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Chapter 5: “Weathering, Soil, and Mass Movements”

5.1: “Weathering”

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Mechanical Weathering

• Occurs when physical forces break rock into small pieces without changing its mineral composition.– Each piece has the same characteristics as the

original rock.

• There are three physical processes in nature that are responsible for mechanical weathering.– Frost Wedging– Unloading– Biological Activity

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Frost Wedging

• Water seeps into the cracks of rocks and eventually it will freeze and expand.

• This processes causes the cracks to enlarge.

• After many freeze-thaw cycles, the rock breaks into pieces.

• Most common in mountainous regions in the middle latitudes.

• Sections of rock that are wedged loose may tumble into large piles called talus.– These typically form at the base of

steep, rocky cliffs.

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Unloading and Exfoliation of Igneous Rocks

• Large masses of igneous rock can be exposed to uplift and erosion of overlying rocks.

• Pressure on the igneous rock is reduced, called unloading, and this causes the outer rock layers to expand.

• These outer layers separate from the rest of the rock mass which creates exfoliation.

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Weathering and Biological Activity

• Activities of plants, burrowing animals, and humans can cause mechanical weathering.– Plants: roots grow into

cracks & are wedged apart as the plant grows.

– Burrowing animals: move rocks to surface to become exposed to weathering.

– Humans: blasting areas in search of minerals or creating new roads; deforestation.

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Chemical Weathering

• Chemical weathering is the transformation of rock into one or more new compounds.

• The most important agent in this type of weathering is water.– Water absorbs gases from the atmosphere and the

ground.

• Chemical Weathering of Granite– Weathering of potassium feldspar produces clay

minerals, soluble salt (potassium bicarbonate), and silica in solution.

– Quartz remains substantially unaltered.

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Chemical Weathering

• Weathering of Silicate Minerals– When silicate minerals undergo chemical weathering,

the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater.

– Produces insoluble iron oxides and clay minerals.

• Spheroidal Weathering– When water enters the joints in a rock, it weathers the

corners and edges most rapidly.• This causes the corners and edges of the rock to be more

rounded.• The rock takes on a spherical shape.

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Spheroidal Weathering

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Rate of Weathering

• Two other factors that affect the rate of weathering are rock characteristics and climate.– 1. Rock Characteristics

• Mineral composition and solubility• Physical features such as joints

– 2. Climate• Temperature and moisture are the most crucial factors.• Chemical weathering is most effective in areas with high

tempeartures and abundant moisture.– 3. Differential Weathering

• Caused by variations in composition• Creates unusual and spectacular rock formations and

landforms