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Rock Structure, Weathering, and g Mass Wasting Chapter 14 Rock Structure as a Landform Control As denudation progresses, landscape features develop according to patterns of bedrock composition and structure. Under similar climatic and topographic conditions, rock denudation rates are generally related to rock type (fastest for sedimentary rock, and is progressively slower for metamorphic and igneous types). Denudation rates are related to rock hardness, but other lithological factors, such as fracturing, jointing, and permeability are also important. 1 Rock Structure as a Landform Control Strike and Dip Most rock layers are not flat, but rather are tilted at an angle. The angle formed between the rock and the horizontal plane is termed the dip (stated in degrees, ranging from 0° to 90°). The line of intersection between the inclined rock plane and a horizontal plane gives the strike of the rock (always measured as an acute angle to the east or west of north). 2 Landforms of Horizontal Strata Extensive areas of the ancient continental shields are covered by thick sequences of sedimentary rock that were deposited in shallow inland seas at various times over the past 600 million years, since the Precambrian Era. Uplifted with little disturbance other than minor crustal warping or faulting, these areas became continental surfaces underlain with horizontal strata. Landforms of Horizontal Strata In arid climates, the normal sequence of landform development on horizontal strata is for a sheer rock wall, or cliff, to form at the edge of a resistant rock layer. p , which flattens At the base of the cliff is an inclined slope, out into a plain beyond. Erosion strips away successive rock layers, leaving a broad platform capped by hard rock layers. Such a platform is usually called a plateau. Landforms of Horizontal Strata Cliff retreat produces a mesa, essentially a small plateau bordered on all sides by steep rock faces. Co t ued retreat Continued et eat o of tthe e su surrounding ou d g ccliffs s reduces educes tthe ea area ea of a mesa, but it retains its flat top (eventually, it becomes a small steep-sided feature known as a butte). 3 Landforms of Folded Rock Strata Areas underlain by gently upwarped or downwarped rock layers form domes and basins. More intense folding produces the wavelike strata of y anticlines and synclines. A sedimentary dome is a circular or oval structure in which strata have been forced upward at its centre. Deep erosion of simple folds produces a ridge-and-valley landscape—weaker rocks, such as shale and limestone, erode away. 4 Landforms of Folded Rock Strata This leaves hard strata, such as sandstone or quartzite, to stand in bold relief as long, narrow ridges. The geometry of the folds will contribute to the differential y tend to be resistance of the rock mass;; strata in a syncline compressed, while in an anticline, extensional forces may weaken and fracture the rock. Where rock was weakened as it was bent upward, it can erode away to form an anticlinal valley - a synclinal mountain can form where the rock has been strengthened by compression. 5 Landforms Developed on other Geological Structures Erosion Forms on Fault Structures Active normal faulting produces a sharp surface break — a fault scarp — that can create a rock cliff hundreds of metres high. Erosion quickly modifies a fault scarp, but because the fault plane extends down into the bedrock, its effect on erosional landforms persists over a long geologic time span. Landforms Developed on other Geological Structures Exposed Batholiths Batholiths, huge intrusions of igneous rock that are formed deep below the Earth’s surface, are sometimes uncovered by erosion of the overlying rock materials. Because batholiths are typically composed of resistant rock, they erode into hilly or mountainous uplands. Batholiths of granitic composition are a major component of ancient shields. Landforms Developed on other Geological Structures Exposed Batholiths Small bodies of granite projecting from underlying batholiths are often found surrounded by ancient metamorphic rocks formed when the granite was intruded. These prominent features are known as monadnocks or inselbergs. 6 Landforms Developed on other Geological Structures Deeply Eroded Volcanoes With continued erosion, an extinct stratovolcano may gradually be reduced to a remnant volcanic neck formed of lava that solidified in the pipe of the volcano. Radiating from it are wall-like dikes created from magma that filled radial fractures around the ancient volcano. 7 Landforms Developed on other Geological Structures Deeply Eroded Volcanoes Shield volcanoes show erosion features that are quite different from those of stratovolcanoes. In the humid climate of Hawaii for example, streams have cut deep, narrow valleys that open out into steep-walled amphitheatres. Over time, the original surface of the shield volcano will be completely dissected, leaving a rugged mountain mass composed of sharp-crested divides and canyons. Landforms Developed on other Geological Structures Impact Structures The typical impact speed of a meteorite, about 50,000 kph, produces a crater that is 10 to 20 times larger than the meteorite’s diameter. Most craters are bowl-shaped depressions around which the ejected material is deposited. Rocks beneath a large crater are significantly altered, and usually consist of a shallow layer of breccia made up of coarse, angular fragments of the original rocks. 8 Weathering Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically due to conditions at or near the Earth’s surface. 9 Weathering In physical weathering, rocks are fractured and broken apart without chemical alteration due to processes such as frost action expansion and contraction caused by changes in temperature, and pressure from roots. In chemical weathering, rock minerals are transformed from those that were stable when the rocks were initially formed into those that are stable under temperature, pressure, and moisture conditions found at the Earth’s surface (oxidation, hydrolysis, solution). Weathering Weathering leads to the production of regolith — a surface layer of rock particles that lies above solid, unaltered rock. Weathering, in conjunction with gravity, also creates a number of distinctive landforms. Physical Weathering Physical weathering, also known as mechanical weathering, produces regolith by the action of forces strong enough to fracture the rock. Fracturing F t i occurs when h stress t is i exerted t d along l zones off weakness in the rock. 10 Physical Weathering Frost Action One of the most important physical weathering processes in cold climates is frost action caused by the repeated growth and melting of ice crystals in pore spaces and cracks in a rock. Water expands when it freezes, resulting in an increase in volume of about 10 percent (over the course of many freeze – thaw cycles, such expansion can fragment even extremely hard rocks). 11 Physical Weathering Salt-Crystal Growth The weathering of rock by salt crystals is similar to the process of ice-crystal growth. During long drought periods, capillary action moves groundwater to the surface of the rock (water is drawn upward through surface tension). Physical Weathering Salt-Crystal Growth As water evaporates from the porous outer zone of the sandstone, tiny crystals of salts, such as halite (NaCl), calcite (CaCO3), or gypsum (CaSO4·2H20), are left behind. Over time, the force created by the growth of these crystals results in grain-by-grain breakup of the sandstone, which crumbles into sand and is swept away by wind and rain. 12 Physical Weathering Unloading Unloading is a process that relieves the confining pressure on a rock mass (when the removal of overlying material brings a rock mass nearer to the surface). This causes the rock to crack in layers that are more or less parallel to the surface, creating a type of jointing called sheeting structure. In massive rocks like granite or marble, thick curved layers or shells of rock successively break free from the parent mass below through the process of exfoliation. 13 Physical Weathering Other Physical Weathering Processes Most rock-forming minerals expand when heated and contract when cooled under normal diurnal temperature cycles - this can create powerful disruptive forces on rocks in deserts and other areas where the surface is subjected to intense daytime heating alternating with cool nights. Daily temperature changes can cause the breakup of a surface layer of rock already weakened by other agents of weathering. Physical Weathering Other Physical Weathering Processes Another mechanism of rock breakup is the growth of plant roots, which can wedge joint blocks apart. The primary role of plants in weathering is biochemical rather than biophysical, and is associated mainly with acidic exudates from mosses and lichens that colonize bare rock surfaces. 14 Chemical Weathering and its Landforms Chemical weathering is a process of mineral alteration. The dominant processes of chemical change affecting silicate minerals are oxidation, hydrolysis, and carbonation. Chemical Weathering and its Landforms Hydrolysis and Oxidation Oxidation and hydrolysis change the chemical structure of primary minerals, turning them into secondary minerals that are typically softer and bulkier and therefore more susceptible to erosion and mass movement. Decomposition by hydrolysis and oxidation changes rockforming minerals into clay minerals and oxides. 15 Chemical Weathering and its Landforms Carbonation and Solution Chemical weathering through acid action is mainly associated with carbonic acid (H2CO3) formed when CO2 dissolves in water. Rainwater, soil water, and stream water all normally contain dissolved CO2. Percolating water is primarily acidified as it moves through the soil and reacts with CO2 generated by decomposition of organic material. Chemical Weathering and its Landforms Carbonation and Solution Carbonic acid slowly interacts with feldspars and other types of minerals to form carbonates; this process of carbonation is an important intermediate stage in the weathering of igneous rocks. Limestone and marble, are particularly susceptible to acid action. In this case, the mineral calcium carbonate (CaCO3) dissolves and is carried away in solution in stream water. 16 Chemical Weathering and its Landforms Limestone Solution by Groundwater In moist climates, the slow flow of groundwater in the saturated zone can dissolve limestone below the surface surface, producing large underground caverns. Some caverns collapse, causing the ground above to sink and a unique type of karst landscape to develop. Chemical Weathering and its Landforms Limestone Solution by Groundwater Limestone caverns are interconnected subterranean cavities in bedrock formed by the corrosive action of circulating groundwater on limestone. A sinkhole is a surface depression in a region of cavernous limestone. 17 18 Mass Wasting Gravity induces the spontaneous downslope movement of rock fragments created by weathering. This movement to lower elevations, which occurs without the action of flowing water, wind, or moving ice, is called mass wasting. Movement of a mass of soil or weathered rock takes place when the internal strength of the material declines to a critical point where it can no longer resist the force of gravity. 19 Mass Wasting Slopes On a typical hill, slope soil and regolith blanket the bedrock. The thicknesses of these surficial materials are q quite variable and depend on the type and calibre of material, as well the gradient of the slope. Although soil rarely extends deeper than 1 or 2 m, residual regolith overlying decayed and fragmented rock can be more than 100 m thick. Soil and regolith may be absent in some places, exposing outcrops of bedrock. Mass Wasting Slopes Residual regolith derived directly from the rock beneath moves slowly down the slope. Accumulations of regolith at the foot of a slope are called colluvium. Beneath the valley floor are layers of regolith, called alluvium, which is sediment transported and deposited by streams. 20 Mass Wasting Soil Creep On most slopes, soil and regolith move extremely slowly downhill through the process of soil creep. Soil creep occurs through the effect of some process of soil disturbance acting under the influence of gravity. Alternate wetting and drying of the soil, growth of ice needles and ice lenses, heating and cooling of the soil, trampling and burrowing by animals, and shaking by earthquakes all produce some disturbance of the soil and regolith. 21 Mass Wasting Earth Flows and Rotational Slumps In humid climates, a mass of moist soil and fine regolith may move down a steep slope in the form of an earth flow. The rate at which an earth flow develops can be slow or rapid, depending on the texture of the material, moisture content, and angle of slope. A slump usually occurs when water percolates deep into a mass of unconsolidated material, resulting in it dropping along a concave slip plane (slow movement along a surface of weakness). 22 Mass Wasting Mudflow and Debris Flood One of the most spectacular forms of mass wasting and a potentially serious environmental hazard is the mudflow. These streams Th t off muddy dd fl fluid id pour swiftly iftl d down canyons iin mountainous regions. Mudflows vary in consistency, from a viscous fluid similar to freshly mixed concrete to fast-flowing, water-saturated mixtures of mud and rock. The more fluid type of mudflow is called a debris flow. 23 Mass Wasting Landslides and Rockfalls A landslide is the rapid sliding of large masses of bedrock or regolith. Landslides, unlike earth flows and mudflows, are triggered by earth t tremors or sudden dd rock k ffailures il rather th th than b by h heavy rains i or fl floods; d they can also result when excavation or river erosion oversteepens a slope. Landslides can range from rockslides of jumbled bedrock fragments to bedrock slumps in which most of the bedrock remains more or less intact as it moves. A Look Ahead Flowing water, in the form of rivers and streams, is the principle agent of erosion and is discussed in the next two chapters. The first deals with water in the hydrologic cycle, in soil, and d iin streams. t The second deals specifically with how flowing water erodes rock and regolith and deposits transported sediment to create a variety of distinctive fluvial landforms. 24