Geochronological principles let geologists order events and estimate ages: the law of superposition (in undisturbed strata the oldest is at the base), the principle of cross-cutting relationships (a feature that cuts another is younger), the use of fossils to correlate rocks of the same age, and the idea of half-life, which gives the absolute age of a rock in years from radioactive decay; relative dating gives the order of events, absolute dating gives the age in years.

What this dot point is asking

Eduqas wants you to use the principles of geochronology to order events and estimate ages. You need the law of superposition (in undisturbed strata the oldest is at the base), the principle of cross-cutting relationships (anything that cuts a rock is younger than it), the use of fossils to correlate rocks of the same age, and the idea of half-life for absolute dating. The key distinction is that relative dating gives the order of events while absolute dating gives the age in years. Reading a sequence of events from a section is a recurring Component 2 skill.

The answer

Relative dating: putting events in order

Relative dating works out which events came first, without giving an age in years. Three principles do most of the work:

• The law of superposition. In a sequence of sedimentary beds that has not been overturned, the oldest bed is at the bottom and the youngest at the top, because each layer was deposited on top of the one before.
• Cross-cutting relationships. Any feature that cuts across a rock must be younger than the rock it cuts. A fault that displaces beds, or a dyke that cuts through them, formed after those beds existed.
• Inclusions. Fragments of one rock contained inside another must be older than the rock that surrounds them (you cannot include a fragment that has not yet formed).

Used together, these let you read a whole history from a single cross-section.

Correlation: matching rocks using fossils

Rocks in different places can be matched (correlated) to show they are the same age. The best tool is fossils: a layer containing a distinctive, short-lived fossil species can be matched to a layer with the same fossil elsewhere, because that species only lived during a particular slice of time. A fossil that is widespread but lived for only a short time is the most useful for this and is called a zone (index) fossil. Correlation lets geologists build a single timeline from rocks scattered across the world.

Absolute dating: the idea of half-life

Relative dating gives only the order. To get an age in years, geologists use radiometric (radioactive) dating, which relies on the idea of half-life.

Because the parent decays to the daughter at a fixed rate, the ratio of parent to daughter in a mineral tells you how many half-lives have passed, and multiplying that by the half-life gives the age in years. Radiometric dating is what calibrated the geological time scale with real ages.

Relative versus absolute

The two approaches work together. Relative dating (superposition, cross-cutting, correlation) gives the order of events cheaply and from a section in the field. Absolute dating (half-life) gives an age in years, usually for igneous rocks whose crystallisation set the clock. Combining them gives both the sequence and the dates.

Examples in context

Example 1. Dating a dyke against its host. A basalt dyke cutting through limestone must be younger than the limestone, because it intruded into rock that already existed. Cross-cutting relationships settle the order at a glance.

Example 2. Correlating coalfields. Coal seams in different basins are matched using the plant and pollen fossils they contain, which lets geologists show that seams hundreds of kilometres apart formed at the same time.

Try this

Q1. State the law of superposition. [1 mark]

• Cue. In an undisturbed sequence of sedimentary rocks, the oldest layer is at the bottom and the youngest at the top.

Q2. A fault cuts through three beds but is covered by a fourth, undisturbed bed. Is the fault older or younger than the fourth bed? Explain. [2 marks]

• Cue. Older: the fourth bed lies across the fault undisturbed, so it was deposited after the faulting (cross-cutting relationships).

Q3. Name the property of a radioactive isotope used in absolute dating. [1 mark]

• Cue. Its half-life (the constant time for half the parent to decay to the daughter).