Weathering,Soil,

Mass Movements,&

ErosionChapter 12 Section 1

Weathering• The breakdown of rocks at the Earth’s surface

• 2 types:

• Mechanical

(physical)

• Chemical

Mechanical Weathering: (disintegration) occurs when rock is split, cracked or broken into smaller pieces of the same material without changing its composition

Types of Physical Weathering•Frost wedging: caused by alternating freezing & thawing

•also called frost action•Water expands as it freezes and widens cracks in rock

•Common where temp. varies from below to above freezing

•Occurs mostly in porous rocks & rocks with many cracks

•Abrasion: rocks rub or bounce against each other when moved by wind, water, ice, or gravity.

•Worn away and broken into smaller pieces

•By water

smooth, rounded particles

•By wind

flat, angular surfaces,

cuts, grooves

•ventifaction

http://video.google.com/videoplay?docid=-7710596677653038828&q=moving+river&hl=en

•Biologic Activity:•plant roots grow into cracks and expand•animals dig holes in the soil

•allow water and air to reach bedrock and weather it•may bring rocks to surface where they weather more quickly

•Exfoliation or jointing:

•As overlying layers of rock are removed, pressure is released and rocks can expand.

•Rocks break along curved joints…parallel to surface

•rock peels off like layers of an onion

• Chemical Weathering: (decomposition) occurs when rock is broken down by chemical action and its minerals are changed into new substances• Occurs almost everywhere because of

presence of water or water vapor• Other agents include acids and oxygen

• Hydrolysis: Chemical weathering by reaction of water with other substances

– Minerals (such as feldspar, hornblende, augite) dissolve into ions and react with water to form clay minerals

– Water’s chemical effects is increased by the presence of acids dissolved in water

Water and Chemical Weathering

•Rainwater is naturally acidic

•carbon dioxide dissolved in water produces carbonic acid

•The acid in rainwater reacts with & dissolves some rocks

•Ex. Marble & Limestone

•Pollution Gases: from volcanoes, cars (nitrogen oxides), or power plants (sulfur dioxides) can make rainwater even more acidic (ACID RAIN)

Caves are carved out of limestone as water flows through them

•Other rocks are very resistant to acid and do not dissolve

•Ex. Quartzite

Organic Lichens: fungi + algae that make acids which can dissolve rock

Acids formed by the decay of dead plants & animals can contribute to chemical weathering.

Oxygen and Chemical Weathering•Oxidation: Oxygen reacts with minerals in rock

•Ex. Iron + Oxygen = Rust (iron oxide)

•Red hematite

•Yellowish-brown limonite

Rates of Weathering•Under normal conditions weathering is slow.

•Several factors affect the rate of weathering.

•Surface area

•Composition of the rock

•Climate

Surface Exposure & Weathering Rate• Breaking rocks into smaller pieces exposes more

surface area and causes them to weather faster.

Composition of Rock• Different types of rocks

weather at different rates.– Determined by:

• the rock’s mineral composition• type of weathering

• More resistant (quartz-based) rocks weather slower than less resistant (feldspar, hornblende, biotite mica, augite, calcite, gypsum-based) rocks

Climate & Weathering Rate

•Warm & Humid (wet):

•more chemical weathering

•higher temp. speeds up chemical reactions

• Arid (dry):

• slow weathering

• Mostly mechanical (by wind)

•Cold & Humid (wet):•more physical weathering

•mostly frost action

Weathering,Soil,

Mass Movements&

ErosionChapter 12 Section 2

What is soil and how does it form?

•Soil is:– the mixture of weathered rock and organic material that covers bedrock and in which plants with roots can grow.

–typically composed of 3 layers or horizons.

Types of Soils•Parent material- the material that

soil comes from

–Residual Soil- parent material is local bedrock (beneath the soil)

–Transported Soil- formed from parent material in different areas and is brought into an area by wind or water (rivers or glaciers)

• Most soils in New England are transported soils

Stages of Soil Formation

•“Mature” soils:•have had a long time to form•have 3 distinct zones

•Can be seen in a soil profile•cross section exposed by digging down to the parent material

Soil Profile• A horizon (topsoil)

– top layer

– rich in organic remains (humus)

– gray-black color

• B horizon (subsoil)– middle layer

– rich in minerals• Washed down from A horizon

– iron, clay, calcium, magnesium

– red-brown color

• C horizon– lowest layer

– slightly weathered parent material

– color depends on parent material

Soil Composition• Soil is

classified by composition (make up).

• 1000s of different types

• Strong link between composition & climate

Weathering,Soil,

Mass Movements&

ErosionChapter 12 Section 3

Mass movements and erosion carry away

weathered materials.

**can only take place after weathering has occurred

Erosion- removal and transport of materials by natural agents such as wind and running water

Erosion

Mass Movements• Mass movements- Movement

of material down slope under the influence of gravity

• Talus- rock fragments that have been weathered from a cliff and pulled down by gravity

• Landslide- general name for the movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff– Most likely on steep slopes– Tend to occur after heavy rains

or during the spring when large amounts of snow are melting

Specific types of mass movements

• Fast moving• Flows

– Earthflows– Mudflows

• Landslides– Avalanches

• Rockslides

• Slow moving

• Creep

• Slump

Creep• Very low velocity (slow), gradual,

steady movement of surface soil

• Movement is imperceptible, but effects can be seen– Fence posts, poles, &

trees to lean downhill

Slump• Blocks of land tilt and move

downhill along a surface that curves into the slope.

• Tends to occur because a slope has become too steep for the bottom of the slope to support the soil at the top.

Flows• Earthflows: soil saturated with

water, flows downhill– Slower and less fluid than mudflow

– Affected by amount of water present, composition of soil, and steepness of the slope

• Mudflows: rapid movement of water that contains large amounts of suspended clay and silt– Can move rocks, boulders, and trees

– Tend to occur in drier regions that experience infrequent but heavy rains

Landslides/Avalanches & Rockslides• High velocity event, material is loose & dry.

Erosion and Landforms

• Rivers, streams, glaciers, wind, and ocean waves and currents are all agents of erosion.– Climate and composition of the rock also play

an important role

• Erosion helps in shaping the landscape– Topography depends on the “balance” between

uplifting forces and erosion

Soil as a Resource• Soil fertility- ability of soil to grow plants

– Proportion of minerals, water & organic matter determines fertility

• Threats to fertility:– Soil depletion- when nutrients are lacking or taken out of

soil to the point where it can no longer grow a usable crop• Reduce depletion by allowing field to lay fallow or by rotating crops

– Often add fertilizers instead

» Runoff affects ecological balance (ex. water quality)

– Salinization- water from irrigation leaves behind minerals as it evaporates

• Eventually, soil cannot support crop growth

Chapter 12 Section 4

Erosion and Soil Conservation•Erosion is accelerated by human activity.

•Farming, construction, mining

•Soil erosion must be controlled using soil conservation methods.

•Plant windbreaks/shelterbelts (trees reduce wind erosion)

•Constructing terraces (“steps”, slows water flow)

•Implementing erosion-reducing farming methods•Contour farming (plow parallel to land shape, slows water flow)

•Strip cropping (alternate rows, little ground cover/complete coverage, planted perpendicular to dominate wind direction)

•No-till (plowing, planting, fertilizing, and weed control done at same time, land not disturbed again until harvest)

windbreak

contour

Strip-cropping

terracing