Weathering is the physical breakdown and chemical alteration of rocks and minerals at the Earth's surface.

Physical disintegration makes the rock particles smaller and looser;

chemical decomposition makes the particles softer by turning them into clays and other soft minerals.

The hard, unaltered, parent rock material which is acted upon by weathering is often called bedrock.

The soft and loose rock particles that result from weathering is called regolith. Regolith is rarely seen, because other organic and hydrologic forces quickly act on regolith and turn it into soil or sediment.

There are two such processes, soil formation and erosion.

Soil is formed when regolith is acted upon by organisms; soil consists of regolith plus organic debris created by organisms.

Sediment is formed by the erosion of regolith; sediment consists of rock particles that are being transported and removed by erosion.

Erosion is the reduction of exposed landforms. Erosion is defined as the "transport and removal of weathered rock material." Erosion is therefore different than weathering, and the two processes must be kept distinct.

Transportation is the movement of eroded material downslope.

There are two types of weathering:

1. Mechanical weathering occurs when physical forces break the bedrock into smaller pieces that retain the chemical and mineral composition of the parent rock. This weathering makes the rock smaller and looser and thus more easily eroded; it is not, however, the more important form of weathering, because it is limited in extent.

* Water (streams, rivers, surf)

* Wind

* Frost action: frost wedging, frost heaving, glaciers

* Pressure release: unloading, exfoliation, sheeting

* Thermal expansion and contraction

* Organism activity

2. Chemical weathering occurs when the bedrock is decomposed by chemical alteration of the parent rock. This weathering is by far the more important because of its pervasive effect on all rocks; it makes rock softer and thus more easily eroded.

* Solution - operates primarily on limestone and the mineral calcite

* Oxidation - operates primarily on mafic igneous rocks and their iron-bearing minerals: olivines, pyroxenes, and amphiboles. Oxidation is the result of the chemically corrosive free oxygen in our atmosphere which may change the oxidation state (valence) of some elements, notably sulfur (S) and iron (Fe), but also manganese (Mn), copper (Cu) and uranium (U).

* Hydrolysis - by far the most pervasive and important form of weathering; hydrolysis turns all hard silicate minerals except quartz into soft silicate clay minerals.

* Hydration occurs as the result of acidic solutions and can transform feldspars to clays, and dissolve carbonates. * Acid rain is the result of oxidation of sulfur in fossil fuel to form SO3 which hydrates to form H2SO4. Acid rain is neutralized by carbonate bedrock, but remains acidic in surface and ground water on granitic bedrock.

Three factors control the rate of chemical weathering:

1. particle size: surface area and joints; water penetrates and affects a larger surface area

2. climate: temperature and precipitation; higher temperatures and more water accelerate chemical reactions

3. parent material: mafic ferromagnesian minerals are the most chemically unstable and weather more rapidly than felsic siliceous minerals, the more chemically stable minerals

Soil - is regolith acted upon by organisms; soil consists of weathered rock material, the regolith, and organic debris from organisms, the humus.

Soil profile: Four soil horizons:

* O - thin layer of organic material

* A - zone of leaching = topsoil

* B - zone of accumulation = subsoil

* C - partially altered parent material (bedrock), the orginal regolith

Below these four soil horizons lies unaltered parent rock, the bedrock.

Five factors control soil formation:

1. Climate: temperature and precipitation

2. Parent material

3. Organic activity

4. Relief and slope

5. Time

Soils are classified into pedalfers and pedocals and lateralites.

Pedocals are soils of arid regions and are characterized by calcite (CaCO3) cementation due to upward movement of water (evaporation). Extreme pedocals may contain salt and borate deposits. - the soil of arid and semi-arid regions (low precipitation)

Pedalfers are soils of humid regions and are characterized by highly leached clays due to downward movement of water. Extreme pedalfers are called laterites (soils of rain forests) which, if cemented, become bauxites. The most insoluble elements are Al3+ and Fe3+.

Here is a laterite soil cross section in Brazil. The deep red color is due to Fe3+ hydroxide minerals in the soil. - the soil of temperate grasslands and forests (moderate precipitation)

Laterites - the soil of the tropical rain forests (high precipitation)

Soil is being degraded and eroded by human activity at an enormous rate. Soil degradation (sometimes called desertification) is caused by a number of factors:

* Removal of the native vegetation by cultivation, clear-cutting, etc.; the native vegetation is drought resistant, and any vegitation, native or cultivated, will hold the soil in place and prevent erosion

* Over-grazing - removes vegetation

* Over-cultivation - removes nutrients, thus preventing vegetation from growing

* Irrigation - causes salinization, which prevents vegetation from growing

* Not using soil conservation techniques, such as contour plowing, strip cropping, wind breaking, crop rotation, drip irrigation, terracing, etc.; these techniques prevent soil erosion

When soil is degraded by the above mechanisms, it is easily eroded by the normal effects of water and wind, and huge amounts of topsoil are lost; e.g., the U.S. Dust Bowl of the 1930's. In the last 20 years, about 11% of the arable land in the world has been lost to erosion caused by human degradation of the soil. Topsoil erosion is one of the five major global environmental problems.

The primary agent of erosion in deserts is surface water, but wind-erosion features are a common sight in deserts because only here is the vegetation sparse enough that wind erosion can occur.

Clay- and silt-sized particles can be carried in suspension by the wind; this transport creates dust storms. Sand is transported by sliding, rolling, and saltation--a jumping motion. The wind cannot transport gravel at all.

Abrasion -- Wind-carried sediment, primarily silt and sand, can abrade rocks. A ventifact is a wind-abraded cobble, while a yardang is a wind-abraded bedrock landform.

Deflation -- Deflation is the removal of finer-grained sediments (sand, silt, clay) from a desert site, leaving behind a ground surface covered by gravel called a desert pavement. These surfaces are very common in deserts, and most U.S. deserts are characterized by desert pavements.

Sand -- Sand is deposited in the form of sand dunes. Dunes are asymmetrical: they have a shallow windward face that is oriented toward the wind direction and a steeper slip face on the opposite side away from the wind. Sand is transported up the windward face by sliding, rolling, and saltation, and then slips down the slip face, so the windward face is being eroded and the slip face experiences deposition, causing the dune to move or migrate. The layers of sand on the slip face are tilted at a high angles, producing cross-bedding, a characteristic of all wind- and water-deposited dunes.

Dunes are typical of desert regions where sorting is by wind, and also of coastal regions where sorting is by wave action in water.

The dunes form different characteristic shapes depending on the amount of sand and the amount of moisture and the strength of the wind.

* A barchan is a small crescent shaped dune with the limbs downwind that is typical of arid regions with limited supply of sand.

* If more sand is available, the barchans may coalesce into transverse dunes which are narrow linear dunes that form at right angles to the prevailing wind direction.

* In more humid coastal regions, typically with some vegetation, a parabolic dune may form which is a crescent-shaped dune with the limbs pointed upwind. These may also be called blowout dunes because they form in areas of abundant sand, where the wind breaks through the vegetation and scours out a pocket of sand depositing it downwind.

* In arid regions with exposed bedrock, little surface relief, and variable wind directions, longitudinal dunes may form. These are parallel to the prevailing wind and may be as much as 200m high and 100 km long.

Silt -- Wind-deposited silt is called loess. Loess can form steep cliffs because it is composed of angular quartz crystals which can stick together and hold a steep surface. Nevertheless, loess is a soft material, easily crumbled by hand. Excellent soils develop in loess, because silt is the finest texture to hold water and allow root penetration. There are two major sources of loess:

* Glacial loess -- Winds blowing across outwash plains pick up silt and deposit it downwind in layers; the loess deposits of the American Midwest and the Ukraine are derived from glacial outwash, and produce these excellent soils, the finest in the world.

* Desert loess -- Desert winds deflate silt and deposit it downwind; the best example is the fertile northern China plain drained by the Yellow River; the silt is derived from the Gobi Desert.

Deserts are located primarily at 30° N and S latitude due to the prevailing downdrafts of cool, dry air at these latitudes that create prevailing high pressure regions. Warm, humid air rises at the equator, creating prevailing low pressure regions here; as it rises and cools, it loses its moisture by precipitation. The now cool and dry air travels to 30° N and S latitudes and then comes down, creating regions of high pressure, cool nights, and dry, hot days. Another way to form deserts is by north-south trending mountain ranges creating rain-shadow deserts on the east side: warm, humid winds from the ocean hit the mountains, rise, and lose their moisture on the west side; after the air passes the mountains, it descends on the eastern side as cool, dry air.

Deserts are defined as regions that receive 0-10 inches (0-25 cm) of rain each year. Despite this small amount of precipitation, water is the major erosive agent in deserts today, eroding and depositing more when it rains once or twice a year than wind erosion and deposition accomplishes the rest of the year.

Desert regions with igneous or metamorphic bedrock also develop characteristic landforms with

* Zones of erosion which have little or no soil or vegetation, high relief and near total outcrop of bedrock.

* Zones of deposition which have low relief, some soil development, and sparse vegetation.

Alluvial fans -- These are fan-shaped deposits of alluvium deposited by flooding ephemeral streams that appear when it rains infrequently in desert mountains

Playas -- These are flat, salty dry lake beds in deserts adjacent to alluvial fans; playas form from playa lakes that appear when it rains; the lakes ultimately dry (the water quicky evaporates or infiltrates), leaving behind a dry, lake bed.

Inselbergs -- These are isolated mountain peaks that have been surrounded and almost completely buried by alluvium.

Plateau -- An uplifted region of horizontal rock, usually sedimentary rock; the plateaus of the American Southwest are particularly famous because the sedimentary strata are brightly-colored and well-exposed in the dry, arid region. As the plateau erodes due to stream erosion, pieces are left isolated as erosional remnants, and these are classified by their dimensions:

* mesa -- broader than it is high, flat-topped hill bounded by cliffs.

* butte -- about as broad as it is high, narrow hill of resistant rock bounded by cliffs.

* pinnacle -- higher than its width

The different minerals that form as weathering products have different characteristic grain sizes that determine the distances that they are typically transported and the environments in which they are deposited.

This produces a sequence of sedimentary rocks that represents the various weathering products of rocks and the distance that these products may be transported from their sources. Because the different grain-size fractions have different chemical compositions, the process produces a strong chemical differentiation in the various rock types.

Coarse products ( boulders and cobbles) require fast moving water (mountain streams) to be moved at all, and so are not transported very far from their sources. These particles may be unweathered and retain their source mineralogy and chemistry.

Intermediate-sized particles (sand) are transported by rivers and wind and deposited at coasts or in deserts. Because quartz (SiO2) is abundant and relatively resistant to chemical weathering it makes up the bulk of sand-sized particles.

Fine particles ( silt and clay) are carried to regions where the water is still (off-shore environments). Clay minerals that are the weathering products of feldspars and ferro-magnesian minerals form the bulk of these particles.

Dissolved material (Ca, Na, K, CO3, SO4, Cl) is carried farthest and deposited where the ocean, sea, or lake is evaporated off. As sea water is evaporated, the sequence of minerals formed is:

1) calcite (CaCO3), 2) gypsum (CaSO4.2H2O), 3) halite (NaCl), and 4) sylvite (KCl).

Such rocks are called evaporites.

Particle name Particle size Rock

Boulder >256mm (10in) Conglomerate

Cobble 64-256mm (2,5-10in) Conglomerate

Pebble 2-64mm (0.1-2.5in) Conglomerate

Sand 0.06-2mm Sand

Silt 0.004-0.06mm Siltstone

Clay < 0.004mm Shale

After sediments are deposited, they are commonly compacted by the weight of overlying sediments. They may be lithified (solidified) by the deposit of a cement or secondary mineral that fills the pores. They may also be lithified by recrystallization of the primary minerals.

* Boulders, cobbles and pebbles may be lithified to form a conglomerate if the particles are rounded, or a breccia if the particles are angular.

* Sand sized particles are lithified to form a sandstone.

* Silt and clay sized particles are lithified to form a mudstone if massive or a shale if fissile (fractures on fine bedding planes).

* Limestone is the rock formed by calcite. Calcite is very near saturation in sea water and so is used as shell material by marine organisms. Most calcite rocks of phanerozoic age (the last 600 million years) are of biological origin. Dolomite (CaMg(CO3)2) may also be formed in this way.

* Evaporites are the rocks formed by chemical precipitation during evaporation of sea water.

In shales in the Grand Canyon, Arizona, USA, alteration in the oxidation potential changes the color of Fe-bearing sediments from greeen to red.

Bedding is a series of visible layers within the rock. It is primarily due to episodic nature of sedimentation where very fine particles are laid down slowly between times of more rapid deposition. Bedding planes are assumed to be originally horizontal or nearly horizontal for water-laid sediments.

For aeolian (wind-laid or sub-aerial) commonly show cross-bedding, because the sand can support steeper dune surfaces in air than in water.

Mud-cracks form because clay minerals may shrink by up to 15% in volume on drying out. Mud-cracks can be preserved and indicate a depositional environment that is near shore and periodically exposed to air.

Ripple marks in sandstones or mudstones are the result of shallow wave action and indicate a very near-shore environment. Asymmetric ripple marks indicate moving water.

Sedimentary Facies - A facies is a distinctive body of sediment or sedimentary rock characterized by distinctive physical or biological features. The most common facies characteristic is the lithology of the rock, e.g. the sandstone facies. Facies of sediments often lie adjacent to each other in a depositional environment.

Marine Transgressions and Regressions - Rising eustatic (worldwide) sea-level or subsidence of the basin will cause the shoreline to move landward; this is termed a marine transgression. If sea-level falls or if sediments accumulate at the edge of the basin, the shoreline will move toward the ocean; this is termed a marine regression. The sedimentary facies move landward or seaward as well as the shoreline. If this continues, sediment facies can begin to overlie each other. A transgressive sedimentary sequence is produced when a transgression causes deeper-water facies to lie on top of shallow-water facies. A regressive sequence is produced when a regression deposits shallow-water facies on top of deeper-water facies.

Sedimentary Structures - formed during sediment deposition, so sedimentary structures are indicators of depositional environment. Examples include: planar bedding, graded bedding, cross-bedding, mud cracks, ripple marks. Know under what conditions each forms.

Fossils - remains and traces of once-living organisms; two types: trace fossils and body fossils; body fossils include casts, molds, replacements, petrification, carbonization, whole preservation, etc.; trace fossils include footprints, burrows, feeding impressions, tunnels, grazing traces, excrement, etc. Fossils preserve the record of life on Earth; fossils are found almost entirely in sedimentary rocks, so strata contain the empirical evidence of both the history of the Earth and the history of life on the Earth.