Chapter 14 Weathering and Erosion

Weathering Processes Weathering - the natural process by which atmospheric and environmental agents, such as wind, rain, and temperature changes, disintegrate and decompose rock at or near the surface. There are two main types of weathering processes—mechanical weathering and chemical weathering. Each type of weathering has different effects on rock.

Mechanical Weathering Mechanical – produces smaller pieces, adds to effectiveness of chemical weathering, leads to development of talus slopes, does not change a mineral’s composition.

Mechanical Weathering Mechanical weathering is strictly a physical process and does not change the composition of the rock. Common agents of mechanical weathering are ice, plants and animals, gravity, running water, and wind. Physical changes within the rock itself affect mechanical weathering.

Types of Mechanical Weathering Ice wedging – water penetrates into cracks in rock and freezes. When the water freezes, its volume increases by about 10% and creates pressure on the surrounding rock. This process eventually splits the rock apart. Must have: Adequate moisture Cracks in rocks Freeze/thaw cycles

Ice Wedging

Heat causes expansion; cooling causes contraction. Thermal expansion Repeated daily heating and cooling of rock; Heat causes expansion; cooling causes contraction. Expanding and contracting causes cracks.

Abrasion Is the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles Abrasion is caused by gravity, running water, and wind.

Spot Question Describe two types of mechanical weathering. Types of mechanical weathering include ice wedging, thermal expansion and abrasion. Ice wedging is caused by water that seeps into cracks in rock and freezes. Thermal expansion causes expanding and contracting which causes cracks. Abrasion is the grinding away of rock surfaces by other rocks or sand particles. Abrasive agents may be carried by gravity, water, and wind.

Organic Activity Plants and animals are important agents of mechanical weathering. Biological wedging – plant roots penetrate into cracks causing cracks to widen. Must have: Climate hospitable for plants Adequate moisture and temperature

Biological Wedging Root wedging widens fractures in rock.

Organic Activity Digging and burrowing activities. Earthworms and other animals that move soil expose new rock surfaces to both mechanical and chemical weathering.

Chemical Weathering Chemical alteration of minerals. Chemical reactions commonly occur between rock, water, carbon dioxide, oxygen, and acids. Most effective in a warm, humid climate. Chemical weathering changes both the composition and physical appearance of the rock.

Types of Chemical Weathering Ion Exchange – H+ replaces other cations. Oxidation - reaction in which elements gain or lose electrons (example: rust). Oxidation commonly occurs in rock that has iron-bearing minerals, such as hematite and magnetite.

CHEMICAL WEATHERING OXIDATION ROCKS / MINERALS CHEMICALLY REACT WITH THE OXYGEN IN THE ATMOSPHERE, CAUSING A DECOMPOSITION ON THE MATERIALS

Types of Chemical Weathering Hydrolysis - any reaction in which water participates. Water plays a crucial role in chemical weathering. Minerals that are affected by hydrolysis often dissolve in water. Water can then carry the dissolved minerals to lower layers of rock in a process called leaching.

CHEMICAL WEATHERING Dissolution - mineral completely dissolves, leaving only ions in solution.

Chemical Weathering The image below shows how water plays a crucial role in chemical weathering.

Carbonation Is the conversion of a compound into a carbonate When carbon dioxide, CO2, from the air dissolves in water, H2O, a weak acid called carbonic acid, H2CO3, forms. H2O + CO2  H2CO3

Organic Acids Acids are produced naturally by certain living organisms. Lichens and mosses grow on rocks and produce weak acids that can weather the surface of the rock. The acids seep into the rock and produce cracks that eventually cause the rock to break apart.

Acid Precipitation Such as rain, sleet, or snow, that contains a high concentration of acids, often because of the pollution of the atmosphere Acid precipitation weathers rock faster than ordinary precipitation does. Rainwater is slightly acidic because it combines with small amounts of carbon dioxide.

Factors influencing Weathering Rates The processes of mechanical and chemical weathering generally work very slowly. Rock Structures – chemical/mineral composition, Physical features Topography Climate

Spheroidal Weathering

Spheroidal Weathering

Differential Weathering Rocks weather at different rates. Softer rocks are less resistant to weathering.

Rock Composition Limestone and other sedimentary rocks that contain calcite are weathered most rapidly. They weather rapidly because they commonly undergo carbonation. Other sedimentary rocks are affected mainly by mechanical weathering processes. For example, shales and sandstones that are not firmly cemented together gradually break up to become clay and sand particles.

Amount of Exposure Surface Area Both chemical and mechanical weathering may split rock into a number of smaller rocks. The part of a rock that is exposed to air, water, and other agents of weathering is called the rock’s surface area. As a rock breaks into smaller pieces, the surface area that is exposed increases.

Greater exposure on smaller pieces

Amount of Exposure Fractures and Joints Most rocks on Earth’s surface contain natural fractures and joints. These structures are natural zones of weakness within the rock. Fractures and joints increase the surface area of a rock and allow weathering to take place more rapidly.

Spot Question How do fractures and joints affect surface area? Fractures and joints in a rock increase surface area and allow weathering to occur more rapidly.

Climate Climates with alternating periods of hot and cold weather allow the fastest rates of weathering. In warm, humid climates, chemical weathering is also fairly rapid. The slowest rates of weathering occur in hot, dry climates. Weathering is also slow in very cold, dry climates.

Topography Ice wedging is more common at high elevations than at low elevations. On steep slopes weathered rock fragments are pulled downhill by gravity and washed out by heavy rains. New surfaces of the mountain are continually exposed to weathering.

Human Activities Mining and construction often expose rock surfaces to agents of weathering. Recreational activities such as hiking or riding all-terrain vehicles can also speed up weathering by exposing new rock surfaces. Rock that is disturbed or broken by human activities weathers more rapidly than undisturbed rock does.

Plant and Animal Activities Rock that is disturbed or broken by plants or animals also weathers more rapidly than undisturbed rock does. The roots of plants and trees often break apart rock. Burrowing animals dig holes into rock and soil. Some biological wastes of animals can cause chemical weathering.

Section 3 Terminology Regolith – blanket of loose, weathered rock debris covering unweathered bedrock. Bedrock - the solid, unweathered rock that lies beneath the regolith. Soil - a loose mixture of rock fragments and organic material that can support the growth of vegetation

Characteristics of Soil The characteristics of soil depend mainly on the rock from which the soil was weathered, which is called the soil’s parent rock.

Soil Composition Soil composition refers to the materials of which it is made. The color of soil is related to the composition of the soil. Soil moisture can also affect color.

More Terminology Soil profile – sequence of soil horizons. Soil horizon – layer within a soil profile that has distinct characteristics. Humus – dark, organic formed in the soil from the decayed remains of plants and animals.

Soil Horizons Horizon O Bedrock

SOIL HORIZONS O-HORIZON A-HORIZON B-HORIZON

O horizon Organic rich, humus Organic mineral matter Organic rich, humus Decaying organic matter releases nutrients

A horizon Organic rich, often dark in color Zone of leaching (lots of chemical weathering) Organic rich, often dark in color Decaying organic matter releases nutrients

B horizon High clay content, reddish color from iron. Zone of accumulation – minerals (clay and iron oxide) are washed down from above. High clay content, reddish color from iron. Able to retain moisture because of clay content

C horizon Parent material – grading from weathered to unweathered. Parent material can be: Bedrock Stream sediments Volcanic ash

Controls of Soil Formation Parent Material Time Climate Plants and Animals Slope

Soil and Climate Climate is one of the most important factors that influences soil formation. Climate determines the weathering processes that occur in a region. These weathering processes, in turn, help determine the composition of soil.

Soil and Topography Because rainwater runs down slopes, much of the topsoil of the slope washes away. Therefore, the soil at the top and bottom of a slope tends to be thicker than the soil on the slope. Topsoil that remains on a slope is often too thin to support dense plant growth.

Erosion erosion a process in which the materials of Earth’s surface are loosened, dissolved, or worn away and transported from one place to another by a natural agent, such as wind, water, ice, or gravity When rock weathers, the resulting rock particles do not always stay near the parent rock. Various forces may move weathered fragments of rock away from where the weathering occurred.

Soil Erosion Ordinarily, new soil forms about as fast as existing soil erodes. Some farming and ranching practices increase soil erosion. Soil erosion is considered by some scientists to be the greatest environmental problem that faces the world today.

Gullying and Sheet Erosion Gullying is caused by furrow plowing. Eventually land that is plowed in this way can become covered with deep gullies. Sheet erosion is the process by which water flows over a layer of soil and removes the topsoil Wind also can cause sheet erosion during unusually dry periods.

Results of Soil Erosion Constant erosion reduces the fertility of the soil by removing the A horizon, which contains the fertile humus. The B horizon, which does not contain much organic matter, is difficult to farm because it is much less fertile than the A horizon. Without plants, the B horizon has nothing to protect it from further erosion. All the soil layers could be removed within a few years by continuous erosion.

Soil Conservation Certain farming techniques can increase the rate of erosion. Land clearing accelerates topsoil erosions. Soil erosion can be prevented by soil conservation methods

Soil Conservation Contour Plowing: soil is plowed in curved bands that follow the contour, or shape of the land. Strip-Cropping: crops are planted in alternating bands slowing the runoff of rainwater. Terracing: construction of steplike ridges that follow the contours of a slope Crop Rotation: farmers plant one type of crop one year and a different type of crop the next.

Gravity and Erosion mass movement the movement of a large mass of sediment or a section of land down a slope Gravity causes rock fragments to move down inclines. Some mass movements occur rapidly, and others occur very slowly.

Mass Wasting and landform development For mass wasting to occur, there must be a slope angle Most rapid events occur in areas of rugged, geologically young mountains As a landscape ages, less dramatic downslope movements occur

Controls and triggers of mass wasting Gravity is the controlling force Important triggers include Saturation of the material with water Diminishes particle cohesion Water adds weight Oversteepening of slopes Stable slope angle (angle of repose) is different for various materials

Causes Important triggers include Oversteepening of slopes Oversteepened slopes are unstable Removal of anchoring vegetation Ground vibrations from earthquakes May cause expensive property damage Can cause liquefaction – water saturated surface materials behave as fluid-like masses that flow

Classification of mass wasting events Generally each event is classified by Type of material involved Debris Mud Earth Rock

Classification of mass wasting events Generally each event is classified by Type of motion Fall (free-falling pieces) Slide (material moves along a surface as a coherent mass) Flow (material moves as a chaotic mixture) The velocity of the movement Fast Slow

Forms of mass wasting Slump Movement of a mass of rock or unconsolidated material as a unit along a curved surface Occurs along oversteepened slopes

A slump with an earthflow at the base

Forms of mass wasting Rockslide Debris flow (mudflow) Blocks of bedrock slide down a slope Generally very fast and destructive Debris flow (mudflow) Consists of soil and regolith with a large amount of water Often confined to channels

Forms of mass wasting

Forms of mass wasting Debris flow Serious hazard in dry areas with heavy rains Debris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars

A lahar from the 1980 Mt. St. Helens eruption

Forms of mass wasting Earthflow Form on hillsides in humid regions Water saturates the soil Commonly involve materials rich in clay and silt

An earthflow on a newly formed slope

Forms of mass wasting Slow movements solifluction the slow, downslope flow of soil saturated with water in areas surrounding glaciers at high elevations Promoted by a dense clay hardpan or impermeable bedrock layer Common in regions underlain by permafrost Can occur on gentle slopes

Forms of mass wasting Slow movements Creep Gradual movement of soil and regolith downhill Aided by the alternate expansion and contraction of the surface material

Some visible effects of creep

landforms a physical feature of Earth’s surface There are three major landforms that are shaped by weathering and erosion—mountains, plains, and plateaus. All landforms are subject to weathering and erosion, which wears down land surfaces.

Erosion of Mountains Young mountain undergoes uplift. Older mountains, undergoes weathering and erosion to wear down the rugged peaks Mountains that are not being uplifted become low, featureless surfaces called peneplains, which means “almost flat.”

Erosion of Plains and Plateaus A plain is a relatively flat landform near sea level. A plateau is a broad, high, flat landform A plateau is subject to much more erosion than a plain. The effect of weathering and erosion on a plateau depends on the climate and the composition and structure of the rock. As a plateau ages, erosion may dissect the plateau into smaller, tablelike areas called mesas. Mesas ultimately erode to small, narrow-topped formations called buttes.

End of Chapter 14