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Content Benchmark E.12.C.5
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Earth Science
Atmospheric Process and Water Cycle
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Physical Science

Students know soil, derived from weathered rocks and decomposed organic material, is found in layers. E/S

Soil is an under-appreciated and often overlooked natural resource. This thin layer of minerals, organic matter, and living organisms affects many areas of our lives. We depend on soil to grow our food. Soils are an essential element in the ecosystem, acting as reservoirs for water and nutrients and they also play an important role in maintaining air quality and filtering contaminants from groundwater. By definition, soil is composed of a mixture of weathered rock and organic matter. In order to understand how soil is formed, its composition, and vital functions we must look at how solid rock becomes smaller sediment and is transformed into fertile soil.

Weathering is the breaking of rocks into smaller pieces. Weathering occurs when rocks of the lithosphere are exposed to the effects of the hydrosphere and atmosphere, or more simply stated when air and water breakdown land. There are two types of weathering; physical (mechanical) and chemical.

Physical weathering is the breaking down of rocks into sediments without changing the chemical composition of the rock. The primary physical weathering mechanisms are; frost action, root pry, abrasion, and exfoliation (Figure 1).

Figure 1: Forms of physical weathering (

Frost action, also known as ice wedging, is the breaking of rocks caused by repeated freezing and thawing (contracting and expansion) of water. Water finds its way into cracks in rocks, then upon freezing, wedges the rocks apart. Water increases its volume approximately 9% upon freezing exerting a powerful force as warped sidewalks, potholes in roads, and burst water pipes will confirm.

Root pry is the biological equivalent to frost action. As trees and plants grow, so to does their root system – in both length and diameter. As the diameter of the root grows, it exerts a force powerful enough to wedge rocks apart.

Abrasion involves rocks rubbing against one another, chipping and breaking small pieces off thus changing the physical characteristics of these rocks. Abrasion occurs in stream beds, glaciers, and wind blown sediments.

Exfoliation results from the pressure unloading of rocks once deeply buried and under great pressure to a condition of reduced pressure at Earth’s surface. This reduced pressure environment causes some rocks to fracture.

To learn more about physical weathering, go to

Chemical weathering is the breaking down of rocks by chemical action producing a change in the composition of a rock resulting in the formation of new minerals. This occurs because at the Earth’s surface rocks and minerals are exposed to different chemical conditions than the ones under which they originally formed. The primary mechanisms of chemical weathering are water, oxygen, and carbon dioxide.

Water is the number one agent of chemical weathering, not surprising considering that water is found in great abundance above (atmosphere), on, and just below (groundwater) the Earth’s surface. Water is an excellent solvent which dissolves many minerals and combines with other substances, altering the chemical makeup of the original rock.

Oxidation is the chemical uniting of oxygen with minerals. In much the same way as oxygen combines with iron in a nail to form iron oxide (rust) so to does oxygen combine with iron-bearing minerals exposed at the Earth’s surface chemically altering them.

Rainwater is naturally slightly acidic due to carbon dioxide dissolving in precipitation as it falls through the atmosphere. Carbonic acid is a weak acid that weakens the rock thus breaking it down in the process.

Formation of Carbonic Acid

Carbon Dioxide + Water ? Carbonic Acid

CO2 (gas) + H20 ? H2CO3

To learn more about chemical weathering, go to

The rate at which the agents of physical and chemical weathering breakdown Earth’s rocks are influenced by climate, exposure, and composition of the rock. Higher temperatures and greater amounts of water present are climatic factors that speed up the rate at which rocks break down. Exposure relates to how much of the rock is open to the elements. As a large boulder cracks the amount of surface area open to the elements increases thus accelerating the rate at which the rock will break down. In other words, the smaller the rock is the greater the ratio of surface area to volume and the faster the rock crumbles. Finally, a key factor to consider is the chemical composition of the rock being weathered – some rocks are simply more resistant to the agents of physical and chemical weathering, affecting the rate at which they decay.

Figure 2: Agents of physical and chemical weathering breakdown Earth’s rocks. From:

For more detailed information on factors that affect the rate of weathering, go to


Did you know that five tons of topsoil spread over an acre is only as thick as a dime? Soil is one of the natural products of weathering. Soil is a mixture of particles of rock (sediments), minerals, and organic matter produced through the process of weathering. Soil actually constitutes a living system, combining with air, water, and sunlight to sustain plant life. An average soil sample is 45% minerals, 25% water, 25% air, and 5% organic matter. Critical in supporting a myriad of forms of plant and animal life, soil contains the required nutrients to cultivate that life.

For a scientific definition of soil, go to

For a list of interesting facts about soil, visit US EPA “What on Earth is soil?” at,

Soil takes a great deal of time to develop – hundreds, thousands or even millions of years. With this known, soil is in point of fact a nonrenewable resource. The evolution of soil is a natural process requiring a significant amount of time and special conditions in order to develop into a fertile and productive reserve. The first step in the evolution of soil is gradual mechanical and chemical weathering of bedrock into a layer of rock debris called regolith. Continued weathering of this layer leads to increasingly smaller and finer particles, ultimately resulting in the formation of soil. As this regolith weathers, environmental factors such as climate, topography, living organisms, and time all play an important role in the rate at which large rocks evolve to become fertile soil (Figure 3).

Figure 3: Bedrock begins to disintegrate as it is subjected to environmental forces.

For more information on soil forming factors, go to

As a result of the weathering process and biological activity, soil horizons (layers) form. A soil profile refers to the layers of soil; horizon O, A, B, C, and D. The uppermost layer generally is an organic horizon. It consists of fresh and decaying plant remains from such sources as leaves, needles, twigs, moss, lichens, and other organic material buildup. This layer is the darkest layer because of the decomposition of organic matter into humus. Following the O horizon is the A horizon which consists of primarily mineral material. Within the A horizon (typically referred to as topsoil); humus, plant roots, insects, worms and finer sediments are found in abundance – making this the most productive layer of soil (and the most evolved – that is different from what it started as, solid rock). Found just below the A horizon is the B horizon, often referred to as subsoil. The B horizon is lighter in color than the A horizon, as this layer lacks much of the humus present in the layer above it. An important process that helps to increase the thickness of the soil is a process called leaching. Leaching resembles what happens in a coffee pot as water drips through the coffee grounds. As water infiltrates the topsoil, it dissolves minerals and carries them downward through the pore spaces in soil, ultimately depositing them in the lower layers of the profile. The C horizon consists of large weathered rocks referred to as the parent material, as this is the rock that the soil formed from. Horizon D is the solid bedrock, and bottommost layer in a mature soil profile (Figure 4).

Figure 4. Cross-section of a soil profile showing
important components of the different horizons. (

For several animations of the development of soil horizons and their characteristics, go to

For additional information on development of soils in a variety of climates, go to

See the Earth Observatory article on The Carbon Cycle at,

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Content Benchmark E.12.C.5

Students know soil, derived from weathered rocks and decomposed organic material, is found in layers. E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly believe that soil is the precursor of rocks. It changes from soil to clay to rock.

Which came first the soil or the rock? It is clear from the discussion on the evolution of soil that the rock comes first. It is the actions of both physical and chemical weathering of the atmosphere and hydrosphere wearing away at the solid rock of the earth that, over long periods of time, create the smaller and finer sediments necessary for soil to develop. Many activities exist that can demonstrate to students, in a relatively short period of time, the process of larger rocks breaking down to become smaller ones. Several activities are listed in the Intervention section of this benchmark.

For additional student misconceptions related to not only this benchmark but across many of the major concepts within science, visit

2. Students incorrectly believe that plants get their food from the soil.

Plants are autotrophs, meaning they create their own food from inorganic substances and energy. The plant takes inorganic substances and water from the soil, not food. Photosynthesis is a complex series of reactions in which light energy is utilized to convert inorganic substances into carbohydrates. See benchmark L.12C.1 For more information related to how plants convert energy and the role soil plays, go to

3. Students incorrectly believe that the soil of the rainforest is very fertile.

Only the top few inches of rainforest soil is very fertile, but take the rainforest away, and the soil itself is not very good at all. The soil’s fertility is derived from the intense biological activity where biomass from dead plants is recycled very quickly because of the warmth and moisture (in a matter of weeks). The high rainfall quickly washes nutrients out of the topsoil unless they are incorporated in the forest plants. As such, minerals are found mainly in the forest plants, not the soil. For additional information on rainforest misconceptions, go to

4. Students incorrectly think that soil is quite young and has been formed in a few years; others think that soil is as old as the Earth.

Almost without exception, soils take hundreds to thousands of years to form. Students will have difficulty with this time scale, as their ability to perceive time and the passage of time is limited. Other students will incorrectly equate the formation of soils with the formation of the earth. It is important that students understand the steps involved in the creation of fertile soil, its essential components and continued evolution. Soil is dynamic, always evolving and responsive the mechanical and chemical weathering, pollution, and misuse.

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Content Benchmark E.12.C.5

Students know soil, derived from weathered rocks and decomposed organic material, is found in layers. E/S

Sample Test Questions

1st Item Specification: Describe the structure of soil, its components, and its formation.

Depth of Knowledge Level 1

  1. The cross section below shows soil layer X, which was formed from underlying bedrock.


Which change would most likely cause soil layer X to increase in thickness?

  1. A decrease in slope
  2. A decrease in rainfall
  3. An increase in biologic activity
  4. An increase in air pressure
  1. As particles of sediment in a stream break into several smaller pieces, the rate of weathering of the sediment will
    1. decrease due to a decrease in surface area.
    2. increase due to a decrease in surface area.
    3. decrease due to an increase in surface area.
    4. increase due to an increase in surface area.
  1. Which activity demonstrates chemical weathering?
    1. Freezing of water in the cracks of a granite boulder
    2. Abrasion of a streambed by tumbling rocks
    3. Dissolving of limestone by carbonic acid
    4. Boulders falling from a cliff and shattering on the rocks below
  1. The four limestone samples illustrated below have the same composition, mass, and volume

Under the same climatic conditions, which sample will weather SLOWEST?

  1. Sample A
  2. Sample B
  3. Sample C
  4. Sample D
  1. Humus, which is formed by the decay of plant and animal matter, is important for the formation of most
    1. round sediments.
    2. soils.
    3. surface bedrock.
    4. minerals.
  1. The cross section below shows a soil profile.


This soil was formed primarily by

  1. erosion by running water over a relatively long period of time.
  2. weathering and biological activity over a relatively long period of time.
  3. erosion by running water over a relatively short period of time.
  4. weathering and biological activity over a relatively short period of time.
  1. The cross section below shows layers of soil.


Which two processes produced the layer of dark brown to black soil?

  1. Compaction and cementation
  2. Erosion and uplift of bedrock
  3. Weathering and biological activity
  4. Melting and solidification of magma

Depth of Knowledge Level 2

  1. The cross section below shows layers of soil.


Which change would most likely cause soil layer X to increase in thickness?

  1. A decrease in slope
  2. An increase in biologic activity
  3. A decrease in rainfall
  4. An increase in air pressure
  1. Two tombstones, located in the same cemetery approximately 10 meters apart, face east. Tombstone A had dates cut into the rock in 1922. Tombstone B had dates cut into the rock in 1892.


Which statement best explains why the dates are more difficult to read on Tombstone A than on Tombstone B?

  1. Tombstone A contains minerals less resistant to weathering than Tombstone B.
  2. Tombstone A has undergone a longer period of weathering than Tombstone B.
  3. Tombstone A has experienced cooler temperatures than Tombstone B.
  4. Tombstone A was exposed to less acid rain than Tombstone B.
  1. Use graphs #1-4 to answer the following question.
Graph 1
Graph 2
Graph 3
Graph 4

Which graph best represents the chemical weathering rate of a limestone boulder as it is broken into pebble-sized particles?

  1. Graph 1
  2. Graph 2
  3. Graph 3
  4. Graph 4
  1. Base your answer to the question on the graph below, which shows the effect that the average yearly precipitation and temperature have on the type of weathering that will occur in a particular region.


Which type of weathering is most common where the average yearly temperature is 5°C and the yearly precipitation is 45 cm?

  1. Moderate chemical weathering with frost action
  2. Moderate chemical weathering
  3. Very slight weathering
  4. Slight frost action

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Content Benchmark L.12.C.5

Students know soil, derived from weathered rocks and decomposed organic material, is found in layers. E/S

Answers to Sample Test Questions

  1. C, DOK Level 1
  2. D, DOK Level 1
  3. C, DOK Level 1
  4. A, DOK Level 1
  5. B, DOK Level 1
  6. B, DOK Level 1
  7. C, DOK Level 1
  8. B, DOK Level 2
  9. A, DOK Level 2
  10. D, DOK Level 2
  11. D, DOK Level 2

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Content Benchmark L.12.C.5

Students know soil, derived from weathered rocks and decomposed organic material, is found in layers. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Soil Science Education Home Page
This site contains extensive links to topics such as; Soils Science Basics, Soil and Society, and Soil and the Environment. Additionally, links to teacher and student demonstrations, activities, and educational resources are provided from the home page.

2. Natural Resources Conservation Service (NRCS)
Students create a scale model of a real soil profile
Additional lesson plans;

3. USDA Forest Service Curriculum links
The forestry home page of activities categorized in to grade bands contains an extensive list of environmental investigations.

What’s Happening Below the Surface?
Part of the Urban Forestry Laboratory Exercises (high school), this investigation examines soil characteristics for samples collected in the field.

4. GLOBE Program (Global Learning and Observations to Benefit the Environment)
The GLOBE project provides many learning activities related to studying the natural resource of soils.

5. From the Surface Down: An Introduction to Soil Surveys for Agronomic Use
The US Department of Agriculture and the Natural Resources Conservation Service developed this 29 page PDF resource. Contained within this document are answers to questions such as: What are soil horizons? How is soil formed? What are the soil forming processes? Soil survey interpretation information and excellent graphics related to soil development and conservation are also provided.

6. National Geospatial Development Center
This site contains a multitude of links to activities, background, animations, and additional educational resources pertaining to soil.

7. Orovada – The Official State Soil of Nevada!
Each state in the US has selected a state soil. Areas with similar soils are grouped and labeled as soil series because of their similar origins, chemical, and physical properties cause the soils to perform similarly for land use purposes. To see a profile and description of Nevada’s state soil;

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