Weathering and Soil
1. Weathering
- Natural earth processes that occur on the surface that work to break down rock are called
weathering
a. Effects of Weathering
i. Weathering breaks rock down into smaller and smaller pieces called “sediments”.
1. Sediments are classified into different sized particles: sand silt clay
2. Sediments gradually change into soil over time
3. The formation of soil depends on the amount of weathering that occurs in a
given location.
ii. Weathering has changed the surface of the earth over millions of years.
1. Mountains have worn away into hills and then flat plains because of weathering.
b. 2 Types of Weathering
i. Mechanical Weathering
1. When rocks are broken apart by [physical processes.
a. Chemical make-up of the rock stays the same.
b. Physical properties of the sediments are the same as the rock from which
the sediments eroded.
c. Causes
i. Growing of plants on rock—Roots grow in small cracks in rocks
and force the cracks further apart.
ii. Burrowing of animals and insects—As animals dig their dens or
tunnels through soil they loosen sediments and push them to the
surface where erosion from wind and water can act on them
iii. Ice Wedging—Water seeps into cracks in rock and then expands
when it freezes. The pressure from the expanded ice forces the
cracks to widen. When the ice thaws, more water can seep into
the wider crack and the process repeats itself.
d. Effects- Mechanical weathering forces rock to break into smaller and
smaller pieces allowing more rock to be exposed to more and more
erosion and weathering. The cycle will continue forever.
ii. Chemical Weathering
1. When chemical reactions dissolve the minerals in rocks or change them into
different minerals.
a. This changes the chemical composition of the rock and makes it weaker.
i. Natural Acids
1. Carbonic Acid is formed when CO2 in the air mix with H2O
a. Reacts with minerals such as calcite which is a
major component of Limestone rock.
2. Natural Acids react with granite and sandstone to form
kaolinite clay over thousands of years.
ii. Plant Acids
1. Roots of decaying plants give weaken rock by giving off
acids that dissolve minerals in the rock.
iii. Oxygen
1. Oxidation occurs as O2 reacts with metallic minerals such
as iron in rock.
a. Hematite transforms into a rust like material called
limonite
c. Effects of Climate
i. Mechanical and Chemical weathering occur everywhere on the planet all the time.
ii. The climate affects the amount of weathering that occurs
1. Climate- the pattern of weather that occurs in a particular area over many years.
a. Chemical weathering occurs at a faster rate in warm wet climates.
Tropical regions see a lot of chemical weathering.
b. Physical weathering occurs at a faster rate in cold, windy climates.
iii. The climate of a region also affects the rate that a type of rock will weather.
1. Marble will weather more quickly in a warm wet climate than it would in a dryer
climate.
2. The Nature of Soil
What is soil? Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water
and air.
a. Formation of Soil
i. Takes thousands of years to form and ranges from very thick in some places where the
weathering process has been active for years and years to very thin where weathering
has just begun to take place.
1. Lots of factors affect the formation of soil such as: climate, slope, types of rock,
vegetation, and the length of time that rock has been weathering.
a. Different kinds of soil develop in tropical regions than polar regions.
b. Composition of Soil
i. Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water
and air.
1. Weathered rock-Form sediments of sand, silt and clay that makes up part of soil.
2. Decayed organic matter from plants and animals turns into a rich, dark colored
material that becomes a source of nutrients plants called humus.
a. Worms, insects borrow in the soil and serve to mix the humus into the
soil.
3. Air / Water - Seeps into the spaces in between the sediments, keeping the soil
aerated and moist.
c. Soil Profiles
i. Horizons- Layers of soil with varying degrees of plant and animal life and mineral
composition.
1. A Horizon -Top layer of soil
a. Covered in “litter” (twigs, leaves and other organic material) that will be
converted into humus over time.
i. Litter prevents erosion and keeps erosion down
b. Dark and rich with lots of nutrients for plants to grow.
2. B Horizon- Below the A layer
a. Lighter in color due to lack of litter and organic matter.
b. Contains less humus and smaller sediments such as silt and clay
c. Contains material that has leached from the A Horizon
i. Leaching is the removal of minerals that have been dissolved in
water. Water seeps down through the A layer, bringing only some
of the dissolved minerals to the B layer.
3. C Horizon- Below B layer.
a. Even lighter in color than the B layer—indicating even less nutrients
b. Bottom of the soil profile
c. Contains coarse sediments and large rocks toward the bottom—gravel
and “bedrock” which is called “parent material”
i. ***** Glaciers deposited soil all across the USA thousands and
thousands of years ago. If we dug down through the soil profiles
and got to bedrock, that rock is NOT the parent material of the
soil in the layers above it. Glaciers moved that soil from it’s
parent material long ago.
d. Soil Types
i. Vary from place to place and are affected by
1. Climate
a. Deserts- soil lacks organic material because not too many things can live
or grow in deserts. Horizons are thinner than in other climates
b. Prairie soils have thick, dark A horizons due to amount of vegetation and
animal life that exist in that climate
c. Temperate forest soils – Thinner A and B horizons that have rich mineral
content due to leaching from abundant rain fall.
d. Tundra and tropical areas have distinct soils because of the rainfall,
vegetation and animal life that each environment can sustain
2. Other Factors
a. Parent Material has an effect on the type of soil that comes from it
i. Clay- comes from basalt
b. Time affects soil development. Soil will be thicker and richer if
weathering has occurred over a long period and vice versa.
c. Slope- There will be more run off if the grade of the landscape sloped,
allowing less soil to build up in thick layers toward the top of the slope.
The bottom of the slope (bottomlands) tend to build up thicker layers of
soil that are rich and dark.
3. Soil Erosion
a. Soil as a Resource
i. Farming / Agriculture is necessary. We need to find ways to keep the top soil (A
horizon) in place because this is where the humus is and where the soil is the most
nutrient rich for plants.
b. Causes and Effects of Soil Erosion
i. Agricultural Cultivation—increasing global populations leads to increased farming.
Plouging churns up the soil, enabling topsoil to be eroded by wind and rain. Over time,
the quality of the soil also worsens.
ii. Forest Harvesting— as forests are cleared or cut back soil is exposed and erosion
increases. Soil quality degrades.
iii. Overgrazing— when ground vegetation is grazed severely, vegetation cannot come back
quickly enough. If roots are exposed to sun and wind they will dry up before they can
recover. New plant material will not grow and consequently less organic material in the
soil. Soil quality degrades and top soil erodes away.
iv. Urban construction—Vegetation is cleared for roads, neighborhoods, shopping centers.
Top soil is exposed to weathering and then erodes away.
c. Preventing Soil Erosion
i. Manage Crops
1. Planting “shelter belts” of trees as a barrier to wind that might erode soil away
2. Plough in different directions to cut down on water erosion
3. Properly manage grazing of livestock
4. No-till farming—dead vegetation is left in fields to add nutrients to soil and hold
soil in place
ii. Reduce Erosion on Slopes
1. Contour farming- Using natural contours of the land reduces erosion because it
slows the flow of water down the slope
2. Terracing- flat areas are cut into slopes so that plants can grow and soil layers
can thicken.
iii. Reduce Erosion at Construction Sites
1. Cover exposed soil with stray, mats, tarps and mulch to decrease erosion.
2. Topsoil added after construction project is over and then is replanted with grass
and other vegetation to keep the soil from eroding.
3. Retaining walls along slopes keeps soil and rocks from sliding down hills.