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OCEANS
AN ILLUSTRATED REFERENCE

Drifting continents and spreading seas

Out of the cloud of dust and gas that condensed to create the Solar System, on the edge of the spiral galaxy we call the Milky Way, the Earth was formed around 4.5 billion years ago. As the third planet out from the Sun, it was neither so close and hot that the surface was covered in a cocktail of poisonous vapors, nor so distant that only frozen wastes prevailed. In fact, it is the only planet we know with an appreciable supply of liquid water on its surface. From hot and tumultuous beginnings, the Earth gradually cooled; as the molten materials separated out, a thin surface crust developed, and slowly the continents and ocean basins were formed. Rain fell and fell, filling up the basins to form the seas and oceans of eons past.

For as long as we can trace back into that distant past, both continents and oceans have been moving, changing, and evolving, at a rate perceptible only in geological time. New crust is continually being formed beneath the oceans; ocean basins grow, only to contract and be again destroyed, leaving ancient ocean water covering a crust many times its junior. Islands rise from the sea, continents grow and divide. The crust and deep interior of the Earth act as a unified system, driven by forces of convection that result from the continued cooling of the planet's core. These processes, known as plate tectonics, form mountain chains and deep-sea trenches, and explain the nature and distribution of earthquakes and volcanoes. This is the great unifying paradigm of the earth and ocean sciences, developed over the past half-century.

The Earth is unique within the Solar System, its liquid oceans and protective atmosphere having survived the tumultuous early years of planetary history. Without water, life as we know it could not exist.

PLATES ON THE MOVE The Ocean Planet

Our search for the origin of life, the universe, and everything goes back to the earliest recorded mythologies. In today's world, science seems to offer the most promising avenue of inquiry into such mysteries, even though the farther back in time we attempt a scientific explanation for the natural evolution of the universe, the less reliable our cognizance becomes. Nevertheless, it is important to establish as accurate a scenario as possible, in order to better understand the development and current nature of the Earth.

Currently, the most widely accepted explanation of the origin of the universe is the Big Bang theory. This derives from astronomical observations that the universe appears to be everywhere expanding, with every galaxy racing away from every other at incredible velocities. Rewinding the clock backward to time zero, it would appear that at what astronomers take as the beginning of time, between 12 and 15 billion years ago, all energy and matter were compressed into an inconceivably dense point, and the universe began in a cosmic explosion, before which moment there was no time, nor space, nor matter, in the sense in which we understand these terms.

Birth of the Solar System

The next few billion years witnessed the expansion and thinning of the universe, coupled with the formation of pockets of concentrated matter from which galaxies were created, each with their own billions of separate stars. Somewhere in the far corner of the universe, a spiral galaxy came into existence, not unlike many others that we can now observe with powerful telescopes. Just over 4.5 billion years ago, a medium-sized star-our own Sun-and a series of planets revolving around it formed along the outer edge of one of the spiral arms.

The Solar System formed from the action of gravity on a rotating cloud of gas and fine dust, known as a nebula. Similar nebulae in outer space far beyond the Solar System are mostly composed of the gases hydrogen and helium, the two elements that make up over 99 percent of our Sun, and a range of dust-sized particles chemically similar to materials found on Earth and the other planets. The diffuse, slowly rotating cloud, originally sub-spherical in shape, contracted under the influence of gravity to become a rapidly spinning, flattened disk, with material concentrated at the center to form a proto-sun.

Under the unceasing pull of gravity, this proto-sun became dense and hot. Temperatures rose to millions of degrees, at which point nuclear fusion-the fusing together of hydrogen atoms under intense heat and pressure to form helium- began. Nuclear fusion releases enormous amounts of energy, and it has continued ever since. This solar energy is the principal, though not the only, source of energy on Earth, fueling the atmosphere and the oceans and giving rise to and sustaining life. Interestingly, the same type of energy, used to destructive effect, powers nuclear bombs.

Grains of dust in the enveloping space around the Sun collided and fused together to form solid bodies, or "planetesimals," ranging from a few meters to thousands of kilometers in diameter. The larger planetesimals finally swept up the smaller ones to form the planets of the Solar System in their present orbits. The four inner planets-Mercury, Venus, Earth, and Mars-are relatively small and made up of dense materials: rocks and metals. Most of the volatile materials were swept to the cold outer reaches of the Solar System, forming the giant planets-Jupiter, Saturn, Uranus, and Neptune, made up of ice and gases. Beyond them lies tiny Pluto, a strange, frozen mixture of methane, water, and rock, while more than three times farther away is the newly discovered planetoid "Sedna" -possibly the most distant object in the Solar System.

Evolving Earth

Somehow, the Earth evolved from a ball of cosmic dust into a habitable planet with clearly differentiated continents and oceans, and an oxygen-rich atmosphere. The first 500 million years of that evolution resemble a lost manuscript telling a tale about which we can only speculate. But there is little doubt that it was a violent story of constant bombardment and intense heat. Astronomers now believe that the Moon originated during this early, tumultuous phase from a cataclysmic impact between Earth and a Mars-sized proto-planet. The impact showered debris into orbit, some of which aggregated to form the Moon. The oldest lunar rocks brought back to Earth are dated at 4.46 billion years, which we can assume was close to the time of impact.

Two other very important events occurred at this stage, with long-lasting significance for the Earth. The first was that the impact is believed to have knocked the Earth askew from a vertical spin axis to its present inclination of about 23°, and may also have had further influence on the nature of the planet's orbit. This change has had a pronounced effect on the Earth's climate ever since. The second was that the heat generated by the impact almost certainly resulted in large-scale melting. Perhaps as much as 50-65 percent of the Earth's total mass melted, and the interior was softened sufficiently to allow differentiation of its components. Heavy metals, principally iron and nickel, sank toward the center, while lighter materials-oxygen, silicon, aluminum-floated to the surface. As the Earth cooled and solidified, it became a layered planet, with a central core, an intermediate mantle, and a thin outer crust, each with very different chemical compositions.

The next crucial phase in the Earth's development was the differentiation of its outer rind into oceanic and continental crust. The composition of these two is very different, as was their formation. Oceanic crust, the more primeval, is made up of dark volcanic rocks, now mainly basalts, which floated to the surface from the molten interior as the Earth was riven and shaken by repeated, violent eruptions. Continental crust, made up of still lighter materials-granites, metamorphic rocks, and sediments-took much longer to develop. Repeated melting and cooling allowed these progressively lighter materials to separate, gradually massing into small "islands"-the cores of primitive continents. Aggressive weathering by torrential rains led to the break-up of these rocks and their redistribution as sediments along the margins of those first, fragmentary landmasses. Countless cycles were repeated, and slowly larger continents formed, their lower density raising them above the surrounding basins. About 7 percent of the present-day continental crust consists of remnants of these early continents, all of them over 2.5 billion, and some up to 3.8 billion, years old. The oldest oceanic crust, by contrast, is little more than 180 million years old, as it is constantly recycled through the mantle.

Oceans and atmosphere

Most important of all to our story, and to the origin of life itself, was the genesis of the oceans that make our blue planet so unique. Like much of the Earth's early history, the details of this momentous occurrence are shrouded in the mists of time. Change has been such a constant factor in Earth's evolution that most of the clues have long since vanished.

The most likely origin of both oceans and atmosphere is from the gases that escaped to the surface during differentiation. Gases emanating from volcanoes today include, among others, water vapor, carbon dioxide, hydrogen, nitrogen, and sulfur dioxide. These are released by the melting of the rocks and minerals in which they are bound. This process, known as degassing, must also have occurred 4 billion years ago, when the first volcanoes began to erupt over a still scorched Earth. Initially these hot gases contributed to the primordial atmosphere-a noxious concoction without the free oxygen that supports most life today. Small amounts of oxygen were released by the action of sunlight on water vapor, but it was not until the appearance of photosynthetic organisms that significant quantities were produced-with great impact on the evolution of life.

Because the Earth is relatively large, the force of its gravity was sufficient to retain an atmosphere. As the planet slowly cooled, the water vapor condensed, and violent storms and endless rainfall lashed its surface, trickling and racing to fill the first ocean basins-with fresh water. Some water was also brought in as a component of extraterrestrial bodies, such as comets, during that early period of intense bombardment.

Carbon dioxide dissolved out of the atmosphere would have made the first oceans rather acidic. These acid waters then reacted with the rocks, releasing neutralizing compounds of calcium and magnesium. The composition of the oceans also changed from largely fresh water to the saline water of today, through further complex interaction between the Earth's rocks, waters, atmosphere, and lifeforms. What challenges scientists is to understand the system of delicate balances that now maintains the oceans and atmosphere in their relatively steady state.

Plate tectonics is an elegant concept that has revolutionized our understanding of the earth-ocean system. Though the movement of the plates is imperceptible to human eyes, the effects are often violent and immediate.

PLATES ON THE MOVE Plate Tectonics and Time

The normal process of science is punctuated by momentous upheavals in thought and the overturning of long-held paradigms. A major scientific revolution occurs only rarely in any one discipline, heralding a great leap forward in understanding and ushering in a new period of intense research that serves to refine and advance the new consensus. Einstein's theory of relativity, Darwin's theory of evolution, and the more recent discovery of the structure of DNA by Francis Crick and James D. Watson are all examples of this process in action. They are some of the seminal ideas and discoveries that have gone on to shape science and the world.

Earth scientists were fortunate to witness the process of just such a revolution in the course of the 20th century, culminating in the paradigm of plate tectonics that gained general acceptance in the early 1960s. This view provides a single unifying theory that can account at the same time for the nature and for many diverse attributes of both oceans and continents, for mountain-building and sedimentary basins, for the morphology of oceans and the mechanism of continental drift, for the location and types of volcanoes and earthquakes across the world, and for much else besides.

Earlier in the century, a quieter revolution had transformed our understanding of geological time, pushing back the limits from the earlier, religious contention that the world began in 4004 BCE. Without the expanded timeframe the new thinking provided, earth scientists' current views on the origin and evolution of the Earth, and on the processes of plate tectonics that have been played out ever since, would have been untenable.

Jigsaw puzzle

The plate tectonic concept is elegant and simple, like many of the great theories that stand the test of time. It holds that the outer layer of the Earth, between 100 and 160km (60-100mi) thick, is made up of a series of large, rigid plates that are in constant motion with respect to one another, jostling and colliding. Plate boundaries are irregular and interlocking, as if in a giant, spherical jigsaw puzzle. Rates of plate motion are imperceptibly slow, being measured in just centimeters per year-roughly the same as the rate of growth of human fingernails.

Not only do the plates move, but their size and shape change constantly as new material is continually added and old material is consumed. This remarkable metamorphosis takes place at plate boundaries, the giant cracks and healed sutures that scar Earth's outer crust, and along which the majority of tectonic activity occurs-earthquakes and volcanic eruptions, hot springs, and heat loss from the Earth's interior. At divergent plate boundaries (spreading centers) new crust is being added, while at convergent margins older crust is consumed or deformed. At transform boundaries, the gigantic, rigid plates simply slide past one another, relatively smoothly but with indescribable force.

An important realization that emerged from the development of plate tectonic theory was that the plates themselves are, in fact, considerably thicker than either the oceanic or continental crust. This crustal material is more like a wrinkled outer rind, tightly bound to the upper mantle. Together, crust and mantle form a relatively cool, and therefore rigid, lithosphere (from the Greek lithos, meaning "stone"). It is this part of the Earth's system that is fractured into a series of distinct lithospheric plates, which ride on a weaker, hotter, partially molten layer known as the asthenosphere (from the Greek asthenes, meaning "weak"). The asthenosphere, like the lithosphere, is around 100-200km (60-125mi) thick.

Why plates move

A second important breakthrough was the understanding of how and why the plates move. Although the details of the mechanism are still being debated and refined by geophysicists, a broad consensus accepts that it is in the main part due to large convection currents within the mantle. Convection is the process, most familiar to us in liquids and gases, whereby hot, less dense material rises and cooler, denser material sinks. It is, essentially, the process that drives the great conveyor belt of ocean circulation and the rapid turmoil of winds in the atmosphere, as much as it does the cooling of coffee in a mug or the rising of smoke up a chimney. At conditions of extremely high temperatures and pressures, however, even the "solid" rocks of the outer Earth can behave as an extremely viscous "fluid" that creeps or flows, and thereby allows convection to occur.

Even after 4.5 billion years, the natural heat contained within the core of the Earth, following the planet's accretion and solidification, is still the principal heat engine driving convection in the mantle. Some heat is also derived from the decay of naturally occurring radioactive elements such as uranium and thorium. Rising plumes of hot mantle occur beneath divergent plate boundaries, building new crust and lithosphere at the mid-ocean ridges. The new matter cools and slowly subsides as it spreads away, eventually sinking back into the mantle at convergent boundaries. It is this slow process of mantle convection, played out over millions of years, that moves the lithospheric plates. Oceans and continents are like passive rafts on the lithosphere, forever moving and changing as the surface manifestation of deeper-seated convection.

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Excerpted from OCEANS by DORRIK STOW Copyright © 2005 by The Brown Reference Group. Excerpted by permission.
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