Chapter 5 Weathering and Soil

Effects of weathering

Spheroidal weathering, Differential weathering, Other evidence of weathering: -Statues and gravestones that have stood for several decades may no longer be sharp -Surface discoloration; Mineral grains in granite may be loosened, cracks enlarged, and the surface discolored and dulled by the products of weathering

How does weathering have an impact on the composition of Earth's atmosphere

Chemical weathering removes carbon dioxide from the atmosphere, allowing it to be transformed into limestone and stored in the crust. -Without chemical weathering, the elevated levels of carbon dioxide in the atmosphere would have long ago made Earth too hot to sustain life

What is the most important factor determining soil type?

Climate

The most important natural source of acid for rock weathering at Earth's surface is...?

Dissolved carbon dioxide (CO2 ) in water to make carbonic acid (H2CO3) -a weak acid that dissociates into the hydrogen ion (H+) and the bicarbonate ion (HCO3-). Even though carbonic acid is a weak acid, it is so abundant at Earth's surface that it is the single most effective agent of chemical weathering

Because soils tend to become thicker with time, the thickness of soils developed on successive basalt flows can be used to...?

Estimate the length of time between eruptions

Which minerals are most vulnerable to chemical attack?

Ferromagnesian minerals such as olivine, pyroxene, and amphibole include other positively charged ions such as Al, Fe, Mg, and Ca. The presence of these positively charged ions in the crystal framework makes these minerals vulnerable to chemical attack due to the weaker chemical bonding between these ions and oxygen, as compared to the much stronger silicon-oxygen bonds. -For example, olivine—(Fe, Mg) 2 SiO 4 —weathers rapidly because its isolated silicon-oxygen tetrahedra are held together by relatively weak ionic bonds between oxygen and iron and magnesium. These ions are replaced by H+ ions during chemical weathering similar to that described for the feldspars.

CO2 + H2O → carbon dioxide + water →

First →H2CO3 carbonic acid then dissociates to →H+ + HCO3- Hydrogen ion + bicarbonate ion

C horizon

Fragments mechanically weathered from bedrock and some partially decomposed -incompletely weathered parent material that lies below the B horizon. -The parent material is commonly subjected to mechanical and chemical weathering from frost action, roots, plant acids, and other agents. The C horizon is transitional between the unweathered rock or sediment below and the developing soil above.

frost heaving

Frost heaving lifts rock and soil vertically -The lifting of rock or soil by the expansion of freezing water. -Process: Solid rock conducts heat faster than soil, so on a cold winter day, the bottom of a partially buried rock will be much colder than soil at the same depth. As the ground freezes in winter, ice forms first under large rock fragments in the soil. The expanding ice layers push boulders out of the ground

Rock chemically weathers much faster in...?

Humid climates than in arid climates -Chemical weathering is largely a function of the availability of liquid water

Main reason seawater is salty...?

If the soluble ions and silica are not precipitated as solids, they remain in solution and may eventually find their way into a stream and then into the ocean. Enormous quantities of dissolved material are carried by rivers into the sea (one estimate is 4 billion tons per year)

Soil

In common usage, soil is the name for the loose, unconsolidated material that covers most of Earth's land surface. However, Geologists call this material regolith -The term soil is reserved for a layer of weathered, unconsolidated material that contains organic matter and is capable of supporting plant growth. -A mature, fertile soil is the product of centuries of mechanical and chemical weathering of rock, combined with the addition and decay of plant and other organic matter. -Soil develops by chemical and mechanical weathering of a parent material.

Iron Oxide

Iron oxide is a weathering product of numerous minerals containing iron, such as the ferromagnesian group (pyroxenes, amphiboles, biotite, and olivine). -The iron in the ferromagnesian silicate minerals must first be separated from the silica in the crystal structure before it can oxidize.

Frost action

Mechanical weathering of rock by freezing water -Occurs as frost wedging or frost heaving -When water freezes at 0 deg. C (32 deg. F), the individual water molecules jumbled together in the liquid align into an ordered crystal structure, forming ice. Because the crystal structure of ice takes up more space than the liquid, water expands 9% in volume when it freezes -This unique property makes water a potent agent of mechanical weathering in any climate where the temperature falls below freezing

Spheroidal weathering

Occurs where rock has been rounded by weathering from an initial blocky shape -It is rounded because chemical weathering acts more rapidly or intensely on the corners and edges of a rock than on the smooth rock faces -Process: Water penetrating along cracks at right angles to one another in an igneous rock produces spheroidal weathering of once angular blocks. The increase in surface area exposed by the cracks increases chemical weathering. -Because of the increased surface area, chemical weathering attacks edges and particularly the corners more rapidly than the flat faces, creating the spheroidal shape

Role of Oxygen

Oxygen is abundant in the atmosphere and quite active chemically, so it often combines with minerals or with elements within minerals that are exposed at Earth's surface -The rusting of an iron nail exposed to air is a simple example of chemical weathering. Oxygen from the atmosphere combines with the iron to form iron oxide

Erosion Processes: Running water and Ice

Running water contributes to weathering and erosion by loosening and removing particles and by abrading rocks during transportation in streams. -Ice in glaciers is a very effective agent of erosion as rocks frozen in the base of a glacier grind down the underlying bedrock. Freezing and thawing of water in cracks in rock is also very effective at mechanically breaking them up

Splash erosion

Soil particles are small and are therefore easily eroded by water and wind. Raindrops strike unprotected soil like tiny bombs, dislodging soil particles in a process called splash erosion -Splash erosion dislodges soil particles, making them available for removal by sheet erosion and rill erosion

parent material of soil

The character of a soil depends partly on the parent material from which it develops. The parent material is the source of the weathered mineral matter that makes up most of a soil

Weathering

The term weathering refers to the processes that change the physical and chemical character of rock at or near the surface. -The tightly bound crystals of any rock can be loosened and altered to new minerals when exposed to air and water during weathering -Weathering breaks down rocks that are either stationary or moving

Chemical Weathering of Feldspar

The weathering of feldspar is an example of the alteration of an original mineral to an entirely different type of mineral as the weathered product. -When feldspar is attacked by the hydrogen ion of carbonic acid (from carbon dioxide and water), it forms clay minerals

_______ is necessary for chemical weathering to take place

Water is necessary for chemical weathering to take place -Oxygen dissolved in water oxidizes iron in rocks. Carbon dioxide mixed with water makes a weak acid that causes most minerals to decompose; this acid is the primary cause of chemical weathering

Why is weathering important to life on Earth?

Weathering processes make the planet suitable for human habitation -The weathering of rock affects the composition of Earth's atmosphere, helping to maintain a habitable climate. Weathering also produces soils, upon which grow the forests, grasslands, and agriculture of the world

The processes that affect rock are ____________?

Weathering, erosion, and transportation

The three most common rocks at Earth's surface are sedimentary, and are:

- Sandstone (from quartz sand) - Shale, or mudstone (from clay minerals) - Limestone (from ions in solution)

Weathering Products of Common Rock-Forming Minerals

-Feldspar→ Clay mineral + Ions (Na+, Ca++, K+), SiO2 -Ferromagnesian minerals (including biotite mica)→ Clay mineral+ Ions (Na+, Ca++, K+ + Mg++), SiO2 , iron oxides -Muscovite mica→Clay mineral + Ions (K +), SiO2 -Quartz→Quartz grains (sand and silt) -Calcite→Ions (Ca++, HCO3-)

Pressure Release

1) A mechanical weathering process in which rocks that formed under pressure expand on being exposed at the surface. The reduction of pressure on a body of rock can cause it to crack as it expands 2) A significant type of mechanical weathering that causes rocks to crack when overburden is removed -Ex: A large mass of rock, such as a batholith, originally forms under great pressure from the weight of several kilometers of rock above it. This batholith is gradually exposed by tectonic uplift of the region followed by erosion of the overlying rock. The removal of the great weight of rock above the batholith, usually termed unloading, allows the granite to expand upward

Sheet joints

1) Cracks that develop parallel to the outer surface of a large mass of expanding rock, as pressure is released during unloading. 2) Cracks that develop parallel to the outer surface of the rock as the outer part of the rock expands more than the inner part -On slopes, gravity may cause the rock between such joints to break loose in concentric slabs from the underlying granite mass

Exfoliation

1) The stripping of concentric rock slabs from the outer surface of a rock mass 2) A mechanical weathering process which removes protruding overlying layers and exposing the weathered down dome of the underlying layers -On slopes, gravity may cause the rock between sheet joints to break loose in concentric slabs from the underlying granite mass -This process of spalling off of rock layers is called exfoliation ; it is somewhat similar to peeling layers from an onion

Differential weathering

1) Varying rates of weathering resulting from some rocks in an area being more resistant to weathering than others 2) describes the tendency for different types of rock to weather at different rates -For example, shale (composed of soft clay minerals) tends to weather and erode much faster than sandstone (composed of hard quartz mineral)

Chemical Weathering of Feldspar to Form a Clay Mineral

1. Rainwater percolates down through soil, picking up carbon dioxide from the atmosphere and the upper part of the soil. 2.The water, now slightly acidic, comes in contact with feldspar in the lower part of the soil. The acidic water reacts with the feldspar and alters it to a clay mineral. 3. The hydrogen ion (H+) attacks the feldspar structure, becoming incorporated into the clay mineral product. When the hydrogen moves into the crystal structure, it releases potassium (K) from the feldspar. 4. The potassium is carried away in solution as a dissolved ion (K+). The bicarbonate ion from the original carbonic acid does not enter into the reaction; . The soluble potassium and bicarbonate ions are carried away by water (groundwater or streams). All the silicon from the feldspar cannot fit into the clay mineral, so some is left over and is carried away as silica (SiO2 ) by the moving water. This excess silica may be carried in solution or as extremely small solid particles

A horizon

A Organic matter mixed with mineral material -a dark-colored soil layer that is rich in organic matter and high in biological activity, both plant and animal. -+The two upper horizons are often referred to as topsoil.

Clay particles

A clay-sized particle can be composed of any mineral at all provided its diameter is less than 1/256 millimeter --Note that we have two different uses of the word clay —a clay-sized particle and a clay mineral -A clay mineral, on the other hand, is one of a small group of silicate minerals with a sheet-silicate structure. Clay minerals usually form in the clay size range.

weathering products

A mineral that crystallized deep underground from a water-deficient magma may eventually be exposed at the surface, where it can react with the abundant water there to form a new, different mineral. A mineral containing very little oxygen may react with oxygen in the air, extracting oxygen atoms from the atmosphere and incorporating them into its own crystal structure, thus forming a different mineral -These new minerals are weathering products. They have adjusted to physical and chemical conditions at (or near) Earth's surface. -Minerals change gradually at the surface until they come into equilibrium, or balance, with the surrounding conditions

Plant nutrients, such as Ca 2+ and K +, commonly supplied by the weathering of minerals such as feldspar, are held loosely on the surface of clay minerals, how do plants receive these nutrients?

A plant root is able to release H + from organic acids and exchange it for the Ca 2+ and K + that the plant needs for healthy growth -Clay minerals occur as microscopic plates and help hold water and plant nutrients in a soil. Because of ion substitution within their sheet-silicate structure, most clay minerals have a negative electrical charge on the flat surfaces of the plates. This negative charge attracts water and nutrient ions to the clay mineral -Too much clay in a soil may pack together closely, though, causing pore spaces to be too small to allow water to drain properly. Too much water in the soil and not enough air may cause plant roots to rot and die

loam

A soil with approximately equal parts sand, silt, and clay -Loamy soils are well-drained, may contain organic matter, and are often very fertile and productive

frost wedging

A type of frost action in which the expansion of freezing water pries a rock apart. -Process: Most rock contains a system of cracks called joints, caused by the slow flexing of brittle rock by deep-seated Earth forces. Water that has trickled into a joint in a rock can freeze and expand when the temperature drops below 0 deg. C (32 deg. F). The expanding ice wedges the rock apart, extending the joint or even breaking the rock into pieces -Frost wedging is most effective in areas with many days of freezing and thawing (mountaintops and midlatitude regions with pronounced seasons). Partial thawing during the day adds new water to the ice in the crack; refreezing at night adds new ice to the old ice

Hematite (Fe2O3)

A type of iron oxide that has a brick-red color when powdered -The brown, yellow, or red color of soil and many kinds of sedimentary rock is commonly the result of small amounts of hematite and limonite released by the weathering of iron-containing minerals

Limonite

A type of iron oxide that is yellowish-brown when powdered; Fe2O3 ·nH2O -If water is present, as it usually is at Earth's surface, the iron oxide combines with water to form limonite , which is the name for a group of mostly amorphous, hydrated iron oxides (often including the mineral goethite), which are yellowish-brown when powdered -The brown, yellow, or red color of soil and many kinds of sedimentary rock is commonly the result of small amounts of hematite and limonite released by the weathering of iron-containing minerals

B horizon (zone of accumulation)

Accumulation of clay minerals, Fe oxides, and calcite -This layer is often quite clayey and stained red or brown by hematite and limonite. Calcite may also build up in B horizons. This horizon is frequently called the subsoil -Within the B horizon, a hard layer of Earth material called hardpan may form in wet climates where clay minerals, silica, and iron compounds have accumulated in the B horizon from eluviation of the overlying E horizon. A hardpan layer is very difficult to dig or drill through and may even be too hard for backhoes to dig through; planting a tree in a lawn with a hardpan layer may require a jackhammer. Tree roots may grow laterally along rather than down through hardpan; such shallow-rooted trees are usually uprooted by the wind

The most effective agent of chemical weathering is..?

Acids -Weak acid, primarily from the solution of carbon dioxide in water, is an effective agent of chemical weathering

Acids

Acids are chemical compounds that give off hydrogen ions (H+ ) when they dissociate, or break down, in water. -The hydrogen ions given off by natural acids disrupt the orderly arrangement of atoms within most minerals -Because a hydrogen ion has a positive electrical charge and a very small size, it can substitute for other positive ions (such as Ca++,Na++,or K+) within minerals. This substitution changes the chemical composition of the mineral and disrupts its atomic structure. The mineral decomposes, often into a different mineral, when it is exposed to acid.

Soil composition

An average soil is composed of 45% rock and mineral fragments (including clay), 5% decomposed organic matter, or humus, and 50% pore space. The rock and mineral fragments in a soil provide an anchoring place for the roots of plants -The clay minerals attract water molecules and plant-nutrient ions, which are loosely held and available for uptake by plant roots -The humus releases weak acids that contribute to the chemical weathering of soil. Humus also produces plant nutrients and increases the water retention ability of the soil -The pore spaces are the final essential component of a fertile soil. Water and air circulate through the pore spaces, carrying dissolved nutrients and carbon dioxide, which is necessary for the growth of plants.

soil horizons

As soils mature, distinct layers appear in them - Soil layers are called soil horizons and can be distinguished from one another by appearance and chemical composition. -Boundaries between soil horizons are usually transitional rather than sharp. By observing a vertical cross section, or soil profile, various horizons can be identified -horizons are caused in part by water movement within the soil

What mineral is completely dissolved by chemical weathering?

Calcite -Calcite goes into solution when exposed to carbon dioxide and water to form a calcium ion and bicarbonate ion: CaCO3 + CO2 + H2O → Ca2+ + 2HCO3- -The carbon dioxide and water combine to form carbonic acid, which dissociates into the hydrogen ion and the bicarbonate ion, so the equation for the solution of calcite can also be written as: CaCO3 + H+ + HCO3- → Ca++ + 2HCO3- -There are no solid products in the last part of the equation, indicating that complete solution of the calcite has occurred -Caves can form underground when flowing groundwater dissolves the sedimentary rock limestone, which is mostly calcite -Rain can discolor and dissolve statues and tombstones carved from the metamorphic rock marble, which is also mostly calcite

Clay minerals properties

Clay minerals are very common at Earth's surface; they are a major component of soil. There are a great number of clay minerals. What they all have in common is that they are sheet silicates. They differ by which ions hold sheets together and by the number of sheets "sandwiched" together -Montmorillonite is one of the more interesting clay minerals. It is better known as expansive clay or swelling clay. If water is added to the montmorillonite, the water molecules are adsorbed into the spaces between silicate layers. This results in a large increase in volume, sometimes up to several hundred percent.

Climate and soil development

Climate is perhaps the most influential factor affecting soil thickness and character. -Temperature and precipitation determine whether chemical or mechanical weathering processes will dominate and strongly influence the rate and depth of weathering. The amount and types of vegetation and animal life that contribute to soil formation are also determined by climate

Benefits of weathering

Creates soil --Soil is produced by rock weathering, so most plants depend on weathering for the soil they need in order to grow. All agriculture depends on weathering -Produces clay, sand, and gravel -Produces minerals -Weathering products transported to the sea by rivers as dissolved solids make seawater salty and serve as nutrients for many marine organisms -Some metallic ores, such as those of copper and aluminum, are concentrated into economic deposits by chemical weathering

E horizon (zone of leaching)

E Leaching by downward percolating water -Organic acids and carbon dioxide produced by decaying plants in the topsoil percolate down into the E horizon , or zone of leaching , and help dissolve minerals such as iron and calcium. -The downward movement of water in the E horizon carries the dissolved minerals, and fine-grained clay minerals as well, into the soil layer below. This leaching (or eluviation ) of clay and soluble minerals can make the E horizon pale and sandy.

Rainwater that has trickled through soil is usually acidic and readily attacks minerals in the unweathered rock below the soil, why is this?

Earth's atmosphere (mostly nitrogen and oxygen) contains 0.03% carbon dioxide. Some of this carbon dioxide dissolves in rain as it falls, so most rain is slightly acidic when it hits the ground. -Large amounts of carbon dioxide also dissolve in water that seeps through soil. The openings in soil are filled with a gas mixture that differs from air -Soil gas has a much higher content of carbon dioxide (up to 10%) than does air, because carbon dioxide is produced by the decay of organic matter and the respiration of soil organisms in the biosphere, such as worms

Exfoliation domes

Exfoliation of rock layers produces rounded exfoliation domes -Exfoliation domes are large, rounded landforms developed in massive rock, such as granite, by exfoliation. -Some famous examples of exfoliation domes include Stone Mountain in Georgia and Half Dome in Yosemite.

When Earth first formed, its atmosphere was probably very much like that of Venus, with much more CO2 . What happened to most of the original carbon dioxide in Earth's atmosphere?

Geologists think that a quantity of CO2 equal to approximately 65,000 times the mass of CO2 in the present atmosphere lies buried in the crust and upper mantle of Earth. -Some of this CO2 was used to make organic molecules during photosynthesis and is now trapped as buried organic matter and fossil fuels in sedimentary rocks. However, the majority of the missing CO2 was converted to bicarbonate ion (HCO3- ) during chemical weathering and is locked away in carbonate minerals (primarily CaCO3 ) that formed layers of limestone rock.

Solid minerals are not the only products of chemical weathering. Minerals such as ____?

Minerals such as calcite dissolve when chemically weathered. -We can expect limestone and marble, rocks consisting mainly of calcite, to weather chemically in quite a different way than granite

Other Processes that mechanically weather rock

Plant growth: Particularly roots growing in cracks, can break up rocks Burrowing animals --Such activities help to speed up chemical weathering by enlarging passageways for water and air. -Extreme changes in temperature: as in a desert environment or in a forest fire, can cause a rock to expand until it cracks --The pressure of salt crystals formed as water evaporates inside small spaces in rock also helps to disintegrate desert rocks

Most important agents for Mechanical Weathering

Pressure release and frost action -Of the many processes that cause rocks to disintegrate, the most effective are pressure release and frost action.

The rate of soil formation is controlled by...?

Rainfall, temperature, slope, and to some extent, the type of rock that weathers to form soil -High temperature and abundant rainfall speed up soil formation, but in most places, a fully developed soil that can support plant growth takes hundreds or thousands of years to form.

Soil texture

Refers to the proportion of different-sized particles, generally referred to as sand, silt, and clay. -Quartz generally weathers into sand grains that help keep soil loose and aerated, allowing good water drainage. Partially weathered crystals of feldspar and other minerals can also form sand-sized grains. Soils with too much sand, however, can drain too rapidly and deprive plants of necessary water. -The size and number of pore spaces, and therefore the ability of a soil to transmit air and water, are largely a function of the texture of a soil

How weathering changes rocks

Rocks undergo both mechanical weathering and chemical weathering -In nature, mechanical and chemical weathering usually occur together, and the effects are interrelated

Mechanical weathering intensity is also related to climate (temperature and humidity), as well as to...?

Slope -Steep slopes cause rock to fall and break up under the influence of gravity. The most intense mechanical weathering probably occurs in high mountain peaks where the combination of steep slopes, precipitation, freezing and thawing, and flowing glacial ice rapidly pulverize the solid rock -Temperate climates, where abundant water repeatedly freezes and thaws, promote extensive frost weathering

Transported soils

Soil not formed from the local rock but from parent material brought in from some other region and deposited, usually by running water, wind, or glacial ice. -Residual soil develops from weathering of bedrock beneath the hills, whereas transported soil develops on top of flood plain deposits (regolith) in the stream valley -Transported soils are generally more fertile than residual soils because the parent material is transported from many different locations; there is more variety in the chemical makeup of the parent material, so a greater variety of minerals and nutrients are supplied to the resulting soil

residual soils

Soil that develops directly from weathering of the rock above them -Residual soil develops from weathering of bedrock beneath the hills, whereas transported soil develops on top of flood plain deposits (regolith) in the stream valley

Strong acids

Strong acids produce a great number of hydrogen ions when they dissociate, and weak acids produce relatively few such ions. -Some strong acids occur naturally on Earth's surface, but they are relatively rare -Sulfuric acid is a strong acid emitted during many volcanic eruptions. It can kill trees and cause intense chemical weathering of rocks near volcanic vents -The bubbling mud of Yellowstone National Park's mudpots is produced by rapid weathering caused by acidic sulfur gases that are given off by some hot springs -Strong acids also drain from some mines as sulfur-containing minerals such as pyrite oxidize and form acids at the surface. Uncontrolled mine drainage can kill fish and plants downstream and accelerate rock weathering.

Whatever processes of mechanical weathering are at work, as rocks disintegrate into smaller fragments, the total ______ ____ increases, allowing more extensive chemical weathering by water and air

Surface area -Mechanical weathering can increase the surface area of a rock, accelerating the rate of chemical weathering. As a cube breaks up into smaller pieces, its volume remains the same, but its surface area increases -By increasing the exposed surface area of rocks, mechanical weathering helps speed chemical weathering

____________ is also a factor in chemical weathering.

Temperature -The most intense chemical weathering occurs in the tropics, which are both wet and hot. Polar regions experience very little chemical weathering because of the frigid temperatures and the absence of liquid water • These processes require water, so they are maximized in warm, moist climates - Limestone in Arizona vs. Limestone in Massachusetts

Consequences of Erosion

The Dust Bowl Era -Advances in farm equipment allowed farmers to practice "intensive row crop agriculture," in which more than 100 million acres of prairie were plowed under and planted in long rows of crops such as corn, soybeans, and wheat. After several years of drought in the 1930s, the row crops failed and the soil was left exposed to the high winds that came whipping across the plains. Huge dust clouds called "black rollers" billowed up, burying vehicles and drifting like snow against houses. The clouds of sediment drifted east, darkening the sky and falling as muddy rain and snow on the East Coast states -The fertile agricultural soils of the Canadian plains and the northern United States took more than 10,000 years to develop on glacial deposits after the thick continental ice sheet melted. These soils and many others around the world are eroding at an alarming rate, much faster than they are being replaced by newly formed soils.

Time and soil development

The character of a soil changes with time -In a soil that has been weathering for a short period of time, the characteristics are largely determined by the parent material. Young soils can retain the structure of the parent rock, such as bedding layers -As time progresses, other factors become more important and climate eventually predominates. Soils forming from many different kinds of igneous, metamorphic, and sedimentary rocks can become quite similar, given the same climate and enough time. In the long term, the only characteristic of the parent rock to have significance is the presence or absence of coarse grains of quartz. With time, soils tend to become thicker

Burrowing organisms and the soil

The chief function of living organisms is to provide organic material to the soil -Burrowing organisms such as ants, worms, and rodents bring soil particles to the surface and mix the organic and mineral components of the soil -They create passageways that allow for the circulation of air and water, increasing chemical weathering and accelerating soil formation. Microorganisms such as bacteria, fungi, and protozoa promote the decomposition of organic matter to humus, and some bacteria fix nitrogen in the soil, making it available for uptake by plants. The interdependency of plants, animals, and soil is a mutually beneficial and delicately balanced system.

Decomposing plants and soil development

The chief function of living organisms is to provide organic material to the soil. Decomposing plants form humus, which supplies nutrients to the soil and aids in water retention. The decaying plant matter releases organic acids that increase chemical weathering of rocks. Growing plants send roots deep into the soil, breaking up the underlying bedrock and opening up pore spaces

Chemical Weathering

The decomposition of rock from exposure to water and atmospheric gases (principally carbon dioxide, oxygen, and water vapor). -As rock is decomposed by these agents, new chemical compounds form -If granite is being chemically weathered, some of the original minerals are chemically changed into different minerals. Feldspar, for example, will change into a clay mineral (with a crystal structure similar to mica) -Chemical weathering results when a mineral is unstable in the presence of water and atmospheric gases. As chemical weathering proceeds, the mineral's components recombine into new minerals that are more in equilibrium.

Where Do Aluminum Cans Come From?

The high temperatures and abundant rainfall in tropical regions produce some of the thickest soil on Earth where extreme chemical weathering occurs -Lush vegetation grows over this soil, but the soil itself is very infertile. As water percolates down through the soil in this hot, humid climate, plant nutrients are dissolved and carried downward, out of reach of plant roots, the soil that remains is composed almost entirely of iron and aluminum oxides. -The aluminum oxides may form rich ore deposits near the surface -As leaching proceeds, nearly pure layers of bauxite (Al(OH)3 ), the principal ore of aluminum, are left near the surface in large deposits that are on average 4-6 meters thick and may cover many square kilometers. Eighty percent of the world's aluminum is mined from large blanket deposits -The bauxite ore is washed, crushed, and then dissolved under high temperature and pressure in a caustic solution of sodium hydroxide -The undissolved residue, mostly iron, silica, and titanium, settles to the bottom, and the sodium aluminate solution is pumped into precipitators, where the previous reaction is reversed -The sodium hydroxide is recovered and returned to the beginning of the process, and the pure bauxite crystals are passed to another process that drives off water to form a white alumina (aluminum oxide) powder -The alumina is then smelted by passing electric currents through the powder to separate the metallic aluminum from the oxygen. Small amounts of other metals may be added to the molten aluminum to form alloys, and the aluminum is cast into blocks that are sent to factories for further processing. The aluminum alloy used for beverage cans contains manganese, which helps the metal become more ductile as it is rolled into the thin sheets from which the cans are formed

Weathering has affected the long-term climate of Earth by changing the carbon dioxide content of the atmosphere through the inorganic carbon cycle

The inorganic carbon cycle helps to regulate the climate of Earth because CO2 is a greenhouse gas, chemical weathering accelerates with warming, and the formation of limestone occurs mostly in warm, tropical oceans. -When Earth's climate is warm chemical weathering and the formation of limestone increase, drawing CO2 from the atmosphere, which cools the climate. When the global climate cools, chemical weathering and limestone formation slow down, allowing CO2 to accumulate in the atmosphere from volcanism, which warms the Earth. -An increase in chemical weathering can also lead to global cooling by removing more CO2 from the atmosphere

Transportation

The movement of eroded particles by agents such as rivers, waves, glaciers, or wind -After a rock fragment is picked up (eroded), it is transported -Weathering processes continue during transportation. A boulder being transported by a stream can be physically worn down and chemically altered as it is carried along by the water

Mechanical weathering (physical disintegration)

The physical disintegration of rock into smaller pieces. -Mechanical weathering breaks up rock but does not change the composition -For example, water freezing and expanding in cracks can cause rocks to disintegrate physically -A large mass of granite may be broken into smaller pieces by frost action, but its original crystals of quartz, feldspar, and ferromagnesian minerals are unchanged

Negative charge on the outside of a platy clay mineral attracts...?

The positive end of a water molecule

Rates of Erosion

The rate of soil erosion is influenced by several factors: Soil characteristics: Coarse-grained soils with organic content tend to have larger pore spaces and can absorb more water than soils dominated by clay-sized particles. Less runoff occurs on the coarser soils, and less of the soil is eroded away Climate: The type of rainfall also influences the amount of erosion. A gentle rain over a long period of time produces less splash erosion than a short, heavy rainstorm. More water can infiltrate the soil during the gentle rainfall, and there is less likelihood of sheet erosion occurring Slope: Slope also plays an important role in soil erosion. Water moves more slowly on gentle slopes and is more likely to percolate down into the soil. The faster-moving water on steeper slopes does not infiltrate and has a greater ability to dislodge and transport soil particles down from the slope Vegetation: A very significant control on soil erosion rate is the amount and type of vegetation present. Plant roots form networks in the O and A horizons that bind soil particles. The leaf canopy protects the soil from the impact of raindrops, lowering the risk of splash erosion. Thick vegetation can reduce the wind velocity near the ground surface, preventing the loss of soil due to wind erosion

Slope and soil development

The slope of the land surface provides an important control on the formation of soil -Soils tend to be thin or nonexistent on steep slopes, where gravity keeps water and soil particles moving downhill -Vegetation is sparse on steep slopes, so there are not many roots to hold the weathering rock in place and little organic matter to provide nutrients -By contrast, soils in bottomlands may be very thick but poorly drained and waterlogged. Vegetation in the bottomlands does not decay completely, and thick, dark layers of peat may form. -The optimal topography for soil formation is flat or gently sloping uplands, allowing good drainage, minimal erosion, and healthy vegetation cover

Soil Erosion

The upper layers of a soil, the O and A horizons, are the most fertile and productive. These are the layers that are most vulnerable to erosion due to land mismanagement by poor farming and grazing practices -Scientists estimate that the Earth has lost about 10% of its productive value (the ability to provide crops, pasture, and forest products) over the last 50 years. If measures are not taken to curb the loss of fertile soils to erosion, an additional 10% of Earth's productivity could be lost in the next 25 years

The O horizon

The uppermost layer that consists entirely of organic material -Ground vegetation and recently fallen leaves and needles are included in this horizon, as well as highly decomposed plant material called humus. The humus from the O horizon mixes with weathered mineral matter just below to form the A horizon

Chemical Weathering of Other Minerals

The weathering of ferromagnesian or dark minerals is much the same as that of feldspars. -Two additional products are found on the right side of the equation—magnesium ions and iron oxides (hematite, limonite, and goethite). -The susceptibility of the rock-forming minerals to chemical weathering is dependent on the strength of the mineral's chemical bonding within the crystal framework

Diamond is the hardest mineral known and is also extremely resistant to weathering because..?

This is due to the very strong covalent bonding of carbon -Diamonds are often concentrated by weathering. --Diamonds are brought to the surface of Earth in kimberlite pipes, columns of brecciated or broken ultramafic rock that have risen from the upper mantle. Diamonds are widely scattered in diamond pipes when they form. At the surface, the ultramafic rock in the pipe is preferentially weathered and eroded away. The diamonds, being more resistant to weathering, are left behind, concentrated in rich deposits on top of the pipes

Weathering is a relatively long, slow process

Typically, cracks in rock are enlarged gradually by frost action or plant growth (as roots pry into rock crevices), and as a result, more surfaces are exposed to attack by chemical agents -Chemical weathering initially works along contacts between mineral grains. Tightly bound crystals are loosened as weathering products form at their contacts -Mechanical and chemical weathering then proceed together, until a once tough rock slowly crumbles into individual grains

Chemical weathering of feldspar and calcite by carbonic and soil acids

Water percolating through soil weathers feldspar to clay and completely dissolves calcite. Soluble ions and soluble silica weathering products are washed away -The weathering process is the same regardless of the type of feldspar: K-feldspar forms potassium ions; Na-feldspar and Ca-feldspar (plagioclase) form sodium ions and calcium ions, respectively. The ions that result from the weathering of Ca-feldspar are calcium ions (Ca++) and bicarbonate ions (HCO3- ), both of which are very common in rivers and underground water, particularly in humid regions

clay mineral

a hydrous aluminum silicate with a sheet-silicate structure like that of mica -Therefore, the entire silicate structure of the feldspar crystal is altered by weathering: feldspar is a framework silicate, but the clay mineral product is a sheet silicate, differing both chemically and physically from feldspar. -Most of the silicate minerals form clay minerals when they chemically weather

Dissolved silica can also precipitate as a solid from underground water as..?

cement -This is significant because calcite and silica are the most common materials precipitated as cement, which binds loose particles of sand, silt, and clay into solid sedimentary rock. The weathering of calcite, feldspars, and other minerals is a likely source for such cement.

Oxygen from the atmosphere combines with the iron to form iron oxide, the reaction being expressed as follows

iron [4Fe(+3)] + oxygen [3O2] → iron oxide [2Fe2O3]

Which rock-forming mineral is least susceptible to chemical attack at Earth's surface?

quartz (SiO2)

Unloading

removal of overlying sediment and rock -The removal of the great weight of rock above the batholith, usually termed unloading, allows the granite to expand upward. Cracks called sheet joints develop parallel to the outer surface of the rock as the outer part of the rock expands more than the inner part -Unloading reduces pressure on granite and causes outward expansion. Sheet joints are closely spaced at the surface where expansion is greatest.

Erosion

the picking up or physical removal of rock particles by an agent such as ocean waves, running water, or glaciers -Weathering helps break down a solid rock into loose particles that are easily eroded -Rainwater flowing down a cliff or hillside removes the loose particles produced by weathering

Properties of clay minerals (cont)

• Clay minerals are flat • Clay minerals absorb water and positively charged ions • Clay minerals virtually prevent water form circulating through them - Compare water in sand, percolating freely, vs. water in mud, causing swelling

products of Mechanical (or Physical) Weathering

• Mechanical Weathering breaks down rocks in fragments • Fragments are named not based on composition but based on size: - Gravel (bigger than 2 mm) - Sand (between 2 mm and 1/16 mm) - Silt (between 1/16 mm and 1/256 mm) - Clay (smaller than 1/256 mm)

Solution of Calcite: CaCO3 + CO2 + H2O → or CaCO3 + H+ + HCO3-

→ Ca2+ + 2HCO3-

Chemical Weathering of Feldspar to Form a Clay Mineral 2KAISi3O8 + 2H+ + 2HCO3- + H20 →

→Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2 →clay mineral + (soluble ions) +silica in solution or as fine solid particles