Transcript Weathering and Soil

Weathering and Soil
Rocks and Weathering
Uniformitarianism - The same geologic processes that work today worked in
the past to change Earth’s surface
What are these processes?
 Erosion- breaking of rocks into smaller pieces and removal of those rocks
and pieces by wind, water, ice or gravity.
 Weathering-Chemical and physical processes that break down rock and
other substances, heat, cold, water, ice and gases contribute
Weathering and Soil
Two Types of Weathering
1.) Mechanical-rock is physically broken
down into smaller pieces
--usually works slowly
--over long periods of time, can wear down
mountains
 Freeze thaw- important in cold climates,
H2O seeps into cracks in rocks, freezes and
expands which forces the rock apart this is
called FROST WEDGING
 Release of pressure-erosions removes
material from surface of a mass of rock,
pressure on rock is reduced which causes
the outside of the rock to crack and flake off
 Plant growth-roots enter cracks of rocks, as
plant grows, so will roots causing cracks to
get larger
 Action of animals-burrowing by moles.
Gophers, prairie dogs and insects loosens
soil and breaks apart rocks
 Abrasion-the grinding away of rock by other
rock particles carried in water, ice or wind
Weathering and Soil
2.) Chemical- process that breaks down rock
through chemical processes
--process works with mechanical weather
--creates holes or soft spots in rocks
--rock breaks into pieces creating larger surface area
for chemical weathering to work
 Water-dissolves rock and carries other substances that
break down rock, ie O, CO2 and other substances
 Oxygen- combines with Iron to form rust, this is called
oxidation
--rust makes rocks soft and crumbly and causes
red/brown color
White lichens cover a blue granite gravestone like
snow near Lake Champlain, New York. Lichens,
symbiotic organisms that combine fungi and algae,
can be powerful weathering agents, secreting
chemicals called chelates that work to break down
rock.
 Carbon dioxide- when dissolved in water forms carbonic
acid which easily weathers some rocks like marble and
limestone
 Living organisms-plant roots produce weak acid that
breaks down rock, ie. lichens
 Acid rain-burning of fossil fuels puts S, C and N in the
air. These compounds react with water vapor, rain
produced is more acidic than normal = acid rain.
--Acid rain rapidly causes chemical weathering
The effects of acid rain can be seen on this close up of
this statues face. The acid rain has reacted with the
mineral grain of the rock which has over time caused
cavities to form as well wearing away the definition of
the facial features.
Weathering and Soil
Rate of weathering depends on two thing
1.) Type of rock-minerals determine how fast the rock weathers
Minerals that dissolve easily = fast weathering
Porosity- a measure of how much of a rock is open space
Permeable- characteristic of material that contains connected air spaces or pores,
that water can easily seep through
-spaces increase surface area of rock allowing weathering to occur more quickly
Weathering and Surface Area
Weathering breaks rock into smaller pieces. While
the pieces are usually irregularly shaped, you can
model the process with cubes. The diagram shows
what would happen if a rock cube broke into
smaller cubes.
2.) Climate
Wet climate = increase physical and chemical weathering
High Temperature = increase chemical weathering
**chemical reactions occur more quickly at high temperatures
Human activities also can increase the rate of chemical weathering
-more people in an area = more pollution = more acid rain (chemical weathering)
Weathering and Soil
How Soil Forms
Soil-the loos weathered material at Earth’s surface in which plants can grow
-mixture of rock, minerals., decayed organic material, water and air
-forms constantly wherever bedrock weathers, continues over a long period of
time.
Composition
Bedrock-solid layer of rock beneath soil
-exposed to air, H2O and living things it weathers
Soil Particle Size-gravel, sand, silt, clay = portion of soil from weathered rock
Humus-dark colored, organic material in soil
-creates spaces in soil for air and water, contains nutrients plants need
Fertility
Fertitility- a measure of how well soil supports plant growth
-rich in humus = high fertility
-sandy soil = low fertility (low humus)
Weathering and Soil
Soil Texture
 depends on particle size
 Sandy soil = course, grainy
 Soil with lots of clay = smooth and silky
 important for plant growth
 Mostly clay holds a lot of water and little
air = plant death
 Sandy soil = loses water quickly = plant death
 Loam- rich fertile soil that is made up of
equal parts clay, sand and silt
 Best for growing MOST plants
Soil pH
pH less than 4 = acidic, reacts with some metals, turns blue litmus paper red
pH greater than 10 is strongly basic, feels slippery turn red litmus paper red
pH of 7 = neither acidic or basic, ie. water
Soil can be acidic (most acidic = 4) or basic, garden plants like pH 6-7.5
Weathering and Soil
Soil Formation
Rock is broken down by weathering and mixes with other materials on surface--- soil is
formed
Soil Horizon
Horizon ATopsoil- dark, crumbly, mixture
of clay, humus, and minerals formed from plants adding organic material and plant roots
weather rock
Horizon BSubsoil-clay,
particles of rock, little
humus
 Rain water washes
materials down
from horizon A
Horizon C- small
particles of
weathered bedrock
Weathering and Soil
Soils of North America
 Soil, fertility, and climate affect the types and numbers of plants that grow in a
region.
Weathering and Soil
How Do Living Things Affect Soil
 Organisms make humus
 Mix soil to make space
for air and water
Forming Humus
 Organic remains = dead
leaves, roots, and plant
materials
 Formed from decomposition
DecomposersMixing Soil
 Burrowing mixes humus with air and materials in soil
 Animal waste
contributes to nutrients
in soil
From Rock to Soil
 The illustrations show a
rock and rich, fertile soil.
What are the three processes that could change rock into soil?
Weathering and Soil
Soil Conservation
Natural resource- anything naturally occurring in the environment that people use
 Soil = one of Earth’s most valuable resources
 everything that lives on land depends directly or indirectly
on soil
 1/8 of land on Earth’s land is farmable
 Soil takes a long time to form, few cm in hundreds
of years
Soil damage and Loss
Loss of Fertility
 Loss of moisture and nutrients– can occur when only one crop is grown
 Desertification- the advance of desert like conditions into areas that previously were
fertile
Loss of Topsoil
 Exposure to wind and water causes erosion
 Plant covering can protect
 Break force of falling rain
 Hold soil together
 Sod- the thick mass of tough roots at the surface of the soil,
holds soil in place
Weathering and Soil
Soil Conservation
Three ways people use land that might cause changes
1.) farming
2.) mining
3.) development- construction of homes, roads and other structures
Soil Conservation-the management of soil to minimize its destruction
 Crop rotation-the planting of different crops in a field each year; corn or cotton one
year followed by oats, barley or rye
 Corn, cotton take up large amounts of nutrients
 Peanuts, alfalfa and beans restore nutrients
 Legumes contain pods of nitrogen fixing bacteria
 Contour plowing-fields plowed along curves of a
slope instead of straight rows
 Slows runoff, prevents soil from washing away
 Conservation plowing-dead weeds and stalks
are plowed into the ground
 Returns nutrients to soil, retains moisture and hold soil in place
Weathering and Soil
Dust Bowl
Parts of Oklahoma and surrounding states that lost soil is the 1930’s were
called the DUST BOWL
Farmers plowed the land removing the sod and exposed soil, drought dried
out the topsoil which turned to dust and blew away = Desertification
Land Reclamation- the process of restoring an area of land to a more
natural, productive state