12.1 Rocks, Fossils & Dating the Earth

The ground beneath your school oval is built from rock, and every rock has a story. Earth scientists read those stories to work out how old our planet is and what has happened to it - and reading that evidence is exactly what ICAS asks you to do, with fossil dating a named topic on Paper D.

The Three Rock Types

All rocks belong to one of three families, named for how they formed:

A simple memory hook: igneous comes from fire, sedimentary comes from settling, and metamorphic comes from morphing - being changed. Notice the middle row of the table, because it answers a classic exam question: fossils are found in sedimentary rock, because living things are buried gently in mud or sand. The fierce heat of molten lava or of metamorphic change would destroy them.

The rock cycle

Rocks do not stay in one family forever. The rock cycle describes how rock material is endlessly recycled:

• igneous rock at the surface is broken down by weathering (wind, rain and ice) and erosion into sediment
• the sediment is deposited, buried and cemented into sedimentary rock
• sedimentary rock buried deep is changed by heat and pressure into metamorphic rock
• metamorphic rock that melts completely becomes magma, which cools into igneous rock again

One full loop can take hundreds of millions of years. ICAS questions on the rock cycle usually hand you a diagram with arrows and labels missing, so learn the processes that drive each arrow - cooling, weathering and erosion, compaction and cementation, heat and pressure, and melting - not just the three rock names.

Fossils: Snapshots of Ancient Life

A fossil is the preserved remains or trace of a living thing from long ago. Fossils form in a specific sequence: an organism dies and is quickly buried by sediment, the soft parts decay, the hard parts (bones, shells, teeth) are slowly replaced by minerals, and the surrounding sediment hardens into rock. Fossils can also be traces - footprints, burrows, even droppings - rather than body parts. Either way, fossils tell us what lived where, what the environment was like (a fish fossil in the desert means the desert was once underwater), and how life has changed over time.

Relative dating: which layer came first?

Relative dating puts events in order without giving them exact ages - it answers which is older, not how old. The key rule is the law of superposition: in undisturbed rock layers, each layer is older than the layer above it, so the oldest layer is at the bottom and the youngest at the top. This is the heart of fossil dating on Paper D.

Worked example. A cliff face shows four undisturbed layers, from top to bottom: Layer 1 (sandy, contains shells), Layer 2 (clay, contains leaf fossils), Layer 3 (limestone, contains fish fossils), Layer 4 (basalt, no fossils). The order from oldest to youngest is Layer 4, Layer 3, Layer 2, Layer 1. The fish lived before the leaves were buried, and the basalt - an igneous rock - hardened before any of the sediment was laid down. If a question asks where a particular fossil sits in time, just find its layer in the stack.

Index fossils

An index fossil is a species that was widespread but lived for only a fairly short time before going extinct. Trilobites and ammonites are famous examples. If the same index fossil appears in rock layers at two dig sites hundreds of kilometres apart, those layers are almost certainly the same age - even if the rock types differ. Think of it as the geological version of matching two photos by the same dated newspaper in the background.

Why dates get revised - a senior extension note

Early scientists estimated the Earth's age at only millions of years; later evidence, including radioactive dating of the oldest rocks and meteorites, pushed the figure to about 4.6 billion years. Scientists revising a date when better evidence arrives is not science failing - it is science working exactly as it should. On Papers I and J this idea connects to evidence about the history of the Earth and the universe itself: conclusions are always held open to new evidence.

Reading a cliff-face or dig-site stimulus

When ICAS shows a cliff, road cutting or dig site, work through it in order: identify each layer, apply superposition to rank them oldest to youngest, use any fossils to say what lived when, and use index fossils to link layers between sites. Watch for traps such as a crack where younger rock has cut across older layers, or a question that quietly assumes the layers were never disturbed.