1 weathering and rock breakdown gly 2010- summer 2015 - lecture 9

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Weathering and Rock Breakdown

GLY 2010- Summer 2015 - Lecture 9

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Weathering

• Two types - Physical, also known as mechanical Chemical, also known as disintegration

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Physical Weathering

• Faster when water is present, but proceeds at slower rates in dry climates

• Many different processes are possible - a few on shown on the following slides

• Breaks a rock into smaller pieces without affecting it chemically

• Increases the surface area of rocks

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Surface Area Increase

Weathering by Expansion

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Boulder Split by Frost Wedging

• Boulder split by the expansion of ice

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Adsorption

• Adsorption of fluid, usually water, can increase the volume, and lead to cracking

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Expansive Soils

• A: A sample of an expansive soil with moderate swell potential

• B: Same soil sample after a small amount of water - Notice the sample has expanded considerably

• C: Same sample 48 hours later, after the sample has had time to shrink to a smaller volume

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Damage Due to Swelling Clays

• Damage that might result from uneven expansion and contraction of soil containing swelling clays

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Building Damage

• Building damaged by expansion and contraction of clay minerals in the soil

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Plant Roots

• (Left) Roots grow into cracks in rock, causing expansion, and enlarging the crack

• (Right) Roots exposed in rock

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Thermal Expansion

• Rocks expand when heated, contract when cooled

• This process has been simulated in the laboratory Despite many rapid heat/cool

cycles, little effect was observed; likely not a very important process in nature

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Development of Exfoliation

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Granite Exfoliation

• Enchanted Rock State Natural Area, south of Llano, Texas

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Stone Mountain, Georgia

• Exfoliation at Stone Mountain, Georgia, about twenty miles from Atlanta

Photo: Jon Cook FAU Junior Field Camp, March, 2004

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Stone Mountain, Georgia

• Exfoliation Domes

Photo: Dr. Anton Oleinik, FAU Junior Field Camp, March, 2004

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Martian Aeolian (Wind) Abrasion

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Chemical Weathering

• Minerals, formed under pressure and/or thermal conditions different than those at the surface, may be unstable

• Chemical weathering changes the chemical composition of minerals that are unstable at the earth’s surface to minerals which are stable

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Chemical Weathering at Surface

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Weathering

Rock writing on chemically

weathered rock

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Hydration

• Water is added to a mineral, creating a new mineral

• Anhydrite, CaSO4, may add two water molecules to create gypsum, CaSO4 2H2O

• Addition of water leads to volume expansion (70% for the above case), which may lead to cracking of the rock

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Hydration of Obsidian

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Dissolution

• Ions or ionic groups are removed and carried away by water

• Ionic bonding is necessary

• Over millions to billions of years, this process is responsible for saltiness of the oceans

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Sunland Park Mall

El Paso, Texas

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Sunland Mall Marble Closeup

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Sunland Mall Marble Closeup

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Oxidation

• Addition of oxygen, often with a change of oxidation state

• Rusting is slow oxidation, while burning is rapid oxidation

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Hydrolysis

• Water, H2O, breaks down to yield ions

H2O H+ + OH-

• Responsible for the conversion of feldspars, the most common minerals in the earth’s crust, to clay, an important part of soil

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Ion Exchange

• Ions in solution exchange with ions held on the surface of minerals K+ (in solution) H+ (on clay) The reaction may later be reversed,

releasing potassium (K), an important nutrient for plants

• Important for the retention and later release of water and nutrients

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Factors Influencing Chemical Weathering

• Heat

• Water

• Time

• Stability of minerals Minerals weather in the reverse order

of Bowen’s Reaction series

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Biological Influences on Weathering

• Animals, plants and bacteria can influence weathering 1) Burrowing animals increase air and water

to surfaces 2) Decay of organic matter produces acids 3) Yellowboy is the product of bacterially

controlled chemical weathering

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Regolith

• When physical weathering dominates, rocks are broken into smaller and smaller pieces called regolith

• Regolith has few of the properties of soil - it is just broken rock

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Soil

• Soil is regolith which has been altered by weathering, and which may have had organic matter added to it

• Soil is essential to life as we know it, since plants need soil to grow, and animals ultimately depend on plants for food

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Parent Rock

• The parent rock is the rock from which regolith is derived

• Depending on their resistance to physical and chemical weathering, different parent rocks may form soil quickly or slowly

• The parent rock also determines how rich in nutrients the soil is

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Factors Influencing Soil Formation

• Climate

• Vegetation

• Topography

• Time

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Topography

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Soil Profile

• Cross-section of soil layering

• The layers represent different weathering zones, and are designated by letters

• Each layer is called a “horizon”

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O-horizon

• Upper layer, rich in organic matter Dead (leaf litter, etc.) Living (bacteria, algae, fungi, insects,

worms)

• Poorly developed in most areas of South Florida

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A-Horizon

• Inorganic mineral matter

• Humus, dark colored decomposed organic matter

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E-Horizon

• Light-colored mineral particles

• Zone of eluviation and leaching

• Eluviation is the washing out of fine soil components from the A-horizon by downward-percolating water

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B-Horizon

• Enriched by precipitation of minerals dissolved from O and A layers

• In arid regions, this may include a caliche layer Carbonate minerals deposited in quantity

because of high rate of evaporation Brief, heavy rains bring the carbonate

downward, and deposit it in B horizon Often impermeable

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C-Horizon

• Parent material which has been slightly weathered

• Retains most of its original appearance

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Example Soil Profile• A horizon extends to

bottom of third mark from top

• B horizon (with various subunits) below the third mark

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Lateritic Soil, Brazil

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Tropical Rainforest Soils

• Very poor soils; nutrients are tied up in the plants

• When a plant dies naturally, its nutrients support new replacement growth

• If the forest is cleared and the plant material removed, the nutrients are also removed

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Soil Taxonomy

• Naming of soils based on their characteristics Physical characteristics - named by

obvious physical characteristics (color, thick/thin, shrinkage/expansion, oxidation, etc)

Texture - description of texture

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Nutrient Depletion – Borneo Vs. Java

• The soil on Java is formed from fresh, nutrient-rich volcanic rock - potassium, calcium, and magnesium rich

• Java has a population density of 460 people per square kilometer

Losing Ground Video

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Java Pictures

• Jakarta at left• Cirebon above• Large cities on Java

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Borneo

• The soils on Borneo are from the weathering of intrusive granitic rock, gabbro intrusions, and andesitic lavas, and are poor in nutrients

• The population density is 2 people per square kilometer

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Borneo Countryside

• A jungle trail and a road leading to a small village in Borneo

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Importance of Soil to Man

• Soil erosion is the number one cause of dollar loss by natural events every year - surpassing hurricanes, earthquakes, tornadoes, etc