Chapter 5 Geol 1114 Exam 2

Factors Affecting Soil Formation: Most soil takes a long time to form. The rate of soil formation is controlled by rainfall, temperature,slope, and the type of rock that weathers to form soil.

-Parent Rock. Additional factors include slope, living organisms, climate, time.

Chemical weathering of other minerals. The weathering of ferromagnesian or dark minerals is much the same as that of feldspar. Two additional products are found on the right side of the equations - magnesium and iron oxides (hematite, limonite, and geothite).

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Factors affecting weathering. The intensity of both mechanical and chemical weathering is affected by a variety of factors.

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Hydrosphere - Water is necessary for chemical weathering to occur. Oxygen dissolved in water oxidizes iron in rocks.

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Soil Classification- different types of soil could form on the same underlying rock.

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The effects of chemical and mechanical weathering are interrelated because they both usually occur at together.

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The fine grained feldspars in the basalt will weather directly to fine grained minerals. Since the parent rock had no coarse grained minerals and no quartz to begin with, the resulting soil may lack sand. Such soil may not drain well, although it can be quite fertile.

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Weathering breaks down rocks that are either stationary or moving.

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The A horizon.

A dark colored soil layer that is rich in organic matter and high in biological activity, both plant and animal. The two uppermost horizons are called Topsoils.

Mechanical Weathering breaks up rock but does not change the composition.

A large mass of granite may be broken into smaller pieces by frost action, but its original crystals of quartz, feldspar, and ferromagnetism minerals are unchanged.

The reduction of pressure on a body of roc can cause it to crack as it expands; pressure release is a significant type of mechanical weathering.

A large mass of rock(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.

Consequences of erosion.

A lot of the areas effected by erosion can not support life as they once would. This essential resource is what the base of all life rests.

Role of Acids: The most effective agent of chemical weathering is acid.

Acids are chemical compounds that give off hydrogen ions when they dissociate, or break down, in water. Strong acids produce a great number of hydrogen ions; weak acids produce relatively few ions.

Role of Oxygen: Oxygen is abundant in the atmosphere and quite active chemically, so it often combines with minerals or with elements within mineral that are exposed at Earth's surface.

An example of chemical weathering - the rusting of an iron nail exposed to air. Oxygen from the atmosphere combines with the iron to form iron oxide. Iron oxide formed in this way is a weathering product of numerous mineral containing iron, such as the ferromagnesian group (pyroxene, 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 be oxidized.

Weathering and Earth Systems -

Atmosphere, Hydrosphere, and Biosphere.

The susceptibility of the rock forming minerals to chemical weathering is dependent on the strength of the minerals chemical bonding within the crystal framework.

Because of the strength of the silicon oxygen bond, quartz is quite resistant to chemical weathering. Quartz is the rock forming mineral least susceptible to chemical attack at Earths surface.

Look along the edge or corners of old stones structures for evidence. Like if the inscriptions on statues or gravestones may no longer be sharp

Building blocks of limestone or marble exposed to rain and atmospheric gases may show solution effects of chemical weathering. Granite and slate gravestones and building materials are much more resistant to weathering due to the strong silicon-oxygen bonds in silicate minerals.

The removal of the great weight of rock about the batholith, usually 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.

Weathering and Diamond Concentration. Diamond is the hardest mineral known and is also extremely resistant to weathering because of the very strong covalent bonding of carbon.

Diamonds are concentrated by weathering. Diamonds are brought to Earths surface in kimberlite pipes, columns of brecciated or broken ultramafic rock that have risen from the upper mantle. Diamonds are widely scattered in diamond pipes where they form.

Aridisols

Dry climates, low in organic matter. Having horizons of carbonate, gypsum and salt. CF are climate

Clay Mineral is a hydrous aluminum silicate with a sheet-silicate structure like that of mica. The entire silicate structure of the feldspar crystals 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.

If granite is chemically weathered, some of the original minerals are chemically changed into different minerals.

Feldspar will change into a clay mineral (with a crystal structure similar to mica).

Andisols

Formed in volcanic ash. CF are parent material.

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.

Frost heaving bulges the ground surface upward in winter, which can often break roads and leave lawns spongy and misshaped after the spring thaw.

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 (mountain tops). Partial thawing during the day adds new water to the ice in the crack; refreezing at night adds new ice to the old ice.

Alfisols

Gray to brown surface horizon. Clay accumulation. med to high in plant nutrient ions. Humid forests. CF- climate and organisms

Parent Rock

Is the source of the weathered mineral matter that makes up most of a soil. Granite will be sandy and as sand sized particles of quartz and feldspar are released from the granite. Feldspar grains weather completely, fine grained clay minerals are formed.

Spheroidal Weathering occurs where rock has been rounded by weathering from an initial blocky shape.

Its rounded because chemical weathering acts more rapidly or intensely on the corners and edges of a rock than on the smooth rock faces.

Earths atmosphere(mostly nitrogen and oxygen) contains .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 dissolve in water that percolates through soil. The openings in soil are filled a gas mixture that differs from air. Soil has a much more higher content of carbon dioxide(10%) than does air, because carbon dioxide is produced by the decay of organic matter and the respiration of soil organisms in the biosphere.

Frost Heaving

Lifts rocks and soil vertically. Solid rock conducts heat faster than soil, son on a cold winter day, the bottom of a partially buried rock will be much colder than soil at the same depth.

Chemical weathering is largely a function of the availability of liquid water. Rock chemically weathers much faster in humid climates in arid climates.

Limestone- (extremely susceptible to dissolution) weathers quickly and tends to form valleys in wet regions. In arid west, limestone is a resistant rock that forms rebids and cliffs. Temperature is also a factor in chemical weathering. The most intense chemical weathering occurs in the tropics(wet and hot). Polar regions experience little chemical weathering.

Silt particles are between clay and sand in size. A soil with approximately equal parts sand,silt,and clay is called a loam.

Loamy soils are well drained, may contain organic matter, and are usually very fertile and productive.

Weathering Products.

Look at powerpoint. Notes: 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 soil horizon containing only organic materail is

O

Ferromagnesian minerals such as olivine, pyroxene, and amphibole include other postiviely charged ions such as Al, Fe, Mg, and Ca. The presence fo these positively chargend ions int eh crystal framework makes these minerals vulnerable to chemical attack due to the weaker chemical bonding between these ions and oxygen, as compared to much stronger silicon-oxygen bonds.

Olivine weathers rapidly because its isolated silicon oxygen tetrahedra are held together by relatively weak ionic bonds between oxygen and iron and magnesium.

Other processes- several other processes mechanically weather rock (but are usually less effective than frost action and frost heaving).

Plant growth-roots growing in cracks can break up rocks, as can burrowing animals. Such activities help to speed up chemical weathering by enlarging passageways for water and air. Extreme temperature change(like in a forest fire or the desert) can cause a rock to expand until it cracks. The pressure of salt crystals forced as water evaporates inside small spaces in rock helps to disintegrate desert rock. No matter which process of mechanical weathering are at work, rocks disintegrate into smaller fragments, the total surface are increases, allowing more extensive chemical weathering by air and water.

Soil. Soil forms an essential interface between the solid Earth (geosphere), biosphere, hydrosphere, and atmosphere.

Regolith-loose, unconsolidated material that covers most of Earths surface. Soil is a layer of weathered, unconsolidated material that contains organic matter and is capable of supporting plant growth. A mature and 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.

Residual and Transported Soil.

Residual Soils develop from weathering of the bedrock beneath them. Transported soils do not develop from locally formed rock but from regolith brought in from some other region(That its not the soil itself that is transported but the parent material from which it formed). Wind deposits called loess form the base for some of the most valuable food producing soils. Transported soils are generally more fertile because the parent material is transported from many different locations there is more variety in the chemical makeup.

Carbon dioxide mixed with water makes a weak acid that causes most minerals to decompose this acid is the primary cause of chemical weathering.

Running water contributes to weathering and erosion by loosening and removing particles and by abrading rocks during transportation in streams. Freezing and thawing of water in cracks in rock is very effective at mechanical breaking them up.

Arid climate's soils are thin and are characterized by little leaching, scant humus, and the upward movement of soil water beneath land surface.

Salts are precipitated within the soil. Sand buildup within in the soil can be found in the desert alkali soils, heavily toxic sodium salts that prevent plant growth.

Differential weathering describes the tendency for different types of rock to weather at different rates.

Shale (composed of soft clay minerals) tends to weather and erode much faster than sandstone (composed of hard quartz mineral). [Layers of resistant rock tend to weather to form steep cliffs while soft layers form shallow slopes of eroded rock debris].

Biosphere. Plants can physically break apart rocks when they grow in cracks. When plants and animals decompose , they become mostly carbon dioxide. While carbon dioxide dissolved in rain makes the water slightly acidic, soil water is much more acidic due to the carbon dioxide provided by decaying plants and the respiration of soil organisms.

Soil is formed by weathering and includes organic matter from decayed plants. Organic carbon compounds and minerals released by weathering provide the nutrients required for plant growth.

As rock is destroyed through weathering, valuable products are produced.

Soil is produced by rock weathering, so most plants depend on weathering for the soil they need in order to grow. In a sense, all agriculture depends on weathering.

Soil Erosion- Soil accounts for an almost insignificant fraction of ll earth materials, its one of the most significant resources in terms of its effects on life.

Soil provides nourishment and physically supports human existence. Soil is abused and the earth has lost 10% of its productive value.

(the size and number of pore space-meaning the ability of a soil to transmit air and water- are largely a function of the texture of soil.

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. Clay minerals occur as microscopic plates and help hold water and nutrients of plants in soil.

Time- the character of soil changes with time.

Soil that has been weathering for a short time, the characteristics are largely determined by the parent material. Young soils can retain the structure of the parent material. In the long term, the only characteristic of the parent rock to have significance is the presence or absence of coarse grain of quartz. With time, soils tend to be thicker. A soil that has been buried by lava flow, volcanic ash, dust, glacial deposits is called a buried soil, paleosol. Such soils are distinctive and can be traced over wide regions.

Chemical weathering initially works along contacts between mineral grains. Tightly bound crystals are loosened as weathering products form at their contacts.

Solid minerals are not the only products of chemical weathering. Calcite dissolves when chemically weathered.

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.

How soil erodes- Soil particles are small and easily eroded by water and wind.

Splash erosion- raindrops strike unprotected soil and dislodge the soil particles. As rain continues, thin sheet of running water forms over the landscape, carrying the dislodged particles away(sheet erosion). Currents hat form in the sheet of water cut tiny channels called rills int eh exposed soils. The rills deepens into gullies. Wind erosion is less significant than that by water. Winds pick up clays, silts, organic matter, and transport them far away.

Some strong acids occur naturally on Earths 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. Strong acids also drain from some mines as sulfur containing minerals such as pyrite oxidize and form acids at the surface.

After centuries, the grains in granite may be loosened, cracks enlarged, and the surface discolored and dulled by the products of weathering.

Surface discoloration is also common on rocks outcrops, where rock is exposed to view with no plant or soil to cover. (Geologists carry rock hammers to break rocks to examine unweathered surface).

Climate- the most influential fact affecting sol thickness and character.

Temp and precipitation-determine whether chemical or mechanical weathering processes will dominate and strongly influence the rate and depth of weathering. Also determines amount and type of vegetation and animal life.

Mechanical Weathering intensity is also related to climate (temperature and humidity) as well as to slope.

Temperature climates where abundant water repeatedly freezes and thaws, promote extensive frost weathering. Steep slopes cause rock to fall and break up under the influence of gravity. Most intense mechanical weathering occurs in high mountain peeks (combination of steep slops, precipitation, freezing, and thawing, and flowing glacial ice rapidly pulverize the solid rock).

Tropical rain forests- the high temperature and rainfall combine to form extremely thick red soils called oxisols, or laterites.

That are highly leached and infertile.

Living organism

The biosphere plays an important role in soil development. The chief function of living organisms is to provide organic material to the soils. Decomposing plants form humus, which supplies nutrients to the soil and aids in water retention. Decaying plant matter releases organic acids that increase chemical weathering of rocks. Burrowing organisms bring soil particles to the surface and mix the organic and inorganic components of the soil. Bacteria, fungi and protozoa=promote decomposition of organic matter in humus.

hematite is the mineral that is formed from the iron oxide (brick red color when powdered). If water is present, hematite combines with water to form limonite (mostly amorphous, hydrated iron oxides, which also includes the mineral goethite; yellow/brown in color)

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.

Solution weathering: Some minerals are completely dissolved by chemical weathering. Calcite foes into solution when exposed to carbon dioxide and water.

The carbon dioxide and water combine to form carbonic acid, which dissociates into the hydrogen ion and the bicarbonate ion. Rain can discolor and dissolve statues and tombstones carved from the metamorphic rock marble, which is mostly calcite.

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.

Organic acids and carbon dioxide produced by decaying plants in the topsoil percolate down into the E horizon, Zone of leaching, and help dissolve minerals such as iron and calcium.

The downward movement of water in the E horizon carries dissolved minerals, and fine grained clay minerals into the soil layer below. This leaching(eluviaton) of clay and soluble mineral can make the E horizon pale and sandy.

Frost Wedging

The expansion of freezing water pries rock apart. 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 C

K-feldspar forms potassium ions. Na-feldspar and Ca-feldspar (plagioclase) form sodium ions and calcium ions.

The ions that result from the weathering of Ca feldspar are calcium ions and bicarbonate ions, both of which are very common in rivers and underground water.

Chemical Weathering (rock decomposition) transform rocks and minerals exposed to water and air into new chemical compounds.

The 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(balance) with the surrounding conditions.

Erosion

The picking up or physical removal of rock particles by an agent such as running water or glaciers. Weathering helps break down a solid rock into loose particles that are easily eroded.

An average soil is composed of 45% rock and mineral fragments(clay), 5% decomposed organic matter(humus) and 50% pore space.

The rock and mineral fragment proved an anchoring place for the roots of plants. Clay minerals attract water molecules and plant nutrients ions. The humus releases weak acids that contribute to chemical weathering of soil; also produces plant nutrients and increases the water retention ability of the soil. The pore space are the final essential components of fertile soil because water and air circulate through poor spaces carrying dissolved nutrients and carbon dioxide(necessary for plant growth).

Slope

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 downhills. Vegetation is sparse on steep slopes(there aren't many roots to hold the weathering rock in place and little nutrients is provided). Soils in bottomlands may be thick and poorly drained and waterlogged. Vegetation here does not decay and thick,dark layers of peat may form. Sloping uplands allows good drainage and minimal erosion and healthy vegetation.

The O horizon.

The uppermost layer that consist entirely of organic material. Ground vegetation, recently fallen leaves and needles, and decomposed plant material(humus) make up this horizon. The humus mixes with weathered mineral matter just below to form the A horizon.

The hydrogen ions given off by natural acids disrupt the orderly arrangement of atoms within most minerals.

These ions have a positive electrical charge, and can substitute for other positive ions. The substitution changes the chemical composition of the mineral and disrupts its atomic structure. The mineral decomposes, often into a different mineral, when its exposed to acid.

Soils in most climates tend to be thick and are generally characterized by downward movement of water through earths materials.

These soils tend to be fertile and have a high content of aluminum an iron oxides and well developed horizons. Marked by downward leaching due to high rainfall and to the acids produced by decay of humus.

Mechanical Weathering (physical disintegration)

This includes several process that break rock into smaller pieces. The change is rock is physical; there is little or no chemical change. Example: Water freezing and expanding in cracks cause rocks to disintegrate physically.

The material leached downward from the E horizon accumulates in the B horizon, zone of accumulation.

This layer is quite clayey and stained red or brown by hematite and limonite. Sometimes calcite. Subsoil. With this horizon, a hard layer of earth material called hardpan may form in wet climates where clay minerals, silica, and iron compounds have accumulated from the eluviation of the overlying E horizon. Hardpan is difficult to dig or drill through.

Soil Horizons: O, A, E, B, C. Soil Horizons are soil layers.

Topsoils are O and A. Then E. Subsoils are B. Then there is C. and after that, unweathered parent material.

After a rock fragment is eroded, it is then transported.

Transportation is the movement of eroded particles by agents such as rivers, waves, glaciers, or wind.

Weathering is a long and slow process.

Typically cracks in rocks are caused by frost acton or plant growth, and as a result more surfaces are exposed to attack by chemical agents.

The most important natural source of acid for rock weathering at Earths surface is dissolved Carbon dioxide in water.

Water and carbon dioxide form carbonic acid, a weak acid that dissociates into the hydrogen ion and the bicarbonate ion. Even though carbonic acid is a weak acid, it is so abundant at Earths surface, that it is the single most effective agent of chemical weathering.

Atmosphere. Oxygen and Carbon dioxide are important for chemical weathering. Water (evaporated from the hydrosphere and distributed as moisture, rain and snow) is critical to both chemical weathering and mechanical weathering.

Weathering has a dramatic 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 weathing, the elevated levels of carbon dioxide in the atmosphere would have long made Earth too hot to sutain life.

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.

Spodosols

acid soils low in plant nutrients ions. accumulation of humus (aluminum and iron). cool humid pine forests - sandy parent material . CF parent material, organism, and climate

Hardspan forms in the

b

Mollisols

black surface horizon rich in organic matter and plant nutrient ions. subhumid to semiariad midlat grasslands. CF are climate and organisms.

most effective agent of chemical weathering at earths surface is

carbonic acid, h2co3

most common end product of the chemical weathering of feldspar

clay minerals

vertisols

clayey soils that swell when wet and shrink when ry. deep cracks. CF parent material

The decomposition of rock from exposure to water and atmospheric gases is called

cw

Rate of erosion. Influenced by soil characteristics, climate, slope, and vegetation and type of rainfall.Coarse grained soils with organic content tend to have larger pore spaces and can absorb more water.

fast moving water does not infiltrate and has a greater ability to dislodge and transport soils particles down from the slope. Plant roots form networks. Thick vegetation reduces wind velocity.

Oxisols

heavily weathered soils low in plant nutrient ions, rich in aluminum and iron oxides, humid tropical climates- also called late rites. CF time and climate

The C horizon.

incompletely weathered parent material that lies below the B horizon. The parent material is commonly subjected to mechanical and chemical weathering from frost acton, plant acids, roots and other agents. This horizon is transitional between the unweathered rock or sediment below and the developing soil above.

Soils with approx equal amounts of sand, silt, and clay along with a generous amount of organic matter is

loam

When rocks that formed deep in Earth become exposed at the Earth's surface, they are altered by what?

mechanical and chemical weathering

Physical disintegration of rock into smaller pieces is called

mechanical weathering

Entisol

no horizon due to young age of parent material or constant erosion. CF are time and topography.

which is not a type of mechanical weathering: frost wedging, pressure release, frost heaving or oxidation

oxidation

Types of Mechanical Weathering

pressure release, frost action, and frost heaving are the most effective. Others include, plant growth, burrowing animals, extreme changes in temperature, and the pressure of salt crystals.

most common end product of the chemical weathering of quartz is

quartz dos not usually weather chemically.

Weathering

refers to the processes that change the physical and chemical character of rock at or near the surface.

ultisols

strongly weathered soils low in plant nutrients ions with clay accumulation. humid temp and tropical acid forest environments. CF are climate time and organisms

Frost Action

the mechanical effect of freezing water on rocks- commonly occurs at frost wedging or frost heaving.

Gelisols

weakly weathered soils with permafrost within 2 meters of the surface. CF is climate

The processes that affect rock are

weathering, erosion, and transportation

Histosols

wet, organic soils with little mineral material. swamps or marshes. CF is topography.

Inceptisols

young soils that have weakly developed horizons and little/no subsoil clay accumulation. CF time and climate