Log entry: environmental studies: earth science

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Chapter 10 Weathering and Mass Wasting

Visualizing Physical Geography by Timothy Foresman & Alan Strahler

Chapter Overview

Weathering

Slopes and Slope Processes

Mass Wasting

Weathering

All the processes that physically disrupt or chemically decompose a rock at or near the Earth’s surface.

Two types of weathering:

1. Physical weathering

2. Chemical weathering

Regolith = a surface layer of weathered rock particles that lies above solid rock, called bedrock. 

Mass wasting = downhill movement of soil, regolith, and bedrock under the influence of gravity.

From rock to soil

Weathering

The vulnerability of rock surfaces to weathering depends on:

How much of the rock is exposed

Existing climate conditions

Ability of water, air, and microscopic organisms to penetrate the exposed surface

Weathering

Joints

Pores

Weathering

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

How might the presence of joints affect the rate of weathering?

Joint-block disintegration

Weathering

When rocks with right angles appear in a landscape, this is a sign of probable _______.

a. granular disintegration

b. joint-block disintegration

c. lack of regolith

d. slow weathering processes

Granular disintegration

Physical Weathering

Breakup of massive rock (bedrock) by physical forces at or near the Earth’s surface (no chemical alteration of minerals)

Frost action

Frost cracking: water expands when it freezes, and fractures rock.

Dominant process in arctic and high-mountain environments.

Weathering

Physical Weathering

Salt-crystal growth = Water evaporates from sandstone pores, leaving salt crystals behind. Crystals grow and disintegrate rock. Important in dry areas.

Growth occurs in arid and semiarid regions.

Weathering

Physical Weathering

Exfoliation = Rock is exposed at the surface, releasing pressure, so rock expands slightly. Layers peel away.

Thermal Action: minerals expand when heated and contract when cooled; different minerals expand and contract at different rates; so internal stresses can crack rocks between mineral crystals

Biological Action: plant roots cracking rock, animal burrowing, etc.

Weathering

Chemical Weathering

The decomposition or decay of minerals in rocks by chemical reactions with water, oxygen, and acids.

Chemical reactions change minerals in rocks

Most effective in warm, moist climates

Chemical weathering includes:

Hydrolysis

Oxidation

Carbonation

Works with physical weathering

Weathering

Chemical Weathering

Oxidation = Oxygen and water react with metallic elements in minerals, making the minerals unstable and causing the rock to degrade.

Hydrolysis: minerals chemically

react with water

Carbonation

Carbonic acid—A weak acid that is formed when carbon dioxide dissolves in water

Dissolves limestone, creating caverns and sinkholes

Weathers buildings, tombstones

Weathering

Slopes and Slope Processes

Colluvium: debris at base of slope

Angle of repose

Slopes and Slope Processes

Angle of repose

DEFINITION: The angle of repose is the minimum angle at which any piled-up bulky or loose material will stand without falling downhill

FACTORS THAT INFLUENCE THE ANGLE OF REPOSE:

Individual material: the different coefficients of friction between different substances

Size and shape of the particles

Moisture: Moist sand has a much higher angle of repose than dry sand

Measuring angle of repose

Angle of repose for various materials

Material (condition)	Angle of Repose (degrees)
Ashes	40°
Asphalt (crushed)	30–45°
Bark (wood refuse)	45°
Bran	30–45°
Chalk	45°
Clay (dry lump)	25–40°
Clay (wet excavated)	15°
Clover seed	28°
Coconut (shredded)	45°
Coffee bean (fresh)	35–45°
Earth	30–45°
Flour (wheat)	45°
Granite	35–40°
Gravel (loose dry)	30–45°
Gravel (natural w/ sand)	25–30°
Malt	30–45°
Sand (dry)	34°
Sand (water filled)	15–30°
Sand (wet)	45°
Snow	38°
Wheat	28°

Source: Clover, Thomas J. Pocket Ref. Littleton, Colorado: Sequoia Publishing, Inc., 1998

Mapping slope stability

Rain

Mountain slopes

Slopes and Slope Processes

Mass Wasting

Earth materials moving down slope due to gravity

Factors such as the steepness of the slope, type of material, and water content determine whether the material:

Creeps

Falls

Slides

Flows downhill

Types of Mass Wasting

Creep

Rock falls

Landslide

Flows



Dry



Wet

Slow

Fast



Mass Wasting

Creep

Soil creep = extremely slow downhill movement of soil and regolith

Solifluction

Soils become thoroughly saturated with water and move slowly downslope

May occur in periglacial environments

Mass Wasting

Falls

Rockfall

Talus-at bottom of cliff

If the size of the rocks that make up a talus slope increases, the angle of the slope with respect to the ground will _______.

a. increase

b. decrease

c. stay the same

d. impossible to determine

Mass Wasting

Slides

Landslide = A rapid sliding of large masses of bedrock on a steep mountain slope or from a high cliff.

Landslides set off by:

Earthquakes

Sudden rock failures

Steep slopes

Mass Wasting

Flows

Mass wasting of Earth materials with high water content results in flows.

May occur in deserts and after wildfires.

Two main types:

Rotational slide

Transitional slide

Mass Wasting

Flows

Earthflow = The moderately rapid downhill flow of water-saturated soil, regolith, or weak shale. Hourglass shape.

Mass Wasting

Flows

Mudflow = A flowing mixture of water and soil or regolith that flows rapidly downhill.

Lahars: mudflow on slope of erupting volcano

Debris flows: mudflows that pick up rocks and boulders; fast moving



Mass Wasting

Induced Mass Wasting

Human activities such as roads and homes

Induced earthflow (septic fields and lawn irrigation)

Scarification-land disturbances to extract mineral resources

Mass wasting

Slides happened in 1995 and 2005 (above)

Slope stability factors

Mass wasting factors

Parent material (sand, soil, rock, etc.)

Moisture content

Type of strain

Rate of movement

Angle of repose factors

Density

Angularity

Shape

Size

Avalanche

Angle of repose plays key role for movement of snow on inclined surface (avalanche)

Slope conditions vary, but generally slope angle is between about 30o and 45o

Types include loose snow, slab

Conditions that cause include heavy snow, disturbance, etc.

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