Geology cheat sheet

Three rock groups

Igneous rocks solidify from molten material; sedimentary rocks form through deposition; metamorphic rocks change under heat or pressure.

Granite → igneous; sandstone → sedimentary; marble → metamorphic.

Plate boundaries

Divergent plates separate, convergent plates approach, and transform boundaries accommodate sideways motion.

Oceanic crust forms at spreading ridges.

Relative age

In an undisturbed sedimentary sequence, lower layers are generally older. A feature cutting another feature is younger than it.

A fault cutting several layers formed after those layers.

Weathering and erosion

Weathering breaks down material in place. Erosion removes and transports it; deposition lays it down.

A river can erode a bank and deposit sediment downstream.

Minerals and rocks

A mineral has a characteristic crystalline structure and composition within defined variation. A rock is an aggregate of minerals or other geological material; the terms describe different levels of organisation.

Quartz is a mineral; granite is a rock containing several minerals.

The rock cycle

Melting, crystallisation, burial, deformation, weathering and deposition provide routes between rock types. The cycle has no compulsory starting point or single direction.

A sedimentary rock can be uplifted and weathered without first becoming metamorphic rock.

Radiometric dating

Radioactive decay follows predictable statistical rates. For an initially closed parent–daughter system, the remaining parent fraction after n half-lives is (1/2)ⁿ.

After three half-lives, one-eighth of the original parent isotope remains.

Earthquake evidence

Seismic-wave arrival times constrain earthquake location and Earth structure. P waves can travel through solids and fluids; S waves do not propagate through liquids as shear waves.

A seismic shadow zone is evidence to interpret, rather than a direct photograph of the core.

Worked example: map gradient

A stream falls 120 m over a horizontal map distance of 6 km. Average gradient is 120 ÷ 6 = 20 m/km. This average can conceal short steep reaches.

Convert the horizontal distance to metres if expressing the slope as a dimensionless ratio: 120/6000 = 0.02.

Common mistake: age and position

Superposition applies to an undisturbed sequence. Folding, overturning, erosion and intrusion can change the apparent order, so combine several relationships before inferring geological history.

An unconformity represents missing time; it is not necessarily one thin rock layer.

Check your understanding

What fraction of a parent isotope remains after two half-lives?

One-quarter, because 1/2 × 1/2 = 1/4.

Curriculum connections

Sources