Transcript Weathering
Weathering Objectives • Distinguish between weathering and erosion. • Identify variables that affect the rate of weathering. Vocabulary – weathering – chemical weathering – erosion – hydrolysis – mechanical weathering – oxidation – frost wedging – exfoliation Weathering Weathering • Changes occur every day to Earth’s rocks and surface features. • Weathering is the chemical and physical processes by which rocks on or near Earth’s surface break down and change. • Erosion is the removal and transport of weathered material from one location to another. Weathering Mechanical Weathering • Mechanical and chemical weathering are the two processes that can wear down rocks and minerals. • Both types of weathering occur at the same time on Earth’s landforms. • Mechanical weathering, or physical weathering, is the process by which rocks and minerals break down into smaller pieces without changing their composition. • Mechanical weathering does not involve any change in a rock’s composition. Weathering Mechanical Weathering Temperature – In many places on Earth’s surface, water collects in the cracks of rocks and rock layers. – If the temperature drops to the freezing point of water, it freezes, expands, exerts pressure on the rocks, and may cause them to split. – When the temperature then increases, the ice in the cracks of rocks and rock layers melts. – Frost wedging is the repeated thawing and freezing of water in the cracks of rocks. Weathering Mechanical Weathering Pressure – Bedrock at great depths is under pressure from the overlying rock layers. – When the overlying rock layers are removed, the pressure on the bedrock below is reduced. – The bedrock surface, formerly buried, is then able to expand, and long, curved cracks known as joints can form that lead to exfoliation. – Exfoliation is the process by which outer rock layers are stripped away over time. – The roots of plants can also exert pressure causing rocks to split. Weathering Chemical Weathering • Chemical weathering is the process by which rocks and minerals undergo changes in their composition as the result of chemical reactions. • Significant agents of chemical weathering include water, oxygen, carbon dioxide, and acids. • Chemical reactions between rocks and water result in the formation of new minerals and the release of dissolved substances. • Some minerals, such as calcite, may dissolve completely. Weathering Chemical Weathering • Temperature influences the rate at which chemical reactions occur. • Generally, chemical reaction rates increase as temperature increases. Weathering Chemical Weathering Water – Water is an important agent in chemical weathering because it can dissolve many kinds of minerals and rocks. – Hydrolysis is the chemical reaction of water with other substances. Weathering Chemical Weathering Oxygen – Oxidation is the chemical reaction of oxygen with other substances. – Iron in rocks and minerals readily combines with this atmospheric oxygen to form minerals with the oxidized form of iron as shown in the following reaction. 2Fe3O4 + ½ O2 3Fe2O3 Weathering Chemical Weathering Carbon Dioxide – Carbon dioxide, which is produced by living organisms during the process of respiration, contributes to the chemical weathering process. – When carbon dioxide combines with water in the atmosphere, it forms a weak carbonic acid that falls to Earth’s surface as precipitation. H2O + CO2 H2CO3 – Carbonic acid reacts with minerals such as calcite in limestone and marble to dissolve rocks and can also affect silicate minerals such as mica and feldspar. Weathering Chemical Weathering Acid Precipitation – Acid precipitation is caused mainly by the oxidation of sulfur dioxide and nitrogen oxides that are released into the atmosphere by human activities. – These two gases combine with oxygen and water in the atmosphere to form sulfuric and nitric acids. – Acid precipitation is precipitation that has a pH value below 5.6, the pH of normal rainfall. Weathering Chemical Weathering Acid Precipitation – Acid precipitation adversely affects fish and aquatic plant populations in lakes. – When lake water becomes too acidic, the species diversity decreases. Weathering Chemical Weathering Acid Precipitation Weathering What Affects the Rate of Weathering? • The natural weathering of Earth materials occurs very slowly. • Certain conditions and interactions can accelerate or slow the weathering process. Weathering What Affects the Rate of Weathering? Climate – The climate of an area—including precipitation, temperature, and evaporation—is a major influence on the rate of chemical weathering. – The interaction between temperature and precipitation has the greatest effect on a region’s rate of weathering. – Chemical weathering occurs readily in climates with warm temperatures, abundant rainfall, and lush vegetation. Weathering What Affects the Rate of Weathering? Climate Weathering What Affects the Rate of Weathering? Climate – Physical weathering occurs readily in cool, dry climates. – Physical weathering rates are highest in areas where water undergoes repeated freezing and thawing. – Because of these differences in their climates, rocks and minerals in Asheville experience a higher rate of mechanical and chemical weathering than those in Phoenix do. Weathering What Affects the Rate of Weathering? Climate – There is a higher rate of mechanical and chemical weathering in Asheville than in Phoenix. Weathering What Affects the Rate of Weathering? Rock Type and Composition – The characteristics of rocks, including how hard or resistant they are to being broken down, depend on their type and composition. – In general, sedimentary rocks are more easily weathered than harder igneous and metamorphic rocks. Weathering What Affects the Rate of Weathering? Surface Area – Mechanical weathering breaks up rocks into smaller pieces. – As the pieces get smaller, their surface area increases. – The greater the total surface area, the more weathering that occurs. Weathering What Affects the Rate of Weathering? Surface Area Weathering What Affects the Rate of Weathering? Topography and Other Variables – Earth materials on level areas are likely to remain in place as they undergo changes. – Materials on slopes have a greater tendency to move as a result of gravity, thereby exposing underlying rock surfaces and thus providing more opportunities for weathering to occur. – Decaying organic matter and living plant roots release carbon dioxide, which combines with water to produce acid, which in turn increases the weathering rate. Erosion and Deposition Objectives • Analyze the impact of living and nonliving things on the processes of weathering and erosion. • Describe the relationship of gravity to all agents of erosion. Vocabulary – deposition – rill erosion – gully erosion Erosion and Deposition Erosion and Deposition • Erosion is the process that transports Earth materials from one place to another. • A number of different agents transport weathered materials on Earth including water and wind. • Erosion can result from the loss of plant cover, which increases the amount of soil lost to wind and water erosion. • Deposition is the process of dropping materials in another location when the movement of transported materials slows down. Erosion and Deposition Gravity’s Role in Erosion • Gravity is associated with many erosional agents, because the force of gravity tends to pull all materials downslope. • Without gravity, glaciers would not move downslope and streams would not flow. Erosion and Deposition Erosion by Running Water • With few exceptions, water has more power to move large particles of weathered material than wind does. • Stream erosion is greatest when a large volume of water is moving rapidly. • Swiftly flowing water can carry material over a greater distance. • Small streams at high elevations flow down to join larger streams at lower elevations draining an area called a watershed. Erosion and Deposition Erosion by Running Water • Rill erosion is the erosion by running water in small channels, on the side of a slope. • Rills commonly form on a slope. • Gully erosion is when a rill channel evolves to become deep and wide. • Gullies, which can be more than 3 m deep, can be a major problem in farming and grazing areas. Erosion and Deposition Erosion by Running Water Coastal Deposition and Erosion – Each year, streams and rivers carry billions of metric tons of sediments to coastal areas. – When a river enters a large body of water, the water slows down and deposits large amounts of sediments which form deltas. – The volume of river flow and the action of tides determine the shapes of deltas, most of which contain fertile soil. Erosion and Deposition Erosion by Running Water Coastal Deposition and Erosion – Ocean currents, waves, and tides carve out cliffs, arches, and other features along the continents’ edges. – The constant movement of water and the availability of accumulated weathered material result in a continuous erosional process, especially along ocean shorelines. – Sand along a shoreline is repeatedly picked up, moved, and deposited by ocean currents. – Sandbars form from offshore sand deposits and can become barrier islands. Erosion and Deposition Erosion by Running Water Coastal Deposition and Erosion – Changing tides and conditions associated with coastal storms can have a great impact on coastal erosion. – Human development and population growth along shorelines have led to attempts to control the ocean’s movements of sand. – Efforts to keep the sand on one beachfront disrupt the natural migration of sand along the shore, thereby depleting sand from another area. Erosion and Deposition Glacial Erosion • Although glaciers currently cover less than ten percent of Earth’s surface, their erosional effects are large-scale and dramatic. • Because they are so dense, glaciers have the capacity to carry huge rocks and piles of debris over great distances. • The erosional effects of glaciers also include deposition. Erosion and Deposition Wind Erosion • Wind is a major erosional agent in areas on Earth that experience both limited precipitation and high temperatures. • The abrasive action of wind-blown particles can damage both natural features and human-made structures. • Shore areas also experience wind erosion. • Wind erosion is relatively insignificant when compared to the erosion accomplished by running water and glacial activity. Erosion and Deposition Wind Erosion Wind Barriers – Planting wind barriers, or windbreaks, is one farming method that reduces the effects of wind erosion. – Wind barriers are trees or other vegetation planted perpendicular to the direction of the wind. – In addition to reducing soil erosion, wind barriers can trap blowing snow, conserve moisture, and protect crops from the effects of the wind. Erosion and Deposition Erosion by Plants, Animals, And Humans • As plants and animals carry on their life processes, they move Earth’s surface materials from one place to another. • The effects of erosion by the activities of plants, animals, and humans are minimal in comparison to the erosional effects of water, wind, and glaciers. Formation of Soil Objectives • Describe how soil forms. • Explain the relationship between the organic and inorganic components of soil. • Identify soil characteristics. • Recognize soil horizons in a soil profile. Vocabulary – soil – soil profile – residual soil – soil horizon – transported soil Formation of Soil Formation of Soil • Soil is essential to life on Earth. • Humans and other organisms are dependent on plants, which grow in soil, for food and other basic needs. Formation of Soil Development of Soil • Except for some steep mountain slopes and extremely cold regions, soil is found almost everywhere on Earth’s surface. • Soil is the loose covering of broken rock particles and decaying organic matter, called humus, overlying the bedrock of Earth’s surface. • Soil is the result of chemical and mechanical weathering and biological activity over long periods of time. • While all soils contain some organic matter, the amount varies widely among different types of soil. Formation of Soil Soil Composition • Soil forms in layers during the process of its development. • The parent rock is the solid bedrock from which weathered pieces of rock first break off. • The smallest pieces of weathered rock, along with living and dead organisms, remain in the very top layer. • Rainwater seeps through this top layer of materials, dissolves soluble minerals, and carries them into the lower layers of the soil. Formation of Soil Soil Composition • Residual soil is soil located above its parent bedrock. • Transported soil is soil that has been moved to a location away from its parent bedrock by agents of erosion, such as running water, wind, and glaciers. • The parent bedrock determines what kinds of minerals a soil contains. • The parent rock and climatic conditions of an area determine the length of time it takes for soil to form. Formation of Soil Soil Profiles • A soil profile is the vertical sequence of soil layers. • A soil horizon is a distinct layer, or zone, within a soil profile. • There are three major soil horizons: A, B, and C. – Horizon A contains high concentrations of organic matter and humus. – Horizon B contains subsoils that are enriched with clay minerals. – Horizon C, below horizon B and directly above solid bedrock, contains weathered parent material. Formation of Soil Soil Profiles Topography – The topography of a region affects the thickness of developing soil. – Soils on slopes tend to be thin, coarse, and infertile. – Soils formed in lower areas, such as in valleys, are thick and fertile. Formation of Soil Soil Types • Because climatic conditions are the main influence on soil development, soils are often classified based on the climates in which they form. • The four major types of soil are polar, temperate, desert, and tropical. Formation of Soil Soil Types Formation of Soil Soil Types Polar Soils – Polar soils form at high latitudes and high elevations in places such as Greenland, Canada, and Antarctica. – These soils have good drainage but no distinct horizons because they are very shallow, sometimes only a few centimeters deep. – Permanently frozen ground, called permafrost, is often present under thin polar soils. Formation of Soil Soil Types Temperate Soils – Temperate soils vary greatly and are able to support such diverse environments as forests, grasslands, and prairies. – The specific amount of rainfall in an area determines the type of vegetation that will grow in temperate soils. – Grasslands, which have an abundance of humus, are characterized by rich, fertile, soils. – Forest soils are characterized by less deep and less fertile soils that contain aluminum-rich clays and iron oxides. Formation of Soil Soil Types Desert Soils – Deserts receive low levels of precipitation. – Desert soils often have a high level of accumulated salts and can support only a limited amount of vegetation. – Desert soils have little or no organic matter and a very thin A horizon, but they often have abundant nutrients. – Desert soils are also light-colored, coarse, and may contain salts and gypsum. Formation of Soil Soil Types Tropical Soils – Tropical areas experience high temperatures and heavy rainfall, leading to the development of intensely weathered and often infertile soil. – The intense weathering combined with a high degree of bacterial activity leave tropical soils with very little humus and very few nutrients. – These soils experience much leaching of soluble materials, such as calcite and silica, but they have high concentrations of iron and aluminum. Formation of Soil Soil Textures • Particles of soil are classified according to size as being clay, silt, or sand, with clay being the smallest and sand being the largest. • The relative proportions of these particle sizes determine a soil’s texture. • The texture of a soil affects its capacity to retain moisture and therefore its ability to support plant growth. Formation of Soil Soil Textures Formation of Soil Soil Fertility • Soil fertility is the measure of how well a soil can support the growth of plants. • Factors that affect soil fertility include: – Availability of minerals and nutrients – Number of microorganisms present – Amount of precipitation available – Topography – Level of acidity Formation of Soil Soil Fertility Soil Color – A soil’s composition and the climate in which it develops are the main factors that determine a soil’s color. – Topsoil is usually dark-colored because it is rich in humus. – Red and yellow soils may be the result of oxidation of iron minerals. – Yellow soils are usually poorly drained and are often associated with environmental problems. – Grayish or bluish soils are common in poorly drained regions where soils are constantly wet and lack oxygen.