Читать книгу Waves and Beaches - Kim McCoy - Страница 17

The Earth formed from rocky and metallic fragments during the construction of the solar system—debris that was swept up by an initial nucleus and attracted together into a single body by the force of gravity. The original materials were cold as outer space and dry as dust; whatever water and gases they contained were locked inside individual fragments as chemical compounds. As the fragments joined, the Earth’s gravity increased, attracting larger and larger objects to impact the Earth. This increasing gravity, combined with the timeless radioactive decay of elements like uranium and thorium, caused the new Earth to heat up. The internal temperature was such that many compounds broke down, releasing their water and gases. Plastic flow could occur. Segregation by density began, and the Earth started to organize into its present layered structure. The heaviest metals sank to the center; the lightest materials migrated outward.

FIGURE 1: The Earth has a surprisingly small amount of liquid water. The liquid sphere (b) represents the amount of all water that circulates in the atmosphere, oceans, lakes, rivers, and groundwater of which 97 percent is in the ocean. The remaining tiny sphere (c) represents fresh water, only 3 percent of all water, for crops, drinking, and industry.

The massive heat in the Earth led to motions of its rocky interior, much like the convection cell patterns seen in a boiling pot of soup. In turn, this process led to plate tectonics, a process where a conveyor belt of basaltic lowlands formed as the upwelling cell reached the surface, forming the ocean basins. As the basaltic plates dove down into the Earth’s interior, they also caused melting, leading to the formation of the lightest rocks—granites—that reached the surface and collected over time into the large blocks now known as continents. At the same time, super-heated water and gases were brought to the surface by volcanic activity. The hot, steamy atmosphere cooled and condensed into liquid water, which flowed into low-lying basins. After a few billion years a global ocean had formed, and the atmosphere was sufficiently dense enough that effective winds could exist to transport the ocean’s water vapor over the land. As soon as the evaporation-condensation cycle (hydrological cycle) could operate, rains fell, and stream erosion began. During one wave of cooling (a billion or so years ago), solid water—snow and ice—appeared as glaciers on mountains and ice caps at the poles. Fragments of continental rock were carried downhill by the running water and deposited into the ocean. In colder regions, glaciers ground away at the underlying rocks and provided fine sediments to the flowing waters below. The coarser particles were deposited close to shore; the finer ones were carried out to deep water, where they formed sedimentary deposits that tended to smooth the seafloor and raise the sea level. The motions of the new atmosphere created the first wind waves, and these waves began the attack on the primordial shorelines. Just as they do now, the waves undermined sea cliffs, bringing down large chunks of rock, which were ground against each other by the moving water to form sand. The sand mined from the cliffs and the sand mined inland by streams were intermingled, sorted by the movement of the water, and redistributed along the shore. The first beaches formed.

As these processes proceeded over millions of years (the segregation of materials in the Earth’s interior plate tectonic motions and new water arriving at the surface still occurs), the level of the ocean rose above the edge of its prior natural basin. The prior edges of the continental blocks have been flooded to an average depth of 600 feet (200 m), causing many shorelines today to be sandy and rocky (see figure 2).

It is well to remember that although the shoreline is important as the place where land and water meet, it is not the rim of the ocean in the geological sense. The true ocean basin begins well offshore where the edge of the continental rock slopes steeply into the watery abyss. In the basin the average water depth is nearly 15,000 feet (4,500 m) and the great waves race along at high speeds; on the shallow shelves these same waves are slowed by the drag of the bottom. Therefore, it is on the shallow continental shelves that many of the phenomena described in this book occur. On these shallow shelves, the waves moving landward from the deep ocean are transformed, where they first feel the bottom. It is here where beaches are created and constantly rearranged; where human constructions must meet and resist the force of the ocean’s waves.

When viewed from space, the surface of Earth appears as primarily liquid, but do not be deceived. There is surprisingly little water on Earth in comparison to Earth’s total volume. Of all the water on Earth, about 97 percent is in the ocean (see figure 1, page 22). The forces of nature engage the Earth’s water as a weapon in a constant battle with the land.

The Sun’s radiation warms the Earth, forming the atmospheric and oceanic circulation patterns. Once in motion, all are influenced by the Earth’s rotation. Warmed tropical air rises and is replaced by cooler air from the north or south. This movement, driven by the heat of the Sun and guided by the rotation of the Earth, causes the major winds. Near the equator the air is relatively still (called the doldrums), but not far to its north or south the trade winds blow steadily to the west. At higher latitudes (40 to 50 degrees) the winds blow to the east.

FIGURE 2: The contact between ocean, seafloor, and continent.

These winds raise waves and provide most of the driving force for the great currents of the Earth. The trade winds give rise to the equatorial currents, flowing close to the surface toward the west until they encounter a land mass that turns them away from the equator. The water masses flow to the north or south and eventually close the loop, forming huge eddies, or gyres, that rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Each ocean has these great rivers of water: those in the Atlantic being known as the Gulf Stream (North America) and Benguela Current (Africa); in the Pacific they are the Kuroshio Current (Japan) and the Humboldt Current (Peru-Chile). The smaller Indian Ocean basin has complex ocean currents that reverse their directions (between the Arabian Sea and the Bay of Bengal) during monsoonal conditions.

Waves come in many kinds and sizes, and for that reason it is best to think of them as a continuous spectrum extending from waves so small that they can hardly be seen to waves so long they are unnoticed in the period of a human lifetime. These subtle ebbs and flows of energy change our climate and affect the polar ice caps, sea level, weather patterns, and global winds; the essence of wave formation. Wave and beach processes only exist with the flow of energy. And today humans are influencing the Earth’s energy flows and climate—its seasons, ice caps, storms, and winds, its sediments and sea level—we have become part of the spectrum.