Ch 5: Weathering, Soil, & Mass Movements

Rocks on the Earth’s surface undergo changes in appearance and composition

Weathering: Physical or chemical changes in rock material exposed at surface Mechanical- physically change rock into smaller pieces Chemical- breakdown of rock by changing its chemical composition

Types of Mechanical weathering

Types of Mechanical Weathering 1. Exfoliation- process where sheets of rock peel or flake away

Example of exfoliation:

Types of Mechanical weathering 2. Frost wedging- occurs when water seeps into rocks and freezes

Ice wedging

Types of Mechanical weathering 3. Biological activity- roots of plants and burrowing animals weather away rock

Weathering by plants

Weathering by sandmartins nesting in cliff

Types of Mechanical weathering 4. Abrasion- collision of rock with one another resulting in breaking and wearing away.

Abrasion refers to the breaking and grinding away of solid rock by collisions with moving particles. Abrasion takes place in many environments: fast-moving streams beaches subject to storm waves desert environments with high winds beneath glaciers that are loaded with fragments of rock.

Water-carried particles eating away at the rock

Storm waves have eaten away at the shoreline

The ocean waves have eroded this beach leaving behind a cliff as the soil was washed away

Observe the effects of mechanical weathering.

Types of Chemical Weathering

Types of Chemical Weathering 1. Hydrolysis: Change in composition of minerals when they react with water

Feldspar combines with H2O to form a common clay called kaolin

Types of Chemical Weathering 2. Carbonation – When some minerals come in contact with carbonic acid, they form a new product

Stalactites caused by carbonation Limestone is eaten away by acid and deposits as it drips from ceiling of cavern

Types of Chemical Weathering 3. Oxidation – When metallic elements combine with oxygen

Oxidation = rust!

Types of Chemical Weathering 4. Acid Precipitation “acid rain”

Types of Chemical Weathering 5. Plant acids – plants secrete acids that erode away the rock

Lichens and mosses grow on rocks and secrete weak acids that dissolve the surface

Rate of weathering depends on 4 things…

1. Rock composition Igneous and metamorphic rocks don’t weather easily Sedimentary rocks do

2. Amount of exposure More exposure it receives, faster it will weather Amount of time and amount of surface area exposed is also important

3. Climate Slow in very hot / very cold climates Fairly rapid in warm, humid climates

Cleopatra’s needle after only one century in New York City

4. Topography Elevation or slope of surface where rock is located affects rate of weathering

Results of Weathering

Results of Weathering: Humus Soil Bedrock Regolith

Humus Dark, organic material, remains of animals and plants

Humus:

Soil Complex mixture of minerals, water, gases, and remains of plants and animals

Regolith A layer of weathered rock fragments (covers much of Earth’s surface)

Bedrock Solid, unweathered rock that lies beneath regolith

Bedrock

Bedrock

Composition of Soil:

The proportion of different soil particle sizes Soil Texture: The proportion of different soil particle sizes Influences the soil’s ability to support plants Consists of 3 main types classified by particle size

Soil Composition Clays: less than .0002 mm in diameter (feldspar) Silts: between .0002mm - .06mm Sand: between .06mm – 2 mm (granite)

The portions of clay, silt, and sands depends on the parent material Parent material or “parent rock” is the rock from which the soil was weathered.

CLAY

SILT

SAND

Soil Texture Diagram Loamy soils are the best for growing plants

Soil profile: A cross-section in which the layers of the soil and bedrock can be seen Each layer is called a horizon. In fully developed residual soil, there are three horizons

Humus & Topsoil A B Subsoil C R Regolith Parent Rock

A horizon (topsoil) B horizon (subsoil) mixture of organic and small rock particles B horizon (subsoil) contains minerals and clay

partially weathered bedrock C horizon (regolith) partially weathered bedrock

Types of Soil Climate is most important factor influencing soil formation

Tropical (wet & warm) climate: Laterite soil (thick and infertile) *rain always is washing away the A horizon but constant rotting vegetation covers the B Horizon

Laterite

Desert (dry) Climate Forms soils from mechanical weathering Soil is thin and mostly of regolith

Temperate climates Temperatures range from cool to warm Rain fall is not excessive Two types of soil are found dependent on amount of rainfall:

Soil types in temperate climates: 2. Pedalfer – soil formed if an area receives 65 cm or more of rain per year (clay, quartz, iron) -found in places east of the Mississippi 3. Pedocal – soil formed if rainfall is less then 65 cm a year (calcium carbonate)

Pedalfer

Pedocal

Soil and Topography The configuration of the land (topography) plays a role in soil formation.

Thinner on slope because water erodes soil & deposits it down the slope

Tends to be thicker here (deposited)

Mass Movements

Mass Movements The transfer of rock and soil down slope due to gravity The combined actions of weathering and mass movement produce most landforms Streams and valleys are the most common landforms on earth Most mass movements end up in a stream or valley

Triggers of Mass Movements Water – heavy rain and snow saturate the ground Oversteepened Slopes – angles 25 to 40 degrees will hold loose particles. If the slope is greater than 40 degrees slips become less likely. Removal of Vegetation – plant roots stabilize Earthquakes

talus: a pile of rock fragments that accumulates at the base of the slope in piles

Types of Mass Movements Geologists classify mass movements based on 3 things: Kind of material that moves How it moves Speed of movement

1. Rockfall: rocks or rock fragments fall freely through the air is the most rapid Common on steep slopes & can trigger other mass movements

2. Slide: a block of material moves suddenly along flat, inclined surface

Hinsdale County, Colorado 700 years old and still moving Slumgullion landslide Hinsdale County, Colorado 700 years old and still moving

Rockslide: slides including segments of bedrock

3. Slump: downward movement of a block of material along a curved surface Leaves a crescent-shaped cliff

A slump is a type of slope failure which involves the rotational movement of soil or rock

4. Flows: mass movements of materials containing large amount of water

Earthflow – moves slowly Mudflow – moves quickly

5. Creep: slowest type of mass movement. (freezing & thawing)