The geosphere: the structure of the Earth, the rock cycle and rock types, the formation and properties of soil, weathering and erosion, and the extraction and sustainable use of mineral resources.

What this dot point is asking

The SQA wants you to describe the geosphere: the layered structure of the Earth, the rock cycle and the three rock types, how soil forms and what gives it its properties, the processes of weathering and erosion, and how humans extract and sustainably use mineral resources. This is the solid-Earth sphere of the Earth's Resources area.

The structure of the Earth

The Earth is layered by density:

• the crust, a thin, rigid outer layer (continental and oceanic);
• the mantle, a thick layer of hot, slowly flowing rock;
• the core, an outer liquid layer and an inner solid layer, mostly iron and nickel.

These layers store the mineral and energy resources humans draw on, and movement in the mantle drives the plate tectonics that recycle the crust.

The rock cycle and rock types

In the rock cycle, rocks at the surface are weathered to sediment, which is buried, compacted and cemented into sedimentary rock. Deeper burial subjects rock to heat and pressure, forming metamorphic rock. Greater heat melts rock to magma, which cools into igneous rock. Uplift then exposes any of these to weathering again, so the cycle has no fixed start or end.

Weathering and erosion

• Physical (mechanical) weathering breaks rock into smaller pieces without changing its chemistry, for example freeze-thaw, where water freezes in cracks, expands and splits the rock.
• Chemical weathering alters the minerals chemically, for example carbonation, where slightly acidic rainwater dissolves limestone.
• Biological weathering is caused by living things, such as plant roots widening cracks.

Weathering produces the raw material for soil; erosion can remove that material, and over-rapid erosion (often caused by human activity) is a serious threat to soil and land.

Soil formation and properties

Soil forms slowly, over hundreds to thousands of years, as weathered rock mixes with organic matter from dead plants and animals broken down by decomposers. A mature soil has horizons (layers) from organic-rich topsoil down to weathered parent rock.

A soil's value for plant growth depends on:

• Mineral content, which supplies nutrients such as nitrogen, phosphorus and potassium;
• Humus (decomposed organic matter), which holds water and nutrients and improves structure;
• Structure and texture (the balance of sand, silt and clay), which control drainage and aeration;
• pH, which affects the availability of nutrients to plants.

Because soil forms so slowly, it is effectively a non-renewable resource on a human timescale, which is why protecting it links directly to sustainable food production.

Mineral resources and sustainable use

Minerals (metal ores, building stone, sand and gravel) are extracted by mining (underground) and quarrying or open-cast mining (at the surface). These are finite, non-renewable resources, and extraction has environmental costs: habitat destruction, dust and noise, water pollution from runoff, and large volumes of waste rock.

Use is made more sustainable by recycling metals (which uses far less energy than extracting new ore), reusing materials, designing products to use less material, finding substitutes, and restoring sites after extraction.

Examples in context

Example 1. Aluminium recycling. Because smelting aluminium from bauxite is so energy-intensive, recycling drinks cans saves around 95 percent of that energy and avoids the mining and red-mud waste of new production. It is a clear example of making a finite mineral resource go further and lowering the environmental cost of using the geosphere.

Example 2. Soil erosion in the Dust Bowl. In 1930s North America, ploughing up native grassland exposed the soil; drought and wind then eroded the loose topsoil in vast dust storms, ruining farmland for years. It shows how removing vegetation accelerates erosion of a slow-forming, effectively non-renewable resource.

Try this

Q1. Name the three main layers of the Earth from the outside in. [1 mark]

• Cue. Crust, mantle, core.

Q2. Explain why recycling metals is more sustainable than extracting new ore. [2 marks]

• Cue. It uses far less energy and avoids the habitat damage, waste and pollution of mining a finite resource.