6.1: 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).

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.

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.

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.

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).