Soil Formation: Rock Cycle, Mineralogy, and Weathering, Slides of Soil Mechanics and Foundations

Download Soil Formation: Rock Cycle, Mineralogy, and Weathering and more Slides Soil Mechanics and Foundations in PDF only on Docsity! CE 240 Soil Mechanics & Foundations Lecture 2 Rock Cycle, and Origin of Soil (Das, Ch. 2) Outline of this Lecture 1.Rock types and rock cycle 2.Basic Mineralogy 3.Weathering of rocks 1. mechanical weathering 2. chemical weathering 4.Origin of soil What is a ‘rock’? In Geology (the science studies rocks), ‘rock’ is defined as the solid material forming the outer rocky shell or crust of the earth. There are three major groups of rocks by its origin: (1) Igneous rocks: cooled from a molten state; (2) Sedimentary rocks: deposited from fluid medium; e.g., products of weathering of other rocks in water; (3) Metamorphic rocks: formed from pre-existing rocks by the action of heat and pressure. Apparently, the igneous rock is the one far more essential and intrinsic since the other two types are relative secondary in origin. The Identification Chart of the Igneous Rocks (Bowen reaction) Hi-resist to weathering Lo-resist to weathering Rock Cycles ost? lthig, PR Sediments Sedimentary Weathering & Erosion WSIYIOWe}3 || Melting a Metamorphism Metamorphic Tr INTERNAL a Igneous Rocks Comparison of surface and subsurface conditions Subsurface • High temperature but constant at which minerals reach equilibrium • high confining pressure (stress) • less water or no water • no oxygen Surface • low temperature, and highly variable • little or no confining pressure (stress) • abundant of water • abundant of oxygen Conclusion can be drawn from the comparison: Rock at the surface will undergo changes ---- This change is called Weathering Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks to form soil or loose particles at or near Earth's surface. Weathering causes deterioration of building materials. It also weakens rocks, a great concern when weathered rocks are used for foundation. Two types of weathering Mechanical weathering: Physical disintegration or degradation of rock pieces without a change in composition --size reduction Chemical weathering: decomposition whereby one mineral species is changed into another through various chemical processes. Water plays a major role, through: 1, provide oxygen, 2 provide mobility for moving ions. Mechanical Weathering (cont.): Mechanical weathering processes include: 1) freezing & thawing (frost wedge) Example: Pottery container with water in winter time, when water is frozen and become ice it can have 9% of relative volume change, i.e., dv/v = 9% = 0.09 This is a very large strain! Similarly if there is water in rock fractures it will force the fracture to propagate into further depth. Mechanical Weathering (cont.): The stress generated by frozen water is about 550 psi, check with Table 6.1 you can find out that this is a value comparable to the tensile strength of most rock types. Or only one order of magnitude less. But remember rocks are experiencing many freezing-thawing cycles before they are finally breaking down. σ The thermal expansion coefficient αL and αV characters how much a mineral change its dimension in response to a unit degree increase in temperature . Using a finite expression ∆L=LαL∆T PL T l L )(1 ∂ ∂ =α PV T V V )(1 0 ∂ ∂ =α Mechanical Weathering (cont.): The linear thermal expansion coefficient αL is on the order of 10-5/ºC, and it could be anisotropic, i.e., αLx ≠ αLy ≠ αLz For example, αmax is in the elongation axis (C- axis). Differential weathering joints are parallel cracks in which rocks on either side are not offset; Sheeting rock layers peel like layers of an onion Another “sheeting” example See also T&L Figure 5.8 Chapter 5: 2) Mechanical weathering Unloading Vertical columns from magma cooling & shrinking “columnar jointing” Chemical weathering Chemical weathering=chemical processes that dissolve and decay earth materials; Chemical weathering needs the rock exposes to air and water. Mechanical weathering could enhance chemical weathering by disintegration, i.e., increase the surface area of rock blocks and debris and this will greatly accelerates chemical weathering. Chemical weathering rate depends on 1. Temperature 2. Amount of surface area 3. Availability of water or natural acid Thus, rocks in tropical environment experience most severe chemical weathering. = So °o ! Se a ! 1-cm cube 6 cm? of Surface area, cm? a oO I nm a 1 = I \ I I 0.5 0.25 0.125 0.0625 Dimensions of cubes, cm kK1 cm>| or 1 unit FIGURE 8.3 Cube dimensions versus surface area. a2 feet —| Volume = @ ft? Area = 24 ft? 64 0.25-cm cubes © 24 cm? of surface 8 0.5-cm cubes 1 foot Volume = 9 ft? Area = 48 ft? Chemical Weathering of silicate minerals by carbonic acid feldspar + water + carbonic acid = clay minerals + dissolved ions 2KAlSi3O8 + H2O + 2H2CO3 = Al2Si2O5(OH)4 + 2K+ + 4SiO2(aq) + 2HCO3- Why is rainwater naturally acidic? Rainwater contains dissolved CO2 from atmosphere. Dissolved CO2 reacts with water to form carbonic acid (H2CO3) CO2 + H2O ⇔ H2CO3 Carbonic acid dissociates to produce hydrogen ion (H+) and bicarbonate H2CO3 ⇔ H+ + HCO3- Chemical weathering (cont.) Common chemical weathering processes are solution (dissolution), oxidation, and hydrolysis. Rock reacts with water, gases and solutions (may be acidic); will add or remove elements from minerals. A 16th-century monastery in Mexico shows the ravages of weathering mostly from wind and wind-driven rain. The rock is volcanic tuff. Karst landscape of Guilin, China, caused dissolved Carbonate rocks. a eee ee Pores eo The other two kinds of chemical weathering Oxidation Oxidation - Oxygen combines with iron-bearing silicate minerals causing "rusting". Iron oxides are produced that are red, orange, or brown in color. Hydrolysis Hydration-reaction between mineral and water. This photo is an outcrop of a glacial till deposit. Glacial till is a heterogeneous mixture of clay to boulder size particles deposited within or beneath glacial ice. The till type on this photo is a dense or basal till with lenses of looser, sandy material (sandy till), the soil type mapped in this area is the Montauk series [the solum (the A and B horizons) has been removed on this photo. photo location: Fearing Hill, Wareham, MA]. A photo of a glacial fluvial deposit (the topsoil and subsoil of a Hinckley soil has been removed) from a gravel pit. This photo shows the horizontal stratified layers of sand and gravel on the top of the photo called the topset beds or delta plain. The inclined or dipping layers of fine and coarse sand (visible on left part of photo) are called the foreset beds or delta slope. The foreset beds were deposited into a glacial lake, the contact of the topset and foreset beds (delta plain/delta slope) marks the former water level of the lake. town of Raynham, MA. Summary Soil come from weathering of rocks. Mechanical weathering is accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition. Chemical weathering involves breaking down rock components and internal structure and forming new compounds. Whereas weathering breaks rocks apart, erosion removes rock debris by mobile agents such as water, wind, or ice.