Evidence from the Geological Record

Layers of rock as a record of past life and environments

In 1815, William Smith completed his geological map of England and Wales, the first of its kind, and demonstrated the principle of faunal succession: each rock stratum contains a unique fossil assemblage that identifies it regardless of where in the country the rock was found. His 48 identified formations could be matched across the country by their fossils. Smith's key insight was that sedimentary rock builds up in layers (strata), trapping organism remains as they die and settle, so the rock record is a time-ordered diary of past ecosystems, with older organisms in lower layers and younger organisms above.

A fossil is the preserved remains, impression or trace of a once-living organism. Most fossils form in sedimentary rock: an organism is buried in sediment (mud, sand), and over long periods the sediment compacts into rock while minerals replace the remains.

Because new sediment is deposited on top of old, the rock forms strata (layers). The law of superposition states that in undisturbed strata, lower layers are older and upper layers are younger, so the vertical order of fossils reflects their order in time.

Fossils lower in undisturbed strata are older, the record reads from bottom (oldest) to top (youngest)

Fossils also reveal the past environment: finding marine fossils (e.g. shellfish) in high mountain strata shows the area was once under the sea and was later uplifted, evidence that environments and ecosystems change over time.

The record is powerful, but it is not complete. Fossilisation is rare and it is biased: it needs rapid burial in sediment, so hard parts (bones, shells, teeth) preserve far more often than soft-bodied organisms, and species that lived in places where sediment was not being deposited may leave no fossils at all. Erosion and later earth movements destroy or disturb strata, leaving gaps in the sequence.

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Putting events in order, without a clock

We just saw that strata record events in order. That raises a question: how do scientists use that order, and match rock layers between different sites? This card answers it → relative dating and index fossils.

Relative dating establishes the order of events, which is older and which is younger, without giving a number of years.

Relative dating uses the law of superposition to order strata and the fossils within them. It tells us the sequence of events (older vs younger) but not an age in years.

Index fossils make relative dating powerful across locations. An index fossil is from a species that was geographically widespread but existed for only a short period, so its presence pins a rock layer to a narrow time range and lets layers at different sites be correlated (matched in time).

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Putting a number of years on the past

We just saw that relative dating only gives order. That raises a question: how do scientists get an actual age in years? This card answers it → radiometric dating, using radioactive decay and half-life.

Radioactive isotopes decay at a fixed, known rate, so measuring how much is left acts like a built-in clock.

Radioactive isotopes (the "parent") decay into stable "daughter" products at a constant rate, measured by the half-life, the time for half the parent atoms in a sample to decay. Radiometric dating measures the ratio of parent to daughter and, using the known half-life, calculates a numerical age.

Each half-life, the amount of parent isotope halves, a fixed clock that gives a numerical age

Different isotopes suit different ages and materials:

• Carbon-14 (half-life ~5730 years): organic material (bone, wood, charcoal) up to ~50,000 years old.
• Potassium-argon / uranium-lead (half-lives of millions–billions of years): very old volcanic and igneous rock, used to date the strata around fossils.

Choose the dating method to match the sample: carbon-14 for recent organic remains; potassium-argon or uranium-lead for ancient rock. Relative and absolute dating are used together to build the geological timescale.

Add the half-life definition and method-selection points to your notes before the check below.

Ice, trapped gas and rock art extend the evidence base

We just saw how radiometric dating puts a number of years on rock and organic remains. That raises a question: what other technologies do scientists use to reconstruct past ecosystems and their environments? This card answers it → ice core drilling, gas analysis, and Aboriginal rock art.

Fossils and rock are not the only archive of the past, ice, trapped air and human-made records also preserve direct evidence of past ecosystems.

Ice core drilling extracts long cylindrical samples from polar ice sheets and glaciers (e.g. Antarctica, Greenland). Snow compacts into ice in distinct annual layers, similar to tree rings, so counting layers down a core gives a date, and cores from deep in an ice sheet can reach back hundreds of thousands of years.

Gas analysis of ice cores examines tiny bubbles of ancient air that became trapped as the ice formed. Measuring the concentration of gases such as carbon dioxide and methane, and the ratio of oxygen isotopes, in these trapped bubbles reveals the composition of the past atmosphere and past temperatures, direct physical evidence of past climate and ecosystem conditions.

Aboriginal rock art is a further evidence source for past ecosystems. Rock paintings and engravings created by Aboriginal and Torres Strait Islander peoples, some dated to tens of thousands of years old, depict animals including extinct megafauna and record past environmental conditions, a cultural record that complements the fossil and geological evidence.

Add ice core drilling, gas analysis and Aboriginal rock art to your notes before the check below.