Geology - The Shorelines - Quick Guides | Geology

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Study GuideGeology–The Shorelines1. BeachesBeaches are one of the most familiar and interesting coastal landforms. They are shaped every dayby waves, currents, and sediment movement.What Is a Beach?Abeachis the area of sand or gravel (and sometimes silt) that covers the shoreline. It stretches fromthe low-water edge up to a clear upper boundary, such as a bluff, cliff, or area with vegetation.Key Parts of a Beach•Beach faceThis is the slopingsurface of the beach that faces the ocean. It is usually the steepest partbecause waves hit it directly and cause the most erosion.•Marine terraceJust offshore from the beach face is a gently sloping platform called the marine terrace. It mayonly be visible during low tide. This surface can be made of sediments left behind byretreating waves, or it may be solid rock that has been worn down by waveaction.•BermThe landward edge of the beach is marked by theberm. This is a raised ridge of sedimentdeposited by high-energy waves during strong storms. Berms are often made of coarsematerial and may contain many shell fragments.

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Study Guide1.1 How Sand Moves Along the Beach1.1.1 Longshore DriŌand Beach DriŌWaves usually strike the shoreline at an angle. This angled movement causes sand to travel alongthe coast in a process calledlongshore drift.As waves wash up the beach face, they carry sand at the same angle as the incoming wave. Whenthe water flows back toward the sea, it moves straight downhill. This creates azig-zag patternofsand movement calledbeach drift.Over time, this repeated motion can move sand and pebbles hundreds of meters along the beacheach day.1.1.2 Longshore Currents and Sediment TransportMost beach sediment is carried by thelongshore currentin the surf zone. Breaking waves createfriction with the sea floor, loosening sediment and keeping it suspended in the water. The current thentransports this material over long distances.Eventually, the sediment is deposited and forms different coastal features, including:•Spits–narrow, finger-like ridges of sand extending from the coast•Baymouth bars–sand deposits that can block a bay from the open ocean•Tombolos–bars of sediment that connect an island to the mainland, forming a smallpeninsula1.2 What Beaches Are Made OfMost beaches are made mainly ofquartz sand. This sand usually comes from rivers, is carried to theocean, and then reshaped by waves andcurrents.Because heavier minerals settle more easily, beaches may also contain dark streaks of densemetallic minerals such asmagnetiteandilmenite.In some areas, beach material may include:•Limestone grains•Basalt fragments

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Study GuideThe exact composition depends on nearby rock types and sediment sources.1.3 Seasonal Changes in BeachesBeaches change throughout the year due to seasonal wave patterns.•SummerGentle waves bring sand from deeper water back toward the shore. This builds wider,smootherbeaches.•WinterStrong storm waves erode the beach and carry sand offshore. The sediment is depositedunderwater as asandbar.When summer returns, waves slowly erode the sandbar and move the sediment back onto the beach,rebuilding it. This cycle repeats year after year.1.4 Human Impact and Beach EngineeringEngineers often try to protect beaches and harbors, but these efforts can disrupt natural sandmovement.Common Engineering Structures•BreakwatersBuilt parallel to the shore to create calm water for boats. However, they can cause excessivesediment buildup and may even block off harbors.•JettiesWalls built on both sides of a harborentrance to reduce sediment buildup and protect againstwaves. Usually, one jetty traps sand, while the beach on the opposite side is “starved” ofsediment and begins to erode.•GroinsShort walls built perpendicular to the shore to trap sand and widen eroding beaches. While theyhelp one area, they often reduce sand supply farther down the coast.

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Study Guide1.5 A Delicate BalanceBeaches naturally exist in a balance between sand supply, wind, and wave energy. When humans tryto control this system, the results are often unexpected. Changes in one place usually createproblems somewhere else, making beach management a challenging task.2. Shoreline FeaturesThecoastis the strip of land located next to the ocean. It includes the beach and the nearby inlandarea. Coastlines can look very different from one another. Some are steep, rocky, and rugged, whileothers slope gently and are covered with sand.2.1 Paleocoasts: Old CoastlinesSome coastlines are calledpaleocoasts. These are ancient coasts that were once underwater. Later,rising sea levels covered them, and tectonic forces eventually lifted them above today’s sea level. Asa result, features that once formed on the ocean floor are now exposed on land.2.2 How Waves Shape Rocky CoastsWavesconstantly crash against the shoreline, and over time, they can change even the hardest rock.•Limestoneslowly dissolves in seawater.•Soft rockswear away easily.•Evenhard rocks like granitecan crack and break due to repeated wave impact.Rocky coastlines are often irregular, with many small bays and inlets. The rocky points that stick outinto the ocean are calledheadlands, and the curved indentations between them are calledcoves.2.3 Coastal StraighteningOver time, wave action tends tosmooth and straighten an irregular coastline.•Headlandsreceive the strongest wave energy, so they erode faster.•Baysare more sheltered, so erosion happens more slowly there.

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