Rocks, weathering and soils

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Lesson notes

Geological Time

Geological time refers to the vast timescales over which Earth's rocks and landscapes have formed and changed.
Processes like weathering and erosion act over millions of years, shaping the land we see today.
Understanding geological time helps explain why some rocks are much older than others.

Types of Rock

Igneous rocks form when molten rock (magma or lava) cools and solidifies.
Sedimentary rocks are made from layers of sediment that become compacted and cemented over time.
Metamorphic rocks form when existing rocks are changed by heat and/or pressure.
Examples include granite (igneous), limestone (sedimentary) and slate (metamorphic).

The Rock Cycle

The rock cycle describes how rocks are continually formed, broken down and reformed over geological time.
Weathering and erosion break down existing rocks into sediment.
Sediment can be transported, deposited and lithified to form sedimentary rock.
Heat and pressure can change rocks into metamorphic rocks, and melting can produce magma for igneous rocks.

Weathering: Physical Processes

Physical weathering (also called mechanical weathering) breaks rocks into smaller pieces without changing their chemical composition.
Freeze-thaw weathering occurs when water freezes in cracks, expands by 9.2%, and exerts pressure that can fracture rock.
Thermal stress weathering results from repeated expansion and contraction of rock due to temperature changes, leading to cracking.
Onion-skin weathering (exfoliation) is the peeling of outer rock layers due to repeated heating and cooling.
Biological weathering includes plant roots prying apart rocks and burrowing animals disintegrating rock.

Weathering: Chemical Processes

Chemical weathering involves reactions between rock minerals and water, atmospheric gases (like oxygen and carbon dioxide), or biologically produced chemicals.
Water is the principal agent of chemical weathering.
Chemical weathering can dissolve minerals or alter them into new minerals.
Biological weathering can also be chemical, for example when lichens produce acids that break down rock.

Soil Formation

Soil forms from the breakdown products of rocks mixed with organic material (humus).
Weathering is the first step in soil formation, creating loose rock fragments.
Organic matter from dead plants and animals enriches the soil.
Soil formation is a slow process that can take thousands of years.

Soil Horizons

A soil profile is a vertical section through soil, showing distinct layers called horizons.
The O horizon is the top layer of organic matter (leaf litter, humus).
The A horizon (topsoil) is a mixture of organic material and minerals, where most plant roots grow.
The B horizon (subsoil) accumulates minerals leached from above.
The C horizon is made of weathered parent rock fragments.
The R horizon is the underlying solid bedrock.

Why Soils Differ

Soils vary because of differences in parent rock, climate, relief (slope), organisms and time.
Climate affects weathering rates and the amount of organic matter.
Steep slopes may have thin soils due to erosion.
Different parent rocks produce soils with different mineral contents and textures.

UK Rock Landscapes

Granite landscapes, such as Dartmoor, form from igneous rock that is resistant to weathering, often creating high moorland with tors.
Limestone landscapes, such as the Peak District or Yorkshire Dales, are shaped by chemical weathering, producing features like caves and pavements.
The type of rock influences the landscape's shape, drainage and vegetation.

Slides

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Practice questions

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1.What is the term for the breakdown of rocks in their original place, without movement?
Easy
- AErosion
- BWeathering
- CTransportation
- DDeposition
2.Which of the following is the main agent of both physical and chemical weathering?
Easy
- AWind
- BWater
- CSunlight
- DGravity
3.Which of the following are types of physical weathering? (Select all that apply.)
Medium
- AFreeze-thaw weathering
- BThermal fracturing
- CPressure release
- DCarbonation
- EHydrolysis
4.Weathering involves the transport of rocks and minerals by agents such as water, ice, wind, and gravity.
Easy
True or false?
5.Match each weathering process with its correct description.
Medium
- Frost wedging
- Ice segregation
- Thermal fatigue
- Expansion of freezing water in cracks
- Growth of ice lenses that pry rock apart
- Repeated stress cycles gradually weaken rock
6.Put the following steps of the rock cycle in the correct order.
Medium
- Weathering breaks down existing rock into sediments
- Sediments are transported and deposited
- Sedimentary rock is formed
- Heat and pressure change rock into metamorphic rock
7.Which type of weathering is most important in subarctic or alpine environments?
Medium
- AChemical weathering
- BPhysical weathering
- CBiological weathering
- DErosion
8.Which of the following statements about frost weathering are true? (Select all that apply.)
Medium
- AFrost wedging is most effective in small tortuous fractures.
- BIce segregation is likely the more important mechanism of frost weathering.
- CFrost wedging is most effective in the tropics.
- DFrost weathering is a form of chemical weathering.
- EIce segregation occurs when supercooled water migrates to ice lenses.

Revision notes

## Rocks, weathering and soils **Weathering** is the *in situ* breakdown of rocks, soils and minerals by water, air, sunlight and living things. It is **not** erosion: erosion moves material away, weathering happens on the spot. ### Types of weathering - **Physical (mechanical)** — rock breaks into smaller pieces with **no chemical change**. - **Freeze–thaw**: water freezes in cracks, expands by **9.2%**, and pries rock apart. Needs daily freeze–thaw cycles and water-saturated rock. - **Onion-skin / thermal stress**: repeated heating and cooling makes outer layers peel off. - **Pressure release**: rock expands when overlying weight is removed. - **Biological**: plant roots, burrowing animals and lichens wedge rocks apart. - **Chemical** — reactions with water, oxygen, carbon dioxide and acids produced by living things. - **Biological weathering** — living organisms cause both physical and chemical breakdown. ### The rock cycle - **Igneous** — formed from cooled magma or lava. - **Sedimentary** — formed from weathered rock fragments and organic material pressed together. Covers **66% of Earth's continents**. - **Metamorphic** — existing rock changed by heat and pressure. ### Soil Weathered rock mixes with **organic material** to make soil. Soil forms in **horizons** (layers). Soils differ because of parent rock, climate, plants and time. ### UK rock landscapes - **Granite** — hard igneous rock, often upland areas. - **Limestone** — sedimentary rock, often forms distinctive scenery. **Exam tip:** Link weathering to the rock cycle — weathered rock becomes sediment, then sedimentary rock.

Weathering: The deterioration of rocks, soils and minerals through contact with water, atmospheric gases, sunlight and biological organisms. It occurs in situ, with little or no movement.
Erosion: The transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity.
Physical weathering: The breakdown of rocks into smaller fragments through mechanical effects such as heat, water, ice and wind, without chemical change.
Chemical weathering: Reactions to water, atmospheric gases and biologically produced chemicals with rocks and soils.
Biological weathering: Weathering caused by living organisms, including both physical and chemical processes.
Freeze–thaw weathering: Physical weathering caused by the formation of ice within rock outcrops; water expands by 9.2% when it freezes, prying cracks apart.
Thermal stress weathering: Weathering resulting from the expansion and contraction of rock due to temperature changes.
Soil: Material left after rock breaks down, combined with organic material.
Soil horizons: The layers within a soil profile.
Rock cycle: The continuous process by which rocks are formed, broken down and reformed; weathering is a crucial part of it.
Igneous rock: Rock formed from cooled magma or lava.
Sedimentary rock: Rock formed from weathered rock fragments and organic material; it covers 66% of Earth's continents.
Metamorphic rock: Rock changed by heat and pressure.

Flashcards

What is weathering?

Weathering is the breakdown of rocks, soils and minerals in situ (on-site) through contact with water, atmospheric gases, sunlight and living organisms.

How does weathering differ from erosion?

Weathering happens in situ with little or no movement, while erosion involves the transport of broken rock and minerals by agents such as water, ice, wind, waves and gravity.

What are the two main types of weathering?

Physical (mechanical) weathering and chemical weathering. Biological weathering is a form of chemical weathering caused by living organisms.

What is physical weathering?

Physical weathering, also called mechanical weathering or disaggregation, breaks rocks into smaller fragments without changing their chemical composition.

What is chemical weathering?

Chemical weathering involves reactions between rocks and soils and water, atmospheric gases, or biologically produced chemicals, which alter the rock's chemical makeup.

What is the principal agent of both physical and chemical weathering?

Water is the principal agent, though atmospheric oxygen and carbon dioxide, and the activities of living organisms, are also important.

What is frost weathering?

Frost weathering is the collective name for physical weathering processes caused by the formation of ice within rock outcrops.

What is frost wedging?

Frost wedging is the widening of cracks or joints in rock caused by the expansion of pore water when it freezes.

By how much does water expand when it freezes?

Water expands by 9.2% when it freezes, which can generate pressures greater than 200 megapascals in theory.

What conditions are needed for frost wedging to be effective?

Frost wedging works best in small, tortuous fractures in rock that is almost completely saturated with water, with daily cycles of freezing and melting.

What is ice segregation?

Ice segregation is a physical weathering process where supercooled water migrates to form ice lenses and needles within rock fractures, gradually prying the rock apart.

At what temperatures is ice segregation most effective?

Ice segregation is most effective in rock whose temperature averages just below freezing, between −4 °C and −15 °C.

What is thermal stress weathering?

Thermal stress weathering results from the expansion and contraction of rock due to temperature changes.

What is the difference between thermal shock and thermal fatigue?

Thermal shock occurs when stresses are so great that rock cracks immediately, while thermal fatigue involves repeated cycles of stress that gradually weaken the rock.

What is block disintegration?

Block disintegration is when rock joints weaken from temperature fluctuations and the rock splits into rectangular blocks, attributed to thermal fatigue.

How do plant roots contribute to physical weathering?

Plant roots can enter cracks in rocks and pry them apart, a process called wedging.

How do animals contribute to physical weathering?

Burrowing animals such as worms can help disintegrate rock by breaking it apart.

What is 'plucking' by lichens?

Plucking by lichens is a form of biological weathering where lichens help disintegrate rock by physically prising grains away.

What is the rock cycle?

The rock cycle is the continuous process by which rocks are formed, broken down by weathering and erosion, and reformed into new rocks.

What are the three main types of rock?

Igneous, sedimentary and metamorphic rocks.

How does sedimentary rock form?

Sedimentary rock forms from the products of weathered rock that are transported, deposited and then compacted and cemented together.

What percentage of the Earth's continents is covered by sedimentary rock?

Sedimentary rock covers 66% of the Earth's continents and much of the ocean floor.

How does soil form?

Soil forms when the materials left after rock breaks down combine with organic material.

What are soil horizons?

Soil horizons are the distinct layers in a soil profile, each with different characteristics.

Why do soils differ from place to place?

Soils differ because of variations in the parent rock, climate, vegetation, relief and the amount of time the soil has had to form.

What is granite and where is it found in the UK?

Granite is an igneous rock that forms underground from cooled magma. In the UK, granite landscapes are found in areas such as Dartmoor and the Cairngorms.

What is limestone and where is it found in the UK?

Limestone is a sedimentary rock made mainly of calcium carbonate. In the UK, limestone landscapes are found in areas such as the Peak District and the Yorkshire Dales.

How does weathering contribute to the rock cycle?

Weathering breaks down rocks into smaller particles, which can then be transported, deposited and eventually form sedimentary rock, making it a crucial part of the rock cycle.

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Rock sample	Mass lost (g)
Limestone	3.2
Granite	0.4
Marble	2.8
Sandstone	0.6