KS3 rocks, weathering and soils: a guide for parents

Rocks, weathering and soils, explained

Geographers sort rocks into three groups by how they formed. Igneous rocks, such as granite and basalt, start as molten rock that cools and hardens, either underground or at the surface. Sedimentary rocks, such as sandstone, limestone and chalk, are built from small pieces of older rock or from the remains of living things that settle in layers and are slowly squashed and cemented together. Metamorphic rocks, such as marble and slate, are older rocks changed by heat and pressure while staying solid. Pupils often meet the rock cycle as a diagram showing that any rock can become any other over very long periods.

Weathering is the breakdown of rock where it sits, with no movement involved. That last point separates it from erosion, where material is worn away and carried off by water, ice, wind or waves. Pupils learn three kinds of weathering. Physical weathering includes freeze-thaw, where water freezes in a crack and forces it wider, and the flaking of rock after repeated heating and cooling. Chemical weathering happens when rainwater, which is slightly acidic because it contains dissolved carbon dioxide, reacts with minerals; limestone slowly dissolves this way. Biological weathering involves living things, such as tree roots prising cracks open.

Pupils also learn that different rocks respond differently to the same weather. Granite is hard and made of tightly locked crystals, so it tends to stand up as high ground, such as moorland. Limestone is slowly dissolved by rainwater, which widens its cracks and can create caves and bare, blocky surfaces. Softer rocks such as clay wear away more easily and usually form low ground. Schools usually choose a place to study these differences, for example a limestone area, a granite upland or a stretch of coast. The programme of study does not name particular places, so your child’s examples may differ from a friend’s at another school.

Soil is the thin layer on top of the land that plants grow in. Pupils learn that it is a mixture of broken-down rock (the mineral part), humus from rotted plants and animals, water, air and living organisms such as worms and bacteria. Soil forms slowly from the parent material below, which is the rock beneath it, and is shaped by climate, relief, plants and time. A soil profile is the vertical slice you see in a pit, with a dark topsoil rich in humus, a paler subsoil below it, then weathered rock and solid bedrock. Pupils also compare sandy soils, which drain quickly, with clay soils, which hold water, and link this to farming.

The wider message in Key Stage 3 is that these are connected systems. Rock type affects the shape of the land, weathering supplies the material that rivers, glaciers and the sea then move, and soil supports the plants and crops that people rely on. This links to the requirement that pupils understand how human activity depends on the effective working of natural systems. Farming that leaves soil bare, for instance, can allow rain and wind to strip away the topsoil that took centuries to form.

Words your child will hear

Igneous

Rock formed when molten rock cools and solidifies, such as granite or basalt.

Sedimentary

Rock made from layers of sediment or the remains of living things, pressed and cemented together.

Metamorphic

Rock changed by heat and pressure without melting, such as limestone becoming marble.

Weathering

The breakdown of rock in place by weather, chemicals or living things.

Erosion

The wearing away and carrying off of material by water, ice, wind or waves.

Humus

The dark, crumbly part of soil made from decayed plants and animals.

Soil profile

The layers of soil seen in a vertical slice, from topsoil down to bedrock.

What the National Curriculum says

The programme of study for Key Stage 3 says:

• Pupils should be taught to understand, through the use of detailed place-based exemplars at a variety of scales, the key processes in physical and human geography
• Physical geography relating to: geological timescales and plate tectonics; rocks, weathering and soils; weather and climate, including the change in climate from the Ice Age to the present; and glaciation, hydrology and coasts

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model answer: explaining freeze-thaw weathering

Common mistakes and how to help

How to help at home

• Read the stone around you. On a walk, look at old walls, church stone, gravestones and paving. Ask which stones look worn, flaky or stained and which look crisp. Older stone often shows weathering clearly, so see if your child can suggest why.
• Make a soil jar. Fill a clear jar a third full with garden soil, add water nearly to the top, put the lid on and shake. Leave it for a day. Pebbles, sand, silt and clay settle in layers, and bits of plant matter float. Let your child say what proportions they see.
• Try ice in a tub. Fill a plastic tub with water to the brim, put it in the freezer and look at the ice next day. It bulges above the rim because water expands as it freezes, which is the force behind freeze-thaw weathering. Use a plastic tub, not a glass container.
• Sort a pebble collection. On a beach or a gravel path, look closely at a handful of pebbles and sort them by colour, texture and crystals, then put them back where you found them. Can your child tell whether a stone is layered like sedimentary rock or glittery with crystals? Looking is enough, and no hammering is needed.
• Compare bare soil and grass. After heavy rain, look at a patch of bare soil and a grassy patch. Is the bare soil washed or muddy? Plant roots hold soil in place, which is why bare farm fields can lose their topsoil.

Related topics

Everything in Key Stage 3 geography