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Chapter 6 - Weathering, Mass Wasting, and Erosion |
| Rocks exposed on the surface are exposed to air and water. The breakdown of materials due to exposure is called weathering; weathered material are subjected to gravitation forces pulling them downhill and transported by erosional forces associated with flowing water, ice, or wind. |
1. Define weathering, mass wasting, and erosion.
2. Define mechanical weathering and explain the roles of frost wedging, unloading, thermal expansion, and organic activity.
3. Define chemical weathering and explain dissolution, oxidation, hydrolysis, and alterations cause by chemical weathering.
4. List and explain factors that control the rate of weathering.
5. Define soil and describe the factors that control soil formation.
6. Describe the soil profile and list its horizons.
7. List the general types of soils.
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| Keywords and Essential Concepts |
1. Define weathering, mass wasting, and erosion.
weathering—the gradual destruction of rock under surface conditions. Weathering may involve physical processes (mechanical weathering) or chemical activity (chemical weathering). Biological activity can also result in weathering the can be construed as mechanical, biological, or both.
mass wasting—a general name for processes by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, landslides, mudflows, debris flows, lahars, topples, rock falls, and avalanches—each with its own geomorphic characteristics, and taking place over time scales ranging from seconds to years.
erosion—the mechanical processes of wearing or grinding away materials on a landscape by the action of wind, flowing water, or glacial ice.
deposition—the process of sediments settling and accumulating from a moving fluid (wind, water, or ice).
sediments—solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice.
evaporation—the physical conversion of a liquid into a vapor.
mineral precipitation—the process of mineral formation directly from a fluid (a water solution and/or hydrothermal gases) such as calcite from seawater or groundwater.
evaporite—a rock composed of salt minerals left behind by the evaporation of salty water (seawater, lake water and groundwater) |
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| Part of the Rock Cycle: Weathering results in the formation of sediments and contributes to the saltiness of seawater. |
The Hydrologic Cycle: water is the most essential component in the weathering and erosional transport of earth materials. |
The landscape and climate of a region plays a major role in the weathering and erosion. This diagram shows the hydrologic setting of California |
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2. Define mechanical weathering and explain the roles of frost wedging, unloading, thermal expansion, and organic activity.
mechanical weathering—All processes that collectively break rocks into smaller pieces. Examples include breaking rocks by water expansion during freezing in cracks, plant root expansion, all forms of mass wasting, and rock particles breaking as they tumble down hillsides and stream beds during floods or get battered by wave action along a shoreline. Examples of mechanical weathering processes include:
erosional grinding—the physical banging and cracking of rock as it is moved by water, wind, or ice, such as waves crashing on a sea cliff, boulders and gravel carried in a fast moving stream, or grinding of rock materials along the bottom of a moving glacier.
frost wedging—the shattering, fracturing, and moving rock and soil caused by the expansion of freezing water turning into ice. Frost wedging is a major force in seasonally wet regions where daytime temperatures rise above freezing and sink below freezing at night.
unloading—expansion of compressed rocks (previously deeply buried) by the removal of overburden, allowing rocks to expand and fracture, commonly resulting in the sheeting off of layers of rocks.
exfoliation—joints or sheet joints are surface-parallel fracture systems in rock often leading to erosion of concentric slabs.
thermal expansion—expansion and contraction caused by daily heating and cooling, particularly effective in arid environments.
organic activity—breakdown and movement of rock and soil caused by expanding tree roots, burrowing, feeding activity, etc.
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| Mechanical weathering is any process that makes big pieces into smaller fragments. |
Wave action grinds rocks into fragments. |
Mass wasting, in this case, a rock fall, breaks big pieces into fragment. |
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| Flowing water transports, grinds fragments, and erodes landscapes. |
Moving ice (glaciers) erode bedrock and carry away large quantities of sediment. |
Frost wedging helps to sculpt unusual landscapes, such as these hoodoos at Bryce Canyon National Park in Utah |
Tree roots grow into cracks and expand; when a tree falls material is ripped up and moved, made accessible to erosion. |
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3. Define chemical weathering and explain dissolution, oxidation, hydrolysis, and alterations cause by chemical weathering.
chemical weathering—the breakdown (decomposition, decay, and dissolution) of rock by chemical mechanisms, the most important means being hydration, hydrolysis, oxidation, carbonation, and ion exchange in solution.
dissolution—the action or process of dissolving or being dissolved, moving soluble components of materials into solution.
leaching—the process of dissolving and removing the soluble constituents of soil or rock near the land's surface; associated with the chemical weathering of rock and the formation of soil.
hydration—the process of combining with water to a molecule; usually reversible by heating or other process.
hydrolysis—the chemical breakdown of a compound due to reaction with water.
oxidation—the process of combining elements with oxygen ions. A mineral that is exposed to air may undergo oxidation.
carbonation—saturation with carbon dioxide (as soda water). |
Chemical weathering products of common minerals
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Common mineral |
insoluble sediment |
soluble content |
| quartz (SiO2) |
quartz sand and silt |
silica (increases in hot water) |
| mafic minerals (olivine, pyroxene, amphibole, biotite) |
clays, hematite and limonite (rust) |
salts of Mg++, Na+, K+, Ca++; soluble iron- oxides Fe++, Fe+++ |
| feldspars |
clays |
salts (Ca, Na, K) |
| gypsum (CaSO4) |
none |
Ca++. SO4-- |
| calcite (CaCO3) |
none |
Ca++, HCO3- (increases in cold water) |
Fate of soluble components of rocks: formation seawater
Effects of physical and chemical conditions on weathering.
| particle size effects |
in a uniform volume of sediment, the the smaller the particle size, the greater the amount of surface area (compare surface area of gravel, sand and silt) |
| oxidation-reduction effects |
the availability of oxygen controls the stability and solubility of minerals; metal oxides precipitate in oxidizing conditions; reducing environments tend to be acidic. |
| acid-base effects |
silica dissolves in basic water and precipitates in acidic water
calcite dissolves in acidic water and precipitates in basic water |
| color |
Oxidation states of iron is the source of most color in sedimentary rocks.
Oxygenated sediments tend to be bright colors - red, orange, yellow, brown
Reduced (acidic) environments tend to be green, blue-gray, or black |
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4. List and explain factors that control the rate of weathering.
| Factors that influence weathering: |
| moisture—water is the "universal solvent" - the availability of water is a major factor in weathering of surface materials. |
| surface temperature—the higher the temperature, the faster chemical reactions affecting soil formation takes place. |
| mineral makeup—mineral (compounds) have a wide spectrum of solubility, oxidation-reduction, and acid-base stability and reaction rates. |
| particle size—large particles have logarithmic scale increases in surface area at materials are broken into smaller and smaller sizes. |
| time—the longer materials are exposed to a weathering condition, the more it will decay into its weathering components. |
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Mechanical breakdown of rocks increases surface area (per unit volume). Increased surface area increases space for chemical weathering processes to take place. |
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5. Define soil and describe the factors that control soil formation.
soil—the unconsolidated mineral and organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. Soil composition reflects the physical and chemical composition of source materials, and the influence of environmental factors.
Environmental factors include:
* climate (water availability and temperature effects)
* macro- and microorganisms living in or on the soil
* sedimentation rates
* duration of exposure to air and water |
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| Chemical weathering of granite. |
Spheroidal weathering |
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| spheroidal weathering |
spheroidally weathered boulders |
regolith—a layer of loose rock debris resting on bedrock, constituting the surface of most land. Regolith can become soil with the introduction of organic residues and ongoing weathering.
colluvium—a general term applied to loose and incoherent surficial deposits, usually at the base of a slope and brought their chiefly by gravity.
alluvium—a general term for unconsolidated sediments deposited by flowing water on stream channel beds, flood plains, and alluvial fans. The term applies to stream deposits of recent times and it does not include subaqueous deposits, such as in lakes or undersea.
tafoni—small, fist- to head-sized, cave-like features found in granular rock such as sandstone, appearing tiny pits, rounded entrances and smooth concave walls. Tafoni usually is found cliff faces, in overhanging vertical places, on large boulders, and rocky outcrops, usually in sandstone or weathered granite. Tafoni features often occur in groups, forming a honeycomb-like form. |
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taphoni weathering
Chitactac-Adams County Park, Gilroy, California |
taphoni weathering
Goat Rock
Castle Rock State Park |
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6. Describe the soil profile and list its horizons.
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| Components of soil |
Example of a soil profile |
Examples of types of soil |
O horizon—zone of intense biological activity, interval with the accumulation of organic residues (humus).
A horizon—zone of leaching of soluble mineral components.
B horizon—zone of accumulation of fine materials and mineral precipitates (mostly clays and calcium carbonate).
C horizon—zone of regolith (a mix of decaying bedrock and rock fragments of all sizes).
bedrock—relatively "fresh" unaltered solid (consolidated) rock below the surface or exposed in rocky outcrops. |
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7. List the general types of soils.
| Soils are classified in a variety of ways, but the most common methods are based on clastic particle size factions, organic content, and major mineral components (calcite, types of clay minerals). |
| Soil Types |
Characteristics |
| Sandy soil |
Composed mostly of sand, it crumbles easily in your hands (not held together by clays), has a sandy, grainy texture; tends to drain well, and typically retains nutrients derived from source rock materials. Grainy sandy soil derived from granite is called grus. |
| Silty soil |
Composed mostly of silt particles. Typically composed of fine quartz particles and organic matter. Typically ideal for agriculture in that it holds moisture and nutrients but also drains fairly well, allowing soil aeration for good rooting of plants. |
| Clay soil |
Composed mostly of clay-sized particles. Typically sticky when wet and has poor drainage (water logging of plants may occur); becomes hard and compact when dry. Typically forms from the weathering of clay-rich rocks (shale and weathered volcanic ash). Some clay soils expand when wet and can crack building foundations and damage roads and other infrastructure. |
| Loamy soil |
Loam is a mix of sand, silt, and clay fractions (component compositions may vary); typically most ideal for agriculture in that it both holds some water yet drains well. |
| Peaty soil |
Soil composed dominantly of dead and decayed organic matter—typically much more organic matter than other soils. Peaty soils form in moist climates, typically marsh or swamp-like conditions. Typically acidic in nature, low in mineral nutrient content, and drains poorly. |
| Chalky soil |
Soil that has a high calcite mineral content, typically found in rocky arid and semiarid regions where calcite precipitates from evaporating groundwater. Naturally alkaline in content, and typically makes poor agricultural soil. If too much calcite is precipitated, the soil will become a rock called caliche. |
| Lateritic soil |
Typically red-colored soil that forms in wet, tropical and subtropical conditions. Lateritic soils tend to be acidic and most of it mineral nutrients essential to plants have been leached out over time (Ca, Na, Mg, K, P, silica, organics, etc.) with the remaining material being enriched in aluminum and iron oxide minerals. Typically very poor for agriculture without artificial treatment with fertilizers. |
| Serpentinite soil |
Soils unique to areas where bedrock consists of ultramafic rocks (rich in magnesium, iron, nickel, and other metals). Serpentinite soils tend to be poor in essential nutrients and rich in heavy metals toxic to most plants, so they are host to unique species specially adapted to these condition. Serpentinite soils are common in many parts of northern California where bedrock of serpentinite occurs. |
| Note that "native plants" are adapted to different, often unique, soil conditions. A region may have different kinds of soils in different settings, and may therefore host a variety of ecosystems associated with the different soils in a region. |
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Mass Wasting
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mass wasting—the downslope movement of earth materials (soil, regolith, rock) caused by gravity. Includes but is not limited to landslides, rock falls, debris avalanches & creep. It does not include surface erosion by running water. It may be caused by natural erosional processes, by natural disturbances, or by human disturbances.
See: Landslide Hazards—An Awareness Guide
landslide— A general term covering a wide variety of mass-movement landforms and processes involving the down slope transport of soil and rock under the influence of gravity. Usually the displaced material moves over a relatively confined zone or surface of shear. Landslides have a great range of morphologies, rates, patterns of movement, and scale. Their occurrence reflects bedrock and soil characteristics and material properties affecting resistance to shear. Landslides are usually preceded, accompanied, and followed by perceptible creep along the surface of sliding and (or) within the slide mass. Slumps, debris flows, rock falls, avalanches, and mudflows are all forms of landslides.
Types of mass wasting include:
* rock falls and topples
* slides (rockslide & mudslides)
* debris flows and floods (including lahars)
* avalanches
* creep Each type has own geomorphic characteristics, and taking place over times scales ranging from seconds to years.
susceptibility—the state or fact of being likely or liable to be influenced or harmed by a particular thing. In geology, the term is used as a means of classifying areas that are prone to natural hazards, such as landsliding, liquefaction, flooding, etc. |
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| Landslides illustrated |
Types of landslides |
Landslide susceptibility map |
rock fall—the relatively free falling or precipitous movement of a newly detached segment of bedrock of any size from a cliff or very steep slope; it is most frequent in mountainous areas during spring when there is repeated freezing and thawing of water in cracks in rock. Movement may be straight down or in a series of leaps and bounds down the slope; it is not guided by an underlying slip surface (like a slump).
talus—a sloping mass or cone-shaped deposit of rock fragments at the foot of a cliff.
angle of repose—the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. Talus slopes reflect the angle of repose of coarse sediments.
avalanche—a mass of snow, ice, rocks, and debris falling rapidly down a mountainside.
rock slide—the usually rapid downslope movement of newly detached segments of bedrock. |
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rock fall
Arroyo Seco Canyon, CA |
talus slope
Arroyo Seco Canyon, CA |
rock slide
Gros Ventre Slide, Wyoming
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avalanche |
slump—a type of landslide where the downward slipping mass of unconsolidated material or rock moves as a unit. A slump block usually displays backward rotation and on a more or less horizontal axis parallel to the slope or cliff from which it descends. Slumps typically form a fault-like escarpment and may occur at the head of a landslide.
debris flow—a moving mass of rock fragments, soil, and mud in which more than half of the particles being larger than sand size (otherwise it would be a mudflow) and with 70 to 90 percent of the material consisting of sediment (the rest is water and trapped gasses). Slow debris flows may only move a few feet per year, whereas rapid ones can reach speeds greater than 100 miles per hour. Debris flows can display either turbulent or laminar flow characteristics.
earth flow—a slow moving downslope viscous flow of fine grained materials that have been saturated with water, and moves under the pull of gravity; a slow moving mass of material, slower than a more fluids debris flow, rock fall, or avalanche.
mudflow—a downhill movement of soft wet mud and rock debris, made fluid by rain or melted snow, and capable of moving downslope at great speed (also see debris flow).
debris flood—a typically disastrous flood, intermediate between the turbid flood of a mountain stream and a debris flow, ranging in sediment load between 40 to 70 percent (the rest is water and trapped gasses). |
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slump
Limekiln Creek Trail
Santa Clara County, CA |
Mission Peak Landslide
Santa Clara County, CA |
Mudslide
Southern California |
Debris flow
Panamint Valley, California |
creep—in earthquake terminology, creep is the slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. In landslide terminology, creep is slow, more or less continuous downslope movement of surface materials (mineral, rock, and soil particles) under gravitational stresses.
solifluction—the slow, downhill movement of soil or other material in areas typically underlain by frozen ground. |
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The Water Cycle
Why is the ocean salty?
Weathering and Erosion: Processes That Break Down Rocks and Landscapes
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Elemental components of salts dissolved in seawater
