Focus Questions

• What are Earth's external processes, and what roles do they play in the rock cycle?
• What are the two types of weathering? In what ways are they different?
• What factors determine the rate at which rock weathers?

Earth's External Processes

• we have been focusing on the the endogenic system - internal processes that shape the Earth's crust (involving flows of heat & material from deep below the crust - powered by radioactive decay) - builds the landscape

• now, our focus will be on the exogenic system - external processes - weathering, mass wasting, & erosion - reduce the landscape

• the external processes are a part of the rock cycle because they are responsible for transforming rock into sediment

• "Denudation" (defn) - all processes that cause a degradation (wearing down) of the landscape: weathering, mass movement, & erosion

• the "dynamic equilibrium model" - landforms adjust toward a condition of equilibrium - e.g., tectonic uplift is balanced by denudation from weathering & erosion (which is influenced by the differential weathering rates of the crustal materials)

• Weathering - disintegration and decomposition of rock at the Earth's surface ..........

Mass Wasting - transfer of rock material downslope under the force of gravity

Erosion - incorporation and transportation of material by a mobile agent, usually water, wind, or ice

.................. loosening & transportation

• these three processes operate together

Weathering - 2 types: Mechanical(Physical) and Chemical

Mechanical Weathering:

• defn - the physical break-up of rock into smaller pieces. Each piece retains the characteristics of the original material

• defn - the processes by which the internal structure of a mineral is altered by the addition and/or removal of elements - i.o.w., the original rock is changed into a different substance

• chemical weathering and physical weathering work simultaneously in nature

• "regolith" - the upper surface of bedrock that is weathered into broken-up rock

• "bedrock" - the parent rock of regolith

• "sediment" - unconsolidated fragmental material (minerals) derived from the parent rock

• weathering is influenced by not only the character of the bedrock, but also by the climate (temp & precip)

• physical weathering predominates in dry, cool climates; chemical weathering dominates in warm, wet climates

• mechanical weathering breaks rock into smaller and smaller pieces, thereby increasing the surface area available for chemical weathering

"crystalization" - when water evaporates from the surface of rocks, salt crystals form from the dissolved mineral salts; the crystals grow & exert enough force to spread apart the individual mineral grains & begin breaking apart the rock

"hydration" - water is added to a mineral which swells & stresses the rock, mechanically forcing mineral grains apart

"frost wedging" - the freeze-thaw action of water breaks rock segments apart - "joint-block" separation can occur along preexisting joints & fractures - can produce discrete blocks of rock

"pressure-release jointing" ("unloading") - as deeply burried plutons experience uplift, the pressure of deep burial is lessened, the granite body responds by shedding layers of rock - curved plates of rock are peeled off - "exfoliation" - creating dome-shaped features - "exfoliation domes"

Thermal Expansion - the daily cycle of temperature (hot - cold) weakens rocks, esp. in deserts due to large diurnal temp. changes

Biological Activity (Organic) - weathering due to the activities of plants (roots), animals (burrowing), and humans

B. Chemical Weathering

• the internal structure of the rock is altered by the addition and/or removal of elements
• the original rock decomposes into substances that are stable in the surface environment
• these substances will remain unchanged as long as the surface environment remains unchanged

• water is the most important agent of chemical weathering
• pure water is nonreactive, however, if material is dissolved in the water it will become reactive
• for example, if oxygen is dissolved in water, materials will be oxidized - e.g., when iron is oxidized to form "iron oxide" (rust)
• carbon dioxide (CO2) dissolved in H2O will produce carbonic acid - a weak acid
• carbon dioxide is dissolved into water as rain falls through the atmosphere; or in the ground, as organic material decays, releasing CO2 in the vicinity of percolating ground water, where it is dissolved into the percolating water
• carbonic acid ionizes to form reactive hydrogen ions (H+) and bicarbonate ions (HCO3-)
• (an "acidic solution" contains the reactive hydrogen ion H+)

(1) dissolution

(2) oxidation - the oxygen dissolved in water oxidizes (combines) with metallic elements in rock to form oxides (e.g., iron oxide (rust)) - the oxide is weaker than the original mineral constituents

(3) carbonation - chemical weathering by a weak carbonic acid (CO2 dissolved in water) that reacts with many minerals (esp. limestone) containing calcium, magnesium, potassium, & sodium

• the acid readily decomposes these (above) rocks, and produces products (ions) that are water soluble
• from this process, large quantities of rock (esp. limestone) are dissolved and carried away by groundwater
• such soluble ions are retained in the groundwater supply

• hydrolysis is the primary process by which the silicates are decomposed

• the felsic minerals are more resistant to this form of weathering than the mafic minerals

• when the weaker minerals are dissolved by hydrolysis, the interlocking crystalline structure is compromised, and granular decomposition takes place

• recall that granite is comprised mainly of K feldspar and quartz
• when attacked by carbonic acid:

• the hydrogen ions attack and replace potassium ions, thereby disrupting the crystalline structure of the mineral
• the potassium then is available as a nutrient for plants, or becomes potassium bicarbonate (KHCO3), a soluble salt that can be incorporated into other minerals or transported in dissolved form to the ocean by streams and rivers

• comprise a high percentage of the inorganic material in soils
• they are the major constituent of the most abundant sedimentary rock - "shale"

• the silica in the feldspar is removed from the feldspar structure as well, and is carried away by groundwater
• the silica can precipitate to form nodules of chert or flint, or be used by microscopic animals in the ocean to build silica shells

• the quartz in granite is very resistant to chemical weathering, and will be released from the structure of the rock once the feldspar has been broken-down
• it is often carried to the ocean where it forms beaches and sand dunes - or "sandstone"

• "spheroidal weathering" - a chemical weathering process in which the sharp edges & corners of boulders & rocks are weathered in thin plates that creates a rounded, spheroidal form

• chem breakdown > with > temps & precip
• the felsic minerals are more resistant to Chem. weathering than the mafic minerals
• differential weathering is affected by: mineral makeup, degree of jointing, exposure to the environment