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Chapter 7 Review: Glaciers, deserts, landscape shaped by wind. Review: Chapter 7 Glaciers, Deserts, Landscape shaped by Wind. Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind. As recently as 15,000 years ago, up to 30 percent of earth’s land was covered by an glacial ice.
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Chapter 7 Review: Glaciers, deserts, landscape shaped by wind Review: Chapter 7 Glaciers, Deserts, Landscape shaped by Wind
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • As recently as 15,000 years ago, up to 30 percent of earth’s land was covered by an glacial ice. • Earth was covered by an ice age • A glacier is a thick ice mass that moves slowly over the land surface. • Glaciers originate on land in places where more snow falls each winter than melts in summer. • The snowline is the lowest elevation in an area that remains covered in snow all year.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers are also important agents of erosion. Like rivers, they accumulate, transport, and deposit sediment • Unlike mountain streams, mountain glaciers advance only a few millimeters to meters a day. • A valley glacier is a stream of ice that flows between steep rock walls from a place near the top of a mountain valley.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Ice sheets are enormous ice masses that flow in all directions from one or more centers; much larger than glaciers. • The only present day ice sheets are those covering Antarctica and Greenland • The Antarctic ice cap holds 80% of the world’s ice and nearly two thirds of the world’s fresh water.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • The movement of glaciers is referred to as flow. • Glacial flow happens in two ways: • Plastic flow • Basal slip • Plastic flow involves movement within the ice • Basal slip is when the entire ice mass actually slips downhill due to gravity. The layers closest to the surface move faster than the layers closer to the top.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • The uppermost zone that does not have plasticity is brittle. We call this zone thezone of fracture. • The zone of fracture experiences tension when the glacier moves. This tension results in deep cracks calledcrevices. • Different glaciers move at different rates. Glaciers may move forward, retreat, or stay in place.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • In the zone of accumulation; above the snowline, the glacier gains ice and forces movement downslope. • The area of the glacier below the snowline is where the glacier melts, losing ice and mass. We call this area of glacial melting the zone of wastage.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers lose ice when large pieces break off their front edge in a process called calving. • Calving creates icebergs when glaciers meet the ocean.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • The glacial budget is the balance between the amount of snow-ice accumulated at the top of the glacier and the amount of loss at the glacier’s foot. • If more ice forms at the top of the glacier than melts at the bottom, than the glacier advances. • If the ice melts faster at the bottom than the glacier accumulates at the top, than the glacier retreats.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers erode the land in two ways: • Plucking • Abrasion • Pluckingoccurs when rocks are broken loose from under the glacier. As the glacier flows, it loosens and lifts the rocks and carries them with the ice flow, plucking them from the Earth. • In abrasion, the load of rock and ice combined acts like sandpaper as it slides over the surface.This leaves telltale lines going in one direction showing the direction of ice flow. These marks are called glacial striations.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • As with all agents of erosion; the rate of erosion is controlled by several factors. • rate of glacier’s movement • thickness of the ice • shape, hardness, and amount of rock fragments in the ice at the glacier’s front edge • the type of surface and hardness of the rock beneath the glacier
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers are responsible for a variety of erosional landscape features. Among these are • Glacial troughs: When a glacier moves through a mountain valley, it widens, deepens, and straightens this valley. The once V shaped valley becomes a U shaped glacial trough. • Hanging Valleys: Main glaciers cut deep U shaped valleys that are deeper than the those carved by smaller side glaciers that feed into the main glacier. • Cirqueis a bowl shaped depression at the head of a glacial valley that is surrounded on three sides by steep rock walls. • Aretes are snaking, sharp-edged ridges produced when cirques form on either side of a divide • Hornsarepyramid shaped peaks produced when several cirques surround a mountain.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers transport huge loads of debris as they slowly advance across the land. When glaciers begin to melt, they deposit these large amounts of sediments and rocks they carry. • The term glacial drift is used to include all sediments of glacial origin. There are two types of glacial drift: • Tillis material deposited directly by the glacier, deposited as the glacier melts and drops it’s load of rock debris. Because everything is dropped at once, till is usually an unsorted mixture of rocks made up of all sizes. • Stratified drift is sediment laid down by glacial melting water. Stratified drift contains particles that are sorted by size and weight of the debris. Some deposits of debris come from streams coming directly from the glacier. Stratified drift often consists of sand and gravel
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers are responsible for a variety of depositional features including: • Lateral moraines are ridges that form along the sides of glacial valleys when the glaciers melt and leave the material it has gathered. • End Morainesform when glaciers stay stationary for long periods of time. Within the glacier, the ice still flows. This flow still carries rock debris to the foot of the glacier like a conveyer belt. Here the debris builds up and produces End Moraines. • Ground moraines form when glaciers begin to recede. The glacier front continues to deliver debris with the ice movement but instead of creating a ridge (as in the end moraine) the retreating foot of the glacier deposits the debris as a rock strewn plain. • Terminal Moraines: Glaciers can periodically retreat and than find balance again and remain stationary for a long time period. A glacier will form an end moraine when stationary than create ground moraines in it’s periods of retreat. This pattern can repeat many times before the glacier completely melts. • The farthest end moraine created in this pattern of stopping and retreating is called theterminal moraine.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Glaciers are also responsible for : • Kettles: Ponds and small lakes called kettles. Kettlesform when blocks of stagnant ice become buried in drift and eventually melt. • Drumlins are long elongated hills composed of glacial till. The steep side of the hill faces the direction the glacier came from and the gentle slope side the direction the ice moved toward. • Eskers are snakelike ridges composed of sand and gravel that were deposited by streams once flowing in tunnels beneath glaciers.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • The ice sheets greatly affected the drainage patterns of major rivers in North America. Before the glaciers: • The Missouri River flowed north toward Hudson Bay • The Mississippi flowed through central Illinois • The Great lakes did not exist • The sheets of glacial ice also triggered changes in the climates of North America beyond their edges. Regions that are arid today became cooler and wetter. • Desert landscapes also reveal the affects of running water and wind. These combine in many ways to provide a wide variety of desert landscapes.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Deserts are very different than landscape is humid areas of the world. Humid areas generally have rounded hills and curving slopes. By contrast, deserts generally have angular rocks, sheer canyon walls, and surfaces covered in sand and small pebbles. • In humid regions, well developed soils support a continuous layer of plant growth on top. In these areas, slopes and rock edges are rounded from chemical weathering. • In contrast, in a desert much of the debris of rock is from mechanical weathering. The minerals that make up the rock debris are unchanged chemically. Because of the dryness of the climate, the rock debris do not break down into rich soil as they do in humid climates.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Chemical weathering is not entirely absent from a desert however. Over long spans of time, clays and thin soils do form. Many of the iron-rich silicate minerals oxidizeproducing rich rust colors in the landscape. • In humid or temperate climates, streams that run year round are a normal fixture. In contrast, in deserts many streams are “ephemeral” lasting only a short time. They run usually only after a rare heavy rain washes over the landscape. We call these ephemeral streams. • These ephemeral streams are especially dangerous because of flash floods that occur. During heavy rains, waters falls so fast that the ground can not absorb it. The lack of vegetation allows the water to run quickly off the land to fill these dry creek beds. • Because arid regions lack permanent streams, they have interior drainage. This means these temporary streams do not flow out to the ocean but end within the desert itself. In the United States the Dry Basin and Range is an example of this. This area includes southern Oregon, all of Nevada, western Utah, southeastern California, southern Arizona, and southern New Mexico.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • When the occasional runoff of water from rainstorms does happen; it is heavily loaded with sediment from the large amount of erosion that happens in a short span of time. Emerging from the mouths of the canyons, the runoff spreads over the slopes at the base of the surrounding mountains and loses it’s velocity and force. When this occurs the water deposits all of it’s sediment suddenly, dropping it within a short distance of space. Most of load is dumped within a short distance of leaving the canyon mouth. The result is a cone of debris known as an alluvial fan at the mouth of the canyon. • On rare occasions of abundant rainfall or snowmelt in the mountains, streams may flow across the alluvial fans to the center of the basin, converting the shallow basin floor into a shallow “playa lake”. Playa lakes last only a few days or weeks before evaporation and infiltration dry them up. The dry flat lake bed that remains is called a playa. Most areas of the country have stream and river systems that drain the land and lead water back to the ocean. • In desert areas, streams dry up long before water reaches the ocean. Water quickly disappears between evaporation and infiltration into the soil.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Some permanent streams and even rivers manage to cross arid areas. The Colorado River, Arkansas River, and the famous Nile River all begin in mountains with abundant rain. These rivers are large enough and with enough flow that they lose little in their crossing of the arid desert landscape. • The Nile River, for example, leaves the lakes and mountains of central Africa and crosses over 3000 kilometers of the famed Sahara desert without a single tributary joining it’s flow. • The point to remember about water in a desert environment, is that although it is rare, it is a major force of change through erosion. Most desert erosion results from running water.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Wind can still be an important force however and contributes to shaping the landscape. In deserts, the soil is dry and dusty and there are few plants with roots to hold the soil together. • Strong desert winds also pick up, move and deposit sediment in great quantities. These are known as dust storms. In the mid 30s, overfarming and drought destroyed much of the midwest. With all the vegetation removed to grow crops; the area became a Dust Bowl when the strong winds picked up the soil and blew it in huge dust storms. • Winds erode in the desert through two ways • Deflationis the lifting and removal of loose particles of clay and silt. Courser sand particles roll or skip along the surface in a process called saltation. These larger sand particles make up the “bed load” much as they do in a stream, rolling along the bottom. • Abrasionis when wind-blown sand cuts and polishes exposed rock surfaces. Blowing sand can grind away at boulders and smaller rocks, sometimes sandblasting them into odd shapes.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Loessis windblown silt that blankets the landscape. Dust storms pick up this material, transport it and deposit it. The thickest and most extensive deposits of loess are in China. • Like running water, wind releases it’s load of sediment when it’s velocity falls and the energy available for transport diminishes. Sand begins to accumulate whenever an obstruction crosses it’s path and the wind diminishes. • Unlike deposits of loess, which forms blanket-like layers over broad areas, winds commonly deposit sand in mounds or ridges called dunes. Dunes can occur whenever an obstruction, such as a rock or small plant, causes the wind’s velocity to slow and particles drop to the ground. Once the sand starts to mound it acts as it’s own windbreak and more and more sand collects.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Dunes are often steeper on the sheltered side and more gently inclined on the side facing the wind. Wind blows sand up the gentler windward side. Once the sand blows over the crest of the dune, the wind slows and the sand drops down. The sheltered side of the dune gradually becomes steeper and the sand eventually slides down the side of the slope. • As sand is deposited on the sheltered side of the dune, it forms layers inclined in the direction the wind blows. These sloping layers are called cross-beds. When the dunes are eventually buried under sediment and become sedimentary rock, the cross beds remain as a record of their origin.
Chapter 8 Review: Glaciers, Deserts, Landscape shaped by wind • Sand Dunes are not random. They occur in a variety of consistent shapes. There are six major types of sand dunes • Barchan Dunes: Solitary sand dunes shaped like crescents are called barchan dunes. • Transverse dunes: If prevailing winds are steady, sand is plentiful, and vegetation is sparse, dunes form in a series of long ridges. These are called transverse dunes because the ridges are perpendicular to the direction of the wind. • Barchanoid Dunes: A common dune form that is between a barchan and transverse dune is the barchanoid dune. These scalloped rows of sand form at right angles to the wind. • Longitudinal dunes are long ridges of sand that form parallel to the prevailing wind. These dunes occur where sand supplies are moderate and the prevailing wind direction varies slightly. • Parabolic Dunes: Parabolic duneslook like backwards barchans. Their tips point into the wind instead of away from it. • Star Dunes: Star dunes are isolated hills of sand mostly found in parts of the Sahara and Arabian deserts. Their bases resemble stars and they usually have three or four sharp ridges that meet in the middle.
