PHYSICAL GEOGRAPHY 2200c Exam 4 Weathering Physical mechanical Mechanical weathering takes place when rocks are broken down without any change in the chemical nature of the rock They are essentially torn apart by physical force rather than chemical breakdown Many things can cause this including pressure in the earths crust that creates faults The most common type is water freezing then thawing Another common type is salt wedging salt deposits from water build up and crack the rock Temperature changes themselves are also an influence Chemical weathering Chemical weathering changes the composition of rocks often transforming them when water interacts with minerals to create various chemical reactions Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment New or secondary minerals develop from the original minerals of the rock The most common types are oxidation hydrolysis and carbonation Oxidation oxygen combines with other elements to form new types of rock Hydrolysis water combines with substances in rocks to form new rocks Carbonation carbon dioxide reacts with certain types of rocks that form a new solution With each of these the new rocks solutions are much softer and easier to break apart and be carried away Frost wedging mentioned previously in mechanical weathering It is caused by the repeated freeze thaw cycle of water in extreme climates When the water freezes into ice it expands which places pressure on the joints in the rock Plants Plants are capable of both mechanical and chemical weathering In chemical weathering mosses and linches have been found to grow on rocks and release chemicals that dissolve minerals from rocks Roots from plants are able to invade cracks with in rocks and increase the size of the cracks There are a number of trees and other plants that release acidic chemicals which can cause weathering of the rocks minerals and soil around them Plants also can have an affect on the way water weathers rocks and soils they can channel water in specific directions that cause greater weathering or they can lessen the impact of rain falling on the ground Carbonic acid The most abundant natural acid It is weak and consists of dissolved carbon dioxide in water Rainwater usually contains some dissolved carbon dioxide and is slightly acidic It weathers calcite minerals limestone Karst Causes Geological formation shaped by the dissolution of a layer or layers of soluble bedrock usually carbonate rock such as limestone or dolomite The result of mildly acidic water like carbonic acid on weakly soluble bedrock The mildly acidic water begins to dissolve the surface along fractures or bedding planes in the limestone bedrock Over time these fractures enlarge as the bedrock continues to dissolve Openings in the rock increase in size and an underground drainage system begins to develop allowing more water to pass through the area and accelerating the formation of underground karst features Surface features Many karst regions display distinctive surface features with cenotes sinkholes or dolines being the most common Cave formations Some karst regions include thousands of caves although evidence of caves large enough for human exploration is not a required characteristic of karst Cave formations are created when acid reacts with limestone or a rock containing 80 or more calcium carbonate Contains stalactites ceiling and stalagmites ground Mass Wasting Driving force mass wasting is the geomorphic process by which soil sand regolith and rock move downslope under the force of gravity Types of mass wasting include creep slides flows topples and falls each with its own characteristic features and taking place over timescales from seconds to years Causes Oversteepening The oversteepness of slopes Usually loose grain particles assume a stable slope called the angle of repose and create different slopes due to the type of particles Oversteepened slopes are unstable Earthquakes the vibrations from earthquakes cause mass wasting as well Water Saturation of the mass material with water this saturation destroys particle cohesions and adds weight to the material Stabilization Many methods have been attempted Removing vegetation draining excess water covering the area retaining walls etc Every action is only successful in the short term gravity and water will always win Groundwater Watertable The area where water fills the aquifer is called the saturated zone or saturation zone The top of this zone is called the water table The water table may be located only a foot below the ground s surface or it can sit hundreds of feet down Aquifer confined unconfined An aquifer is a body of saturated rock through which water can easily move Almost all aquifers are not rivers Unconfined aquifers are sometimes also called water table or phreatic aquifers because their upper boundary is the water table or phreatic surface A confined aquifer is sandwiched between confining beds layers of impermeable materials such as clay which impede the movement of water into and out of the aquifer Because of the confining beds ground water in these aquifers is under high pressure Because of the high pressure the water level in a well will rise to a level higher than the water level at the top of the aquifer In unconfined aquifers the ground water only partially fills the aquifer and the upper surface of the ground water the water table is free to rise and decline The ground water is at atmospheric pressure The height of the water table will be the same as the water level in a well constructed in that unconfined aquifer Aquiclude An impermeable body of rock or stratum of sediment that acts as a barrier to the flow of groundwater Wells springs contamination Permeability porosity The rate of groundwater flow is controlled by two properties of the rock porosity and permeability Porosity is the percentage of the volume of the rock that is open space pore space This determines the amount of water that a rock can contain In sediments or sedimentary rocks the porosity depends on grain size the shapes of the grains and the degree of sorting and the degree of cementation Well rounded coarse grained sediments usually have higher porosity than fine grained sediments because the grains do not fit together well Poorly sorted sediments usually have lower porosity because the fine grained fragments tend to fill in the open space Since cements tend to fill in the pore