Weathering, Erosion, and
Soils
Mandy Meeks, Baldwin Arts and Academics Magnet,
Earth Science
Weathering
What is weathering?
 Weathering refers to surface processes that break down
rock.
 Weathering breaks rock into small loose pieces called
sediment.
 Examples: sand, silt, and clay
 Over millions of years weathering has changed Earth’s
surface.
Mechanical Weathering
 Mechanical weathering occurs when rocks are broken
apart by physical processes.
 The overall chemical makeup of the rock stays the sameeach fragment of rock has the same characteristics as the
original rock.
Mechanical Weathering- Plants
and Animals
 When plants grow in cracks in
rocks, the roots enlarge the
cracks.
 Burrowing Animals loosen
sediment and bring it to the
surface where other weathering
processes act on it.
Mechanical Weathering- Ice
-
Ice Wedging occurs in cold climates where water enters
cracks in rocks and freezes.
-
Water expands as it turns to ice, extending the cracks
and breaking rocks apart.
-
Ice wedging is a problem for roads in highways in the
mountains, causing potholes to form.
Mechanical Weathering: Surface
Area
 Mechanical weathering reduces rocks into smaller pieces,
which increases their surface area.
 This exposes more of the rock to more water and
oxygen, which speeds up the process of chemical
weathering.
Chemical Weathering
 Chemical Weathering occurs when chemical reactions
dissolve the minerals in rocks or change them into
different minerals.
 This changes the chemical composition of the rock and
often makes the rock weaker.
Chemical Weathering- Natural
Acids
 Carbonic Acid is formed when
carbon dioxide and water react
in the air.
 Carbonic acid causes minerals
like calcite to dissolve.
 This causes stones like
limestone to weather away,
forming caves.
 Chemical weathering causes the
mineral feldspar, which is found
in granite and sandstone, to be
broken down into kaolonite,
forming clay.
Chemical Weathering- Plant Acids
 Plant roots and decaying plants give off acids that
dissolve minerals in rocks.
 As the rock breaks apart, nutrients become available
to the plants.
Chemical Weathering- Oxygen
 Oxidation is when some materials chemically react when
exposed to oxygen and water.
 Example: rusting of iron
 Magnatite, a mineral that contains iron, oxidizes to form
limonite.
 Many oxidized minerals have a reddish color.
Weathering and Climate
 Climate is the pattern of weather that occurs in a
particular area over many years.
 In cold climates, freezing and thawing causes many
rocks to break down due to ice wedging.
 Chemical weathering occurs faster in warm, wet
climates.
Weathering and rock type
 Some rocks weather more
quickly than others.
 Example: Marble and
granite
Soils
Formation of Soil
 Soil is a mixture of weathered rock, decayed organic
matter, mineral fragments, water, and air.
 Rock fragments do not become high quality soil without
organic matter.
 Soil can take thousands of years to form.
Factors Affecting Soil Formation
1. Climate
2. Slope of Land
3. Types of Rock
4. Types of vegetation
5. Amount of time rock has been
weathering
Composition of Soil
 Rock and mineral fragments
 Organic matter
 Humus- decayed plant and animal material in soil
Soil Profiles
 Soil is divided up into layers called
horizons.
 All horizons of soil form the soil
profile.
 Soils have three horizons- A (top
layer, lots of humus), B(middle
layer, less humus), and C (bottom
layer, consists of partially
weathered rock).
Erosion
Soil Erosion
 Soil erodes when it is moved from the place where it formed.
 In normal conditions there is a balance between soil erosion and
soil production.
 Agents of erosion include gravity, water, wind, and glaciers.
 Soil erosion is harmful because plants do not grow well when
topsoil has been removed.
Human Causes of Erosion
 Agricultural cultivation
 Plowing, leaving fields bare after harvest
 Forest Harvesting
 Overgrazing
Erosion by gravity
 The greater an object’s mass is, the greater is gravitational
force is.
 Mass movement is any type of erosion that happens as
gravity moves materials downslope.
 Material can move downhill in a large mass (slump) or when
sediments slowly move downhill (creep).
 Other kinds of mass movement include rockslides and
mudslides
Glacier Erosion
 A large mass of ice and snow moving on land under its
own weight is a glacier
 Continental glaciers cover 10% of Earth; they once covered
as much as 28% of Earth.
 Glaciers erode land as they pass over it and deposit
eroded material somewhere else
Wind Erosion
 Wind erodes the Earth via deflation (blowing small loose
sediment around) and abrasion (windblown sediment
striking rock).
 Wind erosion can lead to sandstorms and dust storms.
 Dunes form when sediment is deposited by the wind.
 Many dunes migrate away from the wind unless they are
planted with grass.
 Planting vegetation is one of the best ways to slow wind
erosion.
Water Erosion
 Water that does not soak into
the ground or evaporate but
instead flows across Earth’s
surface is called runoff.
 The amount of rain and the
length of time rain is falling
affect runoff.
 Slope of the land and
vegetation also affects runoff.
Types of Water Erosion
 Water can travel in
channels, carrying
away soil and causing
rills and gullies.
 Water can travel in
sheets, picking up and
carrying away
sediments.
 Water can form
streams
 Sediment carried by a
stream is called the
load.
Groundwater
 Water that soaks into the
ground is called groundwater.
 The water table is the upper
surface of groundwater.
 Carbon dioxide dissolved in
groundwater can form carbonic
acid, causing caves and sink
holes as it weathers away
limestone.
Consequences of Erosion
 Building on steep slopes
 Lots of time and $$ spent
trying to slow down
erosion
 Beachfront developments
are affected by shoreline
erosion.
 Loss of Soil Productivity
 Sediment pollution
 Runoff from pesticides
and fertilizers