Copyright (c) 2005 Pearson Education Canada, Inc.5-1 PowerPoint Presentation Stan Hatfield....

Copyright (c) 2005 Pearson Education Canada, Inc. 5-1

PowerPoint PresentationStan Hatfield . Southwestern Illinois College

Ken Pinzke . Southwestern Illinois College

Charles Henderson . University of Calgary

Chapter 5

Weathering and Soil

PowerPoint Presentation

Stan Hatfield . SW Illinois College

Ken Pinzke . SW Illinois College

Charles Henderson . University of Calgary

Tark Hamilton . Camosun College

Copyright (c) 2005 Pearson Education Canada Inc. 5-2

Earth’s External Processes

Weathering - the mechanical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface

Mass wasting - the transfer of rock and soil downslope under the influence of gravity

Erosion – the physical removal of material by mobile agents such as water, wind, or ice


Weathering

Two predominant types of weathering• Mechanical weathering - breaking of rocks into

smaller pieces by physical forces• Chemical weathering – involves chemical

transformation of minerals in rock into one or more new mineral compounds plus dissolved ions


Mechanical Weathering• Five types of mechanical weathering

– Frost wedging – repeated freezing and thawing of water in fractures and cracks promotes the disintegration of rocks; causes great destruction of Canada’s highways every spring

– Unloading – joints & exfoliation as sheets of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure

– Thermal expansion – alternate expansion and contraction due to heating and cooling, forest fires, winter, solar flares on Mercury & Moon’s Dayside!

– Bioturbation – disintegration resulting from bacteria, plants (roots) and animals: bugs, clams, bears


Frost Wedging!


Unloading Joints, Devonian GraniteGreen Point NS. (Photo by Bruce Railsback)


Sheet Joints: Stone Mountain Gerogia


Thermal Spallation: Fires, Intrusions, Drilling


Bioturbation: Root Wedging & Mining!


Mechanical Weathering

Mechanical weathering increases the surface area exposing more surfaces to chemical weathering.

This can lead to spheroidal weatheing, and curved rocks from original planar joints & fractures.


Chemical Weathering

Chemical Weathering• Breaks down rock components and internal

structures of minerals• Most important agent involved in chemical

weathering is water (responsible for transport of ions and molecules involved in chemical processes)


Rates of WeatheringClimate

Temperature and moisture are the most crucial factors affecting chemical weathering rate

Water (polar molecules) is the best solvent for ions

Chemical weathering is most effective in areas of warm, moist climates, deep red laterite soils

In the tropics rocks are weathered over 100 m deep

This is why bauxites all form in the tropics

Chemical weathering is ineffective in alpine & polar regions because frigid temperatures keep moisture locked up as ice (permafrost, glaciers), regolith

Different times in Earth history, Different climate


Four major processes of chemical weathering• Oxidation

– Any chemical reaction in which a compound, atom or ion loses electrons: Fe 2+ Fe 3+ + e-

– Important in decomposing ferromagnesian minerals in air

• Hydrolysis– Smaller, acidic Hydrogen ions H+ replaces other positive ions

• Hydration– The addition of water to minerals or the solvation of their ions

• Dissolution– Further aided by small amounts of acid in the water

– Soluble ions are retained in the underground water supply

– Ions eventually concentrate in the ocean or playa lakes

Chemical Weathering


Chemical Weathering


Alterations Caused by Chemical Weathering

• Pyrite is the most unstable common mineral weathering to limonite & sulphuric acid (ARD)

• Decomposition of unstable minerals (olivine, glass)• This creates all soils and many sediments• Generation or retention of materials that are stable

– Sheet Silicate Clays, Oxides, Hydroxides

• Physical changes such as the rounding of corners or edges caused by water having greater access to corners by flowing through joints; is called spheroidal weathering


Alterations Caused by Chemical Weathering

Spheroidal weathering of extensively jointed rock.


Weathering Rind on Limestone Boulder


Acid Rock Drainage


Salt Wedging

Salt Wedging – A special type of mechanical plus chemical weathering

Arid landscapes where evaporation > precipitation

Dry coastal areas where seaspray can evaporate

Evaporation concentrates salts in crack & joint tips

Dried salts are hygroscopic (pulls moisture from air)

Chemical weathering wedges between mineral crystals


Rates of Weathering

Rock Characteristics• Rocks containing silicate minerals (granite, gneiss,

sandstone) are relatively resistant to chemical weathering

• Evaporite deposits of halite, gypsum, anhydrite dissolve & can form caves or cause collapse of overlying landscapes

• Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions & form caves (karst)

• Shales are compacted rather than cemented so they hydrate & fall apart readily. They weather recessively.

• Fine grained and glassy volcanic rocks weather readily & tend to generate rich soils quickly

• Silicate minerals weather in the opposite order from their order of crystallization (see Bowen’s Reaction Series)


Dissolution of Joints in Anhydrite, Iowa


Differential Weathering

• Masses of rock do not weather uniformly due to regional and local factors

• Results in many unusual and spectacular rock formations and landforms, such as the hoodoos of Drumheller, Alberta

• The number and spacing of joints can be a significant factor

• Mudstone versus sandstone• Variable cementation or cement solubility


Honeycomb Weathering


Hoodoos & Badlands


Soil

Soil has been called the bridge between life and the inanimate world

• Soil is Life’s (bacteria’s) modification of rocks to make them more useful, supply nutrients, hold water

Soil is an important interface; a common boundary where different parts of a system interact

• Soil is that portion of the regolith (rock and mineral fragments produced by weathering) that supports the growth of plants; it is a combination of mineral and organic matter (humus), water, and air


Soil

Composition (by volume) of a soil in good condition for plant growth.


Controls of Soil Formation

Parent Material• Residual soils – parent material is the underlying

bedrock e.g. Regolith, Laterite, Bauxite• Transported soils – forms in place on parent

material that has been carried from elsewhere and deposited e.g. Alluvial, Glaciomarine


Time and Climate• Time

– Important in all geologic processes

– Amount of time for soil formation varies for different soils depending on geologic and climatic conditions

• Climate– Most influential control of soil formation

– Key factors are temperature and precipitation



Plants and Animals and Slope• Plants and animals

– Organisms influence the soil’s physical and chemical properties

– Also supply organic matter to the soil; mostly plants– Microorganisms like fungi and bacteria play an active role

in the decay of plant and animal remains

• Slope– Steep slopes often have poorly developed soils– Optimum terrain is a flat-to-undulating upland surface– South facing slope in Northern Hemisphere will receive a

great deal more sunshine, which will influence soil character




The parent material for residual soils is the underlying bedrock; soils thin where slopes become steeper.


The Soil Profile

• Soil forming processes operate from the surface downward

• Vertical differences are called horizons – zones or layers of soil

• There are five distinct horizons designated as O, A, E, B, and C


• O horizon – organic matter (humus)• A horizon – organic and mineral matter

– High biological activity

– Together the O and A horizons make up the topsoil

• E horizon – little organic matter– Zone of eluviation and leaching

The Soil Profile


• B horizon – zone of accumulation

• C horizon – partially altered parent material

The O, A, E, and B horizons together are called the solum, or “true soil”

Well developed soil profiles are called mature and indicate environmental conditions have been stable for a long time; some soils lack horizons and are called immature

The Soil Profile


The Soil Profile

Well-developed soils show distinct layers called horizons.


The Soil Profile

Idealized soil profile from a humid, mid-latitude climate.


Soil Types

The characteristics of each soil type primarily depend on the prevailing climatic conditions

Three very generic soil types• Pedalfer

– Accumulation of iron oxides and Al-rich clays in the B horizon

– Organic-rich soil


Three very generic soil types• Pedalfer continued

– Best develops under forest vegetation– Develops in humid, temperate climate

• Pedocal– High accumulations of calcium carbonate– Calcium carbonate cemented soils known as

caliche or hardpan– Less clay than Pedalfer– Associated with dry grasslands and brush

vegetation as seen in the prairies

Soil Types


Caliche: Hardpan soils, Perched water tables


Three very generic soil types• Laterite

– Hot and wet tropical climates

– Intense chemical weathering

– Intense leaching removes soluble material

– Concentrates iron and aluminum; the former giving these soils a distinct red colour

Soil Types


Laterite: Triassic-Modern, Gainesville GA


Generalized descriptions of Canadian soil types as recognized by the Canadian Soil Classification System.

Soil Types


Generalized distribution of Canadian soil types.

Soil Types


Ancient Soils• Paleosols

– Serve as valuable indicators of land areas

– To be preserved, paleosols must reside in an area where it can be buried (lowland areas)

– Oldest are Proterozoic (2 billion years old) and indicate that oxygen levels had reached a critical level to allow the rusting of iron

Soil Types


Paleosol: a fossil or buried soil


• Recycling of Earth materials; part of rock cycle

• Natural rates of soil erosion depend on– Soil characteristics

– Climate

– Slope– Type of vegetation

Soil Erosion


Rates of Erosion• In many regions the rate of soil erosion is

significantly greater than the rate of soil formation• Single cropping agriculture loses cm/yr!• Swidden (slash & burn) & clearing rainforests • Availability of good soils is critical for rapidly

increasing world’s population• Sedimentation and chemical pollution

– Related to excessive soil erosion– Occasionally soil particles are contaminated with

pesticides

Soil Erosion


The Dirty End of Chapter 5

Copyright (c) 2005 Pearson Education Canada, Inc.5-1 PowerPoint Presentation Stan Hatfield....

Documents

Transcript of Copyright (c) 2005 Pearson Education Canada, Inc.5-1 PowerPoint Presentation Stan Hatfield....