Download CHAPTER 10: WEATHERING AND SOIL FORMATION:

CHAPTER 10: WEATHERING AND SOIL FORMATION:
Weathering is the breakdown of rocks into smaller pieces called sediment.
Two types of weathering:
1. Mechanical—physical breakdown of a rock into smaller
fragments, each with the same properties as the original.
WHAT CAUSES MECHANICAL WEATHERING
1. ice wedging: water squeezes into cracks in rocks and expands
making the crack wider and thus wedging the rock apart.
2. wind: nature’s method of sand blasting.
3. water: has abrasive powers. Abrasion is where rocks rub against each
other. This occurs mostly in streams and rivers where fragments
bounce off each other and become rounded.
4. plants/animals: roots can penetrate cracks and physically pry rocks
apart as the root grows in size. Animals burrowing in soil expose
fresh surfaces that will continue weathering.
2. Chemical—chemical reactions dissolve minerals in rocks or
change them into different minerals or into a new substance.
Look for words like dissolved, rusting, mixing, oxidation and decaying
HOW FAST DOES WEATHERING HAPPEN?
It depends on several things:
1. Different rock types weather at different rates
2. The environment—cold, warm, wet dry, humid etc.
WHAT HAPPENS WHEN ROCKS ARE COMPLETELY BROKEN DOWN?
 You have soil!!!
 Soil is a mixture of pieces of broken down rocks and organic
materials
When soil forms, it ends up in a series of layers called horizons
Soil profiles are when the soil is looked at in cross sections. When looking at
soil profiles, you can tell the difference between mature soil formation and
immature soil formation. Mature soils have had enough time to develop
distinct horizons. Immature soils do not have distinct horizons.
Horizon Profile:
“A” horizon—top soil layer
 Also called topsoil
 Is very dark in color
 Contains the most humus (organic soil where plants grow)
and less rock particles
“B” horizon—middle soil layer
 Also called subsoil
 Much lighter than A
 Contains less humus than A

Minerals travel from A to B horizon in
process called leaching
“C” horizon—bottom soil layer
 Contains rock—the parent material of
the soil
 Has very little organic matter
“D” horizon—bedrock which is solid rock…no
dirt
AMAZING EARTH SCIENCE FACTS
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Weathering is the process that rocks are broken down by water, air and organisms.
Mechanical weathering occurs in cold climates-ice wedging
Chemical weathering occurs in warm, humid climates
Soil evolution starts with the weathering of bedrock
Organic material must be present in order to have soil
Soil profile consists of 3 horizons
A-Top soil (most evolved)
B-Less humus, leaching from A
C-Weathered rock
D-Known as solid rock or bedrock
IS IT MECHANICAL OR CHEMICAL WEATHERING?
M for mechanical weathering---stays the same, just smaller pieces
C for chemical weathering---changes into a new substance
________ 1.
________ 2.
the wedging of tree roots along natural joints in
granite
limestone dissolved by carbonic acid
________ 3.
the oxidation of minerals that contain iron
________ 4.
animal burrows dug in rock that let in water and air
________ 5.
________ 6.
repeated freezing and thawing of water that
cracks rock
the action of water, salt, and air on car fenders
________ 7.
acids from plants roots that break up rocks
________ 8.
formation of potholes in streets during severe
winters
raised sections of sidewalk along tree-lined streets
________ 9.
________ 10. a small rock falling from a cliff
________ 11. feldspar mixing with water and producing clay
minerals
________ 12. halite in rocks dissolving in water
________ 13. decaying plants dissolving minerals in rocks
________ 14. tree roots cracking the concrete foundation of a
house
________ 15. iron lawn furniture rusting outside
________ 16. more rapid in tropical climates
________ 17. carbonic acid weathering limestone
________ 18. leaves decaying in the forest