Weathering and Soil

Weathering and Soil

Middle School

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Learning Objectives

• Distinguish between mechanical and chemical weathering and the factors that affect them.

• Explain how weathering, erosion, and deposition change Earth’s surface over time.

• Describe what soil is made of and how it forms from rock.

• Use ongoing processes to explain how landforms have and will continue to change.

Key Vocabulary

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Erosion

The process of moving rock particles from one place to another by wind, water, or ice.

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Mechanical Weathering

The physical breakdown of large rocks into smaller pieces without changing their chemical composition.

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Chemical Weathering

The process that breaks down rock through chemical changes, creating new substances in the process.

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Deposition

The geological process where sediments, soil, and rocks are added to a landform or landmass.

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Soil

The loose, weathered material on the Earth's surface in which plants can successfully grow.

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Humus

Dark, organic material in soil that is formed from decayed plants and animals, providing nutrients.

Key Vocabulary

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Uniformitarianism

The idea that the same geologic processes shaping the Earth today also operated in the distant past.

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Permeable

A material full of tiny, connected air spaces that allow water to easily pass or seep through.

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Geoscience Process

Any natural process that is responsible for shaping the surface of our planet, like erosion or weathering.

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Sediment

Small, broken pieces of rock, soil, sand, or other organic matter that settle in a new location.

Breaking Down and Reshaping Earth

• Weathering is the process that breaks down rock into smaller pieces.

• Erosion carries away the broken pieces by water, ice, and wind.

• Deposition drops the sediment in new locations, building up new landforms.

Multiple Choice

What is the name for the process that breaks down rock into smaller pieces?

• Weathering

• Erosion

• Deposition

• Landformation

Multiple Choice

What is the main relationship between weathering and erosion?

• Weathering builds up land, while erosion carries it away.

• Weathering is caused by wind, while erosion is caused by water.

• Weathering breaks down rock, while erosion moves the broken pieces.

• Weathering drops sediment, while erosion creates new landforms.

Multiple Choice

Over millions of years, what is the most likely effect of wind and water on a tall, rocky mountain?

• The mountain would get taller as new rock is deposited on its peak.

• The mountain would become shorter and more rounded as rock breaks apart and is carried away.

• The mountain would remain unchanged because rock is too strong for wind and water.

• The mountain would split in half, creating a deep canyon.

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Varying Scales of Change

Spatial Scale

• Changes can be microscopic, like a raindrop dissolving a tiny amount of a mineral from a rock.

• Changes can also be massive, such as a river carving out the Grand Canyon over millions of years.

• Most features result from many processes that occurred at different scales over vast periods of time.

Temporal Scale

• Geological changes can happen very rapidly, like a landslide that reshapes a hillside within just a few minutes.

• They can also occur very slowly, like the weathering that wears down a mountain range over millions of years.

• Earth's surface is a product of both fast and slow changes happening over very long periods of time.

Multiple Choice

What are the two primary ways to describe the scale of geological changes?

• By their size and the time they take to happen

• By how much noise they make

• By the temperature at which they occur

• By whether they happen on land or in water

Multiple Choice

How does the temporal scale of a landslide compare to the weathering of a mountain range?

• A landslide is a rapid change, while the weathering of a mountain is a slow change.

• A landslide is a microscopic change, while a river carving a canyon is a massive change.

• A landslide is a massive change, while the weathering of a mountain is a microscopic change.

• A landslide and the weathering of a mountain are both examples of rapid changes.

Multiple Choice

Based on the information about geological change, what is the best explanation for the formation of a feature like the Grand Canyon?

• They are the result of both massive, slow processes like erosion and rapid events like landslides over millions of years.

• They were created entirely by one single, massive event that happened very quickly.

• They were formed only by microscopic changes that are too small to see.

• They were shaped exclusively by slow processes, with no rapid changes involved.

Mechanical Weathering: Physically Breaking Rocks

• ​Mechanical weathering physically breaks rocks into smaller pieces.

• This process does not change the rock's chemical composition.

• It is caused by natural forces like ice, wind, and plants.

Multiple Choice

What is the main effect of mechanical weathering on a rock?

• It physically breaks rocks into smaller pieces.

• It changes the chemical composition of rocks.

• It melts rocks into liquid magma.

• It combines small rocks to form larger ones.

Multiple Choice

What is a key characteristic of the smaller rock pieces that result from mechanical weathering?

• They have the same chemical composition as the original rock.

• They are a completely different type of rock.

• They are much harder than the original rock.

• They have a different chemical makeup than the original rock.

Multiple Choice

If a small plant begins to grow in a crack in a large rock, what is the most likely long-term effect on the rock?

• The plant's roots will slowly break the rock apart.

• The rock will change its chemical makeup.

• The plant will dissolve the rock into the soil.

• The rock will provide food for the plant.

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Chemical Weathering: Changing Rock Composition

• Chemical weathering breaks down rock by changing its chemical composition.

• ​Water dissolves minerals, while oxygen combines with iron to form rust.

• Carbon dioxide in rain forms a weak acid that weathers certain rocks.

• Acid rain from pollution causes very rapid and harmful chemical weathering.

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Multiple Choice

What is the primary effect of chemical weathering on rock?

• It is broken into smaller pieces.

• Its chemical composition is changed.

• It is moved to a new location.

• It is polished smooth by the wind.

Multiple Choice

Which of the following describes a process that causes chemical weathering?

• Ice freezing and expanding in cracks.

• Plant roots growing and breaking the rock apart.

• Water dissolving minerals within the rock.

• Wind blowing sand against the rock's surface.

Multiple Choice

If a city with many limestone statues experiences a large increase in air pollution, what is the most likely long-term effect on the statues?

• The statues would be protected from weathering by the pollution.

• The statues would only be affected by physical weathering.

• The statues would begin to dissolve and break down more quickly.

• The statues would not be affected by the change in air quality.

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The Rate of Weathering

Type of Rock

• The kind of rock plays a big role in how fast it breaks down.

• Permeable rocks have tiny spaces that let water seep in, causing faster weathering.

• Rocks made of minerals that do not dissolve easily will weather very slowly.

Climate

• Climate, the usual weather in a place, changes how fast rocks weather.

• Weathering happens most quickly in climates that are both hot and wet.

• Heat speeds up chemical changes, while rain provides water to break down rock.

Multiple Choice

Which two factors have the biggest influence on the speed at which a rock breaks down?

• The size of the rock and its color

• The type of rock and the climate of the area

• The age of the rock and its location on a hill

• The plants and animals living on the rock

Multiple Choice

What is the relationship between a rock's permeability and the rate at which it weathers?

• Permeable rocks are always softer and easier to break.

• Permeable rocks are made of minerals that dissolve very slowly.

• Permeable rocks allow water to seep in, which speeds up the weathering process.

• Permeable rocks are only found in cold climates.

Multiple Choice

Based on the factors that affect weathering, which of the following scenarios would cause a rock to break down the fastest?

• A permeable rock in a hot and wet climate

• A rock with non-dissolving minerals in a hot and wet climate

• A permeable rock in a cold and dry climate

• A rock with non-dissolving minerals in a cold and dry climate

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What is Soil Made Of?

• Soil is a mix of rock particles, minerals, organic matter, water, and air.

• It forms from bedrock, a solid rock layer that weathers and breaks down.

• Humus is decayed plant and animal remains that make soil rich in nutrients.

• Soil particles are classified by size: gravel, sand, silt, and clay.

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Multiple Choice

Which of the following best describes what soil is made of?

• A mixture of rock particles, organic matter, water, and air

• Only decayed plant and animal remains

• A solid layer of unbroken bedrock

• Just sand, silt, and clay particles

Multiple Choice

What is the relationship between bedrock and humus in soil?

• Bedrock provides the rock particles, while humus provides the nutrients.

• Humus breaks down to form bedrock, which then becomes soil.

• Bedrock and humus are both different sizes of rock particles.

• Humus holds water for the soil, while bedrock provides air.

Multiple Choice

Which of the following correctly lists soil particles from largest to smallest?

• Clay, silt, sand, gravel

• Gravel, sand, silt, clay

• Sand, gravel, clay, silt

• Silt, clay, gravel, sand

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How Soil Forms: A Layered Process

• Soil forms from weathered rock, creating distinct layers called soil horizons.

• The bottom C Horizon sits on bedrock and is mostly made of rock particles.

• The B Horizon (subsoil) contains clay and minerals with very little humus.

• The A Horizon (topsoil) is rich in humus from organisms breaking down matter.

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Multiple Choice

What are the distinct layers of soil that form from weathered rock called?

• Soil horizons

• Bedrock layers

• Humus particles

• Mineral deposits

Multiple Choice

How does the composition of the A Horizon (topsoil) compare to the B Horizon (subsoil)?

• The A Horizon is rich in humus, while the B Horizon has very little.

• The A Horizon is mostly made of rock particles, while the B Horizon is rich in humus.

• The A Horizon contains clay and minerals, while the B Horizon sits on bedrock.

• The A Horizon is made of bedrock, while the B Horizon is made of topsoil.

Multiple Choice

If you were planting a garden that needs nutrient-rich soil, which soil horizon would be the most important for your plants and why?

• The A Horizon, because it is rich in humus from broken-down organic matter.

• The B Horizon, because it contains mostly clay and minerals.

• The C Horizon, because it is closest to the bedrock.

• The C Horizon, because it is made of large rock particles.

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Common Misconceptions

Misconception	Correction
Weathering and erosion are the same thing.	Weathering breaks down rocks, while erosion carries the broken pieces away.
Major changes to Earth's surface always happen slowly.	Rapid events like landslides or floods can also change the Earth's surface.
Soil is just non-living dirt.	Healthy soil is a mix of non-living materials and living organisms.
Rocks are permanent and do not change.	Rocks are always changing through processes like weathering.

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Summary

• Weathering breaks rocks, erosion moves sediment, and deposition drops it in new locations.

• Earth's surface changes at different time and spatial scales.

• Weathering is either physical (mechanical) or chemical, depending on rock and climate.

• Soil forms from weathered rock, organic matter, and living organisms.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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