Deep Time

Geology & Plate Tectonics Fundamentals

Course 1 · Chapter 7 · Deep Time: Fossils, Dating and the Timescale

In the 17th century Archbishop James Ussher added up the generations in the Bible and concluded that the world began in 4004 BC. Within two centuries geologists had evidence that the Earth was many millions of years old, and within three, that it was billions. This is one of the most dramatic changes of scale in the history of science, and it was achieved by careful reading of rocks, not by a single discovery. This chapter covers the two kinds of dating (relative and absolute), fossils as clocks, the geological timescale and its main divisions, and the great extinctions. Ages and dates are from general knowledge and flagged "check" where they matter.

Relative Dating

Superposition

In an undisturbed series of sedimentary layers, the oldest is at the bottom and the youngest at the top. Niels Steno proposed this in the 17th century (check).

Cross-cutting

A fault or an igneous intrusion is younger than the rock it cuts across. If a dyke cuts a sandstone, the sandstone is older.

Unconformities

Surfaces that represent a gap in the record, where rock was tilted or eroded before new layers covered it. Hutton's Siccar Point (Chapter 1) is the classic example.

Relative dating says which of two things is older without saying by how much. It is enough to build a sequence, which is how the first geological timescale was assembled in the early 19th century, before anyone could measure ages in years (check).

Fossils as Clocks

What a fossil is. The preserved remains or traces of ancient life: shells, bones, leaves, footprints, burrows. Fossilisation is rare and favours hard parts buried quickly (check).
Faunal succession. William Smith, a canal surveyor, noticed around 1800 that layers in different places could be recognised by the fossils they contained, and that the sequence was always in the same order (check).
Index fossils. Species that lived for a short time, were widespread and are easy to identify, such as many ammonites and trilobites, make good markers for a particular period (check).
Environment. Fossils also show what a place was like: marine shells mean sea, coal plants mean swamp. This is the present-is-the-key idea again.
Limits. Fossils are found only in some rocks, mainly sedimentary, and the record has large gaps.

Radiometric Dating

Idea (check)	Explanation
Radioactive decay	Some atoms (parent isotopes) change into others (daughter isotopes) at a steady, known rate that is not affected by heat, pressure or chemistry
Half-life	The time for half of the parent atoms in a sample to decay
The clock	Measure the ratio of parent to daughter atoms in a mineral that closed when the rock formed; the ratio gives the age
Different isotopes for different ages	Carbon-14 (half-life about 5,700 years) for organic material up to roughly 50,000 years; uranium-lead (half-life of hundreds of millions to billions of years) for the oldest rocks
Igneous rocks	Best for dating, because crystals lock in the isotopes when they cool; sedimentary layers are dated indirectly using volcanic ash layers above and below

Radiometric dating turned a relative sequence into numbers. The 4.5 billion year age of the Earth comes from lead isotopes in meteorites and the oldest Earth minerals (check). Scientists test the method by comparing different isotope systems on the same rock, which usually agree within small errors (check).

The Geological Timescale

Division (check dates)	Roughly	Notable events
Precambrian (about 88% of Earth's history)	4.5 billion to 540 million years ago	Formation of Earth, first life, oxygen in the atmosphere, first animals near the end
Palaeozoic	540 to 252 million years ago	Shelled animals, fish, land plants, forests, first four-legged land animals
Mesozoic	252 to 66 million years ago	Dinosaurs, first mammals and birds, flowering plants
Cenozoic	66 million years ago to now	Rise of mammals, ice ages, humans

Within these eras are periods such as the Cambrian, Devonian, Carboniferous, Jurassic and Cretaceous. Many period names come from places where the rocks were first described, and several from Britain: the Cambrian from Cambria (Wales), the Ordovician and Silurian from Welsh tribes, and the Devonian from Devon (check).

Mass Extinctions

Five big ones Geologists recognise five major mass extinctions in the last 540 million years (check). The largest, about 252 million years ago at the end of the Permian, is estimated to have wiped out the majority of marine species, probably linked to massive volcanic eruptions and climate change (check). The most famous, about 66 million years ago, ended the non-bird dinosaurs and is linked to an asteroid impact and volcanic activity (check). The causes of each are still studied, and the evidence for a single cause is rarely complete.

A British comparison Britain gave many names to the timescale because geology developed here early and the rocks are varied. Scotland also supplied people: Hutton, who saw deep time at Siccar Point, Charles Lyell, from Angus, whose Principles of Geology popularised the idea and influenced Darwin, and Hugh Miller, the Cromarty stonemason who described Old Red Sandstone fish (check). Edinburgh's museums hold fossils that can be seen free of charge, a good way to check the order of life in the timescale.

Check the details Boundary dates of periods are revised by the International Commission on Stratigraphy, so check the current chart. Popular statements such as "radiometric dating is just guesswork" or "the Earth is exactly 4.5 billion years" both oversimplify: ages come with uncertainties that scientists state, and different methods are cross-checked.

Exercises

Exercise 1

A sandstone is cut by a granite dyke, and a limestone layer lies flat on top of both. Put the three in order from oldest to youngest and name the principle behind each step.

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Exercise 2

A sample contains one quarter of its original parent isotope, which has a half-life of 100 million years. How old is it? Show your working and state the assumption.

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Exercise 3

Explain why the Precambrian is called the "missing" part of the fossil record even though it covers most of Earth's history, and what other evidence geologists use for it.

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Quick Reference

Relative dating: superposition, cross-cutting, unconformities
Fossils: faunal succession; index fossils mark periods
Radiometric dating: parent and daughter isotopes, half-lives, igneous minerals
Timescale: Precambrian, Palaeozoic, Mesozoic, Cenozoic
Extinctions: five major ones, causes still debated