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Engineering Geology CVL 3315 Chapter 5 Volcanoes and Volcanic Hazards Dr. Sari Abusharar University of Palestine Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2014-2015 1 Outline of Presentation Earth’s External Processes Weathering Mechanical Weathering Chemical Weathering Rates of Weathering Soil Controls of Soil Formation 2 Earth’s external processes Weathering, mass wasting and erosion are called external processes because they occur at or near the surface and are powered by energy from the sun They are a basic part of the rock cycle transforming solid rock into sediment Earth’s external processes Weathering –التجويةthe physical breakdown (disintegration) and chemical alteration (decomposition) of rocks Mass wasting –انھيال كتليthe transfer of rock and soil downslope under the influence of gravity Erosion –التعريةthe physical removal of material by mobile agents such as water, wind, ice, or gravity Weathering Weathering occurs constantly and is very important for life on Earth because it decompose the rock to form soils with minerals and elements that eventually we eat. Weathering occurs when rock is mechanically fragmented or disintegrated and/or chemically altered or decomposed. Types of Weathering Mechanical weathering Chemical weathering Weathering Mechanical weathering – breaking of rocks into smaller pieces each retaining the characteristics of the original material Chemical weathering can occur only to those portions of a rock that are exposed to the elements. Mechanical weathering breaks rock into smaller and smaller pieces, thereby increasing the surface area available for chemical attack. Types of Mechanical Weathering Biological activity Thermal expansion Unloading Frost wedging Weathering Frost wedging –توتد صقيعيalternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks Frost wedging As water freezes, it expands, exerting a force great enough to break rock. When frost wedging occurs in a setting such as this, the broken rock fragments fall to the base of the cliff and create a cone-shaped accumulation known as a talus slope ( ميل من ركام من حطام صخري يتجمع )اسفل سفح مرتفع. Weathering Unloading –التفريغexfoliation ( )تقشرof igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure Weathering Thermal expansion ( – )التمدد الحراريalternate expansion and contraction due to heating and cooling • The daily cycle of temperature will weaken the rocks, particularly in places like hot deserts where daily variations may exceed 30°C. • Heating a rock causes expansion, and cooling the rocks causes contraction. • Repeated expansion and contraction of the minerals with different expansion rates will exert stress on the rock’s outer shell. Thermal expansion Weathering Biological activity – Weathering is also accomplished by the activities of organisms, including plants, burrowing animals, and humans. Root wedging widens fractures in rocks and aids the process of mechanical weathering. Weathering Chemical Weathering • Breaks down rock components and internal structures of minerals • Most important agent involved in chemical weathering is water: - responsible for transport of ions and molecules involved in chemical processes Major processes of chemical weathering Hydrolysis التموه Oxidation األكسدة Dissolution الذوبان Weathering Dissolution – Aided by small amounts of acid in the water – Soluble ions are retained in the underground water supply Just as sugar dissolve in water Dissolution One of the most water-soluble minerals is halite which is composed of sodium (Na+) and chloride (Cl-) ions. Weathering Dissolution • Although most minerals are insoluble in water, the presence of even a small amount of acid dramatically increases the corrosive force of water. • An acid solution containing the reactive hydrogen ion, H+. Weathering Dissolution • An acid solution easily attacks calcite CaCO3, the mineral that commonly forms the limestone and marble. • In nature, over periods of thousands of years, large quantities of limestone are dissolved and carried away by underground water forming caves. Missouri’s Meramec Caverns Dissolution Monuments ( )نصب تذكاريةand buildings made of limestone or marble are also subjected to the corrosive work of acids, particularly in areas of Polluted air. acidity & alkalinity pH scale measures the degree of acidity or alkalinity 7- neutral Below 7 is acid Above 7 is alkaline Weathering Oxidation – Any chemical reaction in which a compound loses electrons Iron reacts with oxygen to form iron oxide as seen on these rusted barrels. Weathering Oxidation – – If the water goes into a mine, the water coming out of the mine after this reaction is acid mine drainage that will kill aquatic organism if reaches the fluvial system. Weathering Hydrolysis – The reaction of any substance with water – A hydrogen ion attacks and replaces other positive ions – The silicates, the most common mineral group is decomposed primarily by this process. H2O = H+, OH-, These are very reactive hydrogen and hydroxil ions. It is the H+ ion the one that attacks and replaces other positive ions in the crystal lattice ( الشبكية )البلورية. Weathering Hydrolysis How Granite Weathers Granite is the most abundant continental rock which consists mainly of quartz and potassium feldspar Weathering Hydrolysis • Clay is the most abundant product of the chemical breakdown of potassium feldspar. They are very stable and as a result they make up a high percent of soils. • Some silica is remove from the feldspar structure and carried away by groundwater. • Another component of granite, quartz, is very resistant to chemical weathering and remains. • Eventually it is transported by water into rivers and the sea. Products of Weathering Weathering of Silicate Minerals Weathering Spheroidal Weathering التجوية الكروية • Chemical weathering causes physical changes also. • When water enters joints and fractures, the chemical weathering that occurs along the fractures results into spherical shapes. • This is because the corners are attacked more than the rest of the rock. Spheroidal Weathering Spheroidal Weathering A. Spheroidal weathering is evident in this exposure of granite in California’s Joshua Tree National Park. B. Sometimes successive shells are loosened as the weathering process continues to penetrate ever deeper into the rock Rates of Weathering Advanced mechanical weathering aids chemical weathering by increasing the surface area Factors affecting weathering • Surface area • Rock characteristics • Climate Rates of Weathering Advanced mechanical weathering aids chemical weathering by increasing the surface area Factors affecting weathering • Surface area Increasing the surface area will increase the rate of chemical weathering. Surface area Rates of Weathering Advanced mechanical weathering aids chemical weathering by increasing the surface area Factors affecting weathering • Rock characteristics – Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions – Silicate minerals weather in the same order as their order of crystallization Rock characteristics An examination of headstones reveals the rate of chemical weathering on diverse rock types. The granite headstone (left) was erected four years before the marble headstone (right). granite headstone marble headstone Rates of Weathering Others factors affecting weathering • Climate – Temperature and moisture are the most crucial factors – Chemical weathering is most effective in areas of warm, moist climates Climate Acid rain accelerates the chemical weathering of stone monuments and structures Weathering Differential weathering التجوية المتباينة • Masses of rock do not weather uniformly due to regional and local factors • Results in many unusual and spectacular ( )مثيرrock formations and landforms Differential weathering Climate Differential weathering is illustrated by these sculpted rock pinnacles قمم جبلية منحوتة Soil Soil = mineral + organic mater + water + air • Regolith فتات صخري: the portion of the rock and mineral fragments produced by weathering that supports the growth of plants • Humus دوبال: the decayed animal and plant remains, is a small but essential component. Composition of a “Good” Soil Factors controlling soil formation Parent material Slope Time Factors Plants and animals Climate Soil Factors controlling soil formation • Parent material The source of the weathered mineral matter from which soils develop is called the parent material – Residual soil – parent material is the underlying bedrock ()صخر األساس – Transported soil – forms in place on parent material that has been carried from elsewhere and deposited Soil Factors controlling soil formation • Time – Important in all geologic processes – Amount of time for soil formation varies for different soils depending on geologic and climatic conditions • Climate – Most influential control of soil formation – Key factors are temperature and precipitation Soil Factors controlling soil formation • Plants and animals – Organisms influence the soil’s physical and chemical properties – Also furnish organic matter to the soil • Slope – Steep slopes often have poorly developed soils – Optimum terrain is a flat-to-undulating upland surface Variations in soil development due to topography Soil The soil profile • Soil-forming processes operate from the surface downward. • Vertical differences are called horizons أفق\مستوى, which are zones or layers of soil. • O horizon • A horizon • E horizon • B horizon • C horizon Well-developed soils Well-developed soils show distinct layers called horizons. Soil The soil profile • O horizon – organic matter • A horizon – organic and mineral matter – High biological activity – Together the O and A horizons make up the topsoil Soil The soil profile • E horizon – little organic matter – Zone of eluviation and leaching Eluviation اإلزالة- fine particles are carried away Leaching الغسيل- depletion of soluble material from the upper soil Soil As water percolates downward through E zone, finer particles are carried away. This washing out of the fine soil components is termed eluviation. Water percolating downward also dissolves soluble inorganic soil components and carries them to deeper zones. This depletion ( )نفاذof soluble materials from the upper soil is termed leaching. Soil The soil profile • B horizon – zone of accumulation • C horizon – partially altered parent material The O, A, E, and B horizons together are called the solum, or “true soil” well-developed soil A soil profile showing different horizons A soil profile is a vertical cross section from the surface through all of the soil’s horizons and into the parent material. A. The profile shows a well-developed soil B. The boundaries between horizons in this soil are indistinct, giving it a relatively uniform appearance. Soil Classifying soils • Variations in soil formation over time and distances has led to a great variety of recognized soil types. • Groups have been established using common characteristics. • In the United States, a system was devised, and called the Soil Taxonomy علم تصنيف التربة. Soil Classifying soils • Soil Taxonomy - Emphasis is placed on physical and chemical properties. - Six hierarchical ( )ترتيب ھرميcategories exist. - Descriptive names are derived from Latin and Greek. - 12basic soil orders are recognized. Basic Soil Orders Basic Soil Orders Global soil regions Worldwide distribution of the Soil Taxonomy’s 12 soil orders. Soil Soil erosion • Part of the natural recycling of all Earth materials that we call the rock cycle • Natural rates of soil erosion depend on: – Soil characteristics – Climate – Slope – Type of vegetation How Soil Is Eroded Each drop acts like a tiny bomb. When it is raining, millions of water drops are falling at velocities approaching 10 meters per second (35 kilometers per hour). When water drops strike an exposed surface, soil particles may splash ( )تتناثرas high as 1 meter into the air and land more than a meter away from the point of raindrop impact. Then, water flowing across the surface carries away the dislodged soil particles. Because the soil is moved by thin sheets of water, this process is termed sheet erosion. How Soil Is Eroded A. Soil erosion from this field is obvious B. Gully erosion ( تعرية )اخدوديةis severe in this poorly protected soil in southern Colombia. Soil Erosion Where Vegetation Is Lacking When the land is dry and largely unprotected by anchoring vegetation, soil erosion by wind can be significant. The man is pointing to where the ground surface was when the grasses began to grow. Later wind erosion of the soil lowered the land surface to the level of his feet. A dust storm A dust storm blackens the Colorado sky in this historic image from the Dust Bowl of the 1930s. Soil Soil erosion • In many regions the rate of soil erosion is significantly greater than the rate of soil formation • Sedimentation and chemical pollution – Related to excessive soil erosion – Occasionally soil particles are contaminated with pesticides ()مبيدات حشرية End of Chapter 5