Physical Geology Notes PDF

CHAPTER 1 Weathering and Soil Weathering: It is the disintegration and decomposition of rock at or near the earth's surface. That is divided into mechanical and chemical weathering. Erosion: It is the incorporation and transportation of material by mobile agents suc...

CHAPTER 1 Weathering and Soil Weathering: It is the disintegration and decomposition of rock at or near the earth's surface. That is divided into mechanical and chemical weathering. Erosion: It is the incorporation and transportation of material by mobile agents such as water, wind or ice. Mass Wasting: It is the downslope movement of rock, regolith, and soil under the direct influence of gravity. I- Mechanical weathering: It is the break down rocks into smaller and smaller pieces by physical forces without changing their composition. Effect of physical weathering on rate of chemical weathering: Physical weathering breaks rocks down into smaller pieces thus increasing the surface area over which chemical weathering can occur (see diagram). Types of mechanical weathering: 1- Frost wedging: Frost wedging When water change from liquid state to solid state by freezing (water molecules arrange themselves into a very open crystalline structures), expands because volume increase by 9%. Thus, freezing of 1 Talus slope water filling the fractures, cracks and joints, exerting pressure and expanding them. This type of weathering is common in mountain areas in the middle latitudes where daily freeze-thaw cycles often exist. Rocks are wedged loose and may tumble into large piles called talus slopes that often form at the base of steep rock outcrops. 2- Unloading (Pressure release) or Sheeting: Intrusive igneous rocks (granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand more than the rock below, and thus separate from the rock body. Outwards expansion results in the formation of loose concentric slabs (onion-like layers). The process that generates these onion-like layers is called sheeting. Large structure result from sheeting is termed exfoliation dome. Thermal Expansion It occurs in hot desert areas where daily variations in temperature may exceed 30°C. Heating rocks causes expansion and cooling causes contraction. Repeated swelling and shrinking of minerals with different expansion rates should logically exert some stress on the rock’s outer shell. 2  3- Biological Activity Activities of organisms including plants, burrowing animals and human beings. Plant roots grow into fractures in their search of nutrients and water, as the root grows, they wedge the rock apart. Burrowing animals (earthworms) further break down rock by moving fresh material to the surface, where physical and chemical processes can more effectively attack it. Decaying organisms produce acids that contribute to chemical weathering. Road cuts, quarries and other human constructions play a Tarbuck, E.J. and Lutgens, F.K., role. 1993: II- Chemical Weathering: Chemical weathering involves a chemical transformation of rock into one or more new compounds. It includes dissolution, oxidation and hydrolysis. a) Dissolution: Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. CO2 + H2O => H2CO3 Carbon dioxide + water => carbonic acid Also organic acids are released as organisms decay. Sulfuric acid is produced by the weathering of pyrite and other sulfide minerals. The most soluble mineral in water is halite. Water molecules are polar, oxygen has a negative charge and hydrogen has a positive charge. As the water molecules collide with a halite crystal, oxygen contact and disrupt Na +. The attractive force of water pulls the sodium ions from the crystalline structures. The Cl - is similarly removed by hydrogen. Minerals calcite (CaCO3) which composes the common 3 building stones marble and limestone is easily attacked by weak acidic solution. During this process, the insoluble calcium carbonate is transformed into soluble products. b) Oxidation: Rust occurs when oxygen combines with iron to form iron oxides (e.g. hematite, limonite). 4Fe + 3O2 → 2Fe2O3 Iron was oxides because it lost electrons to oxygen. It occurs in dry environments, but water greatly speeds the reaction. Oxidation decomposes such ferromagnesian minerals as olivine, pyroxene, and hornblende. Oxygen readily combines with the iron in these minerals to form reddish-brown iron oxides called hematite (Fe2O3), or in more extreme cases a yellowish-colored rust called limonite [FeO(OH)]. These products are responsible for the rusty color on the surface of dark igneous rocks, such as basalt. c) Hydrolysis Carbonic acid ionizes (breaks down) into two ions, hydrogen (H+) and bicarbonate (HCO3) -1. H2CO3 → H+ + HCO3- The free hydrogen ions may alter mineral composition by replacing other ions in a mineral’s atomic structure; this reaction is termed hydrolysis. Hydrolysis occurs when minerals react with water to form other products. Potassium feldspar, the most common mineral in rocks on the earth's surface (granite), reacts with water to form a secondary mineral such as kaolinite (a type of clay) and additional ions that are 4 dissolved in water. Clay minerals are the end products of weathering and are very stable under surface conditions. Consequently, clay minerals make up a high percentage of the inorganic materials in soils. K-feldspar + carbonic acid + water → clay + potassium ion + bicarbonate ion + silica 2KAlSi3O8 + 2(H+ + HCO3-) + H2O → Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2 In this reaction H+ replace K+ in feldspar structure. Once removed, the potassium is available as a nutrient for plants or becomes the soluble salt potassium bicarbonate, which may be incorporated into other minerals or carried to the ocean. Quartz, the other main component of granite, is very resistant to chemical weathering, hence it remains substantially unaltered. Rates of Weathering: Rate of weathering is controlled by the following: rock type, rock structure, and climate. a) Rock type The most abundant mineral group is the silicate that shows the following order of weathering (increase in the resistance of weathering from top to bottom): Olivine least resistant Pyroxene Calcium feldspar Amphibole Biotite Sodium feldspar K-feldspar Muscovite Quartz most resistant 5  This arrangement is identical to that of Bowen’s reaction series. Mineral (olivine) that crystallized at high temperature is less resistance to chemical weathering than that crystallized at low temperature (quartz). The early formed minerals are not stable at the earth’s surface, where the temperature and pressure are drastically different from the environment in which they formed. Rocks composed of minerals that are relatively unaffected by chemical weathering will be the most resistant to weathering. For example, quartz is unaffected by dissolution, hydrolysis and oxidation, therefore, rocks composed almost exclusively of the mineral quartz are more resistant than other common rock types. Rocks such as sandstone and quartzite often form resistant ridges separating valleys formed in weaker rocks. Sand on a beach is made up almost exclusively of quartz grains as chemical weathering alters less resistant feldspar minerals and the resulting clays are deposited elsewhere by streams or shoreline currents. The granite headstone (A) was erected six years before the marble headstone (B). Tarbuck, E.J. and Lutgens, F.K., 1984. 6  b) Rock structures Rock structures such as fractures and joints present in the rocks. They are important in weathering because it: i) Breakdown the rock into fragments. ii) Increase the surface area available for chemical reaction. iii) Act as channel ways for elements of the atmosphere, and water to penetrate the rock. Consequently, rocks that contain abundant fractures are typically weathered more rapidly than equivalent unfractured rocks c) Climate Climatic factors, particularly temperature and moisture, are of primary significance to the rate of rock weathering. The optimum environment for chemical weathering is a combination of warm temperatures and abundant moisture. In Polar Regions, chemical weathering is ineffective because frigid temperatures keep the available moisture locked up as ice. In arid regions there is insufficient moisture to foster rapid chemical weathering. Chemical weathering of Cleopatra's Needle (A) before it was removed from Egypt, (B) After a span of 75 years in New York City's Central Park. Tarbuck, E.J. and Lutgens, F.K., 1984. 7  Soil and regolith Regolith is composed of rock and mineral fragments produced by weathering. Soil is that portion of the regolith that supports plant life and includes mineral, organic material, water and air. Soil Profile Soil can be divided into a series of distinct zones or layers (soil horizons) that collectively are termed a soil profile. Each horizon is designated by a letter. Beginning at the top the horizons are: O Horizon: It is only a few centimeters thick, consists of organic matter. The remains of plants are clearly recognizable in the upper part of horizon O, but its lower part consists of partly decomposed organic matter (humus). A horizon: This zone is largely mineral matter, yet biological activity is high and humus is generally present – up to 30%. O and A horizons are called topsoil. Water percolating down through horizon A dissolves soluble minerals and caries them away or down to lower levels in the soil by a process called leaching, so horizon A is also known as the zone of leaching. B horizon: It is called subsoil. It is characterized by the accumulation of soluble ions and fine particles that carried downward (leached) from the A horizon, thus it termed 8 zone of accumulation. Organic material that present in B horizon is less than that present in A horizon but more than that present in C horizon. C horizon: This horizon is composed of partially altered rock debris and contains little organic matter. Soil Types 1- Pedalfer: It came from the Greek pedon (means soil), and Al and Fe. This soil has accumulation of iron oxides and aluminum rich clays in the B zone giving it a brown to red-brown color. It is common in mid-latitude areas (rainfall exceeds 63 cm), most of the soluble materials (e.g.CaCO3) are leached from the soil and carried away by underground water. It is best developed under forest vegetation where enough decomposing organic matter can provide acid conditions for leaching. 2- Pedocal: It came from the Greek pedon (means soil), + cal (calcite). It is characterized by accumulation of CaCO3 giving it white color. It found in drier areas, where rainfall less than 63cm, associated with grassland and brush vegetation. Chemical weathering is less in dry areas; therefore, pedocals contain small percent of clay minerals than pedalfers. A calcite rich layer (called Caliche) may be present in the soil. 3- Laterite: it is developed in hot, wet climates of tropical area, where chemical weathering is intense, and thus soils are usually deeper. Percolating H2O not removes the soluble materials as calcite, but also removes much of the silica, with the result that oxides of Fe, Al become concentrated in the soil (which will have a red color). If the parent rock contained little Fe, the product of weathering is an Al rich accumulation called bauxite, which is the primary ore of Al. 9 Controls of Soil Formation: Soil formation is controlled by, parent material, time, climate, plants and animals and slope. a) Parent Material: the parent material is the source of weathered mineral matter from which soils develop. When a parent material is the underlying bedrock, the soil is called residual soil. On the other hand, transported soil is that developed on unconsolidated sediment. Transported soil develops faster than the residual soil because unconsolidated materials are already partly weathered. * The nature of the parent material affects soils in two ways: - First, the type of parent material will affect the rate of weathering, and thus the rate of soil formation. - Second, the chemical composition of the parent material will affect the soil’s fertility. b) Time: Short time weathering suggests that soil is similar in composition to parent material. Long time weathering will diminish the effect of parent material on the composition of the resulting soil. RULE: the longer a soil has been forming, the thicker it gets and the less it resembles the parent material. c) Climate: Variations in temperature and precipitation determine whether chemical or mechanical weathering will dominate, and the rate and depth of weathering. Example: hot, wet climate forms thick soil faster than cold, dry climate that produces a thin mantle of mechanically weathered debris. d) Plants and Animals: The type and abundance of organisms have a strong influence on the physical and chemical properties of a soil. Plants and animals furnish organic matter to the soil. When organic matter is decomposed, important nutrients are 10 supplied to plants, animals and microorganisms living in soil. Decay of plant and animal remains causes the formation of various organic acids that can accelerate weathering. Microorganisms, mainly bacteria, play a role in the decay of plant and animal remains. The end product is humus, a material that no longer resembles the plants and animals from which it formed. In addition, certain microorganisms aid soil fertility because they have the ability to fix (change) atmospheric nitrogen into soil nitrogen. Earthworms and other burrowing animals act to mix the mineral and organic portions of a soil. e) Slope: On steep slopes, soils are often poorly developed. In such situations the quantity of water soaking in is slight; as a result, the moisture content of the soil may not be sufficient for vigorous plant growth. Further, because of accelerated erosion on steep slopes, the soils are thin or in some cases nonexistent. In bottomlands, soils are usually thick and dark. The dark color results from the large quantity of organic matter that accumulates because saturated conditions retard the decay of vegetation. The optimum terrain for soil development is a flat-to-undulating upland surface. Here we find good drainage, minimum erosion, and sufficient infiltration of water into the soil. Slope orientation, or the direction a slope is facing, is another consideration. In the mid-latitudes of the Northern Hemisphere, a south-facing slope will receive a great deal more sunlight than a north-facing slope. The difference in the amount of solar radiation received will cause differences in soil temperature and moisture, which in turn influence the nature of the vegetation and the character of the soil. References: 1) Tarbuck, E.J. and Lutgens, F.K., 1984: The Earth "An Introduction to Physical Geology". Bell and Howell Company, 2nd Edition, 594pp. 2) Tarbuck, E.J. and Lutgens, F.K., 1993: The Earth "An Introduction to Physical Geology". Macmillan Publishing Company, 4th Edition, 654pp. 11  CHAPTER 2 Mass Wasting Mass wasting is the downslope movement of rock, regolith, and soil under the direct influence of gravity. Mass Wasting Controls: There are several factors that influence mass wasting including, gravity, steepness of slope and water content. 1) Gravity: Gravity is the controlling force of mass wasting because rock particles and soil move downslope by the force of gravity. 2) Water is a factor that: The filling of sediment pore spaces with water lead to: – Destroys cohesion or internal resistance between particles when saturated. – Reduce the frictional force with the underlying substrate. – Adds considerable weight to the mass of material. – Changes the properties of clay; clay becomes "slick" when wetted. 3) Oversteepening of slopes is a factor. Angle of repose is the steepest angle of slope at which loose particles remain stable on the slope. Rock debris is stable at slope angles less than the angle of repose. Angles of repose vary between 25 and 40 degrees depending on the size and shape of the particles. The angular blocks interlock and jam together. As a result, talus typically has a steep angle of repose, up to 45º. In contrast, rounded sand grains do not interlock and therefore have a lower angle of 12 repose. Classification of Mass Wasting Processes: 1- Classification of mass wasting processes based on the type of material. If soil and regolith dominate, we use debris, mud or earth in the description. On the other hand, we use the term rock as a part of the description when mass of bedrock breaks loose and moves downslope. 2- Classification of mass wasting processes based on the type of motion. a) Fall: The movement involves the free-fall of detached individual pieces of any size; it is common on steep slopes. b) Slide: Slides occur whenever material remains fairly coherent and moves along well-defined surface. c) Slump: The term is used when mass of rock or unconsolidated material moving as a unit along a curved surface. 13  d) Flow: Flow occurs when material moves downslope as a viscous fluid. Most flows are saturated with water and typically move as lobes or tongues. 3- Classification of mass wasting processes based on the rate of movement. a) Rock avalanches: They are the most rapid type of mass movements (about 200Km/h). b) Creep: In this movement type the movements of particles are usually measured in millimeters or centimeters per year. Slump: It is the downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface. Slumped material does not travel very fast or very far. The surface of rupture beneath the slump block is characteristically spoon-shaped and concave upward or outward. As the movement occurs, crescent-shaped scarps are formed at the head and the block's upper surface is sometimes tilted backwards. Slump commonly occurs on slopes that have been oversteepened. The material at the top of a slope is held in place by material at the base of the slope. As the material at the base is removed, the material above is unstable and easily to move by gravity. Water percolating downward and along the curved surface may promote further instability through lubrication. Rockslide: It is the downward sliding of blocks of bedrock that have broken loose and slide down a slope. If the material involved is largely unconsolidated, the term debris slide is used 14 instead. It is the fastest and most destructive type of the mass wasting processes. Often occurs in areas where the rock strata are inclined or joints and fractures exist parallel to the slope. Thus, when such a rock unit is undercuts at the base of the slope, it loess support and slide downslope. Rockslide often triggered by an earthquake. Sometimes it triggered when rain or melting snow lubricates the underlying surface. Mudflow: It is a relatively rapid type of mass wasting that involves a flowage of debris containing a large amount of water. It is most characteristic of semiarid mountainous regions. Mudflow tends to follow canyons and gullies. It buildup a fanlike deposits at canyon mouth. Lahars are mudflows on the slopes of volcanoes. Earthflow It is downslope movement of water- saturated soil on hillsides in humid areas as a result of excessive rainfall. Earthflows form tongue-shaped masses with well-defined head scarps. They move relatively slowly than the more fluid mudflows because they are quite viscous. They may remain active for periods ranging from days to years. 15 Creep It is slow downslope movement of soil and regolith. It can take place on even gentle slopes and is extremely widespread. Creep causes fences and utility poles to tilt and retaining walls to be displaced. A primary cause of creep is the alternate expansion and contraction of surface materials caused by freezing and thawing or wetting and drying. Wet clays expand perpendicular to the surface. As clays dry, the contract allows the particles to fall back to slightly lower level due to gravity. Result is the slow movement of material downslope over years of time. References: 1) Tarbuck, E.J. and Lutgens, F.K., 1984: The Earth "An Introduction to Physical Geology". Bell and Howell Company, 2nd Edition, 594pp. 2) Tarbuck, E.J. and Lutgens, F.K., 1993: The Earth "An Introduction to Physical Geology". Macmillan Publishing Company, 4th Edition, 654pp. 3) Thompson, G. and Truk, J. 1997, Introduction to physical Geology, Brooks Cole; 2nd Edition, 432pp. 16