Continents, tectonic plates, and plate boundaries. The Burma plate (not shown) is a small plate between the Indian and Australian plates. 

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... of the two crusts and therefore floats higher than the dense oceanic crust. The Earth's crust, both continental and oceanic, and the rigid uppermost part of the mantle form the planet's lithosphere. The lithosphere is segmented, rather like a poorly sewn soc- cer ball, into rigid plates that shift with the slow motions of the deeper mantle ( fig. 1). The term plate tectonics refers to the interactive motions of the plates and the mantle. The 16 or so major and minor plates vary in size, ranging from the enormous Pacific plate, the largest of the seven major plates, to the much smaller slabs of lithosphere such as the Caribbean ...

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... does concern us about our changing climate is the amount of incoming solar energy that the Earth absorbs, which varies as the result of many factors ( fig. 10). Clouds, for example, reflect incoming sunlight and reduce the amount absorbed; the oceans absorb more sunlight than land; ice-free regions absorb more sunlight than ice-covered regions. Volcanic ash, aerosols, and gases in the upper atmosphere reflect sunlight and can lead to short-term global cooling. Because geologic and atmospheric ...

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... finally attained its most southerly position a little more than 20,000 years ago when the Laurentide ice sheet reached its southernmost limit along Cape Cod and Long Island. The southernmost edge of the ice margin at that time followed approximately the present southern coasts of Long Island, Block Island, Martha's Vineyard, and Nantucket ( fig. 11). Retreating north after a few thousand years, the ice lobe margins next took a position along the present location of the ...

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... high ridges that form the backbone of the upper and mid-Cape are moraines that probably formed by a combina- tion of these two processes. Their locations and orientations indicate that they were deposited by the Buzzards Bay and Cape Cod Bay Lobes of the glacier (fig. 12). To the east of the current shore of the lower Cape, a third moraine associated with the South Channel Lobe probably ran from north to south. Coastal erosion would have removed the moraine long ago, leaving an area of submarine shoals behind. The ice sheets often carried large boulders; as the ice melted, the boulders, called glacial ...

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... such lake, Glacial Lake Cape Cod, lay between the Sandwich Moraine and the Cape Cod Bay Lobe when it had retreated north of the present-day location of Provincetown ( fig. 12). The South Channel Lobe, lying to the east, was still in place and supplying layer after layer of sandy sediment to its outwash plain. Those sediments were deposited in the shallow waters of Glacial Lake Cape Cod, and for this reason they are sometimes referred to as deltaic deposits. Most of the upland of the outer Cape consists of ...

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... Lake Cape Cod, lay between the Sandwich Moraine and the Cape Cod Bay Lobe when it had retreated north of the present-day location of Provincetown Ponds were produced when rising groundwater flooded the kettle hole floors. Figure 12. Glacial retreat (18,000 years ago) and Glacial Lake Cape Cod, Mass.; redrawn from Uchupi and others (1996). ...

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... rise, but rather because coasts are in an almost continual state of change in response to waves and currents as well as sediment avail- ability at the coast, sea-level rise leads to shoreline change by altering these processes and the coast's response to them. This is especially true for shoreline changes observed during the past century ( fig. 13). During the past century, major storms, variations in sediment supply to the coast, and human activity have had direct effects on shoreline change. Large storms coupled with elevated sea level can cause changes in shoreline position that persist for weeks to decades or even longer. Complex interactions (the mechanics of which are ...

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... (1.7 mm/yr) on a global scale in the 20th century with fluctuations occurring throughout the century. From 1993 to 2014, observations from satellite altimeter and tidal gages indicate that the rate of eustatic sea-level rise increased to more than 0.13 in/yr (3 mm/yr), a 50 percent increase compared with sea-level rise during the 20th century ( fig. 14). Given this short record, it is not yet possible to determine with certainty whether this is a natural variation averaged over a decade or a definitive acceleration in sea-level rise due to observed climate warming. Recent studies suggest that the increase is due to equal contributions from ocean thermal expansion and ice-sheet melting ...

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... crust in this region having been depressed by the enormous weight of glacial ice. As the ice retreated, the crust rebounded, and by about 12,000 years ago, falling relative sea level had exposed an enormous coastal landmass comprising glacial deposits and including almost all of present Cape Cod, Martha's Vineyard, Nantucket, and Georges Bank ( fig. 15). Most of the floor of Cape Cod Bay was part of that landmass, but it was a low-lying plain surrounded by higher land to the east, south, and west. The spit at Provincetown was not present-it had yet to form-but most of the lower Cape was much larger than it is today and extended eastward as much as 4 mi (6 km) seaward of the present ...

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... the rate of crustal uplift slowed, rapidly rising global sea levels resulting from enormous volumes of meltwater Figure 15. The changing shape of Cape Cod, Mass., during the past 10,000 years and the development of the hook-shaped spit at Provincetown; redrawn from Uchupi and others (1996). ...

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... present Cape Cod, Martha's Vineyard, Nantucket, and Georges Bank flowing into the oceans from the rapidly melting continental ice sheets became dominant and seas flooded the margins of the ancient glacial landmass. The rate of sea-level rise, fed by melting of the northern ice sheets in North America and north- ern Europe, was extremely rapid ( fig. 13), and by 6,000 years ago, the present outlines of the Cape and its many islands were emerging. The outer Cape was still without Provincetown and was still wider than today on both the eastern and western shores. The coast of Cape Cod Bay had a convex form, sweep- ing southward from High Head, then gradually southwest- ward over the ...

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... of the spit at Provincetown Hook during the past 6,000 years had a striking effect on the bay shore of the outer Cape ( fig. 15). At the beginning of the period, the western coast had extended southwestward from High Head to the pres- ent Billingsgate Shoal in Cape Cod Bay. Unimpeded north- westerly wind waves moved eroded sand southward all along the coast until the growing spit at Provincetown Hook began extending westward and blocking those ...

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... tropical and polar zones are relatively small. In contrast, temperature differences between ocean and conti- nent are pronounced and steady. As a result, the semipermanent high pressure system known as the Bermuda High forms over the relatively cool western North Atlantic and drives a clockwise circulation over the East Coast of the United States (fig. ...

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... pebble disturbs the water surface, gravity acts to restore it to the original condition, and inertia causes the water to over- compensate. The upshot for a water particle is orbital motion, moving in the direction of wave travel under the crest and in the opposite direction under the trough (fig. ...

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... although the energy flows in the form of waves across the water surface, out and away from the original energy source, the water particles themselves move around and around in their orbits, but do not have net motion in the direction of wave travel. Figure 17. Circulation of water particles associated with a simple progressive surface wave. ...

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... there are limits. As wind-generated waves grow in height, they also grow in length ( fig. 17). In fact, they must in order to survive because, when a wave's height reaches one- seventh its length, it becomes unstable and breaks. So wind- generated waves become higher and longer as the wind contin- ues to blow. But there's a catch; the longer the length of a wave is, the faster it travels, and the faster the wave travels, the ...

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... the slowing of wave travel in shallow water is the bending of wave crests termed refraction. When waves approach the shore at an angle (as they usually do) or when water depths vary alongshore (and they usu- ally do, too), the shallower part of a single wave's crest slows relative to the deeper parts, causing the wave crest as a whole to bend ( fig. 18). Typically, the crests of refracting waves tend to more nearly parallel the shoreline as they shoal. However, waves traveling over off- shore shoals and sandbars may refract away from the shore, sometimes producing complex patterns Wave period.-Despite all these changes associated with shoaling, one major characteristic of incoming ...

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... when on some calm August morning, for example, we note long, low, 14-second swell arriving at the Highlands Center wave observation station, we think immediately of tropical storms, because our calculation indicates deep-water wave lengths greater than 1,000 ft (300 m), and that requires an impressive combination of wind speed, duration, and fetch such as only an intense ocean-storm system can produce. Figure 18. Wave refraction associated with shoals. ...

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... the role of water depth increases as depth itself decreases in relation to the length of the traveling waves. A point is finally reached, when the depth is only about a 20th the length of the wave, where water depth alone determines the speed of the waves. Waves whose speeds are controlled only by water depth are known as shallow water waves ( fig. 19). Easterly to northeasterly winds send waves directly onto the shore, and the unsorted mix of waves in a chaotic, fully developed sea ensure turbulent surf at the shore that rapidly erodes beaches, bluffs, and ...

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... and neap tides.-The solar tide, its cycle being faster, would catch up with the lunar tide about every 2 weeks, and the two tides added together would produce a single extra-large tide. This would happen when the Sun and the Moon are aligned with each other at periods of new and full moon ( fig. 21). Such large-range tides are generally known as spring tides, though they are locally sometimes called moon tides due to their association with lunar phases. Note that spring tides have no relation to seasons. Similarly, at half-moon periods (first and last quarter moons), the lunar and solar tides would be opposed to one another ...