Week 24 · Theme 4, Unit 4 Shifting Surfaces, Concept 4.1 Breaking Down and Moving Rocks (B) · 3 lessons
1
Weathering: rocks breaking down
Discovery Education Science P4: Term 2, Unit 4, pp. 110–121
GoalTell mechanical weathering from chemical weathering and give causes of each.
New words
mechanical weathering · breaking rock into smaller pieces without changing what it is made ofchemical weathering · a change in what the rock is made of, so it weakens or dissolvescrack · a thin gap in a rock
Explanation
Weathering breaks rock into smaller pieces, from mountains into boulders, then stones, then sand. There are two kinds. In mechanical weathering the rock only breaks, and every piece is still the same material. Water can seep into a crack, freeze in a cold night and expand, which widens the crack. When this happens again and again, the rock splits. Tree roots grow in cracks and push them wider. Wind that carries sand can scrape rock like sandpaper, and rushing water can make rocks knock into each other. In chemical weathering the rock itself changes. Oxygen in the air can turn iron in a rock into red-brown rust, which makes it crumbly. Water can dissolve some minerals, as it does in limestone caves. Acids from tiny living things such as lichens, or from acid rain, can eat away rock.
Kind
Causes
What happens
mechanical
ice, roots, wind and sand, rushing water, heat and cold
rock breaks into pieces, same material
chemical
oxygen, water, acids
rock changes into something new or dissolves
Worked examples
Ice wedging. Water gets into a crack. On a freezing night it turns to ice and expands. The crack grows wider. In the morning the ice melts and more water seeps in, and the cycle repeats until the rock splits. Parts of the high mountains of Sinai can freeze on winter nights.
Roots at work. A small tree grows in a crack in a rocky slope. Each year its roots get thicker and push the crack wider. This is mechanical weathering, because the rock is only broken.
iron in rock→oxygen and water→rust forms→rock crumbles
Red rock. A rock with iron in it lies in the open air. Over many years the iron meets oxygen and water and turns into red-brown rust. The rock becomes weak and crumbly. This is chemical weathering, because the material changed.
Class activity
Sort the clues. Copy a T-chart with the headings Mechanical and Chemical. Put these in the right column: ice in a crack, rust on a rock, roots in a crack, sand blown against a rock, acid from lichens, water dissolving limestone. Then choose two and draw each one with an arrow showing what happens.
Practice
1
What is the difference between mechanical and chemical weathering?
Mechanical only breaks the rock; chemical changes what it is made of.
2
Why does ice make a crack wider?
Water expands when it freezes, so it pushes the crack apart.
3
Is rust forming in a rock mechanical or chemical weathering?
Chemical.
4
Name two causes of mechanical weathering.
Any two: ice, roots, wind and sand, rushing water, heat and cold.
