Weathering and Erosion
Weathering is the gradual breakdown of pre-existing rock due to natural
processes. These processes can be very quick, but some may take thousands of
years to occur.
 Rocks are weathered by their exposure to the elements- like rain, freeze
and thaw and acid rain.
 They are broken into smaller pieces, and then can undergo chemical
changes.
 Erosion is the process of moving these weathered materials.
Physical Weathering
Physical weathering is processes that break down the rock without
changing the chemical composition.
Types of Physical Weathering:
Frost Action Frost action is the result of the unusual property of water: As water is
cooled, it expands.
 Most other materials expand when they are heated, and contract as they
are cooled.
 Water expands dramatically between the temperature of 4 and 0 degrees
Celsius.
 When water turns to ice, its volume increases by 9%!!!
 This expansion can exert pressure on an object measuring hundreds of
thousands of pounds per square inch.
 Water seeps into cracks and pores of rocks.
 When the temperature drops, the water expands, and this repeated action
can wedge the rock apart.
 This repeated action of freezing and thawing is called frost action.
Abrasion Rocks can also be broken by abrasion.
 Abrasion occurs when rocks are being transported by an agent of
erosion- like moving in a stream or falling down a cliff.
 As these rock fragments bounce around and off of each other, they break
down.
 How do you tell which way a stream flows by looking at a rock in the
stream?
 Rock that faces upstream are angular, the downstream side is smooth.
Exfoliation In certain rock types that contain the mineral feldspar, exfoliation can
occur.
 Water that seeps into the cracks of the rocks bonds to the water, and clay
is formed as this layer is weak; it eventually falls off, along with the rock
layer above the clay.
 This tends to happen faster along the edges of the rock, so an exfoliated
rock tends to have a rounded appearance.
Plant and Animal Action Animals also have an effect on rocks.
 The most common effect posed by animals is burrowing, or creating
tunnels under ground.
 Ants, worms, weasels, mice, etc…can all create an extensive tunnel
system.
 These tunnels allow water and air to reach points below ground, and
begin the weathering process.
 Humans have a dramatic effect on rocks.
 Human technology has allowed us to level mountains, cut out roads and
tunnels, and also created problems such as acid rain.

This acid rain can also weather rocks.
Changes in temperature As a rock is heated by the sun, its temperature can increase dramatically,
and at night it cools down.
 As this process occurs over and over again, the bonds of the rock
molecules can break, leading to cracks in the rock.
 These cracks then allow a path for other agents of weathering.
Chemical
Weathering
Chemical weathering is the breakdown of rock through changes in its
chemical makeup. These changes either break down the rock or weaken the
rock to allow for physical weathering.
Types of Chemical Weathering:
Oxidation The atmosphere is composed of 21 percent oxygen.
 When oxygen combines with an iron rich rock, a process called
oxidation occurs.
 The result is iron oxide.
 This reaction does not weather the rock.
 If, however, water is present at the time of oxidation, the result would be
rust.
 The result is much the same as an iron nail.
 When these bonds are rearranged, the bonds of the rock are weakened.
 If you compare a rusty nail to a regular nail, the unweathered nail is
much stronger.
Dissolution
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
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Dissolving a rock in water is yet another form of chemical weathering.
It is called solution.
Solution occurs when compounds are dissolved into water.
Halite, gypsum, and limestone are most susceptible to solution.
 Most compounds are at least slightly soluble in water, so water is called
the universal solvent.
Carbonation Carbonic acid can devastate rocks made of calcite.
 Rocks such as limestone and dolomite can be entirely dissolved, leaving
extensive caverns.
 In states with bedrock composed of calcite, such as Florida this process
can cause extensive damage to structures above.
 Often, caverns are cut out of the rock, the structures on top are heavy,
and everything on top collapses into the cavern, leaving what is called a
sinkhole.
 This sinkhole can swallow many houses at a time.
Acid Rain Normal rain is at a pH of 6.2.
 Pollution from cars, factories, and coal burning plants are creating rain
so caustic that sometimes the rains at pH of 4.
 If an entire lake was that pH, it would be crystal clear but lifeless.
Factors Affecting Weathering
Climate Climate, which is usually measured in terms of temperature and
moisture, can drastically affect the rate and type of weathering.
 In an area with cold temperature, for instance, frost wedging would be
the dominant form of physical weathering.
 Chemical weathering occurs most rapidly in a high temperature climate.
 In Pennsylvania, the climate reaches both extremes.
 Hot moist summers and cold winters make for a climate that promotes
weathering.
Particle Size The particle size of the material can affect the rate of weathering.
 The smaller the particle size, the faster it will weather.
 The more surface area that is exposed…the more it will weather.
Exposure Rock that is exposed to the atmosphere tends to weather much quicker.
 Bed rock that is covered by soil and vegetation may not weather as
quickly.
Mineral Composition All chemical and physical properties are determined by the mineral
composition of the rock.
 Mineral composition is more important in determining the rate of
chemical weathering.

Those minerals that are most reactive with acids, water, and air are
weathered at a more rapid rate.
 Mineral composition also affects physical weathering.
 Minerals that are soft (hardness lower than six) can be easily abraded
and blasted.
Time Weathering is a slow process.
 When we speak of time in a geologic sense, millions of years are often
used as the units.
 The bedrock of Greenland has barely been weathered, and its age is over
3.5 billion years.
 The longer that a rock has been exposed to the weather, the faster it
weathers.
The Products of Weathering
Sediments The rock that soil comes from is called the parent rock, and soil that
forms from the bedrock directly below is called residual soil.
 If the soil is from material that was transported, the soil is called
transported soil.
 Transported soil often has a different composition that the bedrock
directly below.
Erosion
Erosion is the movement of weathered rock.

Streams, waves, floods, and winds are just a few of the agents of erosion.
Types of ErosionMass Wasting Mass wasting is the downhill movements of material due to gravity.
 This type of movement is going on at all times on most hills, but at
different rates.
 As the slope of the material increases, the rate increases.
 The steepest slope at which the material will remain stable is called the
angle of repose.
 Size, density, and shape of particles determine the angle of repose.
 As the slope's angle increase past the angle of repose, the slope becomes
unstable, and mass wasting occurs.
 If the angle is slightly above the angle of repose, mass wasting is slow,
as in hillside creep.
 If the angle is great, then slides may occur.
Types of mass wasting
Rockfall- rock falls occur when large fragments of rock either fall
from a cliff, or bounce down the side of a hill
Rockslides- rock slides occur on slopes that are less steep when
rocks slide down the entire slope, rather than falling through the air.
Slump- a process in which a mass of bedrock or soil slides down a
slope in one large mass. The mass slides down a curved plane of
weakness. The slump block rotates and comes to rest tilted toward the
cliff.
Mudflow- mudflow is the rapid, downhill flow of a fluid mixture of
rock, soil, and water. Mudflows generally occur when there are long
periods of rainfall, and the water content of the soil builds up. The
watery soil runs downhill, and thickens up as it travels downhill and
picks up rocks and soil.
Erosion by Moving Water
 All precipitation that is not absorbed by the ground is called runoff.
 Runoff can sometimes transport sediment, and often ends up in streams.
 Streams are bodies of running water that flows under the influence of
gravity.
 The running water cuts into the surface of the earth, building a channel.
 This channel is surrounded by a bank on either side.
 Material that is carried by water is called the stream's load.
 The faster a stream, the larger the load it can carry.
 A slow moving stream does not have the energy to carry a large load.
 Large sediments are usually dragged or pushed, a process called traction.
 Saltation is when particles bounce along the stream bed.
 Suspension is the process of sediments picked up by the stream.
 When rocks are dissolved, they are said to be carried by the process of
solution.
 When the load of a stream is a resistant rock, it can abrade the surface of
bedrock.
 This prolonged scratching can gouge rock, and eventually a canyon can
be formed.
 The Grand Canyon was created in this manner.
Ocean Erosion
 The majority of the water in any stream will eventually end up on the
earth's oceans.
 The ocean's waves can be very destructive.
 Formations such as sea cliffs, caves, stacks, and terraces may result.
 Oceanic waves cause long shore current, which can move sediments
along the coast.
Wind Erosion
 Deflation is the process of wind picking up sediment and transporting it.
 Abrasion is the process in which wind blown sediments collide and wear
rocks down.
 When deflation is concentrated, a deflation hollow is formed.
 There are thousands of such hollows on the Great Plains.
 In wet years, they fill in with grass or water.
 In dry years, the grass dies, exposing the hollow to even more deflation.
 When deflation occurs in the desert, the water table may be exposed.
 Vegetation forms around the hollow, and an oasis is formed.
 After deflation occurs for a period of years, only the objects that could
not be transported by wind are left, usually large particles.
 An area with only these particles left is called desert pavement.
 The rock has a flattened surface where it was exposed to abrasion by
wind.
 As sand travels through the desert, being eroded by wind, dunes start to
form.
 The gradual slope that sand is blown along is called the wind face.
 The steep slope that faces away from the wind is called the slip face.
Life Cycle of a Stream
Young Streams
 When a stream is recently formed, it generally flows quickly.
 The stream wears a long v-shaped valley into the ground.
Mature Streams
 In a mature stream, the slope gets eroded over time, and the slope
becomes much less steep.
 The water flows more slowly and sediment accumulates at the bottom of
the valley.
 Water flows around obstacles, rather than over them, and the stream's
path forms curving loops called meanders.
 If the river overflows, the water slows down, deposits its load, and a
broad flat floodplain is formed.
Old Age Streams
 In an old stream, a large peneplain is formed.
 This is the area that used to be the floodplain, that had the stream
meander though it.
 This peneplain has oxbow lakes and cutoffs.
 When a stream meanders, the outside of the meander is heavily eroded.
 That’s where you can find cliffs.
 The inside of the meander is where a sand beach can be deposited.
 The water moves faster toward the outside of the bend because of the
water's inertia.
 Because the water can't "turn" it bashes into the far side.
 Meanwhile, the water on the inside of the bend is moving slower.
 Heavier sediments will fall out of the stream creating a great flat beach
to camp on.
Glaciers
 Glaciers form after long periods of snowfall, usually in the mountains.
 After many meters of snowfall, the snow at the bottom begins to
recrystalize under the great pressure caused by the weight of the
overlying snow.
 As the snow begins to crystallize into solid ice, the pressure continues to
increase.
 This squeezing motion forces the ice to begin flow as a liquid.
 As glaciers move along, the bottom melts, and refreezes.
 As this occurs, the rocks and sediment lying under the glacier are picked
up and frozen into the ice.
 This process is called plucking.
 Sediments the size of boulders can be plucked and carried thousands of
miles.
 Sometimes a plucked rock scrapes on the bedrock as the glacier grids
slowly onward.
 This causes deep grooves in the rock called glacial scratches.
 As the glacier begins to recede, or melt, it deposits these sediments.
 Boulders that are not native to the area that they are deposited on are
called Erratics.
 Large holes or depression that are plucked by glaciers are called Kettles.
 The sediment that is pushed by the glacier, similar to a bulldozer, is then
deposited as the glacier recedes.
 This is called Morraine.
 Morraine State Park is the result of glaciation.
 Glacial deposited sediments are unsorted.