1. Weathering, Soils, & Erosion
Earth Science Unit 7 (Chapter 14)

2. What is Soil
• Soil is a complex mixture of weathered
rock, minerals, organic material (both living &
decaying), water, and air.
• Different soils have different amounts.
• What might change the amounts?
• On average, soil has the following ratios:
– 45% minerals & rocks
– 25% water
– 25% air
– 5% organic material

3. How is Soil Produced?
• Weathering of rocks and minerals
• Deposits of sediments washed/blown to the
site
• Decomposition/actions of living things.

4. Weathering of Rock
• Weathering is the physical or chemical
breakdown of rock into smaller pieces
• These pieces may be large or too small to see!
• Two types of weathering:
– Mechanical
– Chemical

5. Mechanical Weathering
• Physical breakdown of rock
• Chemical composition of the rock does not
change
• Types:
– Exfoliation
– Ice Wedging
– Abrasion
– Plant & Animal Activity

6. Exfoliation
• When rocks come to
the surface, pressure
on them is reduced.
• This can allow cracks
to form, and the rocks
break apart.
• Example: Granite

7. Ice Wedging
• When water flows
into the cracks in
rocks & freezes.
• Why does this
split the rocks?

8. Abrasion
• When rocks broken from other processes
collide and break even more.
• Happens because of gravity, ice, running
water, or wind…wait, how does wind do this?

9. Plant & Animal Activity
• Plant roots act like ice to split rocks
• Animals, mostly ones that burrow, expose
other rocks for further weathering.

10. Chemical Weathering
• Chemical reactions break the minerals in the
rock into different materials
• Types:
– Oxidation
– Hydrolysis
– Carbonation
– Organic Acids
– Acid Rain

11. Oxidation
• When elements
in the rock
combine with
Oxygen, and
break off as new
compounds.
• Usually happens
in rocks with iron
• Produces a red
color

12. Hydrolysis
• When chemicals in the rock react with
water, causing some of the minerals to break
away from the rock.
• Often happens with metals like Potassium &
Aluminum

13. Carbonation
• When CO2 gets into water it makes Carbonic Acid
• This acid can stick to some minerals and form
compounds that are washed away by water.
• This often happens to limestone.

14. Organic Acids
• Some living things produce acids that are
released to the environment.
• These acids can make cracks in rocks and start
the weathering process.
• Examples: Lichens & mosses

15. Acid Rain
• Nitrogen &
Phosphorus from
fossil fuels get into
the air and combine
with rainwater
• This rainwater
weathers rock more
rapidly than normal
rain.

17. Quick Quiz #1
• Why would rocks high in the mountains
weather more quickly than rocks in a plain?
• You have 10 lbs. of rocks. 5 lbs. are in two
rocks, and the other 5 lbs. are in 10 rocks.
Which 5lb. Pile will weather more quickly?
• A stone statue in the desert will weather more
______________than a stone statue in PA.
• This happens when the softer areas of rock
weather away before the harder areas or rock.

18. Formation of Soil
• When new rock is exposed, soil begins to
form.
• This newly exposed rock is known as Bedrock.

19. Formation of Soil
• Weathering begins to break the bedrock into
smaller rocks.
• This layer of partially weathered rocks, above
the bedrock, is known as Regolith.

20. Formation of Soil
• The top of the Regolith is weathered even
more, producing small particles that begin to
form soil.
• Sediments from water or wind may add to the
soil.
• Also, organic material begins to add to the
soil.

21. Formation of Soil
Immature soil
Regolith Bedrock
Young soil
Mature soil

22. Soil Characteristics
• The main characteristics of a soil are
determined by the bedrock it formed
from…this is the Parent Material.
• Parent material can affect soil texture and soil
color.
• Example: quartz-rich rocks produce sandy soil.
• Example: iron-rich rocks produce red soil.

24. Soil Characteristics
• The size of soil particles also affects the
characteristics of a soil.
• Soils are usually categorized by the amounts
of each type of particle that they have.
• Particles are:
– Clay: less than 0.0002mm
– Silt: 0.0002mm – 0.05mm
– Sand: 0.05mm – 2mm

27. Effects of Particle Sizes
• The size of soil particles affects how much
water & air the soil can hold.
• The smaller the particles, the less the soil can
hold.
• However, if the soil is made entirely of larger
particles, the water will run right through the
soil!
• We measure particle sizes by measuring
Porosity.

28. Porosity:
The measure of
the volume of
pores and
distance of
pores per volume
of soil

29. Effects of Particle Sizes
• Infiltration: the rate at which water from
precipitation or other sources flows into the
soil

30. Saturation
The amount of water and air
that can move through soil
Water Water
High permeability Low permeability

32. Soil Profiles
• The formation of soil in
different areas produces
different layers.
• These layers are called
Horizons.
• The composition and depth
of horizons is measured by
looking at a Soil Profile.

33. Soil Horizons
Immature soil
O horizon
Leaf litter
A horizon
Topsoil
B horizon Regolith
Subsoil
C horizon Young soil
Parent
material
Mature soil

34. Soil Horizons
• O Horizon:
• Top Layer/Surface litter layer
• Mostly plant debris
(leaves, branches) and animal
waste
• Brown/black in color

35. Soil Horizons
• A Horizon
• The Topsoil Layer
• A mixture of decomposing
organic matter called Humus
• And…
• Small soil particles from the
bedrock
• Contains most plant roots
and soil organisms.

36. Soil Horizons
• B Horizon (Subsoil Layer)
• A transitional layer of parent
material, organic
material, and minerals
leeched from the A horizon.
• Leaching is when water
reacts with minerals in the
upper soil layers, carrying
them to the B horizon.

38. Soil Horizons
• C Horizon (parent material):
• This is the regolith…which
is…?
• The partially weathered
rocks from the bedrock.
• This layer usually has no
organic material.
• Below the C horizon is the
bedrock.

39. Typical Pennsylvania Soil Profile

40. Other Soil Profiles

41. Different Soil Profiles in Rice Fields

42. What changes a Soil Profile?
• What determines the composition of a soil?
• Mostly the bedrock, erosion, & organic
stuff…but what determines these?!?
• Climate!
• Climate mostly affects the layers of a soil, but
not the materials found in the soil.
• In general, climates that have all 4 seasons
have all the layers of soil.

43. Soil Layers & Climate

44. Tropical Rainforest Soils
• Large amounts of rain leech minerals from the
upper layers to the B horizon, which becomes
very thick.
• Almost no mechanical weathering…why?
• Decomposition is high…why?
• Would this be good for farming?

45. Tropical Soil Profile

46. Desert/Arctic Soils
• Very little rainfall, so almost no chemical
weathering.
• Little life, so not much organic material.
• Soil is mostly regolith (C horizon) with a little
organic material at the top (O horizon)

47. Desert Soil Profile

48. Temperate Soils
• This is where we live.
• Varying seasons provide all kinds of
weathering.
• Lots of life, so there’s lots of organic material.
• Soils in these areas usually have all 4 layers.

49. Temperate Soil Profile

51. Soil Erosion
• What is erosion?
• The movement of weathered materials from
one location to another.
• So…isn’t this the same as weathering?
• No! Weathering helps to build soil, soil
erosion destroys soil.

52. What Causes Soil Erosion?
• Wind
• Gravity
• Water
• Glaciers
• Human Actions can speed up the process!

53. Wind Weathering

54. Wind Erosion

55. Gravity
Weathering

56. Gravity Erosion

57. Water Weathering

58. Water Erosion

59. Glacier Weathering

60. Glacier Erosion

61. Humans & Erosion
• So, how do humans affect this Soil Erosion
thing?
• Development for housing/buildings
• Farming…this is the big one

62. Humans & Erosion
• Farming can expose a lot of lose soil to
erosion.
• This soil is the O and A horizons…why are
these important?
• These contain most of the organic material in
the soil, which plants and animals need to
survive.

63. Farm Erosion

64. Is Erosion Really a Problem?
Areas of serious concern
Areas of some concern
Stable or nonvegetative areas

65. What can we do to prevent Erosion?

66. Contour Plowing
• Hillsides are plowed in curves that follow the
shape of the land.
• Prevents water from flowing straight down.

67. Strip Cropping
• Crops are planted in alternating bands.
• This way, the whole area is not filled with row
crops that don’t cover all the land.

68. Terracing
• Hills are carved into flat “steps”.
• This prevents water from flowing rapidly down
the hill.

69. Crop Rotation
• Planting a crop to harvest 1 year, and one to
leave in place the next year.
• Often combined with strip and contour
farming.

70. Quick Quiz #2
• In your experiments, the variable you
measured is known as the _______________
variable.
• This region would have a large B horizon in the
soil, full of minerals leeched from the upper
layers.
• These soil particles are the smallest.
• The measure of the rate of water movement
into the soil.