Earth's Ocean: The Geology of the Sea
Story by Alex Brylske
Photos by Joseph C. Dovala
Note: This article is the last of a four-part series that examines the primary disciplines of oceanography. Geological oceanography is a discipline of ocean science that seeks to explain a lot about how the oceans came to be, their physical expanse and how they continue to change.
On first impression, the term "geology" seems to have little to do with the ocean. After all, geology is the study of rocks, right? Well, yes and no. Geology is literally the study of the Earth, and the major feature of our planet is its ocean. Geological oceanography is a discipline of ocean science that seeks to explain a lot about how the oceans came to be, their physical expanse and how they continue to change. For example, where did the water in the ocean come from? How did the world ocean come to be shaped the way it is? Are the seas continuing to grow or are they shrinking? Why is what goes on deep inside the Earth so vital to the health and stability of the ocean? These are only a few of the questions geological oceanographers grapple with in their quest to understand the ocean. Geological processes occur at rates that are imperceptible, if not incomprehensible, to humans. So, we will never live long enough to witness the kinds of changes that occur to the ocean on a geological scale. But, as divers, if we really want to understand how more than 70 percent of our Earth operates, then an elemental appreciation of geological oceanography is essential.
The Genesis of the Land and Sea
Our Earth was formed by the accumulation of gas and larger particles resulting from the explosion of a star or supernova, the same stuff that formed the sun. As a result, from its surface to its core, Earth's density was at first relatively homogenous. Eventually, this amorphous mass of gas and particles started to consolidate and cool. But cooling was hindered by a continual bombardment of meteors and other space debris, which kept the surface of our planet a molten inferno. During this time high temperature gases were absorbed into the molten rock (magma) and churned deep within the Earth. The tremendous pressure deep inside the Earth held the gases within the magma until it reached the surface, depressurized and violently released the gases into the atmosphere. These gases are termed excess volatiles, and the major constituents are water, nitrogen (the most abundant gas in the atmosphere), carbon dioxide and hydrogen chloride. Additional heating from gravitational compression and the decay of radioactive elements kept the Earth in a semimolten state for millions of years.
Because of its high density, most of the Earth's iron was pulled toward the core as the result of gravity. This sinking created even more heat through friction. Conversely, lighter minerals such as silicon, magnesium, aluminum and various oxygen-based compounds floated toward the surface. The surface eventually cooled, forming a crust. For more than 100 million years this differentiation of elements and compounds, termed density stratification, occurred. And as Earth segregated into the layers of core, mantle and crust, the volatiles were released through volcanic activity in a process called outgassing. Because the Earth's interior was hotter and the magma contained more volatiles than today, volcanic activity at this time on Earth was far more intense than anything seen today.
Because of the tremendous outpouring of gasses, clouds soon covered the Earth, and little sunlight penetrated the thick cloud cover. Like modern-day Venus, anyone approaching the Earth from space 4.5 billion years ago would have seen nothing but a sphere blanketed by clouds punctuated by continuous lightning. But after millions of years, the upper atmosphere cooled enough to produce rain. At first, the rainfall boiled away by the still superheated surface, but eventually it cooled enough for the water to collect in basins and dissolve minerals from the rocks. In the granddaddy of all storms, these heavy rains are estimated to have lasted about 10 million years. (Remember that the next time you complain about a rainy day.) This rain, combined with the continued outgassing from volcanic activity, resulted in the massive accumulation of water that's now our world ocean.
But that's not the whole story. Some of Earth's water originated literally from the heavens. Research suggests that millions of tiny comets, diminutive ice balls, colliding with the Earth probably contributed significantly to Earth's accumulating reservoir of water.
The next phase of Earth's history is still in dispute. The traditional view is that rock masses have always protruded through the ocean surface forming continents. Recent evidence, however, suggests otherwise. Some believe that water may have covered the entire surface of the Earth for more than 200 million years before any continent emerged. Regardless, about 4 billion years ago, when the rain ceased and the skies cleared, a global ocean covered the blue marble called Earth to a depth of nearly two miles, the sea was in place. (We know this because of the discovery in Greenland of sedimentary rock, which requires water to form, nearly 4 billion years old.) But surprisingly, the ocean continues to grow even today. About 0.025 cubic miles (0.1 cubic km) of new water is added to the sea each year, mostly from the continued outgassing of volcanic events and ice from tiny comets that continually pelt the Earth from space.
Perhaps the most amazing fact regarding the world ocean isn't what's present, but what's not. The quantity of water in the sea is so vast that it defies comprehension. The world ocean, which contains 97 percent of all water on Earth, holds a volume of about 108,000 cubic miles (450,000 cubic km). Polar ice caps account for another 1.9 percent, groundwater contributes 0.5 percent, all the lakes and rivers of the world only another 0.02 percent, and 0.001 percent is held in the atmosphere as vapor. But that still isn't the amazing part, from a geological perspective. The fact is that most of the water on our planet remains sequestered within the mantle as excess volatiles. In fact, it's estimated that Earth's surface waters account for only 10 percent of the planet's entire water reservoir, 90 percent is still locked deep within the Earth.
The composition of Earth's early atmosphere was also very different from present, and it wouldn't have created a very inviting place for humans (or any other organisms that required oxygen). The early atmosphere was composed primarily of carbon dioxide and water along with lesser amounts of carbon monoxide, nitrogen, hydrogen, hydrogen chloride, ammonia and methane. About 3.5 billion years ago, the atmosphere began to change into its present composition of primarily nitrogen and oxygen. Much of the carbon dioxide, 1,000 times more than is in our current atmosphere, dissolved into the sea water, forming carbonic acid and combining with rocks. Water vapor saturated the air to such an extent that the atmospheric pressure was many times greater than it is today. (Something to remember the next time you complain about the humidity.)
For the next 2 billion years, simple, single-celled microorganisms were the only life form to populate the Earth. But their size belied their importance. Dependent upon the presence of water, it was their life processes that altered the composition of the atmosphere through photosynthesis. These phytoplankton, the earliest green plants, greatly accelerated the rate of oxygen production in what is now called the "oxygen revolution," and paved the way for oxygen-dependent organisms like us. Furthermore, the increase in atmospheric oxygen lead to the production of an ozone shield which protected life on Earth from the devastating effects of the sun's ultraviolet radiation.
The Living Earth
As you may remember from elementary school, our planet is made up of several concentric spheres within a sphere, each markedly different from the other. The outer layer or crust consists of relatively light material, and is separated from the deeper mantle by a region known as the Moho discontinuity. The crust varies in thickness from 19-25 miles (30.4-40 km) beneath the continents to a mere 2.4 to six miles (3.8-9.6 km) beneath ocean basins. Figure 1 depicts the Earth's layers. First is the lithosphere, the Earth's cool and rigid outer layer. It's made up of the continental and oceanic crust along with the uppermost cool and rigid portion of the mantle.
Next is the asthenosphere, the hot, slowly flowing layer of the upper mantle, which extends to a depth of 430 miles (688 km). The asthenosphere is characterized by its ability to deform, also known as plasticity, under great stress. The third layer is the lower mantle that extends to the core. Although the asthenosphere and lower mantle have similar chemical composition, they're very different in physical properties. The asthenosphere is deformable, while the lower mantle is rigid.
Finally comes the core, which is divided in two. The upper core is a viscous liquid, while the inner core is solid. The core is an extremely hot place, with temperatures of about 9,900 degrees Fahrenheit (5,482 degrees Celsius). Some suggest that the core may be even hotter, as much as 12,000 degrees F (6,649 degrees C), which is hotter than the surface of the sun. What keeps this iron core solid at such high temperature? It is, of course, the tremendous pressure.
The inner and outer core has yet another effect on our planet. As the smaller, solid inner core is isolated from the mantle by the liquid outer core, it spins faster than the rest of the planet. In fact, it gains a full turn about every 300 years. And it's this differential between the rotation of the Earth and the inner core that creates our planet's magnetic field.
Because the asthenosphere is deformable, the continents sink into it until they reach what's termed isostatic equilibrium. This equilibrium is an easy concept to understand for divers because it's determined by the same principles of buoyancy familiar to all of us. As the asthenosphere is essentially a fluid, Archimedes' Principle applies here just as it does in the ocean above or in your own swimming pool. But not only do continents float on the Earth's surface; they also drift.
Sail On, MightyContinent
Because of the apparent natural fit between the continents, it was proposed as far back as the mid-19th century that they were once welded into a single large landmass. (The most obvious example of the apparent fit is between the continents of South America and Africa.) The most ardent supporter of this theory was German meteorologist Alfred Wegener who in 1915 published his theory in a book "Origin of Continents and Oceans." Unfortunately, the scientific community soundly ridiculed Wegener's theory, in part, because of his explanation of the mechanism that propelled the continental drift. He proposed that continents "plowed" through the crust of the ocean basins, driven by centrifugal force and tidal action. Later calculations showed that this was impossible, so Wegener's theory was dismissed. But it wasn't destined to remain in the dustpan of history. In the 1960s, the riddle was solved regarding how continents moved and Wegener's theory about the movement of continents was exonerated. Today, the theory of continental drift is accepted as a tenet of Earth science, and it explains how ocean basins are formed.
Researchers from Scripps Institution of Oceanography and the Lamont-Doherty Laboratory proved that what drives the drifting continents takes place far from land. The mechanism is termed sea-floor spreading, and a shortened version of the theory goes like this: Because the asthenosphere is fluid, and extremely hot, large-scale convective currents can develop within it. (See Figure 2B.) These molten currents rise toward the Earth's surface at regions termed mid-ocean ridges. Where the currents flow toward the Earth's core, deep ocean trenches are created. Thin oceanic crust forms at the mid-ocean ridges and absorbs, literally melts, back into the mantle at the trenches. This process of "absorption" is actually known as subduction. (See Figure 2A.)
A relatively recent example of this sea floor spreading phenomenon can be seen where Africa meets Asia, the Middle East. There, about 40 million years ago, a new area of upwelling began within the asthenosphere. The tension split the crust, forming a classic rift valley (as seen throughout the eastern African continent). As the two continents moved away from each other, a new sea formed, the Red Sea. Along the center line of its bottom is a mid-ocean ridge forming new oceanic crust, which continues to push the continents apart.
The mid-ocean ridges are responsible for a phenomenon that surprised early oceanographers of the 19th century. When they started sailing across the Atlantic they found, as expected, that the sea grew deeper. However, as they continued and approached the middle, the bottom rose in what was later determined to be the largest mountain range on Earth.
Unfortunately, the oceanic ridges are blocked from view. If they weren't, they'd be undeniably the Earth's most incredible features. These spreading centers comprise a continuous mountain stretching 40,000 miles (64,000 km), more than one and a half times the Earth's circumference, girdling the Earth like the stitching on a baseball. At some places the oceanic ridge projects above the sea's surface forming islands. Examples are Iceland, the Azores and Easter Island.
Conversely, the trenches created at the deep subduction zones result in the ocean's deepest regions, such as the Marianas Trench. The enormous friction occurring at the subduction zones is also responsible for extensive volcanic activity. In fact, subduction along the Pacific Plate is so widespread that the area is termed the "Ring of Fire," and explains why volcanic activity is so common in regions as distant as Southeast Asia, Japan, Alaska and the Pacific Northwest.
The Earth's outer crust, although rigid, is not unbroken. Like a cracked hardboiled egg, the outer shell of the Earth is divided into segments called plates. These plates ride on the semi-solid asthenosphere. These lithospheric plates average about 60 miles (96 km) in thickness and are bound by mid-ocean ridges or deep sea trenches. There are six major plates and several smaller ones that are in constant, though imperceptibly slow, movement. (They move at a rate of about 2 inches [5 cm] per year.) As depicted in Figure 2, plates separated by mid-ocean ridges, moving apart from one another, are termed divergent boundaries. Plates separated by deep ocean trenches, or mountain chains like the Himalayas, moving toward each other are termed convergent boundaries. A third type of boundary neither converges nor diverges, but moves laterally. Here, no crust is created nor destroyed, but the plates are separated by major faults. These are termed transform boundaries, and the most famous is the San Andreas Fault of Southern California. This mechanism of movement on the grandest scale, called plate tectonics (tekton from the Greek word for builder), allows continents to literally drift along the surface of the asthenosphere propelled by convective currents, pushing at the spreading sea floor and pulling at the subduction zones.
Although the face of the Earth has changed many times since it cooled enough for the crust to solidify, we're in the midst of a continental journey that began about 600 million years ago. (This was the same period, incidentally, when the first multicellular organisms began to appear). At that time most of today's Southern Hemisphere landmasses were fused into a super continent now known as Gondwanaland. North America, northern Europe and most of Asia remained separate landmasses, but eventually merged to form the continent Laurasia. By 350 million years ago, tectonic movement caused North America and Northern Europe to collide. By about 250 million years ago, Laurasia and Gondwanaland joined, forming the supercontinent Pangaea. Surrounding Pangaea was the single super-ocean known as Panthalassa.
Between 200 and 180 million years ago, Pangaea split again into the northern continent of Laurasia and the southern continent of Gondwanaland. Laurasia broke up, forming the North Atlantic. Thirty million years later, Gondwanaland began to separate, forming the Indian and South Atlantic oceans, the latter being only 100 million years old.
When something happened so long ago, it may appear irrelevant to us today. But in the case of the breakup of Pangaea, its significance to us cannot be overstated; there were several fundamentally important consequences. For one, global temperatures fluctuated as the continent moved around relative to the angle of the sun. Then, as coastlines increased, so did the continental shelves around the margins of the newly formed continent. These coastal waters provided a stable and conducive environment for the evolution of species. But these were not static environments. As the plates continued to move, there were concomitant changes in the environment, and thus organisms had to adapt (evolve) or die. Separation of the continents promoted further isolation of organisms whereby different populations could adapt to the new habitats and environmental conditions. Such isolation eventually led to the evolution of new species. (This is thought to be the driving force behind the high species diversity found in the waters of Southeast Asia.)
Vents in the Bottomof the Sea?
Discovered only in 1977 by famed ocean explorer Robert Ballard, hydrothermal vents are perhaps the most exciting features along oceanic ridges. While making an exploratory dive in the Woods Hole Oceanographic Institution submarine, Alvin, at a depth of 1.9 miles (3 km) along the oceanic ridge East Pacific rise (near the Galapagos Islands), Ballard and his associate, J.F. Grassel, sighted these vents for the first time. What they saw were rock chimneys up to 66 feet (20 m) high, spewing dark, mineral-laden water at incredible temperatures of 660 degrees F (349 degrees C). (Only the extreme pressure at this depth prevents the superheated water from immediately flashing into steam.) For obvious reasons, the structures were dubbed "black smokers," and they've fascinated marine geologists ever since. (Lower temperature vents called "white smokers" have also been found.)
As Figure 3 shows, these vents occur when water descends through fissures and cracks in the sea floor, and contacts the very hot rocks associated with active sea floor spreading. There the water is superheated, dissolves minerals in the rocks, and escapes through the vents by convection. Hydrothermal vents have been found not only in the East Pacific, but also along the Mid-Atlantic Ridge east of Florida, the Sea of Cortez (Gulf of California) and the Juan de Fuca ridge off the Washington-Oregon coast. It's now believed that hydrothermal vents may be common features, especially in areas of rapid sea-floor spreading. A vent has even been discovered at the bottom of the freshwater Lake Bikal in southern Siberia. Given that the entire world ocean cycles through the hot oceanic crust at spreading centers every one to 10 million years, these vents are considered critical in maintaining the chemical balance of sea water.
Hydrothermal vents are also important from a biological perspective. They are the sites of one of the few biological communities on Earth not dependent upon the sun (photosynthesis). Instead, through a process termed chemosynthesis, bacteria use the energy of chemicals emanating from the sea floor to construct organic compounds. These compounds serve as the base of the food chain. Furthermore, vent communities may have played the most vital role in the evolution of life on Earth. Today, many dispute the long-held idea that life began in the shallow tidal pools of Earth's early oceans. Instead, in one of life's remarkable ironies, a growing number of scientists hold that life may have started in these inferno-like and chemically-rich communities of the deep ocean far from the light of day.
The Water Planet:
A Closing Thought
So what may we conclude about the importance of water and, particularly, of the ocean? While there has been much conjecture about the profusion of life on other planets, life, at least as we know it, may not be very common after all. Although water itself may not be scarce, as ice has been found on several planets in our own solar system, this doesn't mean that the conditions by which life can arise are as common. This is because planets where water remains in a liquid state are probably quite rare. In fact, the obstacles to the formation of large permanent oceans of liquid water are considerable. Consider the following:
To maintain a permanent ocean, a planet would have to be at exactly the right distance from a stable star (sun). If the path of the planet were just a little too close, too far or even too elliptical, water would freeze or boil away. And for an ocean to occur, the materials that make up the planet must include water and substances capable of forming an external crust. Moreover, some mechanism such as volcanism would have to exist to allow out gassing and the eventual condensation of rain. Finally, of course, the planet would have to be large enough for its gravity to prevent the atmosphere and the ocean from flinging off into space.
But while the obstacles to the existence of liquid water and development of an ocean are formidable, the obstacles to the emergence of life are even more so. For example, without a magnetic field, a planet might not be able to deflect harmful radiation that would disrupt genetic instructions within cells. Without a moon to create tides, life forms that evolved in the ocean might never make the transition to land. The atmosphere itself would have to be clear enough for some light to penetrate, but still allow enough moisture for rain to form, as well as winds that drive oceanic currents. Lastly, there would have to be some way of further protecting life forms from ultraviolet radiation, such as our ozone layer. This is a tall order.
As the four articles in this series have pointed out, all life on Earth, including us, exists because there is an ocean. The fact is simple: No ocean, no life. So, there was indeed ample reason for the first astronauts who first observed the Earth from afar to describe it as the "Blue Planet." Or, as author Arthur C. Clarke has so aptly remarked, "How odd it is that we call our planet Earth when it is so obviously Ocean."