Rocks, soils and fossils

Rocks are made of minerals. They form in three ways, so there are three types: igneous, sedimentary and metamorphic. Over millions of years rocks are broken down, moved and made into new rocks: the rock cycle. Soil is made from tiny pieces of rock mixed with rotted living things, and fossils are the remains of living things kept in rock.

Key facts

  • Igneous rocks (granite, basalt) form when molten rock cools. They are made of interlocking crystals: slow cooling underground gives large crystals, fast cooling at the surface gives small ones.
  • Sedimentary rocks (sandstone, limestone, chalk) form from layers of sediment that are squashed and cemented together. They are often porous and may contain fossils.
  • Metamorphic rocks (marble, slate) form when rocks are changed by heat and pressure without melting.
  • Weathering breaks rock where it is (freeze–thaw, acid rain, roots); erosion carries the pieces away; deposition drops them as layers of sediment.
  • Soil contains small pieces of rock, humus (rotted plant and animal material), air, water and living things. Sandy soil drains quickly; clay soil holds water; loam is a good mixture.
  • A fossil forms when a living thing is buried quickly in sediment and its hard parts are slowly replaced by minerals. In layers that have not been disturbed, the lowest layers are the oldest.

Worked examples

Example 1

Sort these rocks into igneous, sedimentary and metamorphic: granite, chalk, marble, sandstone, basalt, slate.

  1. Granite and basalt are made of crystals that formed as molten rock cooled: igneous.
  2. Chalk and sandstone are made of grains and layers of sediment: sedimentary.
  3. Marble (from limestone) and slate (from mudstone) were changed by heat and pressure: metamorphic.

Answer: igneous: granite, basalt; sedimentary: chalk, sandstone; metamorphic: marble, slate

Example 2

Which layer of rock is the oldest? Which fossil is younger?

  1. Sediment is laid down in layers, each on top of the one before. So the bottom layer, D, is the oldest.
  2. The top layer, A, is the youngest.
  3. The fossil in C is above the fossil in D, so it is younger.

Answer: D is the oldest layer; the fossil in layer C is younger than the one in layer D

Example 3

100 ml of water is poured onto each of two soils. 80 ml drains through the sandy soil and 30 ml drains through the clay soil. How much water does each soil hold?

  1. Sandy soil: 100 − 80 = 20 ml
  2. Clay soil: 100 − 30 = 70 ml
  3. Clay has tiny particles with tiny gaps, so it holds water; sandy soil has big grains with big gaps, so water drains through.

Answer: sandy soil holds 20 ml; clay soil holds 70 ml

Example 4

Put these in order to show how a fossil forms: the soft parts rot; the animal dies; minerals replace the bones; it is buried by sediment; the rock is worn away and the fossil is found.

  1. An animal dies and is quickly buried by mud or sand, so it is not eaten.
  2. Its soft parts rot away, but bones and shells are left.
  3. Over millions of years, minerals replace the bones and the sediment around them becomes rock.
  4. Later, the rock is worn away and the fossil is found.

Answer: the animal dies; it is buried by sediment; the soft parts rot; minerals replace the bones; the rock is worn away and the fossil is found