Agents of Erosion and Deposition

Shoreline Erosion and Deposition

Shoreline Erosion and Deposition The results of erosion can often be dramatic. For example, this sinkhole formed in a parking lot in Atlanta, Georgia, when water running underground eventually caused the surface of the land to collapse. Section 1

Wave Energy When waves crash into rocks over long periods of time, the rocks are broken down into smaller and smaller pieces until they become sand. Waves usually play a major role in building up and breaking down the shoreline. A shoreline is the boundary between land and a body of water.

Wave Energy, continued 2 As the wind moves across the ocean surface, it produces ripples called waves. The size of a wave depends on how hard the wind is blowing, the distance over which it blows (fetch) and how long the wind blows. The wind that results from summer hurricanes and severe winter storms produces large waves that cause dramatic shoreline erosion. This photo, taken while the Virginia Beach Erosion Control and Hurricane Protection Project was underway, shows the significant difference between the unimproved area (top of photo) and the area of the widened beach berm already completed.

Wave Energy, continued 3 Wave Trains Waves travel in groups called wave trains. When wave trains reach shallow water, the bottom of the wave drags against the sea floor, slowing the wave down. The upper part of the wave moves more rapidly and grows taller, and begins to curl and break. These breaking waves are known as surf. The time interval between breaking waves is called the wave period.

Wave Energy, continued 4 The Pounding Surf Tremendous energy is released when waves break. Crashing waves can break solid rock and throw broken rocks back against the shore. Breaking waves also wash away fine grains of sand, which are picked up by the waves and wear down and polish coastal rock. The process continues until rock is broken down in smaller and smaller pieces that eventually become sand. Breaking waves crash against the rocky shore, releasing their energy.

Wave Erosion Shaping a Shoreline Wave erosion produces a variety of features along a shoreline. Much of the erosion responsible for coastal landforms takes place during storms. Sea cliffs are formed when waves erode and undercut rock to produce steep slopes. The next two slides show some of the major features that result from wave erosion.

Wave Deposits Beaches are areas of the shoreline made up of material deposited by waves. Some beach material is also deposited by rivers. Waves carry a variety of materials, including sand, rock fragments, dead coral, and shells. The colors and textures of beaches vary because the type of material found on a beach depends on its source.

Wave Deposits, continued 2 Wave Angle and Sand Movement Waves moving at an angle to the shoreline push water along the shore and create longshore currents. Longshore currents move sand in a zigzag pattern along the beach.

Wave Deposits, continued 3 Offshore Deposits When waves erode material from the shoreline, longshore currents can transport and deposit the material offshore, which creates landforms in open water. A sandbar is an underwater or exposed ridge of sand, gravel, or shell material. A barrier spit is an exposed sandbar connected to the shoreline. A barrier spit, such as Cape Cod, Massachusetts, occurs when an exposed sandbar is connected to the shoreline.

Wind Erosion and Deposition

The Process of Wind Erosion Saltation is the skipping and bouncing movement of sand or other sediments, caused by wind or water. Moving sand grains knock into one another, bounce up into the air, fall forward, and strike other sand grains, causing them to roll and bounce forward.

The Process of Wind Erosion, continued 2 Deflation is a form of wind erosion in which fine, dry soil particles are blown away, removing the top layer of fine sediment or soil and leaving behind rock fragments that are too heavy to be lifted by the wind. Deflation may cause desert pavement, which is a surface consisting of pebbles and small broken rock. Scooped-out depressions in the landscape are called deflation hollows. Desert pavement, such as that found in the Painted Desert in Arizona, forms when wind removes all the fine materials.

The Process of Wind Erosion, continued 3 Abrasion is the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles. Abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks. The blowing of millions of sharp sand grains creates a sandblasting effect, helping erode, smooth, and polish rocks. Picture shows the powerful effect of wind generated abrasion is the Double Arch from Arches National Park.

Wind-Deposited Materials Loess is a deposit of windblown, fine-grained sediment. Because wind can carry fine-grained material much higher and farther than it carries sand, loess deposits are sometimes found far from their source.

Wind-Deposited Materials, continued 2 Dunes When the wind hits an obstacle, the wind slows down, depositing the heavier material. The material collects, creating an additional obstacle and eventually forming a mound that buries the original obstacle. The mounds of wind-deposited sand are called dunes. A dune keep its shape, even though it moves.

Wind-Deposited Materials, continued 3 The Movement of Dunes Different wind conditions produce dunes in various shapes and sizes. A dune usually has a gently sloped side and a steeply sloped side, called a slip face.

Erosion and Deposition by Ice

Glaciers—Rivers of Ice A glacier is a large mass of moving ice. They are capable of eroding, moving, and depositing large amounts of rock materials. Glaciers form in areas so cold that snow stays on the ground year-round. Because glaciers are so massive, the pull of gravity causes them to flow slowly, like “rivers of ice.”

Glaciers—Rivers of Ice, continued 2 Alpine Glaciers form in mountainous areas. One common type of alpine glacier is a valley glacier. Valley glaciers form in valleys originally created by stream erosion. As these glaciers slowly flow downhill, they widen and straighten the valleys into broad U shapes. Valley Glacier: a valley flowing glacier. These glaciers may be the combination of several smaller glaciers joining and flowing together down a large valley.

Glaciers—Rivers of Ice, continued 3 Continental Glaciers are huge, continuous masses of ice that can spread across entire continents. The largest continental glacier in the world covers almost all of Antarctica. This ice sheet is approximately one and a half times the size of the United States, and is more than 4,000 m thick in some places.

Glaciers—Rivers of Ice, continued 4 Glaciers on the Move When enough ice builds up on a slope, the ice begins to move downhill. Thick glaciers move faster than thin glaciers, and the steeper the slope, the faster the glaciers will move. Glaciers move in two ways: sliding and flowing. A glacier slides when its weight causes the ice at the bottom to melt. A glacier flows as ice crystals within the glacier slip over each other.

Glaciers—Rivers of Ice, continued 5 Glacier movement is affected by climate. As the Earth cools, glaciers grow. About 10,000 years ago, a continental glacier covered most of North America.

Landforms Carved by Glaciers Continental glaciers and alpine glaciers produce landscapes that are very different from one another. Continental glaciers smooth the landscape by scraping and eroding features that existed before the ice appeared. Alpine glaciers carve out large amounts of rock material and create spectacular landforms.

Landforms Carved by Glaciers 2

Glacial Landscape Features

Types of Glacial Deposits As a glacier melts, it drops all the material it is carrying. Glacial drift is the general term used to describe all material carried and deposited by glaciers. Glacial drift is divided into two main types, till and stratified drift.

Types of Glacial Deposits, continued 2 Till Deposits Unsorted rock material that is deposited directly by the ice when it melts is called till. Unsorted means that the till is made up of rock material of different sizes. The most common till deposits are moraines. Moraines generally form ridges along the edges of glaciers.

Moraines

Types of Glacial Deposits, continued Stratified drift is a glacial deposit that has been sorted and layered by the action of streams or meltwater. Streams carry sorted material and deposit it in front of the glacier in a broad area called an outwash plain. Sometimes, a block of ice is left in an outwash plain when a glacier retreats. As the ice melts, sediment builds up around the block of ice, forming a depression called a kettle.

The Effect of Gravity on erosion and Deposition

Angle of Repose Gravity is an agent of erosion and deposition. It influences the movement of water and ice, and it causes rocks and soil to move downslope. Mass movement is the movement of any material, such as rock, soil, or snow, downslope.

Angle of Repose, continued 2 Material such as rock, soil, or snow moves downhill until the slope becomes stable. The angle of repose is the steepest angle at which loose material will not slide downslope. The angle of repose is different for different surface material. Size, weight, shape, and moisture level determine at what angle material will move down-slope. If the slope on which material rests is less than the angle of repose, the material will stay in place. If the slope is greater than the angle of repose, the material will move downslope.

Rapid Mass Movement Rock falls happen when loose rocks fall down a steep slope. The rocks can range in size from small fragments to large boulders. Mass movements, like rock falls, happen suddenly and rapidly, and can be very dangerous.

Rapid Mass Movement, continued 2 Landslides are sudden and rapid movements of a large amount of material downslope. .

Slumps The most common type of landslide is a slump. Slumping occurs when a block of land becomes detached and slides downhill

Rapid Mass Movement, continued 3 Mudflows are rapid movements of large masses of mud. Mudflows happen when a large amount of water mixes with soil and rock. The water causes the slippery mass of mud to flow rapidly downslope. Mudflows commonly happen in mountainous regions when a long dry season is followed by heavy rains. Mudflow-damaged house along the Toutle River. The height of the mudflow is shown by the "bathtub-ring" mudlines seen on the tree trunks and the house itself. Caused by eruption of Mt. St. Helens May 18, 1980.

Rapid Mass Movement, continued 4 Lahars are mudflows caused by volcanic eruptions or heavy rains on volcanic ash. Lahars can travel at speeds grater than 80 km/h and can be as thick as cement. On volcanoes with snowy peaks, an eruption can suddenly melt a great amount of ice. Water from the ice liquefies the soil and volcanic ash to produce a hot mudflow that rushes downslope. A lahar overtook this area on the island of Kyushu in Japan.

Slow Mass Movement Creep is the slow mass movement of material downslope. Although rapid mass movements are visible and dramatic, slow mass movements happen a little at a time. However, slow mass movements occur more frequently, and more material is moved collectively.