Chapter 7:WEATHERING AND EROSION

Objectives• Describe the two major kinds of rock

weathering.• Identify three end products of weathering.• Explain the difference between weathering,

erosion, and mass wasting.• Describe how ice, water, and air transport

regolith across Earth’s surface.• Define and give examples of mass wasting

by slope failure and/or sediment flow.

Weathering-The First Step in the Rock Cycle

Weathering-The First Step in the Rock Cycle

• How rocks disintegrate– Weathering

• The chemical and physical breakdown of rock exposed to air, moisture and living organisms

– Regolith• A loose layer of

fragments that covers much of Earth’s surface

– Soil• The uppermost layer of

regolith, which can support rooted plants

The rock in the photo has weathered in place with little erosion, forming soil

Weathering: Two Types

• Mechanical engineering• The breakdown of rock

into solid fragments by physical processes

• Chemical composition of rock NOT altered

• Chemical weathering• The decomposition of

rocks and minerals by chemical and biochemical reactions

• Joints– A fracture of rock , along

which no appreciable movement has occurred

– Sheet jointing or exfoliation

– Frost wedging

• Abrasion– The gradual wearing

down of bedrock by the constant battering of loose particles transported by wind, water or ice

The jointing in these rocks has exposed new surface area which has broken and smoothed due to wind, water and ice.

Weatheringexfoliation and frost wedging

Frost wedging and biomechanical weathering

Chemical weathering• Dissolution

– The separation of materials into ions in a solution by a solvent, such as water or acid

– Rainwater acts as weak solution of carbonic acid

– Anthropogenic actions influence acidity of rainwater

The marble grave marker has been attacked by acidic rain because of the calcite composition. The grave marker on the right, while old, has not been dissolved because of its granite composition

Chemical weathering: ion exchange and the chemical breakdown of feldspar

Factors affecting weathering• Tectonic setting

– Young, rising mountains weather relatively rapidly

– Mechanical weathering most common

Factors affecting weathering• Rock composition

– Minerals weather at different rates

• Calcite weathers quickly through dissolution

• Quartz is very resistant to chemical and mechanical weathering

• Mafic rocks with ferromagnesian minerals weather more easily

Factors affecting weathering

• Rock structure– Distribution of

joints influence rate of weathering

• Relatively close joints weather faster

Factors affecting weathering

• Topography– Weathering

occurs faster on steeper slopes

• Rockslides

Factors affecting weathering• Vegetation

– Contribute to mechanical and chemical weathering

– Promotes weathering due to increased water retention

– Vegetation removal increases soil loss

Vegetation can both hold waterAnd increase weathering. If removedRocks may also be vulnerable to abrasion

Factors affecting weathering

• Biologic activity– Presence of

bacteria can increase breakdown of rock

Factors affecting weathering

• Climate– Chemical weathering is

more prevalent in warm, wet tropical climates

• Mechanical weathering less important here

– Mechanical weathering is more prevalent in cold, relatively dry regions

• Chemical weathering occurs slowly here

Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering, i.e. New York area

Factors affecting weathering:

color dots on map match colors on chart

Products of Weathering• Clay

– Tiny mineral particles of any kind that have physical properties like those of the clay minerals

– Clays are hydrous alumino-silicate minerals

Products of Weathering

• Sand– A sediment made of

relatively coarse mineral grains

• Soil– Mixture of minerals with

different grain sizes, along with some materials of biologic origin

– Humus– Partially decayed

organic matter in soil

Erosion and Mass Wasting

Erosion is the removal of weathering products from the source and most often occurs by water

• Erosion– The wearing away of bedrock and transport of

loosened particles by a fluid, such as water– Example: Sediment moved along the bottom of

a stream

Erosion and Mass Wasting• Erosion by wind

Particles of sand are transported close to the surface.finer particles of silt and clay can be transported great distances

• Erosion by ice– Glacier

• A semi-permanent or perenially frozen body of ice, consisting of recrystallized snow, that moves under the pull of gravity

Wind-blown fine sedimentssuch as this dust cloud canBe transported across oceans

Erosion and Mass Wasting

Left: deposits of unsorted glacial till from glacierRight: rock polished and striated by glacier

Erosion by ice:glacier removes, breaks and transports rock piecesglaciers scour valleys and deposit piles of debris as

moraines

Erosion and Mass Wasting• Mass wasting

– The downslope movement of regolith and/or bedrock masses due to the pull of gravity

• Slope failure– Falling, slumping or sliding of relatively coherent masses

of rock

Erosion and Mass Wasting:Rock slide, rock fall, and slumping result in downhill

transport of broken rock

Erosion and Mass Wasting

• Flow: If water or air combines with the downward movement, the regolith can “flow” downhill

• Creep– The imperceptibly slow downslope flow of regolith

• Unstable slopes move very slowly over long periods of time

Erosion and Mass Wasting

Why do major landslides occur near plate boundaries?

• Tectonics and mass wasting– World’s major

historic landslides clustered near converging lithospheric plates

• High mountains undergo rapid weathering

• Earthquakes near plate

boundaries can triggerlandslides

This massive slide was triggered byA magnitude 9 earthquake in Alaska

near a subduction zone.

Critical Thinking

• On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why?

• Why are some granite bodies extensively jointed, while others are essentially joint free?

Critical Thinking

• On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why?

• Why are some granite bodies extensively jointed, while others are essentially joint free?