Q1.
Have you seen heaps of sand, soil, and stones lying at the base of mountains as shown in Fig. 5.10a? These are called sediments. How have these formed?
Answer
Yes. These sediments are formed by the slow breaking down of large rocks — a process called weathering, which involves both physical and chemical changes.
| Kind of change | What causes it | What it does to the rock |
|---|---|---|
| Physical | Temperature changes due to climatic conditions | The rock expands by day and contracts by night until it cracks |
| Physical | Growing roots of trees | Roots push into cracks and widen them until pieces break off |
| Physical | Freezing of water inside cracks | Water expands on freezing and splits the rock apart |
| Chemical | Water, or chemicals present in water, reacting with the rock | The composition of the rock itself changes — as when black basalt, which contains iron, forms a red layer of iron oxide (Fig. 5.10b) |
Why it happens: Large rocks break into smaller pieces, those pieces break into still smaller ones, and gravity brings them down to the base of the cliff, where they collect as heaps of stones, soil and sand. Continued over thousands of years, weathering eventually leads to the formation of soil.
Did you know? The red colour of the sediment layer in Fig. 5.10b is the same iron oxide that makes an iron nail look brown when it rusts. Nature is doing on a mountain what rain does to a gate.