Chapter 15 Weathering Mass Wasting Overview Weathering Rock

Chapter 15 Weathering & Mass Wasting

Overview • Weathering & Rock Openings • Weathering processes • Mass Wasting • Human-Induced Mass Movement

Denudation • Denudation is the overall effect of the disintegration, wearing down, and removal of rock material, thus changing the landscape. –This is accomplished by 3 processes • Weathering – larger rock into smaller components –Atmospheric & biotic • Mass wasting – short-distance down-slope movement of broken rock material –Gravitational influence • Erosion – removal, transportation, and deposition of fragmented rock material –Greater distances than mass wasting

• Different rock types weather at different rates • • • Igneous is hardest and most resistant Metamorphic is next step down Sedimentary weathers more easily and quickly than the other two types

Weathering & Rock Openings • Weathered rock is bedrock that has become exposed at the surface. –Different color and texture from non-weathered rock of the same type –Surface rock is “looser” than deeper rock –Cracks or crevices allow for weathering agents to change rock features

Types of Rock Openings • Microscopic – very numerous; responsible for extensive weathering • Joints – cracks that develop from stress; most common structural feature in rock; not always seen as a major feature of the landscape, commonly a few meters in size –Master Joints – joints in bedrock that run long distances through considerable thickness of rocks • Faults – breaks in bedrock that show signs of displacement; sometimes appear as major landscape features • Lava Vesicles – holes that develop in cooling lava as a result of gas being unable to escape to surface • Solution Cavities – holes formed in carbonate rocks (limestone) as minerals are dissolved and carried away by water; sinkholes can form as a result

Erosion along joint lines.

Types of Weathering • Mechanical weathering • • Also known as physical weathering Most occurs at or near the surface of a rock/landform Mechanical weathering results in rock fracturing Different types of physical weathering

Frost Wedging • • • Water percolates into joints in the rock Freezing of water causes expansion Stress from frozen expansion causes rock to break apart

Salt Wedging • • Similar process to frost wedging Key difference is the evaporation of water from joints leaves behind salt deposits • • • Salt crystals form and grow over time As the crystals grow, they increase pressure within the rock formation, thus resulting in fracturing Common among coastlines & arid regions

Temperature • • Temperature changes weather rock much more slowly than wedging Changes are very small • Temperature weathering functions through the forced expansion and contraction of rock materials • • This changing state in temperature weakens the strength of mineral grains in the rock Intense heating from wildfires can increase the effects of temperature weathering

Exfoliation • • Curved layers of bedrock “peel” away from bedrock Plutons rise through uplift • The feature that results is known as an exfoliation dome • The process gently smooth’s the landscape • Mainly occurs in granite formations

Chemical Weathering • • Decomposition of rock by chemically altering its minerals Requires moisture (H 2 O) • • • Most effective in humid regions Reactions are more rapid in high heat areas 3 main types of chemical weathering • • • Oxidation Hydrolysis Carbonation

Oxidation • Occurs when oxygen atoms combine with metallic elements in rock minerals to form new substances • • Iron Oxide (rust) is one example Common signs are a reddish-brown color to rock surfaces

Hydrolysis • The process where minerals chemically react with water. • • Newer compound produced is always weaker and softer than the original Breaks down silicates Igneous rocks are particularly susceptible Granular disintegration takes place

Carbonation • Occurs when carbonic acid forms as a result of carbon dioxide being dissolved in water. • • Carbonic acid dissolves minerals in the rock Common among limestone & marble formations

Biological Influence on Weathering