Unit 8 : Weathering, Erosion, and Soils
A. Soil
1. Soil is the result of weathering of rocks and includes weathered particles : sand, silt, and clay
2. A soil texture triangle is used to classify soil texture using the different proportions of sand, silt,
and clay in a sample
B. Physical Weathering
1. Physical (or mechanical) weathering – occurs when physical forces break rocks into smaller and
smaller pieces without changing the rock’s mineral composition
a. Ex. tearing a sheet of paper
2. Frost wedging – rock breaks into pieces after many freeze-thaw cycles
a. most common in mountainous regions in middle latitudes
3. Unloading – large masses of igneous rock may be exposed through
uplift and erosion of overlaying rocks
a. the outer rock masses separate and break into smaller pieces
4. Biological activity – activities of organisms such as
plant growth, burrowing animals, and humans can
cause physical weathering
C. Chemical Weathering
1. Chemical weathering – transformation of rock into
one or more new compounds
a. Ex. Burning a sheet of paper
2. Water promotes chemical wreathing by absorbing
gases from the atmosphere and the ground
a. these dissolved substances then chemically
react with various minerals
b. ex : iron oxide is rust that forms when iron
containing objects are exposed to water
D. Rates of Weathering
1. Physical weathering speeds up the process of chemical weathering by increasing the surface area
of the exposed rock
2. High temperatures and abundant
moisture creates the most favorable
climate for chemical weathering
3. Different parts of a rock mass
weather at different rates depending
on mineral composition and the
number of cracks
E. Erosion
1. Erosion – process by which Earth materials are carried away and redeposited by wind, water,
gravity, or ice
a. plucking –lifting up and moving fragments of rocks
b. abrasion – scouring actions of particles carried by the wind
c. Humans move things from place to place due to construction, deforestation, and
agricultural practices
2. Water
a. The higher the speed of the water, the greater the mass of material will be moved
b. Coastlines are constantly eroded by waves causing loss of
coastline
c. Water forms river systems by eroding rock and moving
sediment downstream
d. Water will create canyons like the Grand Canyon
e. Erosion by water is the most common form of erosion
3. Wind
a. Causes erosion by plucking and abrasion
b. Erosion through wind will cause landforms such as sand
dunes
4. Ice
a. Glaciers move sediments by abrasion or plucking
b. Landforms left by glaciers include deep rounded valleys, bowl valleys, and sharp pointed
mountain ridges
5. Gravity
a. Creates landslides, mudslides, and avalanches
b. Gravity is constantly pushing materials down which causes loose sediments to erode
F. Precautions
1. Landslide
a. Be aware of landslide susceptible areas in your neighborhood and recognize landslide
warning signs
b. If you see signs of a landslide contact your local authorities, your neighbors, and evacuate
c. If you are caught in a landslide, curl up into a ball and protect your head
2. Earthquakes
a. If your areas is prone to earthquakes : secure tall or heavy furniture to the wall, and latch
cupboards so that plates and glass do not fall out
b. During an earthquake, stay indoors, move away from windows and anything that can fall
on you and duck
c. If you are outdoors, move quickly and safely into an open space away from electrical
lines, trees, and buildings
3. Tsunami
a. Plan an evacuation route, follow local authorities directions, and seek higher ground
4. Sinkholes
a. Sinkholes are caused by rock that is dissolved by weak natural acids found in rain and
pore spaces in soil
b. Check for limestone deposits and be aware of past sinkholes in the area
5. Groundwater pollution
a. Test groundwater before consumption or use
6. Flooding
a. Be aware if you live in a flood plain area
b. Move to higher ground
c. Do not walk, swim, or drive through swift moving water