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Chapter 13-17

Chapter 13-17.docx

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Department
Geography
Course Code
GG101
Professor
Rich Petrone

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Chapter 13: Weathering - Geomorphology: the science that analyzes and describes the origin, evolution, form and spatial distribution of land forms - Denudation: the exogenic system, powered by solar energy and gravity, wears down the landscape through the processes of landmass; weathering, mass movement, erosion, transportation, and deposition - Differential Weathering: different rocks offer differing resistance to weathering processes and produce a pattern on the landscape - Dynamic equilibrium model: Considers slope and landform stability to be consequences of the resistance of rock materials to the attack of denudation processes. Agents of change include moving air, water, waves and ice - Slopes are shaped by the relation between rate of weathering and break up of slope materials and the rate of mass movement and erosion of those materials. - Geomorphic threshold: Occurs where there is a struggle against gravity where the point at which potential energy overcomes resistance against movement - Slopes that form boundaries of landforms have several general components: waxing slope (near the top), - Free face: presence of this means an outcrop of resistance rock that forms a steep scarp or cliff, - Debris slope: receives rock fragments and materials from above. It reflects local climate - Waning slope: a concave surface along the base of the slope that forms a pediment, or broad, gently sloping erosional surface - Slopes seek an angle of equilibrium among the operating forces - Weathering: processes disintegrate both surface and subsurface rocks into mineral particles or dissolve them in water. - Regolith: the upper layers of surface material that undergo continual weathering and create broken up rock - Weathered bedrock is the parent rock from which regolith forms - Sediment: the small, unconsolidated, fragmented material that develops after weathering - Important in weathering processes are joints, the fractures and separations in the rock. Jointing opens up rock surfaces on which weathering processes operate. Factors that influence weathering include character of the bedrock (hard or soft, soluble or insoluble), climatic elements (temperature, precipitation, freeze-thaw cycles), position of water table, slope orientation, surface vegetation and its subsurface roots, and time. - Physical weathering: the break up of rock into smaller pieces with no alteration of mineral identity. The physical action of water when it freezes and thaws is a powerful agent in shaping the landscape - Frost action breaks apart any rock - Working in joint, expanded ice can produce joint-block separation through the process of frost-wedging - Another process of physical weathering is salt crystal growth: as crystals in rock grow over time and force apart mineral grains break up rock - As overburden is removed from a granitic batholith, the pressure of deep burial is relieved - The granite slowly responds with pressure release jointing, with layer after layer of rock peeling off in curved slabs or plates. - Sheeting: slabs slip off in the process - Exfoliation dome: exfoliation process creates an arch-shaped or dome- shaped feature on the exposed landscape - Pressure release jointing: layers of rock peels off in curved slabs or plates, thinner at the top of the rock structure and thicker at the sides - Chemical Weathering: the chemical decomposition of minerals in rock - Spheroidal Weathering: chemical weathering occurs in cracks in the rock. As cementing and binding materials are removed, the rock begins to disintegrate and sharp edges and corner become rounded - Hydration occurs when a mineral absorbs water and expands, thus creating a strong mechanical force that stresses rocks - Hydrolysis: breaks down silicate minerals in rock, as in the chemical weathering of feldspar into clays and silica. Water participates in many chemical weathering reactions - Oxidation: the reaction of oxygen with certain metallic elements, the most familiar example being the rusting of iron, producing iron oxide - The dissolution of materials into solution is considered chemical weathering. - For instance, mild acid in rain water will cause carbonation, wherein carbon combines with certain minerals, such as calcium, magnesium, potassium, and sodium - Karst topography refers to distinctively pitted and weathered landscapes - Sinkholes that form at the surface and are circular form and may extend to form a karst valley. - A sinkhole may collapse through the roof of an underground, forming a collapse sinkhole. - The formation of caverns is part of karst processes and groundwater erosion. - Limestone caves feature many unique erosional and depositional features. - Mass movement: any movement of a body of material, propelled and controlled by gravity - Angle of Repose: loose sediment grains represents a balance of driving and resisting forces on a slope. - Mass movement of Earth’s surface produces some dramatic incidents, including: - Landslides: a large amount of material failing simultaneously - Mudflows: material in motion with a high moisture content - Soil Creep: a persistent movement of individual soil particles that are lifted by the expansion of soil moisture as it freezes, by cycles of wetness and dryness, by temperature variations, or by the impact of animals - Solifluction: a form of mass movement characterized by a slow movement of soil material downslope under the influence of gravity. It creates lobe-shaped features in environments that experience cycles of freeze-thaw - Gelifluction: a class of solifluction that occurs in areas of permafrost. It is characterized by the movement of soil downslope under the influence of gravity over a permafrost layer, also produces lobe-shaped features. - Human mining and construction activities have causes massive scarification of landscapes Chapter 14: River Systems and Landforms - Fluvial processes are stream-related - Hydrology is the science of water and its global circulation, distribution, and properties- specifically water at and below Earth’s surface - Water dislodges, dissolves, or removes surface material in the erosion process, in which weathered sediment is picked up for transport, and moved to new locations - Sediments are laid down by another process, deposition - Alluvium is the general term for the clay, silt, sand, gravel or other unconsolidated rock and mineral fragments deposited by running water as sorted or semi-sorted sediment on a floodplain, delta or stream bed - Base level is the lowest elevation limit of stream erosion in a region. A local base level occurs when something interrupts the stream’s ability to achieve base level, such as is created by a dam or a landslide that blocks a stream channel - Drainage Basin is an open system that is part of the basic fluvial system - Drainage divides define the drainage basin catchment area of the drainage basin - In an drainage basin, water initially moves down slope in a thin film called Sheet flow - This surface runoff concentrates in rills, or small scale downhill grooves, which may develop into deeper gullies and a stream course in a valley - High ground that separates one valley from another and directs sheet flow is an interfluve - Extensive mountain and highland region act as continental divides that separate major drainage basins - Some regions have internal drainage that does not rach the ocean, the only outlets being evaporation and subsurface gravitational flows - Drainage density is determined by the number and length of channgels in a given area and is an expression of a landscape’s topographic surface appearance - Drainage pattern refers to the arrangement of channels in an area as determined by the steepness, variable rock resistance, variable climate, hydrology, relief of the land, and structural controls imposed by the landscape. - Seven basic drainage patterns are generally found in nature - Dendritic - Trellis - Radial: results when a stream flows off a central peak or dome - Parallel: associated with steep slopes - Rectangular: formed in a faulted and jointed landscape, which directs stream courses in patterns of right-angle turns - Annular: produced by structural domes, with concentric patterns of rock strata guiding stream courses - Deranged: areas with disrupted surface patterns, such as the glaciated shield region - Stream channels vary in width and depth - The water that flows in stream channels varies in velocity and in the sediment load it carries - All of these factor may increase with increasing discharge - Discharge a stream’s volume of flow per unit of time, is calculated by multiplying the velocity of the stream by it’s width and depth for a specific cross section of the channel. - Most streams increase discharge downstream - Some exotic streams originate in a humid region and flow through an arid region, such that discharge decreases with distance - Hydraulic action is the work of turbulence in the water - Running water causes hydraulic squeeze and release action to loosen and lift rocks and sediment - As this debris moves along it mechanically erodes the streambed further, through a process of abrasion - Solution refers to the dissolved load of a stream, especially the chemical solution derived from minerals such as limestone or dolomite or from soluble salts - The suspended load consists of the fine-grained, clastic particles held aloft in the stream, with the finest particles not deposited until the stream velocity slows nearly to zero - Bed load is the coarser materials that are dragged, pushed and rolled along the streambed by traction or that bounce and hop along by saltation - If the load in a stream exceeds it capacity, sediments accumulate through deposition on the bed and the stream channel builds through aggradations - With excess sediment, a stream becomes a maze of interconnected channels that form a braided stream pattern - Where the stream is gradual, stream channels develop a sinuous form called a meandering stream - The outer portion of each meandering curve is subject to the fastest water velocity and can be thee site of a steep undercut bank - The inner portion of a meander experiences the slowest water velocity and forms a point bar deposit - When a meander neck is cut off as two undercut banks merge, the meander becomes isolated and forms an oxbow lake - Every stream develops its own gradient and establishes a longitudinal profile - A graded stream condition occurs when a channel has just enough energy to transport its sediment load; this represents a balance among slope, discharge, channel characteristics, and the load supplied from the drainage basin - An interruption in a stream’s longitudinal profile is called a nickpoint - This abrupt change in gradient can occur as the stream flows across hard, resistant rock or after tectonic uplift episodes - Flood plains are settled, raising ussues of human perception of hazard - The flat low lying area along a stream channel that is subjected to recurrent flooding is a floodplain - It is formed when the river overflows its channel during times of high flow - On either bank of some stream, natural leeves develop as by-products of flooding - On the floodplain, back swamps and yazoo tributaries may develop - The natural leeves and elevated channel prevent the yazoo tributary from joining the main channel so it flows parallel to the river and through the back swamp area - Entrenchment of a river into its own floodplain form alluvial terraces - A depositional plain formed in a body of water at the mouth of a river is called a delta - When the mouth of a river enters the sea and is undated by seawater in a mix with freshwater, it is called estuary. - Floods occur when high water overflows the natural riverbank along any portion of a stream - Both floods and the adjacent flood plains are rated statistically for the expected time interval between floods. - A 10 year flood is the greatest level of flooding that is likely to occur
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