Weathering
Evidence of Weathering
- Weathering is the process that breaks down rocks into
smaller and smaller fragments
- Rock breaks down into soils
- changes Earth’s surface
Mechanical Weathering
- breaks apart rocks without changing their chemical
composition
- can be caused by:
o growing plants
 root growth wedges rocks apart
 lichen – helps create soil
o ice
 ice wedging
 cycle of freezing and thawing
 noticeable in mountain regions
o mineral crystal growth
o lightning
o heating and cooling (expansion and contraction)
Gravity
Erosion – the process that wears away surface materials and moves them from one place
to another
- major factors:
o gravity
o glaciers
o wind
o water
- All agents of erosion deposit sediments when erosion energy decreases
- Deposition – dropping off of sediments
Gravity
- force of attraction that exists between all objects
- mass movement
o when gravity causes sediments to move downslope
o slow process typically
o can be fast
 slump – mass movement that occurs when loose materials or rock
layers slip down a slope
 strong rock or sediment lies over weaker materials
 can’t support the stronger materials
 slip downslope in a large mass
 happens usually after heavy long rains or an earthquake
 creep – sediments slowly “inch” their way down a hill
-
 common in areas of freezing and thawing
 good sign is objects that lean
 rockslides – fast and destructive
 large blocks of rock break loose from a steep slope and
tumble
 common in mountainous areas or steep cliffs
 occur usually after heavy rains or earthquakes but can
happen at anytime without warning
 mudflows
 thick mixture of sediments and water flowing downslope
 usually occur in dry areas with a thick layer of dry sediments
 water mixes with the dry sediments making it thick
Erosion prone land
o Steep slopes
o Ways to make it less erosive
 Plant vegetation
 Terracing
 Wall building
Glaciers
Glacier – moving mass of ice and snow
- when snow and ice pile so high the pressure causes partial melting of the ice at
the bottom
- allows the glacier to move downhill slowly
Continental glaciers
- cover up to 28% of Earth in the past
- ice ages – when glaciers covered much of the land
o most recent – 2-3 million years ago
o 20,000 ya the sheets began to melt
o Present day – glaciers cover about 10% of Earth – mostly near poles
Valley glaciers
- occur in high mountains where temp is low enough to prevent snow from melting
during summer
- ex. – Tibet
Glacial Erosion
- move over land pushing loose materials
- sediments pile along its sides, are pushed in front or are carried underneath it
- weathers rock that isn’t loose as well
- friction causes glaciers to melt then refreeze in cracks in rocks which increases
weathering process
- plucking
o some rocks are refrozen into glacier and are picked up
o results in boulders, gravel, & sand added to the bottom
- grooves
o gouges in land from dragged rocks
o deep, long, & parallel scars
- striations
o shallow scars on rocks
Evidence of valley glaciers
- plucking usually occurs at top or near top of mountain
o Cirques
 Bowl-shaped basins in sides of mountains
o Arete
 2 or more glaciers erode a mountain summit from different
directions
 Ridge is formed
o Horn
 Sharpened peak on an Arete
Glacial Deposition
- Till
o Mixture of different sized sediments
o Dropped off from base of glacier
o Includes boulders, sand, clay, and silt
 Types of tills:
 Moraine
o When till is deposited in front of a glacier
o Material left behind by glacier
o Deposited in middle and along sides
o Terminal moraine – deposits at the end of a glacier
- Outwash
o Material deposited by meltwater from a glacier
o Heavier sediments drop out first
o Esker
 Winding ridge of sand and gravel
 When meltwater forms a river within the ice of a melting glacier
Wind
Wind erosion
- wind picks up sediments and moves them from place to place
- two types:
o deflation
 blows across loose sediments
 removes smaller pieces (sand, silt & clay)
 heavier stuff left behind
o abrasion
 windblown sediments that strike rock and scrap and wear it away
 like sand-blasting
- sandstorms
o sand that gets picked up and blown creating a cloud above land
o most occur in deserts
- dust storms
o soil dries out and is easily picked up
o smaller particles than sand
o can be carried to far places due to it light mass
Reducing wind erosion
- windbreaks
o walls of trees or vegetation planted
- roots
o vegetation discourages erosion due to its adhesive nature
Depostion of wind
- Loess
o Wind deposits of fine-grained sediments
o Fine like talcum powder
- Dunes
o Sediments that build up in an area over time
o Wind increases as you move inland
o Breaks in wind due to objects slow down energy and allow deposits to
settle
o Repetitive nature accrues more material until dune is formed
o Wind blows sand from back side to front and therefore they are
constantly moving positions
o Shape depends on type of wind
 Crescent
 Transverse
 Parabolic
 Star
Chemical Weathering
- when water, air and other substances react with the minerals in the rocks
- changes the chemical composition
o water
 H and O atoms react with some substances
o Acids
 Carbonic acid
 Water and CO2 mix in atmosphere
 Weak
 Reacts with calcite
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Found in limestone
Creates stalagmites and stalactites in caves
Produces clay after carbonic acid hits granite
o Oxygen
 Oxidation – rusting
 Iron and water exposed to air
 Reddish color of soil
Climate & Weathering
- Climate
o The pattern of weather that occurs in a particular area over many years
o Can help the speed of weathering
 Mechanical weathering – colder climates
 Chemical weathering – warm, wet climates
Soil
Formation of soil
- layers of rock & mineral fragments produced by weathering
- plants and animals add organic matter to mix
- the combination of two creates soil
Soil
- the mixture of weathered rock, organic matter, mineral fragments, water and
air
- a material that supports vegetation
- different types of weathering affects the formation of soil
Composition
- organic material decays and turns into dark colored matter called humus
o serves as a source of nutrients for plants
o provides nitrogen, phosphorus, potassium, & sulfur
o helps hold water
- can take thousands of years to form
Profile
- different layers in soil
- horizon – each individual layer in soil
o generally there are 3
 A – top layer
 Litter – composed of leaves, twigs, and other organic
material
 Will change to humus
 Helps prevent erosion and hold water
 Key to successful plant growth
 B – layer below A
 Lighter in color
 Contains less humus
Contains elements washed down from A
o Leaching – the removal of minerals that have been
dissolved in water
C – layer below B
 Bottom of soil profile
 Mostly partially weathered parent rock
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Climate effects
- chemical weathering
o faster in areas where there is a lot of rainfall
 rain forests
 deciduous regions
 equator
o slower in areas where there is little rainfall
 deserts
 tundras
- time
o amount of time weathering has occurred will affect the type of soil
- slope
o affects erosion and loss of soil
Erosion
- less vegetation causes faster erosion on land
- increase in population increases field use
- plowing increases rate of erosion
- tropical regions face lots of run off from soil
o leaves little fertile soil
o cycle of planting and growth lasts a few years
- farmers act
o shelter beds
o grazing animals in dry climates
o terraced slopes
o planting trees to protect understory (tropics)