Выветривание горных пород

INTRODUCTIONThe Earth’s surface is changing slowly over time. Rocks are a geomaterial, and they present several types of problems. They may present undesirable behaviors, such as low strength, disaggregation, high plasticity, slaking, and many other characteristics. Scientists’ explanations for how these land features came to be; these typically rely on a deep understanding of geological forces, like weathering and erosion, that help shape the Earth slowly over time.
No rock is stable or immune to weathering. Rocks show consistent reduction of the rock strength and conditions of discontinuity. The discontinuity spacing however increases from moderate weathering degree to complete weathering degree. Weathering is the breaking down or wearing away of rocks where they are. Weathering is distinguished from erosion by the fact that the latter usually includes the transportation of the disintegrated rock and soil away from the site of the degradation.
This process does not happen because rocks move or collide with each other. Many pathways and agents are involved in weathering, but most can be grouped into three main processes: mechanical, chemical and biological weathering.
The aim of the paper is to provide an analysis of the process of the weathering of rocks.
The objectives are:
1) to define the notion of the weathering of rocks;
2) to examine and explain the peculiarities and specific features of the types of rock weathering.

PART 1. THE NOTION OF WEATHERING
Weathering is the process of disintegration of rock from physical, chemical, and biological stresses. Weathering is influenced by temperature and moisture (climate). As rock disintegrates, it becomes more susceptible to further physical, chemical, and biological weathering due to the increase in exposed surface area. During weathering, minerals that were once bound in the rock structure are released.
The degree of weathering that occurs depends upon the resistance to weathering of the minerals in the rock, as well as the degree of the physical, chemical, and biological stresses. A rule of thumb is that minerals in rocks that are formed under high temperature and pressure tend to be less resistant to weathering, while minerals formed at low temperature and pressure are more resistant to weathering. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses generally decrease with depth. Weathering of rocks occurs in place, but the disintegrated weathering products can be carried by water, wind, or gravity to another location (i.e., erosion or  mass wasting) [4, p. 146-152].
There are three major types of weathering, although most textbooks only distinguish two. The first type is physical weathering and is defined as the mechanical breakup of rock. The second type of weathering is called chemical weathering. This is the most important process in soil formation and involves chemical changes during the breakup of rock. The last of the weathering types (not always distinguished in texts) is biological weathering. This involves the actions of plants and animals and is really just a combination of physical and chemical weathering. The main thing to remember about these types of weathering is that they all reduce rock into sediment. Physical weathering does this with little loss in volume. Chemical weathering may result in a significant loss in volume.
Weathering of rocks has several products. They are:
(1) regolith, a collective term for sediment regardless of how it was deposited, as well as layers of pyroclastic materials and the residue formed in place by weathering;
(2) soil, consists of weathered materials, air, water, and organic matter;
(3) humus consists of carbon derived by bacteria decay of organic matter and is highly resistant to further decay. It is important source of plant nutrients and it enhances moisture retention;
(4) residual soil, if a body of rock weathers and the weathering residue accumulates over it, the soil so formed is residual, meaning that is formed in place [1, p. 84-85].
PART 2. PHYSICAL WEATHERINGPhysical weathering leads to physical disintegration of rocks without chemical changes taking place. It occurs when physical forces break Earth materials into smaller pieces.
Physical weathering occurs everywhere, but is especially prevalent in areas of the Earth that are either very hot (e.g., deserts) or very cold (e.g., mountains, tundra). During thermal expansion and contraction, the volume of rocks changes in response to heating and cooling. In desert, where the temperature may vary as much as 30°C in one day, rocks expand when heated and contract as they cool. Rock is poor conductor of heat, so its outside is heated up more than its inside; the surface expands more than interior, producing stress that may cause fracturing.
It is felt by many geologists that this causes rocks to “sheet” off in a process called exfoliation. Spherical weathering results in rounded granite boulders atop mountains [5, p. 375].
Fire causes very rapid expansion. During the forest fire, rocks may heat very rapidly, especially near the surface, because they conduct heat so poorly. The heated surface layer expands more rapidly than the interior, and thin sheets paralleling the rock surface become detached.
Another type of physical weathering is called unloading. Granite forms well below the surface of the Earth in areas of fairly high pressure. When exposed at the Earth’s surface, the rocks no longer feel the confining pressure and may tend to shatter because of the reduced pressure load. Unloading is really a problem in new mine shafts. Some granites (other rocks too, but granite is about the worst) will explode in what is called a rock burst [5, p. 383]. In other words, rock sometimes expands violently in response to the pressure released by removal of the mine rock. These rock bursts kill 20 miners/year. It is also known as pressure release. Excavations of only 7 or 8 m in the granite quarries have produced sheet joints with enough force to knock the tracks off of the heavy quarrying equipment.
One more type of physical weathering is called abrasion. It includes wearing or grinding by small sedimentary particles carried by wind, water, or ice.
In cold climates, water is the major agent behind physical weathering. Liquid water expands when it freezes, so any water within cracks, fractures and joints exerts tremendous force when it freezes. Rocks can be literally split apart as the temperature drops. Generally speaking, frost action involves the repeated freeze and thaw of water in the cracks and pores of rocks. This results in frost wedging, a very effective process for widening and extending cracks and thereby breaking rocks into smaller pieces. When water freezes, it expands in volume by 9% which exerts great force on the walls of containing cracks and pores. Mountains are particularly good areas to see the results of this frost heaving. The piles of rock that occur along the base of mountains (called scree or talus) was mostly derived from frost heave [7, p. 221-222].
Growing crystalls exert enough force to widen cracks and crevices or dislodge particles in porous, granular rocks such as sandstone. Even in crystalline rocks such as granite, salt crystal growth may pry loose individual minerals. It is similar to frost wedging. Most salt crystal growth occurs in hot, arid areas, although it probably affects rocks in some coastal regions as well.
Physical weathering produces smaller bits of rock, but it doesn’t actually change the composition of the rock. You would be able to recognize bits of granite or basalt or rhyolite. The most important thing it does is increase the relative surface area of the rock. The surface area is the amount of contact area in an rock that is exposed to water. Water is the principle agent behind chemical weathering so the more surface area, the more contact area for chemical weathering. Or to put it more succinctly, the higher the surface area, the faster chemical weathering occurs.


PART 3. CHEMICAL WEATHERINGChemical weathering refers to the processes that decompose rocks and minerals. In some instances, minerals are chemically altered such that new minerals are formed. Other minerals may completely dissolve and their ions taken into solution.
Important agents of chemical weathering include atmospheric gases, especially oxygen, water, and organic acids produced by plants and decaying organic matter.
Chemical weathering proceeds from the surface of a rock inward. As a result, when a weathered rock is broken open, an outer weathered rind and relatively unweathered interior are commonly present. The rate of chemical weathering is controlled largely by climate, particle size, and mineralogy of the parent material rock. Water enters rocks along fractures, so the more fractures, the more opportunities for chemical weathering.
The term oxidation, in chemical weathering refers to reactions with oxigene to form an oxide (one or more metallic elements combined with oxygen) or if water is present, a hydroxide (a metallic element or radical combined with OH). Most oxidation is carried out by oxygen dissolved in water.
Oxidation is important in the alteration of ferromagnesian silicates suc as olivine, pyroxenes, amphiboles and biotite. Iron in these minerals combines with oxygen to form the reddish iron oxide hematite (Fe2O3 ) or the yellowish or brown hydroxide limonite [FeO(OH).nH2O]. The oxidation of iron sulfide such as the mineral pyrite (FeS2 ) is commonly associated with coal, so in mine taiilings pyrite oxidizes to form sulfuric acid (H2SO4 ) and iron oxide. Acid soils and waters in coal-mining areas are produced in this manner and present a serious environmental hazard [2, p. 301].
Hydrolysis, is the chemical reaction between the hydrogen (H+) ions and hydroxyl (OH-) ions of water and a mineral’s ions. In hydrolysis, hydrogen ions actually replace positive ions in minerals thus changing the composition of minerals and liberating soluble compounds and iron that then may be oxidized.
Hydrogen ions attack the ions in orthoclase structure, and some liberated ions are incorporated in developing clay minerals, while others simply go into solution. On the right side of the equation is excess silica that would not fit into the crystal structure of the clay mineral. This dissolve silica (SiO2) is an important source of cement that binds together the particles in some sedimentary rocks [6, p. 69].
Because chemical weathering affects particle surfaces, the greater the surface area, the more effective the weathering. It is important to realize that small particles have larger surface areas compared to their volume than do large particles. As a rock is divided into smaller and smaller particles, its surface area increases but its volume remains the same. Small particles have more surface area compared to their volume than do large particles [4, p. 164].
Chemical processes proceed more rapidly at high temperatures and in the presence of fluids. So chemical weathering is more effective in the tropics than in arid and arctic regions because temperatures and rainfall are high and evaporation rates are low.
PART 4. BIOLOGICAL WEATHERINGAnimals, plants, and bacteria all participate in the mechanical and chemical alteration of rocks. Plants and animals release acid forming chemicals that cause weathering and also contribute to the breaking down of rocks and landforms.
Microbial activity breaks down rock minerals by altering the rock’s chemical composition, thus making it more susceptible to weathering. One example of microbial activity is lichen; lichen is fungi and algae, living together in a symbiotic relationship. Fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering [3, p. 113-132].
Burrowing animals, such as worms, termits, reptiles, rodents, many others, constantly mix soil and sediment particles and bring material from depth to the surface where further weathering occurs.
The roots of plants, especially large bushes or trees, can grow into small spaces and gaps in rock. When these roots grow, they exert pressure on the rock around them, causing the gaps to widen or even crack. Plant roots can also weather rock through chemical processes. When dead roots decompose, they release carbon dioxide; this is sometimes converted into carbonic acid, which chemically breaks down rock into soil.
Some bacteria derive nutrition by taking a combination of nitrogen from the air and minerals – such as silica, phosphorous and calcium – from rock. By removing these minerals, the rock is weakened and is further subject to other weathering forces such as wind and water. Lichens, symbiotic colonies of fungi and microscopic algae that grow on rock, also contribute to weathering. The fungi in a lichen produce chemicals that break down the minerals in the rock. The algae, like the bacteria, use these minerals for nutrition.