Chapter 16 – Wind, Waves, and Currents Ms. Martel

16.1 – Wind as an Agent of Change Like rivers and glaciers, winds cause changes to Earth’s surface. Winds pick up and move sediment, they also drive sediment against rocks and other materials. Winds also play a role in the formation of ocean waves, which erode shorelines.

Windblown Rock Material Windblown erosion and weathering are most pronounced where sands, silts, and clays lie loose and dry. Optimum conditions for wind erosion are found in great deserts, such as the Sahara and the Mohave.

When strong, steady winds lift great amounts of silt and clay from the topsoil, a dust storm results. Dust in the atmosphere can be transported over great distances, even across oceans. Sand grains are much larger and heavier than clay and silt particles, and require 18 km/hr winds in order to move them.

Deflation Deflation is the removal of loose rock particles by the wind. In many desert areas the sands and clays formed by weathering are blown away by winds, leaving pebbles and boulders. The resulting surface is called desert pavement. In semiarid regions, deflation has formed thousands of hollows called blowouts. Most of these are shallow and small, but some are many thousand meters long and about 100 m deep. If the bottom of the blowout reaches the water table, the wet ground stops deflation. The growth of vegetation also halts deflation.

Abrasion Windblown silt and clay particles are too small, and often too soft, to wear away most rocks. Sand grains driven by winds grind and scour most surfaces they hit. Blasts of desert sand grind boulders and small rocks into shapes called ventifacts.

Loess Wind can deposit sediment as well as remove it. Loess is made of un-layered silt-sized particles that are generally yellowish in colour. Loess particles are angular in shape and have been weathered from many different minerals and rocks. Loess particles hold together so well that when loess erodes pieces of it split off vertically to form clifflike slopes.

Sand Dunes Sand dunes are hills of sand deposited by winds. Sand dunes are found wherever there are strong winds and enough loose sand, such as in the Sahara. Sand dunes also form on sandy river floodplains in semiarid climates and on sandy beaches.

Most sand dunes are made of quartz sands, but there are exceptions. Dunes range in size from a few meters high to more than 100 m high and many 1000 m long. Most sand dunes are made of quartz sands, but there are exceptions. Dune sand may contain grains of additional minerals, such as feldspar, mica, and magnetite. The shape of a dune seems to depend on the supply of sand, the strength and steadiness of the winds, and the amount of vegetation present. Shapes of sand dunes include: Barchans Parabolic Transverse Longitudinal

16.2 – Waves in the Sea An ocean wave is a rhythmic rise and fall of the water’s surface. When ocean waves reach the shore, they erode and shape the shoreline. Ocean waves are generally produced in 3 ways: Undersea earthquakes Gravitational pull of the moon Most commonly winds

Winds and Waves When a gusty storm wind blows over open water in an ocean or a lake ripples form. If the wind continues to blow, or if it blows over a long distance, the ripples grow larger until they become waves. The height of a wind formed waves depends on 3 things: Wind speed Length of time wind blows And the fetch The fetch is the length of the open water over which the wind blows. The fetch of a lake is shorter than that of an ocean. Thus lake waves are not as high as ocean waves.

Features of Water Waves Wave height is the difference between a wave’s high point, or crest, and its low point, or trough. Wavelength is the distance from one crest to the next. Strong winds produce waves with long wavelengths. The period of a wave is the time it takes one wavelength to pass a given point. To find the speed of a wave, divide its wavelength by its period.

Wave Motion You may have seen objects bobbing in the water as waves passed beneath them. Waterborne objects are not carried forward with a wave; instead they move in place in a circular motion. The water in the wave moves in the same way. As depth increases, the water moves in smaller and smaller circles. As each water particle moves, it bumps into another particle, passing its energy along. In this way, the wave’s energy is transferred through the water.

Wave Refraction Most waves approach the shoreline at an angle. The swinging or bending is called refraction. Refraction occurs because the end of the wave closest to shore scrapes bottom first and slows down. The end that is still in deeper water continues at its normal speed and catches up. Thus the wave ends up nearly parallel to the shore. MAKING WAVES MINI LAB

Breakers Waves usually approach the shoreline smoothly until they reach water so shallow that they touch the bottom. This occurs where the water depth is about half the wavelength. As a wave scrapes bottom, the circular motion of the wave is distorted, and the lower part of the wave slows down. The upper part of the wave moves ahead until there is no longer enough water to support it. The crest falls over and breaks into surf

Shoreline Currents Waves, like the winds that form them, may come from any direction. Thus many waves approach the shoreline at an angle. The motion of water up the beach is called swash. Most of the water runs back down the beach under the next wave in a gentle current called backwash.

The water behind the line of breakers is also pushed toward shore by waves and pulled back by backwash. This movement forms a so called longshore current, which runs almost parallel to shore. Longshore currents are important in the movement of sand and in the formation of sandbars.

Rip currents are strong surface currents that flow away from the beach. They may form where too much water builds up in the area where surf forms. Rip currents are dangerous to swimmers. A rip current may carry a lot of sand, which makes it visible. Waves around the rip current may be steeper than the surrounding waves.

16.3 – Shoreline Features Waves and Erosion Breaking storm waves may strike rock cliffs with a force of thousands of kg per square meter. When waves strike he headlands of a deep- water shoreline, they may cut away the rock up to the high-tide level, forming a notch. If the materials overhanging the notch collapse, a sea cliff results. Cliffs made of soft materials such as soil and sand wear away very quickly. Cliffs made of harder rock materials, notches may deepen until it becomes a sea cave. Sea arches may form when waves cut through vertical cracks in narrow headlands.

Beaches Geologists define a beach as the area of shore between the high-tide level and the low-tide level. The makeup of the beach depends on both the available material and the slope of the shoreline. Most of the sand on beaches has been deposited by rivers. Most beach sands are grains of durable minerals, such as quartz and some feldspars.

Sandbars Along irregular shorelines, longshore currents carry away most of the sand and pebbles eroded from the headlands. Often the current carries enough sand to form a sandbar across the mouth of the bay. The sandbar seems to grown right out of the end of the headland, this structure is called a spit. Sandbars usually protect the water behind them from strong winds and waves. These protected areas are called lagoons

Sandbars may also form on coasts with straight shorelines. These bars are not attached to the shore. They are called barrier islands. Barrier islands, sandbars, and beaches are not permanent features. The sand that composes them are constantly being removed by waves, storms, and longshore drift.

Irregular Shorelines Most irregular shorelines appear to have formed when the coastal areas were flooded by the sea. This flooding occurred either because the land sank or the sea rose. Glacial valleys were formed below the present-day sea level. When the glaciers retreated, the sea flooded parts of the valleys, forming long, deep, steep-sided bays called fjords. The tributary valleys of fjords are hanging valleys.

Regular Shorelines The shorelines of the west coast of North and South America are comparatively regular. They do not have many deeply indented inlets or drowned valleys, although they do not have a number of shallow coves. The reason for their regularity is that these shorelines are on the boundary line between two sets of plates.