Chapter 5 Atoms to Minerals

Chapter 12
Weathering, Soil, and Erosion
New rock forms from
molten minerals deep within the earth.
Over time, rock can work its way to surface where
it can be exposed to water and oxygen.
These chemicals can begin to break this rock
down in the form of weathering.
There are two types of weathering, mechanical
(where things physically break the rock) and
chemical (where rocks are dissolved!)
Mechanical Weathering
Ice and Water
In most places, water seeps between
rock joints(cracks) and can freeze.
Remember, when water freezes, it
This expanding ice can push and
break rock apart.
This process is called frost
wedging. .
Clay will absorb water and
expand and shrink as well
Another form of mechanical weather occurs
when broken rock bumps into other rock.
These collisions break rock down in a process called
This occurs with rock moving in rivers, blowing wind,
and even rolling down hills.
Over time, the particles become
smaller and smaller.
Plants and Animals
The roots of plants can break
apart rock over time.
Small animals and even
earthworms can burrow
This creates tunnels that
allows water to penetrate
below the surface.
Upward Expansion
Rocks like granite form
deep within the earth.
Once the rock cools it
remains under great pressure below the surface. .
As time passes, the rock above can erode away
eventually exposing this rock.
Once at the surface, this rock is no longer under
pressure. It begins to crack, and weathers away
quickly in a process called exfoliation (skin)
Chemical Weathering
Water and Chemical
Water and water vapor are everywhere.
Chemical weathering by
water is called hydrolysis.
Many rock minerals react with
water and break down into clays..
Water can form dissolved acids.
Carbonic acid forms from water reacting to
the carbon dioxide in the air and falls as rain
As this carbonic acid seeps into the ground, it
can dissolve many types of minerals into clay.
When carbonic acid comes into contact with the
mineral calcite, it can dissolve away large areas
underground creating caves.
Pollution from factories and car exhaust can mix with
rain creating acid rain. This can be harmful to plants
and animals over long periods of time
Oxygen and Chemical Weathering
Rocks that contain high levels of iron can turn a
reddish color over time.
This is caused by the oxygen in the air and water.
Since the reaction is due to oxygen with the iron
molecule, the process is called oxidation.
Rates of Chemical Weathering
Surface Exposure
The rate at which a rock weathers depends on
how much of its surface is exposed.
Once a rock starts to break apart, more of it is
exposed to water and oxygen.
Composition of Rock
Some rock is more resistant to weathering than
other rock.
In some places you can see which rock is more
resistant by looking to see which
layer overhangs the others
In areas where it is warm and wet, chemical
weathering occurs at a greater rate.
In dry and cold areas, mechanical weathering has
more of an effect on the rock because of ice wedging
12.1 Questions
1. Give three examples of mechanical weathering
frost wedging, abrasion, plants/animals, upward expansion
2. Give two examples of chemical weathering
Hydrolysis (acid rain) and oxidation
3. Give two examples of what might increase the
rate weathering will occur.
Increased surface, composition and climate warm/wet
4. What kind of weathering creates caves?
12.2 Soil
Weathering has taken
place ever since the earth
It has taken down
mountains and given us the
landforms we see today.
But the most important thing weathering
gives us is the life supporting resource - soil.
How Soil Forms
The material that soil
forms from is called its parent material.
If the soil forms from the
material below it, it is
called a residual soil.
In some places, weathered
material is transported by
rivers or wind. This called
transported soils.
Soils form at different rates depending on the
parent material and the climate..
Scientists use the term
mature to describe how
developed a soil is.
To know a soil’s maturity,
we study the soil’s profile
or cross section.
In mature soil profiles
there are three distinct
zones called soil horizons
Soil that is in the top “A” horizon, is called top
soil. This soil tends to be dark due to an organic
material called humus.
Over time most clay
material can seep down
into the “B” horizon or
subsoil. Other materials
like iron also wash
down into this zone
The lowest zone or “C” horizon, is mostly
just slightly weathered parent material.
Soil Composition
Soils are made up of different size particles.
The smallest are the clays, then the silts and
finally the sand size particles.
Soils that are mostly sand hold lots of
water but dry out quickly.
Soils that are mostly clay
do not absorb water well.
What determines the composition of a soil?
Many factors like…
- time
- parent material
- plants and
- topography
- climate
Soil regions
12.2 Questions
1. The material that soil forms from is called its
parent material.
2. In some places, weathered material is
transported by rivers or wind. These are known as
transported soils.
three distinct zones
3. In mature soils there are _____
called soil horizons.
4. Soils are made up of three particle sizes; clay,
silts and _______
Mass Movements and Erosion
Anticipatory Response
1. Very slow movement of soil down a hill slope is creep.
2. Blocks of land tilt and moving downhill is “Tilt”.
3. Rapid movement of water that contain large amounts
of clay is a landslide.
4. Rivers, streams, glaciers, wind, and ocean waves are
agents of erosion.
Our Goal: To understand the various types of Mass M_________
during which ero________ reshapes Earth’s surface.
Mass Movements & Erosion
As rock weathers, it
doesn’t stay in the same
Rain and gravity cause
weathered materials to
move downward.
These are known as - mass
Erosion is the removal and
transport of materials with
wind or running water
Mass Movements
Soil protects bedrock from being weathered.
Mass movements however continually
pull downward exposing the underlying rock..
La Conchita, 2005
Rocks that weather away
from steep cliffs are known as
Landslides can include
soil/rock that slide down steep hills.
Creep is a very very slow movement of soil down a hill
slope caused from the presence of water in the soil.
Trees, fence posts and other object will start to tilt from
this movement.
Sometimes blocks of land tilt & move downhill
Slump happens fast (even at once).
Earth Flows (“medium paced”)
During an earth-flow, a mass of weathered material has
been saturated with water & flows down as 1 piece.
Earthflows can last days to years depending on the
material and water present.
Mud Flows
A mudflow is the rapid movement of water with
large amounts of clay and silt.
Some can travel at speeds of 60 m.p.h.!
Volcanic lahars can
send large volumes of
mud downhill quickly.
Erosion and Landforms
Rivers, streams, glaciers, wind, and ocean waves
are agents of erosion.
Their forces combined with plate tectonics shape our
The topography of an area depends on the balance
between all these forces.
The rates (speed) at which all this takes place is
further influenced by climate.
An extinct volcano can weather
to the point of exposing its
internal neck.
12.3 Questions
1. This is a very slow movement of soil down a hill
slope caused by the presence of water in the soil.
2. This occurs when blocks of land tilt and move
downhill (quickly).
3. This a rapid movement of water that contain
large amounts of clay and silt and can travel at
speeds of 60 miles per hour!
mud flow
4. Rivers, streams, glaciers, wind, and ocean
agents of erosion.
waves are known as ________
Soil as a Resource
Soil as a Resource
Soil is very important because it
supports plant life. This then
supports animal life and human
Soil is a renewable resource but
it only covers 25% of the earth.
Many places like polar regions
and deserts are void of soil.
World populations are growing,
and soil rich areas are being
expanded into.
Soil Fertility
Soil fertility is the ability for soil to grow plants.
The proportions of minerals, water and organic
matter determine the types of plants will grow in soil.
Potatoes grow better in one type of soil than wheat
Soil Depletion
When plants grow and die, they return
the nutrients they use back to the soil.
When crops are harvested, the
nutrients stay with the crop.
Over time, the soil will become
depleted of its nutrients.
Farmers know they can rotate their
crops, letting their fields fallow
(replenish themselves) by using
other fields for awhile.
Other option is to add fertilizers to
the soil to restore needed nutrients
Desert soils can be watered or irrigated.
The dry air causes the
water to evaporate
quickly, leaving behind
minerals and dissolved
Salinization occurs when
years of watering has left
too many minerals and
salts in the soil.
Erosion and Soil Conservation
Erosion by wind and water occur naturally.
Human activities, like farming, construction,
mining have left many areas unprotected. This
leads to faster erosion.
Fortunately, farmers have developd better farming
techniques to prevent soil erosion.
These include…
Contour farming
Terraces (levels)
(alternating planting)
12.4 Questions
1. Soil is a renewable resource but it only covers
25 of the earth.
2. The proportions of _______,
minerals water and organic
matter determine the types of plants will grow in soil.
3. __________
Salinization occurs when dry air causes water to
evaporate quickly, leaving behind minerals and
dissolved salts.
4. Wind breaks, contour farming, terraces and stripcropping are forms of soil ___________

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