs. Planet Ocean

To begin, let’s consider the dimensions of the seafloor in relation to the rest of our planet and address a common misconception. I’m talking here about the notion that we should call our home Planet Ocean.

The idea that our planet should have been named after its water (i.e., Ocean) instead of its land (i.e., Earth) appears to have originated in 1963 with University of Virginia oceanographer G. Carlton Ray (Ray 1965; see quoteinvestigator 2017). Of course, British author Sir Arthur Clarke (1917—2008) popularized the notion when—according to British atmospheric chemist James Lovelock (1919—2022)—he said, “How inappropriate to call our planet Earth when it is clearly Ocean” (Lovelock 1979, 1990). It’s not clear when or where Clarke said this. And as much as I respect these gentlemen’s work and would love to live on Planet Ocean, the fact of the matter remains that Earth is mostly rock. The ocean makes up a tiny percentage of our planet compared to its solid part, a mere 0.14 percent of Earth’s volume and only 0.025 percent of its mass. Yes, the world ocean covers 71 percent of Earth’s surface, but the geosphere dominates our planet. So Earth remains an apt name for our rocky planet.

We must also consider that not all of the seafloor belongs to the ocean proper. A significant part of it, about 6.3 percent, is attached to the continents (i.e., the continental shelf). Oceanographers define ocean basins as the region of the seafloor from the shelf break (where the seafloor rapidly drops beyond the continental shelf) to the deepest locations in the world ocean (and all parts in between). By that measure the ocean basins cover about 64.7 percent of Earth’s surface (approximately 127,275,090 square miles, or 329,640,970 square kilometers, km2). Thus, from the outset, you should be aware that the seafloor—the part of Earth’s surface submerged by ocean—includes part of the continents (e.g., Harris et al. 2014).

7.1.1 A Two-Part Crust: Oceanic vs. Continental

The distinction between continental and oceanic seafloor is more than academic. Scientists have long known that the ocean basins—the depressions in Earth’s crust—must be composed of a different type of rock than the continents—the elevations in Earth’s crust. 

For basins to exist as “depressions” in Earth’s surface and for continents to exist as “elevations,” their rocks must be different, in accordance with a physical principle know as isostasy—the equilibrium “height” of Earth’s crust as it floats on the mantle. Thinner and denser rocks (i.e., ocean basins) sink into Earth’s surface, and thicker and less dense rocks (i.e., continents) rise above it. 

Prior to acceptance of the theory of plate tectonics, scientists lacked a good explanation for how the heavier rock formed (e.g., Holmes 1945). Once deep-sea sounding revealed vast mountain ranges, ships began collecting seafloor rocks, and humans visited the seafloor in person using submersibles, scientists realized that the source of the heavier rock on the seafloor was undersea volcanism.

As it turns out, the crust of the ocean basins, the oceanic crust, consists of a dark, often vesicular (filled with holes) rock called basalt that forms from lava, which is molten rock that spills onto Earth’s surface. In addition to making up most of the seafloor, basalt can also be found above water. If you’ve ever been to Kilauea or Mauna Loa on the island of Hawaii, the shores of the Columbia River in Washington or Oregon, or Pisgah Crater in the Mojave Desert near Barstow, California (or any of a number of other locations around the world), you’ve likely seen basalt. When lava erupts beneath the ocean and interacts with seawater, a curious structure called a pillow basalt forms. It’s a pillow-shaped basalt (though I doubt you’d get a good night’s sleep with your head resting on it). Many parts of the seafloor are covered in pillow basalts, especially in regions with volcanic activity. 

The continental crust generally consists of granite-like rocks, similar to the “granite” countertops found in many homes (which mostly come from metamorphic rocks formed from squeezing and baking granite and other rocks). Granite forms from the slow cooling of magma, which is molten rock that remains below Earth’s surface. In contrast to basalt, granite appears lighter in color, features larger crystals, and has a lower density than basalt. California’s mountains and deserts and the Cascade and Rocky Mountains feature extensive outcrops of granite (as do many other locations around the world).

The density differences between basalt and granite account for the shape of Earth’s surface. Geologists describe Earth’s crust as having a global bimodal topography—that is, having two main features: depressions (ocean basins) and elevations (continents). Think of a two-humped camel. If we graph the area of different elevations on Earth—a type of graph called a hypsographic curve—we see in mathematical detail what our eyes have already told us: Earth’s surface is composed of two kinds of crust.

I like to think of Earth as a kind of two-dough, deep-dish pizza where the bottom represents the heavier oceanic crust and the high edges represent the lighter continental crust. It might not be a great analogy, but I hope that the next time you eat pizza (shallow or deep), you’ll think about Earth’s crust.

7.1.2 Coastal vs. Oceanic

One final distinction bears importance in the chapters ahead. Oceanographers define the waters overlying the edges of the continents as coastal waters, or the coastal ocean. Coastal waters interact very closely with the land and the continental crust they cover (i.e., the continental seafloor), which tend to be quite shallow. Waters beyond the continents’ edges are classified as oceanic waters, or the open ocean. Oceanic waters have very limited interaction with land and the seafloor, which reaches its greatest depths in oceanic waters.

This broad classification of the waters of the ocean also defines two types of oceanography: coastal oceanography, the study of the coastal ocean, and blue water (or open ocean) oceanography, the study of the open ocean. Scientifically speaking, oceanographers consider coastal oceanography to be the more difficult topic of study because terrestrial processes complicate the already complicated ocean environment. Such distinctions prove useful in considering human interactions with the world ocean, as most human impacts occur in the coastal ocean. 

7.1.3 Earth’s Geographic Continents

To gain familiarity with the surface of our planet, and especially the seafloor, it’s helpful to review the major geographic features of our globe. Here we take a view that might be enjoyed by astronauts aboard the International Space Station.

Looking out the spacecraft’s viewport, you will see brown, green, and white patchworks of land. Thanks to your fourth-grade teacher, you recognize these aboveground segments of Earth’s crust as the seven continents. A query to an onboard computer helps identify them.

Asia, Earth’s largest landmass, makes up nearly 30 percent of the continental landmass and hosts some 60 percent of the world’s 8 billion people. It’s also the site of the world’s highest mountains, the Himalayas, which feature the highest mountain on Earth, Mount Everest, whose peak is some 5.49 miles (8.84 km) above sea level. That’s roughly the walking distance from Fullerton College to Disney California Adventure Park. 

Africa, the second-largest continent and where modern humans originated, is distinguished by the presence of the Sahara Desert. The world’s largest hot desert, the Sahara stretches from the Red Sea to the Atlantic and frequently showers the Atlantic Ocean with golden plumes of dust caught in the trade winds. 

The third-largest continent, North America, includes Greenland, the world’s largest island, Canada, the US, Mexico, and Central America. For the record, the US ranks fifth in use of Earth’s resources (i.e., ecological footprint per capita), an improvement over previous years (Global Footprint Network 2023). 

South America stands out for its range of snow-capped peaks that stretch south to north along its western edge. These are the Andes, the longest continental range in the world. 

Antarctica, an island-continent, holds more than 60 percent of Earth’s freshwater. A lone volcano, Mount Erebus, pokes its fiery head out of the ice and puffs gray smoke that quickly vaporizes in the cold, incessant winds. The Antarctic is surrounded by the tumultuous Southern Ocean. 

Europe, attached to Asia, ranks second to last in area. From a geological perspective, Europe and Asia constitute the continent of Eurasia, reducing the number of major continents to six. Nevertheless, Europe boasts the third-largest human population, about 10 percent. And it’s home to many of the world’s finest institutions dedicated to studying the world ocean. So we’ll recognize it on its own here.

Australia has fewer people than any other continent except Antarctica. Though small in size and number of inhabitants, Australia boasts the largest structure ever built by any living beings, larger than anything humans have built: the Great Barrier Reef, some 1,400 miles (2,253 km) long and quite visible from space. Sadly, we may be experiencing the last decades of this natural wonder, as warming ocean waters threaten the corals and algae that built the reef. 

7.1.4 Earth’s Geographic Oceans

A view high above Antarctica reveals the Southern Ocean as a gateway to the three major ocean basins: 

  • The Pacific Ocean, the oldest, largest, and deepest
  • The Atlantic Ocean, the youngest and most widely studied
  • The Indian Ocean, the smallest of the major ocean basins, but traveled upon by humans for millennia 

From this vantage point the Southern Ocean appears as a wheel surrounding Antarctica, with spokes in each of the three ocean basins. Though oceanographers once viewed the North Atlantic as the main driver of ocean circulation, they now see the Southern Ocean as the gatekeeper for the world ocean circulation. Regardless, the interconnectivity of the ocean basins, including the Arctic, underscores the idea of one ocean, the world ocean. 

Whipping over Earth’s northern pole, we see a partially frozen ocean surrounded by continents, the Arctic Ocean. In recent decades the Arctic’s sea ice has thinned and diminished in extent—one sign of a warming planet. Similar signs of warming can be found on glacier-capped Greenland, home to 10 percent of the frozen water on our planet. Sapphire-blue lakes dot its surface with greater frequency, evidence of melting. Of all places on Earth, the Arctic has experienced the worst effects of human-caused climate change (Post et al. 2019; Kumar et al. 2020). Scientists predict the Arctic could experience ice-free summers as early as 2035 (Guarino et al. 2020).

7.2 A Few Words About “Seafloor”

Before we go any further in this chapter, I’d like to make clear a couple things about the term “seafloor.” It’s not as simple as it sounds.

First, “sea” typically refers to the waters of the ocean. From this we might conclude that the ocean ends at the seafloor. However, seawater actually penetrates the seafloor, creating a region known as the subseafloor environment. We can’t be certain, but there’s evidence the subseafloor environment may extend as deep as a mile or more beneath the seafloor (Ciobanu et al. 2014; D’Hondt et al. 2019; Heuer et al. 2020). So while we casually think of the ocean as the seawater between the surface and the top of the seafloor, in truth, part of the ocean—and perhaps all of it on geologic timescales—circulates beneath the seafloor. 

Second, and perhaps more important, we typically think of a floor as something that is flat, horizontal, or level. But where the sea’s “floor” is concerned, nothing could be further from the truth. The seafloor is rough on nearly every scale you can think of. As Pizer (1967) comments: 

Oceanographers displayed little imagination when they chose “ocean floor” as the designation for the bottom of the world’s 140 million square miles of salt water. . . . The forces that created the ocean floor laid out the wildest roller coaster ride imaginable.

In truth, the seafloor is as varied as the surface of the continents, if not more so. The seafloor hosts mountains and trenches and a whole bunch of other geologic features. You’ll get a good workout should you decide to walk across it.

7.3 Marine Geomorphometry

The World Ocean Floor Map (1977) painted by Austrian Heinrich Berann (1915—1999) under the guidance of American oceanographers Bruce Heezen (1924—1977) and Marie Tharp (1920—2006) has long served as the standard bearer for maps of the seafloor. But the emergence of new methods has given rise to a new field calleds marine geomorphometry, the quantitative study of the seafloor (see LeCours et al. 2016). Geomorphometry takes advantage of advances in satellite-derived bathymetry, geographic information systems (GIS), digital bathymetric models, and spatial analysis software to quantitatively identify and statistically analyze various features of the seafloor.

In 2014 Australian geoscientist Peter Harris and colleagues published the first digital map of seafloor geomorphic features (Harris et al. 2014). Using quantitative tools, their analysis provides for the first time an assessment of the numbers and sizes of different seafloor features and offers new insights into the characteristics of seafloor features.

7.4 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. 

7.4.1 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. 

7.4.2 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). 

7.4.3 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).

7.4.4 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).

7.5 Deep-Sea Mining

While plastics and their impacts on marine life have captured the public’s attention in recent years, another potentially destructive human activity is gaining momentum. Ocean mining, the extraction of mineral resources from the seafloor, raises the potential for serious impacts on ocean habitats, marine organisms, and even people. The possible harmful consequences of ocean mining deserve our attention.

7.5.1 Deep-Sea Metals

Though we rarely think about it, the manufacture of smartphones, green technologies, batteries, and many other devices requires minerals. At least 14 different minerals go into a smartphone. While many are present in tiny quantities, each is required for a smartphone to function properly (USGS 2017). Some 1.43 billion smartphones were sold globally in 2021 (Statista 2022). The rapidly increasing demand for renewable energy technologies (e.g., solar panels, wind turbines) and battery technologies (e.g., electric cars, electrical storage) has led to concerns about shortages in the supply of minerals. Making matters worse, the supply chain for minerals involves countries around the world, raising issues of security, diplomacy, regulation, ecology, and human welfare (Ali et al. 2017). 

Worried about global shortages, governments and industries have increasingly looked toward the seafloor as a source of minerals. The deep seafloor exceeds land deposits in terms of abundance for nearly all minerals (e.g., Hein et al. 2013). Four types of deep-sea mineral deposits have received the most attention:

  • marine phosphorites, phosphate-rich sedimentary rocks formed in oceanic upwelling regions with high biological productivity, mostly continental margins, but also some oceanic seamounts (Filippelli 2011)
  • polymetallic nodules, also known as manganese nodules, tennis-ball-sized globes of different metals that precipitate slowly around a nucleus (a shell or bone fragment, for example) on the seafloor (Cuyvers et al. 2018)
  • polymetallic crusts, similar to nodules in composition, forming as a coating on rocks along the flanks and summits of seamounts (Cuyvers et al. 2018)
  • seafloor massive sulfide deposits, essentially, the sulfur-rich chimneys and deposits of hydrothermal vents (Cuyvers et al. 2018)

As reported by Mining-Technology.com (Davies 2019), “The deep sea could contain more cobalt, nickel and rare earth minerals than all land-based reserves combined forecast . . . to account for 15% of global supply by 2050.” 

7.5.2 Status of Deep-Sea Mining

Until now, deep-sea mining has remained in a testing phase. But pressure is mounting to allow commercial deep-sea mining to begin (e.g., Lyons 2021). In June 2021, the Micronesian island nation of Nauru (the third-smallest nation in the world next to Vatican City and Monaco) invoked a United Nations Law of the Sea rule known as the two-year rule, which would require a decision on seafloor exploitation by mid-2023 (Reid and Lewis 2021). As Blanchard et al. (2023) describe it, “A lot of work has yet to be done to evaluate how environmental considerations will be embedded into the DR , if deep-sea mining does indeed go ahead.”

There seems little doubt mining can be carried out successfully. In 2017 Japan became the first country to excavate polymetallic sulfides from an inactive hydrothermal vent in the Okinawa Trough (Okamoto 2018). And in 2020 Japan was able to retrieve more than 1,400 pounds (649 kilograms) of copper and nickel-enriched crust from the seabed near Minami-Tori-shima Island (JOGMEC 2020). A number of countries and companies have obtained leases and manufactured equipment for deep-sea mining.

At the same time, efforts are underway to address potential environmental concerns. The International Seabed Authority, which works under the UN Law of the Sea treaty, continues to work on adoption of environmental regulations that would reduce environmental impacts. But uncertainty remains, particularly with regard to scientific understanding of deep-sea ecosystems and their response to disturbances from mining (Beaulieu et al. 2017). The European Union Parliament recently called for a moratorium on deep-sea mining “until its effects on the environment are better understood and can be managed” (Lewis 2021). 

7.5.3 Impacts of Deep-Sea Mining

Oceanographers have reasons for concern. A 1989 seafloor “plowing” experiment in the Peru Basin so damaged the habitat that it had still not recovered 26 years later (Lledo et al. 2019). Track marks remained visible and microbial activity was severely reduced (Vonnahme et al. 2020). In some places, the sediments had become so compacted that animals could no longer inhabit them (Ackerman 2020). Though scientists acknowledge the limitations of extrapolating results from one region to another, their experiments underscore the need to proceed with caution. 

Oceanographers have barely begun to explore the diversity of organisms inhabiting these regions. A 2016 study using remotely operated vehicles (Chapter 4) within the Clarion-Clipperton Zone—an abyssal plain southeast of Hawaii targeted for collection of polymetallic nodules—found seven species “new to science” and higher than average biodiversity (Amon et al. 2020). Such discoveries underscore how little scientists know about these regions of the ocean.

Seafloor impacts are not the only ones that concern scientists. A group of oceanographers recently expressed concerns about the impacts of deep-sea mining on organisms living within the water column. Plumes of sediments kicked up by mining operations may threaten midwater ecosystems, which “represent more than 90% of the biosphere contain fish biomass 100 times the annual fish catch” (Drazen et al. 2019). Sediments may impair the breathing of organisms, bury organisms directly, and impair the ability of organisms to find food or communicate (Christiansen et al. 2020). Locations targeted for mining boast some of the clearest waters in the world ocean and the impacts of increased sedimentation may be high (Smith et al. 2020). Increased ocean noise has also been cited as a concern (Christiansen et al. 2020).

7.5.4 Should We Mine the Deep Seafloor?

While some ask, “Should we mine the deep seafloor?” (Beaulieu et al. 2017), others respond, “Seabed mining is coming—bringing mineral riches and fears of epic extinctions” (Heffernan 2019). Indeed, deep-seafloor mining appears inevitable (e.g., Hein et al. 2010).

Whether inevitable or not, some people believe that deep-sea mining could be carried out in a manner that minimizes its impacts to organisms and the marine environment. At least one company—DeepGreen Metals—has partnered with research institutions to address scientific concerns (Moore 2020). Underscoring the need for this type of research, Smith et al. (2020) write:

The deep sea contains many of the most pristine, poorly studied, and evolutionarily remarkable ecosystems on our planet—in situ scientific knowledge addressing the full scales and intensities of seabed mining should be obtained and properly applied to sustain biodiversity and ecosystem functions in the deep sea if mining is to proceed.

Deep-sea mining—dubbed the “new gold rush” by one author (Tasoff 2017)—appears on the brink of rapid expansion once regulations and investments in technologies take hold. When it does, the riches will be there for those with the money and technology to find and extract them.

On the other hand, some argue that the need for deep-sea mining has been overhyped (Miller et al., 2021; Earle and Kammen 2022). Green energy and other metal needs can be met without deep-sea metals. They point to the fact that even some automakers support a moratorium on deep-sea mining. As Earle and Kammen state in the opening of their article, “Seldom do we have an opportunity to stop an environmental crisis before it begins. This is one of those opportunities.” 

7.6 Chapter References

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