Exogenic Processes PDF

LESSON 8 EXOGENIC PROCESSES Presented by Group 1&2  Objectives This chapter should enable your students to: Appreciate that by disintegrating and decomposing rock, weathering prepares rock fragments for erosion, transportation, and deposition. Explain the principal differences am...

LESSON 8 EXOGENIC PROCESSES Presented by Group 1&2  Objectives This chapter should enable your students to: Appreciate that by disintegrating and decomposing rock, weathering prepares rock fragments for erosion, transportation, and deposition. Explain the principal differences among the various physical and chemical weathering processes. Discuss how variations in climate, rock type, and rock structure influence weathering rates. Provide examples of the topographic effects of differential weathering and erosion. Understand the role of gravity in mass wasting. Use appropriate terms to describe the types of material involved in slope failures. Categorize various mass wasting events as slow or fast. Describe the various ways by which materials can move downslope. Recognize some of the landforms and landscape features created by mass wasting. Identify ways in which people influence weathering and mass wasting and  Endogenic Geomorphic Processes endogenic geomorphic processes of tectonism and volcanism, which arise from within Earth, are opposed by the relief-reducing exogenic geomorphic processes, which originate at the surface. These exogenic processes break down rocks and erode rock fragments from higher energy sites, transporting them to locations of lower energy. NATURE OF EXOGENIC PROCESSES Earth's surface provides a harsh environment for rocks. Most rocks originate under much higher temperatures and pressures and in very different chemical settings than those found at Earth's surface. Thus, surface and near-surface conditions of comparatively low temperature, low pressure, and extensive contact with water cause rocks to undergo varying amount of disintegration and decomposition  This breakdown of rock material at and near Earth's surface is known as WEATHERING. Rocks weakened and broken by weathering become susceptible to the other exogenic processes-erosion, transportation, and deposition. A rock fragment broken (weathered) from a larger mass will be removed from that mass (eroded), moved (transported), and set down (deposited) in a new location. Together, weathering, erosion, transportation, and deposition actually represent a chain or continuum of processes that begins with the breakdown of rock. Gravity. induced downslope movement of rock material that occurs without the assistance of a geomorphic agent, as in the case of a rock falling from a cliff, is known as MASS WASTING.  WEATHERING Environmental conditions at and near Earth's surface subject rocks to temperatures, pressures, and substances, especially water, that contribute tophysical and chemical breakdown of exposed rock. Broken fragments of rock, called clasts, that detach from the original rock mass can be large or small. These detached pieces continue to weather into smaller particles. The several types of rock weathering fall into two basic categories. PHYSICAL WEATHERING known as mechanical weathering, disintegrates rocks, breaking smaller fragments from a larger block or outcrop of rock. CHEMICAL WEATHERING decomposes rock through chemical reactions that remove ions from the original rock-forming minerals.  PHYSICAL WEATHERING The mechanical disintegration of rocks by physical weathering is especially important to landscape modification in two ways. First, the resulting smaller clasts are more easily eroded and transported than the initial larger ones. Second, the breakup of a large rock into smaller ones encourages additional weathering because it increases the surface area exposed to weathering processes. High elevation helps drive erosional stripping of the overlying rocks, and ultimately through this removal of overlying weight- the unloading process the upper part of the granite is exposed at the surface, where it experiences the low pressure of the atmosphere  PHYSICAL WEATHERING The successive removal of these outer rock sheets is known as exfoliation, and each concentric broken layer of rock is an exfoliation sheet. The term exfoliation dome designates an unloaded, exfoliating outcrop of rock with a dome-like surface form. Those scientists cited widespread existence of split rocks in arid regions as evidence of the effectiveness of this thermal expansion and contraction weathering.  Freeze-Thaw Weathering In areas subject to numerous diurnal cycles of freeze/ thaw, water repeatedly freezing in fractures and small cracks in rocks contributes significantly to rock breakage by freeze-thaw weathering, Salt Crystal Growth The development of salt crystals in cracks, fractures, and other void spaces in rocks causes physical disintegration in a way that is similar to freeze-thaw weathering. With salt crystal growth, water containing dissolved salts accumulates in these spaces. Hydration In weathering by hydration, water molecules attach to the crystalline structure of a mineral without causing a permanent change in that mineral's composition.  CHEMICAL WEATHERING Chemical reactions between rock-forming minerals and other matter at Earth's surface also work to break down rocks. In chemical weathering, ions from a rock are either released into water or recombine with other substances to form new materials, such as clay minerals. New materials made by chemical weathering are more stable at Earth's surface than the original rocks.  Oxidation Water that has regular contact with the atmosphere contains plenty of oxygen. This chemical union of oxygen atoms with another substance to create a new product is oxidation. Solution and Carbonation Under certain circumstances, some rock-forming minerals dissolve in water. The chemical weathering process of carbónation is a common type of solution that involves carbon dioxide and water molecules reacting with, and thereby decomposing, rock material. Hydrolysis In the weathering process of hydrolysis, water molecules alone, rather than oxygen or carbon dioxide in water, react with chemical components of rock- forming minerals to create new compounds, of which the H+ and OH- ions of water are a part.  VARIABILITY IN WEATHERING How and why types and rates of weathering vary, at both the regional and local spatial scales, are of particular interest to physical geographers. Climate, the type of rock (lithology), and the nature and amount of fractures or other weaknesses in the rock are major influences on the effectiveness of the various weathering processes.  CLIMATE FACTORS In almost all environments, physical and chemical weathering processes operate together, even though one of these categories usually dominates. ROCK TYPE Some rock types are more resistant to weathering than other rock types. Commonly, a rock that is strong under certain environmental conditions may be easily weathered and eroded in a different environmental setting.  STRACTURAL WEAKNESSES In addition to rock type, the relative resistance of a rock to weathering depends on other characteristics, such as the presence of joints, faults, folds, and bedding planes that make rocks susceptible to enhanced weathering. DEFFERENTIAL WEATHERING AND EROSION Wherever several different rock types occupy a given landscape, some will naturally be more resistant and others will be less resistant to the weathering processes operating there. Because erosion removes small, weathered rock fragments more easily than large, intact rock masses, areas of diverse rock types undergo differential weathering and erosion.  MASS WASTING Mass wasting, also called mass movement, is a collective term for the downslope transport of surface materials in direct response to gravity. Mass wasting operates in a wide variety of ways and at many scales. A single rock rolling and tumbling downhill is a form of this gravitational transfer of materials, as is an entire hillside sliding hundreds or thousands of meters downslope, burying homes, cars, and trees. Some mass movements act so slowly that they are imperceptible by direct observation and their effects appear gradually over long periods of time.  MATERIALS AND MOTION Physical geographers categorize mass wasting events according to the kinds of Earth materials involved and the ways in which they move. A thicker unit of the same type of material is referred to as earth. Debris specifies a given mass of sediment that contains a wide range of grain sizes, at least 20% of which is gravel. Mud indicates saturated sediment composed mainly of clay and silt, which are the smallest particle sizes.  SLOW MASS WASTING Slow mass wasting has a significant, cumulative effect on Earth's surface. Landscapes dominated by slow mass wasting tend toward rounded hillcrests and an absence of sharp angular features.  Creep Most hillslopes covered with weathered rock or soils undergo creep, the slow migration of particles to successively lower elevations. This gradual downslope motion often occurs as soil creep, primarily affecting a relatively thin surface layer of weathered rock particles. Solifluction The word solifluction, which literally means "soil flow," refers to the slow downslope movement of water-saturated soil and/or regolith. Solifluction is most common in high latitude or high-elevation tundra regions that have permafrost, a subsurface layer of permanently frozen ground. Above the permafrost layer lies the active layer, which freezes of during winter but thaws during summer.  FAST MASS WASTING Four major kinds of mass wasting usually occur so quickly-from seconds to days-that people canwatch the material move. The actual speed of movement varies with the situation and depends on the quantity and composition of the material, the steepness of slope, the amount of water involved, the vegetative cover, and the triggering factor.  Avalanches An avalanche is a type of mass movement in which much of the involved material is pulverized-that is, broken into small, powdery fragments-and then flows rapidly as an airborne density current along Earth's surface. Slides In slides, a cohesive or semicohesive unit of Earth material slips downslope in continuous contact with the land surface. Water plays a somewhat greater role in most slides than it does in falls. Flows Mass wasting flows are masses of water-saturated unconsolidated sediments that move downslope by the force of gravity. Flows carry water in moving sediments, whereas rivers carry sediments in moving water. Compared with slides, which tend to move as cohesive units, flows involve considerable churning and mixing of the materials as they move. Thank you! LESSON 8