Weathering

Weathering- chemical (altered or dissolved minerals) and physical (fragmented rocks) processesthat break down rocks IN PLACE at the surface.

Erosion- loosen and move soil and rock downhill or downwind.

Some rocks weather more rapidly than others

Different minerals - feldspar crystals corrode at earth's surface, quartz crystal do not. calcite completely dissolves

Structure of rock - shale splits easily along bedding planes.

Climate - High temperatures and heavy rainfall speed chemical weathering; Physical weathering my be active in climates with little chemical weathering (e.g., deserts)

Soils - fragments of rock, clays formed by chemical weathering and organic material. Soil retains water and hosts organisms which speed weathering

Length of Exposure - More time, more weathering

Chemical Weathering - some minerals dissolve and others combine with water, oxygen, andcarbon dioxide to from new minerals

Feldspar to Kaolinite Clay

Feldspar + Carbonic Acid + Water -> Kaolinite + dissolved ions

Dissolved Carbon Dioxide makes water slightly acidic (e.g., Coca Cola)

Physical weathering and high temperature makes the process faster by increasing surface area and reaction rate

Chemical weathering makes physical weathering faster by making the rock easier to break

Other Silicates

Amphibole and mica weather to clays

Olivine (fast), pyroxene (fast) and quartz (slow) completely dissolve.

Iron Silicates to Iron Oxides (rust/red soils of Georgia)

Dissolution of Carbonates (Solid completely dissolves)

calcite + carbonic acid -> calcium ion + bicarbonate ion

Chemical Stability - tendency of a compound to spontaneously react to become a different chemical compound. Chemical stability is related to environment (e.g., temperature, water). At the Earth's Surface, Chemical Stability of Silicates is roughly inversely proportional to Bowen's reaction series. Olivine and Ca-Feldspar are unstable. Quartz is stable.

Minerals that form at High Temperature and High Pressure (olivine) are unstable at the earth's surface.

Fe, Mg, Ca rich and simple silicate structures (isolated, chain) are less stable. More ionic bonds which are not as strong as covalent bonds.

Calcite is other common rock forming mineral that weathers rapidly

Physical Weathering

Natural Zones of Weakness - Bedding Planes, fractures, joints

Activity by Organisms - tree roots

Frost Wedging - expansion of freezing water.

Mineral Crystallization - calcite, gypsum grow in cracks and pry them open (pyramids)

Exfoliation and Spheroidal Weathering - cracking and splitting off of layers from an outcrop or boulder (like layers of an onion).

Soils: Residue of Weathering - on moderate or gentle slopes a layer of loose, heterogeneous weatheredmaterial over bedrock. Weathered and unweathered parent rock, clays, metal oxides,and organic matter.

Soil Profile

A-Horizon - top layer, contains clays, quartz & organic matter

B-Horizon - precipitation of soluble minerals from A-Horizon, iron oxides, sparse organic matter

C-Horizon - slightly altered bedrock mixed with clays

Climate, Time, and Soil Groups

Wet: Laterites - thick soils, upper layer is completely leached of soluble and easily weathered minerals. Not productive for farming.

Dry: Pedocals - thin soils, much unweathered minerals, soils rich in calcium, low in organics

Temperate: Pedalfers - abundant, insoluble minerals, clays, and iron alteration products. Good for agriculture.

Weathering, Erosion and Humans

Agriculture, building, road construction accelerates erosion because they break up the soil and eliminates erosion-resistant natural plant cover

Soils may be long term sources of pollution from salt, pesticides and petroleum products.

2 billion tons of topsoil are lost to erosion each year in U.S. and Canada. Soils take thousands of years to form.

Mass Wasting

Mass Movement - downhill movementsof masses of soil, rock, or mud due to gravity.

What Makes Masses Move? Mass movements occur when the force of gravity exceeds the strengthof slope materials. Mass movements can be triggered by earthquakes, floods, or volcaniceruptions

Unconsolidated Materials

Angle of Repose - characteristic slope of a pile of loose material. A slope larger than the angle of repose will eventually collapse. 35° for dry sand.

Surface Tension - angle of repose also varies with amount of moisture. A little moisture creates surface tension which binds particles together

Consolidated material - steeper slope angles but can still collapse if oversteepened or vegetation removed. Material held together by cohesion and adhesion.

Water Content - Saturated ground has a lower internal friction (less cohesion and adhesion) and particles move past each other. Pressure of water in pores can separate grains and cause the material to move like a fluid. Root systems hold soil together and limit water invasion

Steepness and Instability of Slopes - Steeper the slope the stronger the pull of gravity. Bedding planes may be zones of weakness.

Triggers for Mass Movements - heavy rainstorms, earthquakes, volcanos or steepening of slope.

Classification of Mass Movements - nature of the material, speed of movement, and nature of movement(slide or flow).

Rock Mass Movements - rapid downhill movement of bedrock, usually on mountains.

Rockfall - free fall from cliff or steep mountainside. Pile of debris (Talus)

Rockslide - slides down a slope as a unit.

Rock Avalanches - flows with internal deformation. Very destructive because large volumes (0.5 million cubic meters) and high velocities (km/hr).

Unconsolidated Mass Movements - Mixtures of soil, broken up bedrock, vegetation (Debris). Usually slower than rock movements. Less Topography

Creep - very slow (mm/yr), bends trees or fence posts, cracks house foundations.

Earthflows - fluid movement of fine grained material (soils or weathered shales)

Debris flows - fluid movement of rock fragments and mud

Mudflows - fluid movement of fine material and large amounts of water. Fast (km/hr). Common in hilly arid regions after a heavy rain.

Debris Avalanches - fast downhill movement of water saturated soil and rock usually in humid mountainous regions.

Slump - slow slide of unconsolidated material that travels as a unit along a concave upwards basal surface.

Debris Slides - rock material and soil moves as one or more units along planes of weakness, such as waterlogged clay layer.

Natural Causes of Landslides (1925 Gros Ventre, Wyoming)

Heavy rain and snow melt.

Undercutting of overlying sandstone by Gros Ventre River

Slid down slippery, water saturated shale bed below sandstone

Slide materials formed a dam on the river creating a lake. Two years later the lake overflowed, breaking the dam and flooded the valley below.

Human Activities that Promote Slides

Oversteepening slopes - road cuts or other construction

Exposing weak units (clays) to water - road cuts or other construction, dams (e.g., Vaiont, Italy in 1963).

Preventing Landslides - Most natural mass movements cannot be prevented, but we can control construction and land development to minimize losses

Avoid construction in mass movement prone areas. Look for signs of old slides

Don't oversteepen slopes or remove all the vegetation

Engineer water drainage so slope materials don't become water logged