Unit 6 - Weathering

weathering

the wearing down of rock by chemical and physical means

erosion

the wearing down of rock through movement agents

mechanical weathering

Alters the size/shape of the rock, disintegrates rock and keeps the same characteristics as the parent rock

chemical weathering

the changing of a rock’s composition and characteristics due to chemical reactions.

ice wedging

freezing of water expands by 10% pushing rock apart, and then melts. Best in regions where there is freezing and thawing seasonally, so it can repeatedly ice wedge.

Organic Activity

Root wedging

Animals

root wedging

tree roots pry rocks apart

animals

borrow or scour away at rock

Abrasion

water/wind picks up grains of sand that pelt.

wind - creates pits, polishes

water - erodes rock, makes it rounded

sheeting, joints, exfoliation

• When overlying pressure is removed, rock expands and cracks creating a joint.

• When cracks are parallel to the surface, they form layers of rock. this is called sheeting.

• When sheets of rock fall off, it’s called exfoliation.

Oxidation

oxygen, usually in the presence of water, reacts with metal causing it to change color. ex. oxygen + iron = rust.

hydrolosis

water interacts with mineral, and turns it into a clay, changing composition to include H or OH. ex. water +feldspar = clay mineral

carbonation

CO₂ dissolved in rain water produces carbonic acid (pH of 5.6). This acid reacts with calcitic minerals (ex. limestone and marble). ex. calclite + carbonic acid = calcium bicarbonate.

spheriodal weathering

water, CO₂, and organic acids seeps through rock and causes chemical weathering.

organic acids

lichen and moss grow on rock, and produce acids that eat away at surface of rock

acid precipiation

sulfur and nitrous oxides dissolve in rainwater, producing nitrous or sulfuric acid that dissolves away at rocks (pH <5).

rock composition

• Bowen’s reaction series - mafic rocks weather more easily than felsic

• sedimentary rocks dissolve more easily than igneous and metamorphic

• Sed rock glue - silica = strongest, and calcite = weakest

climate

• precipitation/high humidity - increases mechanical and chemical weathering

• High temperature speeds up chemical weathering, breaks bonds

• Cold temperature associated with mechanical weathering. ex. ice wedging

surface area

more exposure for chemical weathering

Topography

• high altitude - associated with mechanical weathering due to cold temperatures.

• slope - more exposure to weathering

Explain how mechanical weathering can lead to chemical weathering

mechanical weathering breaks down rock into smaller pieces, giving more surface area for chemical weathering.

Describe how a young mountain range might be different from an old mountain range

Old mountain ranges are more rounded because they have experienced more weathering than young mountain ranges, which are more pointy and jagged.

Soil horizons

• O - loose particles, decomposed organic matter, humus, dark. organics, litter from trees, etc

• A - minerals mixed with humus . contains topsoil (humus)

• E - light-colored mineral particles, zone of eluviation, and leech. leaching

• B - Accumulation of clay transported from above. zone of accumulation/deposition of minerals

• C - partially altered parent material. bedrock just starting to weather and break apart, regolith

pedalfer

Temperate forest areas, thick horizons, acidic, rich in aluminum and iron from silicates.

northern forest soils, in area of humidity, iron, acidic, fertile, dark brown/black, distinct horizons

laterite

high precipitation, warm temp, lots of chemical weathering, leeched soil nutrients depleted, iron → reddish in color, no real A horizon

tropical environments, heavy rains cause major leaching to occur and create soils with lots of iron and aluminum

pedocal

semiarid/steppes, weathering of calcite → rich in calcium, basic, white colored → caliche, fertile

prarie soils, in Central US, from low humidity regions, calcium carbonate, basic, fertile,  

regolith

Polar regions or arid, mechanical weathering is dominant, C horizon.

in desert regions, no topsoil, rocky

Provide differences between residual soils and transported soils

• residual - soil from rock that weathered in place, formed from bedrock below

• transported - soil from rock that eroded, does not represent bedrock below

Describe the difference between young soils and mature soils

• Young - immature, hasn't formed the three layers (A, B, C)

• old - has had enough time to mature and form distinct layers.

Describe preventative methods to reduce soil erosion (ie-terracing)

• planting windbreaks - belts of trees are planted along the edge of a field to slow wind erosion

• contour plowing - crops are planted in rows parallel to the contour of the land to prevent rapid flow of water downhill

• terracing - the slope of a hill is flattened by building small terraces or ridges that resist erosion

• strip cropping - alternate crops are planted next to each other, rather than having soil there.

• no till - an area is plowed, planted, and fertilized all at once, so the ground is not disturbed again until harvest.

• replanting trees - lumber companies replant trees after cutting som down, so there will be roots to keep the soil together.

Describe each of the different mass wasting movements: flows, slides, and falls. Be able to provide specific processes for each

1. flows - individual particles move in their own path as a fluid.

1. creep - slowest of all

   1. 1-2 cm/yr

   2. most damage

   3. caused by freezing/thawing

2. solifluction - in permafrost regions, water builds in thawed layer over a frozen layer

   1. teardrop shape (indicator)

3. mudflow or debris flow - a mix of water, mud, and debris

1. slides - whole block moves as one whole chunk, usually from different materials layered

   1. slump - slow slide creates a curved slope called a scarp

   2. rockslide - up to 40 mph
      

2. falls - particles don't make contact with the ground as it's descending (free fall)

List things that increase the likelihood of a landslide/mass wasting event

1. water

   1. adds weight

   2. lubricates slope

   3. breaks cohesion

2. weight of top of slope

3. undercutting at base of slope - waves, rivers, road cuts

4. Lack of vegetation

5. Thickness of loose material and strength

6. earthquakes

7. Bedding orientation

Soil

organic material, weathered rock, air, water, → must support life

Mass Wasting

movement of any loose material by gravity alone that occurs on slopes, leading to various types of landslides, including flows, slides, and falls.