Читать книгу Waves and Beaches - Kim McCoy - Страница 19

The crust of the Earth is slowly but constantly shifting; the continents act much like great rafts of rock, floating on the viscous interior of the Earth and reshaped by plate tectonics. Consequently, if a load is added to the top of the raft—by a huge volcanic outpouring of lava or the accumulation of a great mass of ice, for example—the raft will sink a little and the sea level will appear to rise. By the same reasoning, as erosion removes land mass and large ice sheets melt, the load is lightened and the land rises. This rising of the land as ice sheets melt is referred to as post-glacial rebound. For example, a number of embayments on the Alaska coast that were used as harbors a century ago are now too shallow to be navigable because that part of the continent has risen. Seventy million years ago England’s famous White Cliffs of Dover were a shallow seafloor. Many types of forces are at work—everything changes.

Other forces deep in the Earth also cause the great blocks of continental rock to move up and down and the ocean level to rise and fall. These major crustal movements occur very slowly, but as they do, the shoreline—which is especially sensitive to such changes—advances and retreats. Many geologists classify coasts according to whether they are submerging or emerging from the sea and whether erosion of rock or deposition of sediment has the upper hand. For example, much of the central California coast is rising, from Monterey to Mendocino. This movement is evidenced by the existence of terrace-like remnants of old sea bottom now well above sea level. Along the Northwest coast (i.e., Puget Sound) and Northeast coast (i.e., Hudson River Valley to Maine), large segments are described as drowned topography, meaning that the land has sunk relative to sea level. Because the original topography was largely hills and valleys, in both of these areas the shoreline is very irregular. Beaches tend to be narrow, short, and rocky; they do not form an important part of the coast.

Most of the East Coast from New Jersey to Florida is nearly straight because the submerged land has a long gentle slope that extends from many miles inland to the edge of the continental shelf 100 miles (160 km) offshore. Neglecting the rapid changes of the past century, this coast is stable, as is the coast of the Netherlands, and has not changed its elevation with respect to the ocean for a long period of time. Such coastlines are characterized by an almost continuous line of sandy barrier islands with great, wide beaches. Between these elongated islands and the mainland is a series of shallow bays and lagoons. So the basic shape, or the geomorphology, of a coast is the result of the ocean’s history of interactions with the land. If there is an ample sand supply and if enough time elapses without a major change in elevation, the beach will become an influential part of the coast.

Most coasts have a rather complicated geologic history. Relative to sea level, they have at various times emerged and submerged again, each time retaining some features left from the previous iteration. Moreover, because the marine processes are usually interrupted before they are complete, there are relatively few examples of “finished” work. The geologist is thus forced to observe changing situations and guess how the process started and what forms it will eventually produce. The principal concern becomes determining the mechanism that causes the changes and the rate at which the changes are taking place. Then, perhaps, the future can be forecast.

FIGURE 5: The anatomy of a beach, showing the locations of coast, berm, shoreline, beach face, and sandbar.

Before moving on, it’s important to define three terms—shoreline, coast, and beach—that often cause confusion. A shoreline is the line of contact between water and land. A coast is a large physical geographic feature often extending several miles inland from the shore and several hundred miles along it. By comparison a beach is a relatively small feature of the coast, near the water’s edge, whose limits are defined by the effects of waves (see figure 5).

A beach is an accumulation of rock fragments subject to movement by ordinary wave action. A tide will extend the process up and down a beach face. Beaches may be composed of any kind or color of rocky material, ranging in size from boulders to fine sand. Because most of the beach material along the most heavily populated part of the US coast consists of a light-colored sand—which is created as a result of the weathering of granitic rock into its two main constituents, quartz and feldspar—most of us tend to think of beaches as stretches of white sand. Some white beaches in Florida are made of finely sorted quartz sand as in Destin, Florida, whereas others are composed of carbonates: by-products from the waves grinding up coral reefs, shells, and other organisms. But many Pacific island beaches are made of black sand, formed by the disintegration of dark volcanic rocks. Many English beaches are composed of small flat stones called shingle, formed from the destruction of sea cliffs made of sedimentary rock, and many Alaska beaches consist of large cobbles. And for a hundred miles along the coast of Baja California, Mexico, the beach (see photo above) is made of two materials: a flat sandy portion that is exposed only at low tide, while immediately above and behind the sand, great cobble ramparts rise to a height of 30 feet (10 m) or more. Our idea of a beach depends on what we have been exposed to. In this book, for convenience, all beach material will be called sand, although it is recognized that all the features described may be formed in pebbles or shingle or cobbles. We will not, however, explore in detail the complex study of sand, its composition, grain sizes, and mechanisms of formation, as it is outside of the scope of this book.

In Baja California each high tide erodes the cliff, permitting endless waves to churn cobbles into sand. Baja California, Mexico. Kim McCoy

Moving ahead, we can think about beaches in two ways: (1) as small closed systems in which the sand moves either onshore or offshore at the whim of the waves, or alongshore in accordance with currents; and (2) as geologic units of considerable size.