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
• 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.
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
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• 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
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
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?