Chapter Five Weathering, Soil, and Mass Movements
Weathering • Weathering is a basic part of the rock cycle and a key process in the Earth system. • Weathering: The breaking down and changing of rocks at or near Earth’s surface. • There are two types of weathering: • Mechanical Weathering • Chemical Weathering • Even though they are different, they at are work at the same time.
Weathering Mechanical Weathering • Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition. • Each piece has the same composition. • Breaking a rock into smaller pieces increases the surface area of the rock. • In nature, three physical processes are especially important causes of mechanical weathering: • Frost wedging • Unloading • Biological activity
Weathering • Frost Wedging: • When liquid freezes, it expands by about 9%, exerting a tremendous outward force. • When water freezes in the cracks of rocks, it enlarges the cracks. • This process is known as frost wedging. • It is most common in mountainous regions in the middle latitudes. • Sections of rock that are wedged loose may tumble into large piles called talus, which typically form at the base of steep, rocky cliffs.
Weathering 2. Unloading: • Large masses of igneous rocks may be exposed through uplift and erosion of overlying rock. • When this happens, the pressure exerted on the igneous rock is reduced. • This is known as unloading. • Unloading causes the outer layers of the rock to expand more than the rock below. • Slabs of outer rock separate like layers of an onion and break loose in a process called exfoliation. • Especially common in granite. • It often produces large, domed shaped rock formations. • Ex: Stone Mountain in Georgia and Liberty Cap in Yosemite National Park.
Weathering 3. Biological Activity: • The activities of organisms, including plants, burrowing animals, and humans can also cause mechanical weathering. • Examples: • Tree roots. • Burrowing animals move rocks to the surface. • Decaying organisms produce acids which cause chemical weathering. • Humans accelerate weathering through deforestation and blasting.
The process that occurs when physical forces break rock into smaller pieces without changing the rock’s chemical composition is called • Differential Weathering • Chemical Weathering • Mechanical Weathering • Erosion
Which of the following weathering processes involves the constant freezing and thawing of water? • Unloading • Frost Wedging • Exfoliation • Spheroidal Weathering
Which of the following is not associated with mechanical weathering? • Frost Wedging • Unloading • Biological Activity • Reactions with Oxygen
What is responsible for the formation of exfoliation domes? • Frost Wedging • Biological Activity • Reactions with Oxygen • Unloading
What type of mechanical weathering is most common in mountainous regions in the middle latitudes? • Frost Wedging • Biological Activity • Oxidation • Unloading
When water freezes, its volume • Decreases slightly. • Increases. • Stays the same. • Decreases greatly.
Weathering Chemical Weathering • Chemical weathering is the transformation of rock into one or more new compounds. • The new compounds remain mostly unchanged as long as the environment in which they formed does not change. • Water is the most important agent of chemical weathering. • Water promotes chemical weathering by absorbing gases from the atmosphere and the ground. • These dissolved substances then chemically react with various minerals. • Oxygen dissolved in water reacts easily with certain minerals , forming oxides. • Ex: Iron-rich minerals get a yellow to reddish-brown coating of iron oxide (rust) when they react with oxygen.
Weathering • Water also absorbs carbon dioxide when rain falls through the atmosphere. • Water that seeps into the ground also picks up carbon dioxide. • The dissolved carbon dioxide forms carbonic acid, which reacts with many common minerals. • Carbonic acid is a weak acid found in carbonated drinks.
Weathering • Water can also absorb the sulfur oxides in the atmosphere produced by the burning of fossil fuels (coal and petroleum). • Through a series of chemical reactions, these pollutants are changed into acids that cause acid precipitation. • Acid precipitation accelerates the chemical weathering of stone monuments and structures.
Weathering • When granite (feldspar/quartz) is exposed to water containing carbonic acid, the feldspar is converted mostly into clay minerals. • As the feldspar is converted into clay, the quartz grains are released from the granite. • Sometimes it is then transported by rivers to the ocean where the tiny particles are carried far from shore and the quartz crystals are deposited near the shore where they become the main component of beaches and sand dunes.
Weathering • When silicate minerals undergo chemical weathering, the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater. • The three remaining elements are aluminum, silicon, and oxygen which usually combine with water and produce clay minerals.
Weathering • Chemical weathering can change the physical shape of rock as well as its chemical composition. • Ex: When water enters along the joints in a rock, it weather the corners and edges most rapidly. As a result, the corners and edges become more rounded. • This process is called spheroidal weathering. • The rock takes on a spherical shape. • Spheriodal weathering sometimes causes the outer layers of a rock to separate from the rock’s main body. • Similar to exfoliation except that they are chemically weathered.
Which of the following is not related to chemical weathering? • Frost wedging. • Dissolution • Reactions with oxygen • Reactions with water
The gradual rounding of the corners and edges of angular blocks of rock is called • Exfoliation • Unloading • Spheroidal Weathering • Mechanical Weathering
Which of the following is the result of chemical weathering? • A rock that has been changed into one or more new compounds. • A rock that has been broken into tiny pieces. • A rock that has been split in two. • A rock that has lost its outer layers.
The chemical weathering of feldspar produces • Quartz. • Iron oxide • Clay minerals • Calcium carbonate
Whenever the characteristics and chemical composition of weathered materials have been altered, they have undergone • Chemical weathering. • Mechanical weathering. • Mass movement. • Frost wedging.
The atmospheric gas that forms a mild acid when dissolved in water is • Carbon Dioxide • Oxygen • Aluminum • Sulfur
Weathering • Mechanical weathering affects the rate of chemical weathering. • By breaking rocks into smaller pieces, chemical weathering is increased due to the increased amount of surface area of the rock. • Two other factors that affect the rate of weathering are: • Rock characteristics • Climate
Weathering • Rock Characteristics: • Physical characteristics of rock (cracks) are important in weathering because they influence the ability of water to penetrate rock. • Mineral composition also affects the rate of weathering. • Ex: Granite vs. Marble; granite is relatively resistant to chemical weathering while marble is not very resistant to chemical weathering (reacts with weak acids).
Weathering 2. Climate: • Climatic factors, especially temperature and moisture, have a strong effect on the rate of weathering (mechanical and chemical). • The climate most favorable for chemical weathering has high temperature and abundant moisture. • Slow in arid and polar regions.
Weathering • Different parts of a rock mass often weather at different rates. • This process, called differential weathering, has several causes. • Differences in mineral composition in one cause. • More resistant rock protrudes as pinnacles, or high peaks. • Variations in the number and spacing of cracks in different parts of a rock mass is another cause of differential weathering.
What would cause the inscription on a marble gravestone to become harder and harder to read over time? • Frost wedging • Mechanical weathering • Exfoliation • Chemical weathering
Which of the following is not a factor that affects the rate of weathering in rocks? • Biological evolution • Climate • Rock characteristics • Amount of exposed surface area
Which of these factors affects the rate of weathering? • Climate • Chemical composition of the exposed rock. • Surface area of the exposed rock. • All of the above.
Rock features such as the sculpted pinnacles seen in Bryce Canyon National Park are the result of • Exfoliation. • Differential Weathering. • Unloading. • Frost Wedging.
Chemical weathering would be • Most effective in a warm, dry climate. • Most effective in a cold, dry climate. • Most effective in a warm, humid climate. • Equally effective in any climate.
If granite and marble were exposed in an area with a hot and humid climate, • The granite would weather most rapidly. • The marble would weather most rapidly. • Both rocks would weather at the same rate. • Neither rock would become weathered.
Soil • Soil is an important product of weathering. • All life depends on a dozen or so elements that come from Earth’s crust. • Weathering produces a layer of rock and mineral fragments called regolith. • Soil is the part of the regolith that supports the growth of plants. • Three important characteristics of soil are its: • Composition • Texture • Structure
Soil • Soil Composition: • Soil has four major components: • Mineral Matter (Broken-down rock) • Organic Matter (Humus – decayed organisms) • Water • Air • The percentages of the four major components varies greatly. • In most soils, organic matter (humus) is an essential component for plants nutrients and the soil’s ability to retain water.
Soil 2. Soil Texture: • Most soils contain particles of different sizes. • Soil texture refers to the proportions of different particles sizes. • To classify soil texture, the U.S. Department of Agriculture has established categories based on the percentages of clay, silt, and sand in soil. • Texture strongly influences a soil’s ability to support plant life.
Soil 3. Soil Structure: • Soil particles usually form clumps that give soils a particular structure. • Soil structure determines how easily a soil can be cultivated and how susceptible it is to erosion. • Soil structure also affects the ease with which water can penetrate the soil and thus influences the movement of nutrients to plant roots.
Which of the following is not a major component of soil? • Mineral matter • Air • Humus • Earthworms
A soil’s texture is determined by • Mineral composition • Type of humus • Water content • Particle size