Physical Geography - Version 1 Unit 12 Weathering & Soils

, Figure Highly Chemically Weathered Truck in Rhyolite , Nevada . Image by Jeremy is used under a license . UNIT 12 WEATHERING SOILS Goals Objectives of this unit Explain why rocks formed at depth in the crust are susceptible to weathering at the surface . Describe the main processes of weathering and the types of materials that are produced when mechanical weathering . Describe the main processes of chemical weathering , and the products of chemical weathering of minerals such as feldspar , ferromagnesian silicates , and calcite . Discuss the relationships between weathering and soil formation , and the origins of soil horizons and some of the different types of soil . GEOGRAPHY

WHAT IS WEATHERING ?

Weathering is what takes place when a body of rock is exposed to the weather , in other words , to the forces and conditions that exist at Earth surface . Except for volcanic rocks and some sedimentary rocks , most rocks are formed at some depth within the crust . There they experience relatively constant temperature , high pressure , no contact with the atmosphere , and little or no moving water . Once a rock is exposed at the surface , which is what happens when the overlying rock is eroded , conditions change dramatically . Temperatures vary widely , there is much less pressure , oxygen and other gases are plentiful , and in most climates , water is abundant . Weathering includes two main processes that are quite different . One is the mechanical breakdown of rock into smaller fragments , and the other is the chemical change of the minerals within the rock to forms that are stable in the surface environment . Mechanical , also known as physical weathering , provides fresh surfaces for attack by chemical processes , and chemical weathering weakens the rock so that it is more susceptible to mechanical weathering . Together , these processes create two very important products , one being the sedimentary and ions in solution that can eventually become sedimentary rock , and the other being the soil that is necessary for our existence on Earth . Mechanical Weathering Intrusive igneous rocks form at depths of several hundreds of meters to several tens of kilometers . Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths more than several hundreds of meters . Most metamorphic rocks are formed at depths of kilometers to tens of kilometers . Weathering can not even begin until these rocks are uplifted through various processes of mountain building , most of which are related to plate tectonics , and the overlying material has been eroded and the rock is exposed as an outcrop . The important agents of mechanical weathering are The decrease in pressure that results from removal of overlying rock . Freezing and thawing of water in cracks in the rock . Formation of salt crystals within the rock . Cracking from plant roots and exposure by burrowing animals . When a mass of rock is exposed by weathering and removal of the overlying rock , there is a decrease in the confining pressure on the rock , and the rock expands . This unloading promotes cracking of the rock , known as exfoliation . GEOGRAPHY

Granitic rock tends to exfoliate parallel to the exposed surface because the rock is typically homogenous , and it does have predetermined planes along which it must fracture . Sedimentary and metamorphic rocks , on the other hand , tend to exfoliate along predetermined planes . FROST WEDGING Frost wedging is the process by which water seeps into cracks in a rock , expands on freezing , and thus the cracks . The effectiveness of frost wedging is related to the frequency of freezing and thawing . Frost wedging is most effective in a climate like Canada . In warm areas where freezing is infrequent , in very cold areas where thawing is infrequent , or in very dry areas , where there is little water to seep into cracks , the role of frost wedging is limited . In many parts ofthe Sierra , the transition between freezing nighttime temperatures and thawing daytime temperatures is frequent , tens to hundreds of times a year . A common feature in areas of effective frost wedging is a talus slope , a deposit of fragments removed by frost wedging from the steep rocky slopes above Figure Example of Frost Wedging , Notice How the is Cleanly Fractured . Image is used under a license . TION Exfoliation , separation of successive thin shells , or , from massive rock such as granite or basalt it is common in regions that have moderate rainfall . The thickness of the individual sheet or plate may be from a few millimeters to a few meters . GEOGRAPHY

Some geologists believe that exfoliation results when rocks formed at depth are exposed at the ground surface the previous compressional forces would decrease and thus allow the rock to expand by fracturing parallel to the surface . Quite often , however , the fractures are not parallel to the ground surface , and this circumstance is taken as an indication of some other method of formation . Large daily variations in temperature , especially pronounced in deserts , were also credited with producing exfoliation , the expansion from heating during the day followed by contraction from rapid cooling at night was thought to cause the separation of thin slabs from large blocks of rock at the surface . The study of thin shells that separate from rock exposed to the weather reveals a common cause of the separation of the slow development of clay minerals , which involves an increase in volume . The outer surface of exposed rock dries rapidly after wetting , but the moisture that penetrates minor crevices stays until some decay is started , and the resultant swelling causes flaking roughly parallel to the outer rock surface . Figure Exfoliated Granite Domes in the Alabama Hills , Lone Pine California . Image is used under a license . GEOGRAPHY

Biologic Weathering The effects of plants and animals are significant in biological weathering . Roots can force their way into even the tiniest cracks , and then they exert tremendous pressure on the rocks as they grow , widening the cracks and breaking the rock . Although animals do not normally burrow through solid rock , they can excavate and remove huge volumes of soil , and thus expose the rock to weathering by other mechanisms . Figure Example of Biological Note How the Roots are Separating the Rock . is used under a license . MASS WAS TING Mass wasting , also known as slope movement or mass movement , is the geomorphic process by which soil , sand , regolith , and rock move downslope typically as a solid , continuous or discontinuous mass , largely under the force of gravity , but frequently with characteristics of flow as in debris flows and of mass wasting include creep , slides , flows , topples , and falls , each with its characteristic features , and taking place over from seconds to hundreds of years . Mass wasting occurs on both terrestrial and submarine slopes and has been observed on Earth , Mars , Venus , and Jupiter moon Io . GEOGRAPHY

When the gravitational force acting on a slope exceeds its resisting force , slope failure ( mass wasting ) occurs . The slope material strength and cohesion and the amount of internal friction between the materials help maintain the stability and are known collectively as the shear strength . The steepest angle that a slope can maintain without losing its stability is known as its angle of repose . When a slope made of loose material possesses this angle , its shear strength perfectly the force of gravity acting upon it . Mass wasting may occur at a very slow rate , particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilized the surface . It may also occur at very high speed , such as in or landslides , with disastrous consequences , both immediate and delayed . Factors that change the potential of mass wasting include the change in slope angle , weakening of material by weathering , increased water content changes in vegetation cover , and overloading . Sandwiched between a steep , unstable hillside ( with the La Ranch Company situated on the plateau directly over the community ) and the Pacific Ocean , La has been the site of recent major mudslides . In the image notice the different slides that have occurred . The arrows below the red line show additional slumps of perhaps an insight for future movements . Figure Landslides of La , Outlined in Blue is the 1995 Slide , Yellow is the 2005 Slide . Image courtesy of Airborne Corporation , El , California is in the public domain . GEOGRAPHY

Pin It ! Landslide Hazards ! Watch this Video about landslide hazards . Chemical Weathering Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions . The kinds of changes that take place are highly specific to the mineral and the environmental conditions . Some minerals , like quartz , are virtually unaffected by chemical weathering , while others , like feldspar , are easily altered . In general , the degree of chemical weathering is greatest in warm and wet climates and least in cold and dry climates . The important characteristics of surface conditions that lead to chemical weathering are the presence of water ( in the air and on the ground surface ) the abundance of oxygen , and the presence of carbon dioxide , which produces weak carbonic acid when combined with water . That process , which is fundamental to most chemical weathering , can be shown as follows CO then , water carbon dioxide carbonic acid then carbonic acid hydrogen ion carbonate ion Here we have water , plus carbon dioxide in the atmosphere , combining to create carbonic acid . Then carbonic acid ( comes apart ) to form hydrogen and carbonate ions . The amount of in the air is enough to make only very weak carbonic acid , but there is typically much more in the soil , so water that through the soil can become significantly more acidic . There are two main types of chemical weathering . On the one hand , some minerals become altered to other minerals . For example , feldspar is altered , by hydrolysis , to clay minerals . On the other hand , some minerals dissolve completely , and their components go into solution . For example , calcite ( is soluble in acidic solutions . The hydrolysis of feldspar can be written like this ( OH ) Ca plagioclase carbonic acid kaolinite dissolved calcium carbonate ions This reaction shows calcium plagioclase feldspar , but similar reactions could also be written for sodium or potassium . In this case , we end up with the mineral kaolinite , along with calcium and carbonate ions in solution . Those ions can eventually combine ( probably in the GEOGRAPHY

ocean ) to form the mineral calcite . Other silicate minerals can also go through hydrolysis , although the results will be a little different . For example , pyroxene can be converted to the clay minerals chlorite or smectite , and olivine can be converted to the clay mineral serpentine . Whenever water reacts with another chemical compound , the process is called hydrolysis . Hydrolysis differs somewhat from hydration , although the two can occur together . Hydration is the bonding of whole water molecules to an ion ( a charged atom or molecule ) usually a metal ion . Hydrolysis , on the other hand , involves an actual chemical reaction ofthe water molecule itself with another reactant . Aluminum ion , for example , can bond with six water molecules to form the hydrated aluminum ion . In water , however , the hydrated ion can undergo hydrolysis some of the hydrated molecules contribute a hydrogen ion to the solution , making the solution acidic . Solutions of ions often become either acidic or basic because of hydrolysis , too . In general , negative ions ( form basic solutions if they hydrolyze , because the negative charge on the ion attracts the positively charged hydrogen ion ( away from water , leaving the basic hydroxide ion ( behind . Similarly , positive ions ( form acidic solutions if they hydrolyze , because the positive charge on the ion attracts the negatively charged hydroxide ion away from water , leaving the acidic hydrogen ion behind . Hydrolysis of these ions only occurs , however , if the ion originally came from a weak acid or base , or the salt of a weak acid or base . A salt is an ionic chemical compound derived from an acid or base , often as the result of a neutralization reaction . Ions do not hydrolyze if they are from strong acids or bases , such as chloride ion from hydrochloric acid or sodium ion from sodium hydroxide ( a base ) or their salts . Oxidation is another very important chemical weathering process . The oxidation of the iron in a ferromagnesian silicate starts with the dissolution of the iron . Picture below , a granitic rock containing biotite and amphibole which have been altered near to the rock surface to limonite , which is a mixture of iron oxide minerals . GEOGRAPHY

Figure Example or Oxidation on the links ofa chain near the Golden Gate Bridge in San it was continuously exposed to moisture and spray , causing surface breakdown , cracking , and flaking of the metal . image is used under a Alike license . For olivine , the process looks like this , where olivine in the presence of carbonic acid is converted to dissolved iron , carbonate , and silicic acid olivine ( carbonic acid ) dissolved iron dissolved carbonate dissolved silicic acid In the presence of oxygen , the dissolved iron is then quickly converted to hematite 02 dissolved iron bicarbonate oxygen hematite carbonic acid The equation shown here is for olivine , but it could apply to almost any other ferromagnesian silicate , including pyroxene , amphibole , or biotite . Iron in the sulfide minerals ( pyrite ) can also be oxidized in this way . The mineral hematite is not the only possible result , as there is a wide range of iron oxide minerals that can form in this way . The results ofthis process are illustrated below , which shows a granitic rock in which some of the biotite and amphibole has been altered to form the iron oxide mineral limonite . GEOGRAPHY

Water contains many weak acids such as carbonic acid . This weak but abundant acid is formed when carbon dioxide gas from the atmosphere mixes with rainwater . Sulfur dioxide and nitrogen gases create other types of acid rain that act as chemical weathering agents . Some sources of sulfur dioxide are power plants that burn coal as well as volcanoes and coastal marshes . Sulfur gases react with oxygen and rainwater to form sulfuric acid . Although relatively weak , acid abundance and effects produce noticeable damage to vegetation , fabrics , paints and , rocks . Below is a picture of the , in the British Museum . In the , the curators at the museum decided there was something wrong with the surface of the marbles , and applied a caustic and abrasive chemical , leaving them as we see them today . There may have been damaged from the polluted air of London at that time , but they removed any vestiges of the original painting that had been applied to the carvings . a license . GEOGRAPHY

FORMATION OF SOIL Weathering is a key part of the process of the soil formation , and the soil is critical to our existence on Earth . In other words , we owe our existence to weathering , and we need to take care of our soil . Many people refer to any loose material on Earth surface as soil , but to earth scientists , soil is the material that includes organic matter , lies within the top few tens of centimeters of the surface , and is important in sustaining plant growth . a SAND a ?

Figure Size Comparison for Sand , Silt Clay . Image by OER team is used under a BY license . Soil is a complex mixture of minerals ( approximately 45 ) organic matter ( approximately ) and empty space ( approximately 50 , filled to varying degrees with air and water ) The mineral content of soils is variable but is dominated by clay minerals and quartz , along with minor amounts of feldspar and small fragments of rock . The types of weathering that take place within a region have a major influence on soil composition and texture . For example , in a warm climate , where chemical weathering dominates , soils tend to be richer in clay . Soil scientists describe soil texture in terms of the relative proportions of sand , silt , and clay , as shown in the soil triangle below . The sand and silt components in this diagram are dominated by quartz , with lesser amounts of feldspar and rock fragments , while the clay component is dominated by the clay minerals . The soil texture triangle gives names associated with various combinations of sand , silt , and clay . A or sandy soil is one comprised primarily of particles . A textured or clayey soil is one dominated by tiny clay particles . Due to the strong physical properties of clay , a soil with only 20 clay particles behaves as sticky , gummy clayey soil . The term loam refers to a soil with a combination of sand , silt , and particles . For example , a soil with 30 clay , 50 sand , and 20 silt is called a sandy clay loam . GEOGRAPHY

Soil forms through the accumulation and decay of organic matter and both mechanical and chemical weathering processes described above . The factors that affect the nature of the soil and the rate of its formation include climate ( especially average temperature and precipitation amounts , and the consequent types of vegetation ) the type of parent material , the slope of the surface , and the amount of time available . Sand ( Figure 1266 Soil Texture Triangle . Image by is used under a Alike Un license . Climate Soils develop because ofthe weathering of materials on Earth surface , including the mechanical breakup of rocks , and the chemical weathering of minerals . Soil development is facilitated by the downward percolation of water . Soil forms most readily under temperate to tropical conditions ( not cold ) and where precipitation amounts are moderate ( not dry , but not too wet ) Chemical weathering reactions ( especially the formation of clay minerals ) and biochemical reactions proceed fastest under warm conditions , and plant growth is enhanced in warm climates . Too much water ( in rainforests ) can lead to the leaching of important chemical nutrients and hence to acidic soils . In humid and poorly drained regions , swampy conditions may prevail , producing soil that is dominated by organic matter . Too little water ( in deserts and ) results in very limited downward chemical transportation GEOGRAPHY

and the accumulation of salts and carbonate minerals ( calcite ) from water . Soils in dry regions also suffer from a lack of organic material Parent Material Soil parent materials can include all different types of bedrock and any type of unconsolidated sediments , such as glacial deposits and stream deposits . Soils are described as residual soils if they develop on bedrock , and transported soils if they develop on transported material such as glacial sediments . But the term transported soil is misleading because it implies that the soil itself has been transported , which is not the case . When referring to such soil , it is better to be specific and say , soil developed on unconsolidated material , because that distinguishes it from soil developed on bedrock . parent material , such as granite , sandstone , or loose sand , leads to the development of sandy soils . material , such as shale or basalt , generates soils with little sand . Parent materials provide important nutrients to residual soils . For example , a minor constituent of granitic rocks is the mineral apatite , which is a source of the important soil nutrient phosphorus . Basaltic parent material tends to generate very fertile soils because it also provides phosphorus , along with significant amounts of iron , magnesium , and calcium . Some unconsolidated materials , such as deposits , make for especially good soils because they tend to be rich in clay minerals . Clay minerals have large surface areas with negative charges that are attractive to positively charged elements like calcium , magnesium , and iron , and important nutrients for plant growth . Slope Time Soil can only develop where surface materials remain in place and are not frequently moved away by mass wasting . Soils can not develop where the rate of soil formation is less than the rate of erosion , so steep slopes tend to have little or no soil . Even under ideal conditions , the soil takes thousands of years to develop . As an example , most of northern California was still up until , and so , at that time , conditions were still not ideal for soil development even in the southern regions . Therefore , soils in northern California , are relatively young and not well developed . The same applies to soils that are forming on newly created surfaces , such as recent deltas or sand bars , or in areas of mass wasting . GEOGRAPHY

Soil Horizons The process of soil formation generally involves the downward movement of clay , water , and dissolved ions , and a common result of that is the development of chemically and texturally different layers known as soil horizons . The typically developed soil horizons are the layer of organic matter A the layer of partially decayed organic matter mixed with mineral material the ( leached ) layer from which some of the clay and iron have been removed to create a pale layer that may be than the other layers the layer of accumulation of clay , iron , and other elements from the overlying soil the layer of incomplete weathering the parent material or bedrock Another type of layer that develops in hot arid regions ( such as in the Mojave Desert ) is known as ( pronounced ) It forms from the downward ( or in some cases upward ) movement of calcium ions , and the precipitation of calcite within the soil . When well developed , cements the surrounding material together to form a layer that has the consistency of concrete . Figure Soil Horizons . The Horizon Would be Below the Horizon . Image by is in the public domain . Like all geological materials , the soil is subject to erosion , although , under natural conditions on gentle slopes , the rate of soil formation either balances or exceeds the rate of erosion . Human practices related to forestry and agriculture have significantly upset this balance . GEOGRAPHY

Soils are held in place by vegetation . When vegetation is removed , either through cutting trees or routinely harvesting crops and tilling the soil , that protection is either temporarily or permanently lost . The primary agents of the erosion of unprotected soil are water and wind . Soil Erosion Water erosion is accentuated on sloped surfaces because water has greater eroding power than still water . Raindrops can disaggregate exposed soil particles , putting the finer material ( clays ) into suspension in the water . unchanneled flow across a surface carries suspended material away , and channels erode right through the soil layer , removing both fine and coarse material . Wind erosion is exacerbated by the removal of trees that act as and by agricultural practices that leave bare soil exposed . Tillage is also a factor in soil erosion , especially on slopes , because each time the soil is lifted by a cultivator , it is moved a few centimeters down the slope . UNIT 12 SUMMARY Rocks weather when they are exposed to surface conditions , which in most cases are quite different from those at which they formed . The main processes of mechanical weathering include exfoliation , salt crystallization , and the effects of plant growth . Chemical weathering takes place when minerals within rocks are not stable in their existing environment . Some ofthe important chemical weathering processes are hydrolysis of silicate minerals to form clay minerals , oxidation of iron in silicate and other minerals to form iron oxide minerals , and dissolution of calcite . The main products of weathering and erosion are grains of quartz ( because quartz is resistant to chemical weathering ) clay minerals , iron oxide minerals , rock fragments , and a wide range of ions in solution . Soil is a mixture of fine mineral fragments ( including quartz and clay minerals ) organic matter , and empty spaces that may be partially filled with water . Soil formation is controlled by climate ( especially temperature and humidity ) the nature and lithology of the parent material , the slope ( because the soil ca accumulate on steep slopes ) and the amount of time available . Typical soils have layers called horizons that form because of differences in the conditions with depth . GEOGRAPHY