Notes · GeologyUK · A-Levels

Time and change

This chapter is about geological time and how it is measured. It sets out the principle of uniformitarianism and the concept of deep time, the principles of relative dating used to sequence events, the geological time scale of eons, eras and periods, the radiometric methods that give absolute ages from radioactive decay, and the correlation of rock sequences by lithology and by fossils.

5 sections~19 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 1

T·0666 / 13

Exam profile

AO1 · Describe the principles of relative and absolute dating and the geological time scaleAO2 · Apply dating principles to sequence events and calculate radiometric ages from half-livesAO3 · Interpret a cross-section to deduce a geological history and evaluate the reliability of dating methods

Operators:describeexplaincalculatededuceinterpretevaluate

basic level

AS-Level expects you to apply the principles of relative dating, know the order of the geological periods, and outline radiometric dating.

higher level

The full A-Level requires you to sequence complex histories, calculate radiometric ages, and evaluate the reliability and limits of dating methods.

Depth

Text

Contents · 5 sections

Time and change

Uniformitarianism and deep time#

FoundationWJEC/Eduqas A level Geology (Component 2) — Time and change

The immensity of geological time

Fig. 1Deep time: the Earth is about 4600 million years old; abundant animal life appears only in the last ~541 million years and humans in the last sliver.

Uniformitarianism is the principle that the geological processes we observe today — weathering, erosion, deposition, volcanism, plate movement — are the same processes that operated in the past, so that 'the present is the key to the past'. It means that a geologist who understands how a modern river deposits cross-bedded sand can recognise an ancient river deposit from the same features, and that the slow rates measured today can, in principle, be extrapolated back through time. It is the founding assumption of historical geology, first argued by James Hutton in the late eighteenth century and popularised by Charles Lyell.

The great consequence of uniformitarianism is deep time: the recognition that, because geological processes are generally very slow, the features of the Earth require immense spans of time to form. Hutton, seeing an angular unconformity where tilted, eroded rocks were overlain by younger horizontal beds, realised that it recorded deposition, deformation, erosion and renewed deposition — a cycle demanding far more time than the few thousand years then allowed. His famous remark that he could see 'no vestige of a beginning, no prospect of an end' captured the vastness of geological time, now known to be about 4.6 billion years.

Uniformitarianism must be applied with care, and the modern version is more subtle than 'the present is exactly like the past'. Rates have varied — the early Earth was hotter and more volcanically active, the atmosphere had little oxygen, and life has changed the planet — and some processes, such as the great meteorite impacts, are rare and catastrophic rather than slow and uniform. The principle is best understood as 'the same physical and chemical laws have always operated', while allowing that rates and conditions have changed, a balance the specification expects students to appreciate.

Deep time reframes everything else in the course. It makes sense of how slow weathering can level mountains, how plate movements of a few centimetres a year can open and close oceans, and how the fossil record documents the evolution of life over hundreds of millions of years. Grasping the scale — that the whole of recorded human history is a thinner sliver of geological time than a single page in a very long book — is essential to interpreting the geological record honestly, and it underpins both the relative and the absolute dating that follow.

Worked example

Reasoning from the present to the past

A rock shows symmetrical ripple marks identical to those forming today on a modern beach. Using uniformitarianism, explain what can be inferred and what caution is needed.

Apply the principle
Because the same processes operate today, symmetrical ripples forming under to-and-fro wave motion today imply the ancient ripples also formed under shallow-water wave action.
Infer the environment
The ancient rock was therefore deposited in a shallow-water, wave-agitated setting such as a shoreline.
Add the caution
Uniformitarianism assumes the same physical laws, but rates and exact conditions may have differed, so the inference is of process and environment, not of an identical setting.

Result: The ripples indicate shallow-water wave action (present as key to the past), interpreted as process and environment rather than an exactly identical setting.

Typical mistakes

Stating uniformitarianism as 'nothing has ever changed'; rates and conditions have varied, while the physical and chemical laws have not.
Underestimating geological time; the Earth is about 4.6 billion years old and abundant animal life is only about the last 541 million years of it.

Active revision

Explain how the presence of an angular unconformity demonstrates that a long span of time and several geological events are recorded at one locality.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — geological time (British Geological Survey)

Relative dating and the sequencing of events#

StandardWJEC/Eduqas A level Geology (Component 2) — Time and change

Relative dating in a cross-section

Fig. 2A cross-section for relative dating: superposition orders the beds, an unconformity marks a gap, and a cross-cutting dyke is younger than every bed it crosses.

Relative dating puts geological events in order — older or younger — without assigning them a number of years. It rests on a set of simple, powerful principles. The principle of superposition states that in an undisturbed sequence the oldest beds are at the bottom and each bed is younger than the one below; the principle of original horizontality states that beds are deposited nearly horizontally, so tilted or folded beds have since been deformed; and the principle of lateral continuity states that a bed originally extended in all directions until it thinned out or met a barrier, so beds on opposite sides of a valley can be correlated.

Cross-cutting relationships extend the reasoning to intrusions and faults: any feature that cuts across a rock must be younger than the rock it cuts, so a dyke or a fault that slices through a sequence is younger than all the beds it crosses. The principle of included fragments (inclusions) states that a fragment must be older than the rock that contains it, so pebbles in a conglomerate, or blocks of country rock caught in a granite, are older than the rock enclosing them. Together these principles let a geologist establish an order for beds, intrusions, faults and folds at a single locality.

Unconformities are especially important because they represent gaps in the record. An unconformity is a buried surface of erosion or non-deposition separating older rocks below from younger rocks above, and it records a time when the area was uplifted and eroded rather than accumulating sediment. An angular unconformity, where the beds below are tilted and truncated while those above are horizontal, records a whole cycle of deposition, deformation, uplift, erosion and renewed deposition, and so represents a long and eventful gap in time.

The principle of faunal succession, that fossil species succeed one another through time in a fixed and recognisable order, allows relative ages to be compared between distant localities and underlies correlation; it is developed with radiometric dating and correlation later in the chapter. Applying all these principles together, a geologist can take a cross-section and reconstruct its full history — the order of deposition, the folding and faulting, the intrusions and the erosion surfaces — which is one of the defining skills of the subject and a certain examination task.

Worked example

Reconstructing a geological history

A cross-section shows, from the bottom up, tilted beds A-B-C, then an erosion surface, then horizontal beds D-E, and finally a dyke that cuts through everything. Reconstruct the sequence of events.

Order the lower beds
By superposition, A is oldest, then B, then C; their tilting means they were deposited horizontally and later folded/tilted.
Read the unconformity
The erosion surface above C is an angular unconformity, recording uplift and erosion of the tilted beds before D-E were deposited.
Order the upper beds and the dyke
D then E were deposited horizontally on the erosion surface (superposition); the dyke cuts all the beds, so by cross-cutting it is the youngest event.
State the full history
Deposit A, B, C; tilt them; uplift and erode (unconformity); deposit D, E; intrude the dyke.

Result: Sequence: deposit A-B-C, tilt, uplift and erode (unconformity), deposit D-E, then intrude the cross-cutting dyke.

Typical mistakes

Forgetting that a cross-cutting dyke or fault is younger than every bed it crosses, even the beds above the level where it appears to stop.
Treating an unconformity as a single event; an angular unconformity records deposition, deformation, uplift, erosion and renewed deposition.

Active revision

Given a cross-section with folded beds cut by a fault, which is in turn cut by an unconformity overlain by flat beds, list the events in order from oldest to youngest.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — relative dating (British Geological Survey)

The geological time scale#

StandardWJEC/Eduqas A level Geology (Component 2) — Time and change

The Phanerozoic time scale

Fig. 3The Phanerozoic eon (last 541 Ma): the Palaeozoic, Mesozoic and Cenozoic eras, bounded by the end-Permian (252 Ma) and end-Cretaceous (66 Ma) mass extinctions.

The geological time scale is the calendar of Earth history, built up by relative dating and calibrated in years by radiometric dating. Its largest divisions are eons, subdivided into eras, which are subdivided into periods (and periods into epochs). The vast span before abundant animal life, from the Earth's formation about 4600 million years ago to about 541 million years ago, is the Precambrian; it makes up roughly seven-eighths of all geological time but, because its rocks are largely barren of useful fossils, is subdivided only coarsely.

The eon of abundant, shelly life is the Phanerozoic (the last 541 million years), and it is divided into three eras with evocative names: the Palaeozoic ('ancient life'), the Mesozoic ('middle life', the age of the dinosaurs) and the Cenozoic ('recent life', the age of mammals). The boundaries between these eras are drawn at the two greatest mass extinctions: the Palaeozoic ends and the Mesozoic begins at the end-Permian extinction about 252 million years ago, and the Mesozoic ends and the Cenozoic begins at the end-Cretaceous extinction about 66 million years ago, which killed the non-bird dinosaurs.

Within the eras come the periods, whose order and names must be known. The Palaeozoic periods run Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian; the Mesozoic periods are the Triassic, Jurassic and Cretaceous; and the Cenozoic comprises the Paleogene, Neogene and the Quaternary, in which we now live. Each period is defined by its characteristic fossils and rocks — the Carboniferous by its coal swamps, the Jurassic by its ammonites and marine reptiles — so a period name is also a shorthand for a stage in the history of life and environments.

The time scale is the framework onto which every other topic is hung: a fossil, an orogeny, a coal seam or an ice age is placed by naming its period. It is worth remembering the scale of the divisions — that the whole of the Cenozoic 'age of mammals' is shorter than the Cretaceous alone, and that the Precambrian dwarfs the entire Phanerozoic — because a sense of proportion prevents the classic error of treating the well-populated recent past as typical of all of Earth history. Fluency with the periods and their order is assumed throughout the examination.

Worked example

Placing an event in the time scale

A coal seam contains fossil ferns and lies below rocks with dinosaur remains and above rocks with early land plants. Assign the coal, and the dinosaur-bearing rocks, to periods and eras.

Place the coal
Extensive coal formed in the humid, forested swamps of the Carboniferous Period, in the Palaeozoic Era.
Place the dinosaurs
Dinosaurs are characteristic of the Mesozoic Era (Triassic, Jurassic and Cretaceous periods), so the overlying dinosaur-bearing rocks are Mesozoic.
Check the order
The superposition (early plants below, coal, then dinosaurs above) is consistent with the time scale: Palaeozoic below, Mesozoic above.

Result: The coal is Carboniferous (Palaeozoic); the dinosaur-bearing rocks above are Mesozoic — consistent with superposition.

Typical mistakes

Muddling the order of the periods, especially the Palaeozoic six (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian).
Forgetting that the Precambrian, though poorly subdivided, makes up about seven-eighths of geological time.

Active revision

List the three eras of the Phanerozoic and their periods in order, and state which mass extinction marks each era boundary.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — the geological timechart (British Geological Survey)

Absolute (radiometric) dating#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Time and change

Radioactive decay and half-lives

Fig. 4Radioactive decay: the parent falls by half each half-life (50% at one, 25% at two, 12.5% at three) as the daughter accumulates; the curves cross at one half-life.

Radiometric dating gives an absolute age in years by using the steady, clock-like decay of radioactive isotopes. A radioactive parent isotope decays to a stable daughter isotope at a rate that is constant and unaffected by temperature, pressure or chemistry, characterised by the half-life — the time for half of any quantity of the parent to decay. After one half-life half the parent remains, after two half-lives a quarter, after three an eighth, and so on, while the daughter accumulates in step. Measuring the ratio of parent to daughter in a mineral therefore reveals how many half-lives, and so how many years, have passed since the mineral formed and 'started the clock'.

The decay is described by the exponential law in which the fraction of parent remaining equals one-half raised to the power of the number of half-lives elapsed. Rearranged, the age is the half-life multiplied by the number of half-lives, which for a non-integer number is found with logarithms. The clock starts at the moment the mineral crystallises (for igneous rocks) or is reset by metamorphism, trapping the daughter atoms; so radiometric dates most reliably date the crystallisation of igneous rocks, which is why datable volcanic ash bands within a sedimentary sequence are so valuable.

Different methods suit different ages, because a method is useful only over a range of a few half-lives around its own half-life. Uranium-238 (half-life about 4.5 billion years) and potassium-40 to argon-40 (about 1.25 billion years) and rubidium-strontium date the oldest rocks and the deep-time framework; carbon-14 (half-life about 5730 years) dates only organic material from the last few tens of thousands of years, and so is used in Quaternary geology and archaeology, not for dating rocks millions of years old. Choosing an appropriate method for the expected age is an examinable point.

Radiometric dating transformed geology by putting numbers on the relative time scale, but it must be used critically. It requires a closed system — no parent or daughter gained or lost by later weathering, heating or fluid flow — or the date will be wrong; it dates the mineral's crystallisation, not necessarily the geological event of interest; and every date carries an experimental uncertainty. Cross-checking several methods and several minerals, and combining absolute dates with relative dating and correlation, is how reliable ages are established, and evaluating this reliability is a classic AO3 demand.

N = N_0 \left(\tfrac{1}{2}\right)^{t/T}

Radioactive decay law

N is the amount of parent remaining, N0 the original amount, t the elapsed time and T the half-life. After each half-life the parent halves.

t = T \times \frac{\log(N_0 / N)}{\log 2}

Age from the parent fraction

Rearranging the decay law gives the age t: the half-life T multiplied by the base-2 logarithm of the ratio of original to remaining parent (computed here with common logarithms).

Worked example

Calculating a radiometric age

A volcanic ash is dated using potassium-40 (half-life 1.25 billion years). Measurement shows that 20% of the original potassium-40 remains. Calculate the age of the ash.

Find the parent fraction
N / N0 = 0.20, so N0 / N = 5.
Find the number of half-lives
Number of half-lives = log(N0/N) / log 2 = log(5) / log(2) = 0.6990 / 0.3010 = 2.32.
$\frac{\log 5}{\log 2} = 2.32\ \text{half-lives}$
Multiply by the half-life
Age = 2.32 x 1.25 billion years.
$t = 2.32 \times 1.25\ \text{Ga} = 2.9\ \text{Ga}$
Interpret
The ash crystallised about 2.9 billion years ago, in the Precambrian; the potassium-argon method suits such an old rock because the age is a few half-lives.

Result: The ash is about 2.9 billion years old (2.32 half-lives of potassium-40).

Typical mistakes

Using carbon-14 to date rocks millions of years old; with a half-life of only about 5730 years it is useless beyond a few tens of thousands of years.
Forgetting that the clock dates crystallisation and assumes a closed system; later heating or weathering that adds or removes isotopes gives a false age.

Active revision

A mineral contains parent and daughter isotopes in the ratio 1:3 (so a quarter of the original parent remains). The half-life is 1.3 billion years. Calculate the age of the mineral.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — dating rocks (British Geological Survey)

Correlation and the reliability of dating#

StandardWJEC/Eduqas A level Geology (Component 2) — Time and change

Correlation by a zone fossil

Fig. 5Biostratigraphic correlation: a bed containing the same zone fossil in two separate columns marks the same time, correlating the sequences regardless of rock type.

Correlation is the matching of rocks of the same age between different localities, which is how a local sequence is fitted into the global time scale. There are several bases. Lithostratigraphic correlation matches distinctive rock units by their lithology — a particular sandstone, a marker bed of volcanic ash — but it can be misleading because the same rock type can form at different times in different places, so it is reliable only over short distances. Chronostratigraphic correlation matches rocks of the same age regardless of rock type, and it is what we ultimately want.

The most powerful tool for correlation is biostratigraphy, using fossils and the principle of faunal succession. Because species appear, spread and become extinct in a fixed order through time, a rock's fossils identify its position in that succession and hence its relative age, anywhere in the world. A zone fossil (index fossil) is a species used to define and recognise a particular interval of time; a rock containing that species is assigned to its zone, and two rocks with the same zone fossil are the same age even if they are on different continents and made of different rock.

A good zone fossil has a demanding set of characteristics: it should have existed for only a short period of time (so it pins the age precisely), have been geographically widespread (so it can be found in many places), have been abundant and easily preserved (so it is commonly found), and be easily identifiable and distinctive. Rapidly evolving, free-swimming or floating marine organisms make the best zone fossils, which is why graptolites are used for the Ordovician and Silurian, ammonites for the Mesozoic, and trilobites for the Cambrian; a slowly evolving, geographically restricted organism makes a poor one.

Reliable dating combines all these tools rather than trusting one. Relative dating orders the events, biostratigraphy correlates them between localities and ties them to the succession of life, and radiometric dating from datable igneous bands calibrates the whole scheme in years; where several independent methods agree, the age is secure. Evaluating the reliability of a correlation or a date — recognising the ambiguity of lithostratigraphy, the requirement of a closed system for radiometric dates, and the value of cross-checking — is exactly the critical skill the specification rewards in this chapter.

Worked example

Evaluating a zone fossil

A geologist wishes to correlate marine mudstones between Wales and Scotland. Explain why graptolites would be more useful than the surrounding mud, and what makes them good zone fossils.

Reject lithostratigraphy
The mudstone itself is unreliable for long-distance correlation because similar mud can form at different times in different places.
Use biostratigraphy
Graptolites were free-floating, so they spread rapidly across whole oceans, and they evolved quickly, so each species marks a short interval of time.
Apply the criteria
Being widespread, short-lived, abundant and distinctive, graptolites let a bed be assigned to a precise zone and matched between Wales and Scotland.

Result: Graptolites correlate the sequences because, being widespread, short-lived, abundant and distinctive, they are excellent zone fossils, unlike the ambiguous mudstone.

Typical mistakes

Assuming the same rock type is the same age everywhere; a sandstone can form at different times in different places, so lithostratigraphy alone is unreliable.
Choosing a long-lived, geographically restricted fossil as a zone fossil; a good zone fossil is short-lived, widespread, abundant and distinctive.

Active revision

Explain why an ammonite makes a better zone fossil than a coral, referring to the characteristics of a good zone fossil.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — fossils and correlation (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Time and change

5 sections~19 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 1

Time and change

Key points

Reasoning from the present to the past

Key points

Reconstructing a geological history

Key points

Placing an event in the time scale

Key points

Calculating a radiometric age

Key points

Evaluating a zone fossil

Sources

Time and change

Key points

Reasoning from the present to the past

Key points

Reconstructing a geological history

Key points

Placing an event in the time scale

Key points

Calculating a radiometric age

Key points

Evaluating a zone fossil

Sources