Changes Around Us: Physical and Chemical · Lesson 5 of 6
Weathering and Erosion
“Follow slow changes from cracking rocks to soil, transported sediments, and new rock.”
• Explain physical and chemical weathering of rocks using familiar causes. • Connect weathering, iron oxide formation, and soil formation. • Distinguish weathering from erosion by wind and flowing water. • Explain how transported sediments settle and can become rock over long periods. • Use evidence to discuss landslides and the consequences of soil erosion.
Slow changes shape the land around us
A candle changes while you watch it, but rocks may change too slowly for you to notice in one visit. Look below a cliff or along a riverbed: small stones, sand, and soil show that rock has been broken down and moved. The same ideas of physical and chemical change apply, even when the process takes many years.
A rock is not necessarily one pure substance. It contains materials that can respond differently to water, temperature changes, and other conditions. One part may crack into smaller pieces while another reacts chemically. This is why a broad natural process can include both kinds of change.
The physical breaking down and chemical alteration of rocks at or near Earth’s surface.
Loose material such as rock fragments, pebbles, sand, and fine soil particles that can accumulate or be transported.
Physical weathering breaks rocks into smaller pieces
In physical weathering, a rock breaks apart without the breaking itself forming a new substance. The fragments are smaller, but their material has not been changed by that fracture. Several natural actions can enlarge existing cracks until pieces separate.
Heating and cooling can make materials expand and contract. Repeated temperature changes can create stresses that help widen cracks. Growing roots can also push into cracks; as the roots become thicker, they may force the sides apart. Neither action needs the broken rock to become another substance.
Water entering a crack provides another route. If it freezes, it occupies more space than the liquid water did. That expansion can push on the sides of the crack. Repeated freezing and thawing can widen the crack and help detach a fragment. The water’s change of state and the rock’s fracture are physical changes in this explanation.
Problem
Water freezes in a rock crack and a piece breaks off. Why is this an example of physical weathering?
- 1.The freezing water expands and pushes against the rock.
- 2.The rock breaks into fragments, changing their size and shape.
- 3.The fracture does not itself turn the rock material into a new substance. This part of weathering is physical.
Chemical weathering changes rock materials
Water does more than push or carry material. Water and chemicals dissolved in it can react with the materials in rocks. Such reactions form new substances and alter the rock’s composition. This is chemical weathering.
Consider a dark rock called basalt that contains iron-bearing material. Prolonged exposure to moisture and air can produce iron oxide at the surface, creating a reddish layer. The new oxide explains why this is chemical change. It connects weathering to the brown rust discussed earlier: both involve formation of iron oxides.
Weathering contributes to soil formation by breaking rocks into smaller material and chemically altering some of their components. It is therefore incomplete to call the entire formation of soil from rock only physical or only chemical. Different stages can involve both. Soil formation is also slow; grinding one rock does not instantly reproduce the natural soil-forming process.
Problem
Why is a red layer on weathered basalt not explained simply as the rock becoming smaller?
- 1.Breaking into pieces changes size but does not explain a new surface material.
- 2.Iron-bearing material in the rock reacts over time in moist conditions to form iron oxide.
- 3.Formation of that new substance makes this part of weathering chemical.
Erosion moves weathered material
Breaking a rock where it stands and moving its fragments to another place are related, but they are not the same action. Wind and flowing water can wear down and carry rock fragments, sand, and soil. This movement and wearing away are central to erosion.
The wearing away and movement of rock, soil, or sediment from one place to another by natural agents such as wind and flowing water.
Think of rainwater flowing over exposed soil. It can pick up particles and carry them downhill. In a river, moving water and carried particles wear against stones, gradually smoothing and rounding them. A landslide can also move soil and rock downhill. The breaking, wearing, and movement described here are physical changes; they do not require new substances to form.
Weathering can supply loose material for erosion to move. You can therefore observe both at the same site: a cliff breaks down, and rainwater carries some fragments away. Always identify the particular stage. “Natural” or “slow” does not tell you whether a change is physical or chemical.
Problem
A pebble is worn smooth while travelling in a river. What kind of change is involved?
- 1.Flowing water and collisions with carried material wear away its rough edges.
- 2.Its shape changes, and small pieces may separate and be carried away.
- 3.These stages are physical erosion: the pebble’s material need not become a new substance.
Transported sediment eventually settles
Moving water and wind can carry sediment, but their ability to carry it decreases when they slow down. In calmer water, such as a lake or parts of an ocean, transported particles settle. This settling is called deposition. Over long periods, deposited sediments can accumulate, compact, and become rock.
The settling and accumulation of material that has been transported.
Many of these changes develop over thousands of years and cannot be reversed in ordinary life. Returning one pebble to its former location would not restore all the material worn away or rebuild a cliff. At the same time, “a new rock” means a newly formed object or arrangement of materials, not automatically a new chemical substance. Examine the processes rather than the word “new” alone.
Weathering breaks down or chemically alters rock where it is. Erosion wears and carries material away. A sequence can involve both, and weathering itself can contain physical and chemical stages.
The landslide project asks how damage and erosion might be reduced. Use the lesson to frame the discussion: what loosens material, what carries it away, and what conditions allow it to remain stable? Compare the roles of vegetation, exposed soil, and flowing water, using teacher-selected evidence. Planning protection on actual unstable slopes belongs to trained specialists; the learning task is to understand the processes.
Quiz
Which is an example of physical weathering?
Why can freezing water widen a rock crack?
What makes the reddish basalt surface an example of chemical weathering?
Which description best distinguishes erosion?
Why does sediment often settle in calmer water?
How should soil formation from weathered rocks be described?
Practice Problems
- Explain three causes of physical weathering: temperature changes, growing roots, and freezing water.
- Compare a rock breaking into fragments with an iron-oxide layer forming on basalt.
- Explain why the whole soil-forming process can involve both physical and chemical changes.
- A cliff has loose pieces at its base; rain later carries some downhill. Identify weathering and erosion separately.
- Explain how river pebbles become rounded and how this relates to physical change.
- Trace a sediment particle from a weathered rock to a lake deposit. Name each stage.
- Why does the formation of a new rock object not by itself prove that a new chemical substance formed?
- Use the landslide project to discuss the roles of exposed material, roots, and flowing water. State what evidence you would collect.
- Explain why many slow landscape changes cannot be reversed by ordinary means.
Key Takeaways
• Weathering can break rocks physically and alter them chemically. • Temperature changes, roots, and freezing water can widen cracks without forming new substances. • Iron oxide forming on basalt is an example of chemical weathering. • Weathering contributes to soil formation; erosion wears and transports loose material. • Sediment settles when transport slows and can form rock over long periods. • The type of change depends on its processes, not on whether it is natural, slow, or difficult to reverse.