Download Geoscience Earth`s external processes Frost wedging Unloading

Earth's external processes
Geoscience
Chapter 4 notes
“Weathering, Erosion, and Mass
Wasting”
Weathering
Two kinds of weathering
Weathering – the disintegration and
decomposition of material at or near the
surface
Erosion – the incorporation and
transportation of material by a mobile agent,
usually water, wind, or ice
Mass wasting – the transfer of rock material
downslope under the influence of gravity
Frost wedging
• Mechanical weathering
• Breaking of rocks into smaller pieces without altering the
chemical composition
• Processes of mechanical weathering
• Frost wedging
• Salt wedging
• Unloading/exfoliation/sheeting
• Thermal Expansion
• Biological activity
Unloading and exfoliation
of igneous rocks
1
Exfoliation/Sheeting
Shark’s tooth, Mt Bierstadt, CO
Sheeting/Exfoliation
Chemical Weathering of Granite & Silicate Minerals
Chemical Weathering
• Alters the internal structures of minerals by
removing or adding elements
• Most important agent is water
– Oxygen dissolved in water oxidizes materials
– Carbon dioxide dissolved in water forms
carbonic acid and alters the material
Spheroidal Weathering
• Granite contains mainly quartz and
orthoclase as well as other silicate minerals
When chemically weathered:
• Orthoclase (K-feldspar) produces clay
minerals, soluble salt (potassium
bicarbonate), and silica in solution
• Quartz remains unaltered
• Silicate minerals produce insoluble iron
oxides (red, rusty material) and clay
• End result: small quartz grains, salt, and
clay
Spheroidal Weathering at Elephant
Rocks
• The gradual rounding
of the corners and
edges of angular
blocks due to chemical
processes
2
Rates of weathering
• Mechanical weathering aids chemical weathering
by increasing surface area
Rates of weathering
• Important Factors that influence the rate of
weathering
• Rock Characteristics
• The kinds of minerals and the type of rock
determines how fast the rock will weather
• Sedimentary rocks will weather faster than igneous
and metamorphic rocks
Rates of weathering
Rates of weathering
Important factors (cont.)
• Climate
• Temperature and moisture are the most crucial
factors
• Chemical weathering is most effective in areas of
warm temperatures and abundant moisture
Differential weathering
• Caused by variations in rock and mineral
composition
• Creates unusual and spectacular rock formations
and landforms
Differential Weathering – Phipp’s Arch
Chemical Weathering
3
Weathering creates ore deposits
• Differential
Weathering (dike)
South San Juan
Wilderness, CO
Effects of Weathering: Fresh roadcut
Process called secondary enrichment
1. Concentrates metals into economical deposits
2. Takes place in one of two ways
a. Removing undesired material from the
decomposing rock, leaving the desired
elements behind
b. Desired elements are carried away and
deposited
Examples
1. Bauxite, the principal ore of aluminum
2. Many copper and silver deposits
Same roadcut 10 years later
Same roadcut 25 years later
Mass Wasting
• The downslope movement of rock, regolith,
and soil under the direct influence of gravity
– Regolith: weathered rock and mineral
fragments
• Gravity is the controlling force
4
Mass Wasting (cont)
• Important triggering factors are:
1. Saturation of the material with water
a. Destroys particle cohesion
b. Water adds weight
2. Oversteepening of slopes
a. Unconsolidated granular particles assume a
stable slope called the angle of repose
- slope’s angle of repose is generally
25-40 degrees depending on the material
b. When the slope angle is greater than the
angle of repose, the slope is
“oversteepened” and becomes unstable
Triggering Factors (cont)
3. Removal of anchoring vegetation
- plants keep the soil and ground stable
4. Ground vibrations from earthquakes
Types of mass wasting processes
1. Generally each type is defined by
a. The material involved
1. Debris
2. Mud
3. Earth/land
4. Rock
b. The movement of the material
1. Fall (free-fall of pieces)
2. Slide (material moves along a well-defined surface)
3. Flow (material moves as a viscous fluid)
c. The rate of the movement
1. Fast
2. Slow
Forms of mass wasting
•
•
Slump
1. Fast/Rapid
2. Movement along a curved surface
3. Due to oversteepened slope
Rockslide
1. Fast/Rapid
2. Blocks of bedrock move down a slope
3. Forms Talus Slopes
- a pile of rock and/or debris at the base of a
slope
Slumping in regolith
5
Rio Grande River near Taos New Mexico
Slumping of a mesa top
Shows how stream valleys widen due to Slumping
and Mass Wasting episodes
Talus Slope
Rock Avalanche
• http://www.youtube.com/watch?v=ZVYGJ
YnJTi0&feature=results_main&playnext=1
&list=PL7E10FDF17F691CB7
6
• Debris flow (mudflow or mudslide)
1. Fast/Rapid
2. Flow of debris with water
3. Often confined to channels
4. Serious problem in dry areas with
heavy rains
5. Debris flows composed mostly of
volcanic materials on the flanks of
volcanoes are called lahars
• Earthflow (landslide)
1. Fast/Rapid
2. On hillsides in humid regions
3. Water saturates the soil
4. Liquefaction - a special type of earthflow
sometimes associated with earthquakes
Rock Flow
Gros Ventre debris flow in Wyoming
Debris flow
7
Earthflow
• Creep
1. Slow movement of soil and regolith
downhill
2. Causes fences and utility poles to tilt
• Solifluction
1. Slow
2. In areas underlain by permafrost
- permanently frozen ground
3. Upper (active) soil layer becomes
saturated and slowly flows over a frozen
surface below
Ground subsidence in
Alaska due to solifluction
8