Lesson 2: Geologic Time

2.1 How old is Earth?

Scientists were not alive in ancient times, yet they have worked out much of Earth's history by studying two records:

1. The fossil record: all fossils preserved in rocks, ice or amber and discovered.
2. The rock record: all the rock layers and formations of Earth's crust.

Together they show past environmental and climate changes and help to arrange the major events in order.

2.2 The geologic time scale

• We divide ordinary time into units (years, months, days, hours).
• In the same way scientists divide Earth's 4.6 billion year history into units, based on rock and fossil evidence.
• This division is the geologic time scale.
• It helps us understand the sequence of:

1. great geologic changes over millions of years;
2. the appearance of different living organisms;
3. major mass extinctions.

Units, from the largest to the smallest:

Eons → Eras → Periods → Epochs

• An eon is the largest unit. Each eon is divided into eras.
• Each era is divided into periods, each with distinct climate conditions.
• Each period is divided into epochs.

Reference outline (approximate, millions of years ago):

The Phanerozoic Eon has three eras: Paleozoic (about 541–252 million years ago), Mesozoic (about 252–66 million years ago; the age of dinosaurs) and Cenozoic (about 66 million years ago to the present; the age of mammals).

Making a timeline. To model Earth's history, choose a scale such as 1 cm = 500 million years. A line for 4.6 billion years then needs 4600/500 = 9.2 cm. Mark events: formation of Earth (4.6 billion years ago, left end), first life (about 3.5 – 4 billion years ago), extinction of dinosaurs (66 million years ago, only 0.13 cm from the right end), and the appearance of modern humans (a few hundred thousand years ago, just a tiny fraction of a millimetre). Such a scale shows how recent the human era is.

2.3 Reading Earth's history from rocks and fossils

Studying geologic structures, the sequence of rock layers and the fossils helps us infer events that happened in the past. Examples:

1. The Red Sea mountains and Uweinat mountains are made of igneous (granite) and metamorphic rocks. Igneous rocks like granite form deep within Earth by the cooling of magma; metamorphic rocks form from other rocks under great heat and pressure. So these mountains are among the oldest parts of Earth's crust, formed deep inside the Earth.
2. Basalt covering limestone at Abu Rawash. Basalt is an igneous (volcanic) rock; limestone is a sedimentary rock that forms on the floor of a sea. So the area was first under the sea (limestone formed), the sea retreated, and later volcanic eruptions covered the limestone with basalt.
3. Marine fossils on the peaks of the Himalayas. Marine fossils form only in the seas. Their presence on mountain peaks shows the area was an ancient sea floor, which was uplifted when tectonic plates collided to form the mountain range.
4. A fossil type found in one layer worldwide and then absent from higher layers, such as the dinosaurs, indicates the extinction of those organisms (a sudden global mass extinction, probably due to a meteorite impact or severe climate change).

2.4 Determining the age of rocks

There are two methods: relative dating and absolute dating.

2.5 Relative age

The relative age tells whether a rock is older or younger than another rock without giving its age in years. It depends on:

1. The law of superposition

• In an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top.
• (Each new layer is laid down on top of the previous layers.)

2. Index fossils

Index fossils

• are fossils of organisms that lived for a short period of geologic time but were widely distributed geographically, then became extinct.
• Examples are trilobites and ammonites.

How to use them:

• If the same index fossil is found in two rock layers far apart, the two layers have the same relative age, even if the rocks above and below them are different.
• The layer containing the index fossil is used as a reference: the layer below it is older and the layer above is younger.

Why are index fossils good for dating? Because their short existence limits the layer's age to a narrow range of time, and their wide distribution allows matching of layers in different places.

Worked example 1

• In rock sequence 1 (from bottom to top) the layers are A, B, C, D.
• Fossil X is found in layer B and fossil Y in layer D.
• Which organism appeared first?
• B is below D, so B is older, so organism X lived earlier than organism Y.

Worked example 2

• In an area, layer P contains trilobite fossils; in a distant area, a layer Q contains the same trilobite species.
• We conclude that P and Q formed at the same time, even if P is limestone and Q is sandstone.
• A layer above P that holds fossils of a later species is younger than P.

• Relative dating tells us the order of events, not the number of years.
• It is used when rocks cannot be dated in years, such as when there are no suitable radioactive minerals in a sedimentary rock.

2.6 Absolute age

The absolute age is the real age of a rock or fossil in years. It is found using radioactivity.

• Some minerals contain unstable (radioactive) elements such as uranium-238 and carbon-14.
• Their nuclei give off radiation and change into more stable elements.
• A radioactive isotope decays at a constant rate which is characteristic of that element and is not changed by heat, pressure or chemical reactions.
• That makes it a reliable clock.

Half-life: the time required for half the quantity (half the nuclei) of a radioactive element to decay.

• After 1 half-life, 1/2 remains; after 2 half-lives, 1/4 remains; after 3, 1/8; after 4, 1/16; and so on.
• After n half-lives the fraction left is (1/2)ⁿ.

By measuring how much of the radioactive element remains in the rock and how much of its decay product has formed, scientists can calculate how much time has passed since the rock formed.

Examples of isotope pairs:

• Carbon-14 (half-life about 5730 years) is used for dating organic remains (bones, wood, shells) up to about 50 000 years old. Living things take in carbon-14 from the air while alive; when they die the intake stops and the carbon-14 decays.
• Uranium-238 (half-life about 4.5 billion years) decays eventually into lead-206; it is used for very old rocks and minerals.

Worked example 3: carbon-14

A wooden object has only 25% of its original carbon-14. How old is it? (Half-life = 5730 years.) 25% = 1/4 = (1/2)², so 2 half-lives have passed. Age = 2 × 5730 = 11 460 years.

Worked example 4: fraction remaining

How much of a 80 g sample of a radioactive isotope remains after 3 half-lives? Fraction = (1/2)³ = 1/8; mass = 80/8 = 10 g. (After the first half-life 40 g; after the second 20 g; after the third 10 g.)

Worked example 5: age of a rock

A rock contains 12.5% of the original amount of a radioactive isotope whose half-life is 100 million years. How old is the rock? 12.5% = 1/8 = (1/2)³, so 3 half-lives have passed. Age = 3 × 100 = 300 million years. Reality check: any age you calculate for a rock must be less than the age of Earth (about 4.6 billion years). For uranium-238, whose half-life is 4.5 billion years, a rock that formed with Earth would have lost only about half of its uranium.

Worked example 6: the reverse problem

A fossil bone is 17 190 years old. How much of its original carbon-14 remains? Number of half-lives = 17 190 ÷ 5730 = 3. Fraction left = (1/2)³ = 1/8 = 12.5%.

• Zircon crystals. Tiny zircon crystals from Jack Hills in Australia are very hard, and they contain uranium that decays into lead; this makes them an accurate "geologic clock".
• Some are about 4.4 billion years old.
• Studies show Earth is about 4.6 billion years old.

2.7 Comparing relative and absolute dating

• Critical thinking. Relative dating is needed when no radioactive material suitable for measurement is present (for example in many sedimentary rocks), or when only the order of layers is needed.
• Often both methods are used together.

• Index fossil vs transitional fossil. An index fossil is a fossil of an organism with a short time range and a wide distribution, used to date and match rock layers.
• A transitional fossil shows features of two groups and supports evolutionary change (Archaeopteryx).

2.8 Exploiting resources and extinction (issue)

• Human exploitation of natural resources, such as hunting and destroying habitats, can cause species to die out much faster than natural processes.
• Studying past mass extinctions helps us understand the risks today.

2.9 Summary of Lesson 2

• Geologic time scale: eons, eras, periods, epochs (largest to smallest).
• Rock and fossil records reveal events such as mountain building and volcanic activity.
• Relative age: law of superposition, index fossils.
• Absolute age: radioactive isotopes and half-life; fraction left = (1/2)ⁿ.
• Earth is about 4.6 billion years old.