• When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth

Grand Canyon

Chapter 4: Geologic Time—Concepts and Principles

Relative Geologic Time Scale

• The relative geologic time scale has a sequence of – eons – eras – periods – epochs – but no numbers

indicating how long ago each of these times occurred

• Relative Dating – putting rock layers and events in order relative to when they occurred.

• Absolute dating which results in specific numerical dates for rock units or events– Such dates are calculated from the natural rates of

decay of various natural radioactive elements present in trace amounts in some rocks

Concept of Geologic Time

• The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale– The most recent

geologic time scale model is a dual scale

– a relative scale and an absolute scale

Fig. 4-1, p. 62

Geologic Time Scale -- Geologic Time Scale -- todaytoday

• Attempts to give an age to the earth– James Usher (1581-1665) in Ireland – calculated the age of Earth based on genealogies in

Genesis – Announced that Earth was created on

October 22, 4004 B.C.• A century later it was still considered heresy to say

Earth was more than about 6000 years old.

Changes in the Concept of Geologic Time

– Georges Louis de Buffon (1707-1788)• calculated how long Earth took to cool gradually

from molten iron balls– Earth about 75,000 years

Changes in the Concept of Geologic Time

Others calculated the rate of sediment deposition

Also the rate of salt build-up in the oceans from the continental rivers

Ages in millions to billions of years

In 1953, the dating of meteorites was accomplished. Age ~ 4.5 by

• Six fundamental geologic principles are used today in relative dating

1. Principle of superposition– Nicolas Steno (1638-1686)

• In an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top

• (note: Steno lived contemporaneously with Usher)

– This method is used for determining the relative age of rock layers (strata) and the fossils they contain

Relative-Dating Principles

Illustration of the principles of superposition

2. Principle of original horizontality– Nicolas Steno

• Sediment is deposited in essentially horizontal layers

– Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification

Relative-Dating Principles

Illustration of the principles of original horizontality

Principle of Lateral ContinuityNicholas Steno

Sediment extends laterally in all direction until it thins and pinches out or terminates against

the edges of the depositional basin

Principle of Cross-Cutting Relationships James Hutton (1726-1797

An igneous intrusion or a fault event must be younger than the rocks it intrudes or cuts across

North shore of Lake Superior,

Ontario Canada

• A dark-colored dike has intruded into older light colored granite.

Cross-cutting Relationships

• The dike is younger than the granite.

Templin Highway, Castaic, California

• A small fault displaces tilted beds.

Cross-cutting Relationships

• The fault is younger than the beds.

Principle of inclusionsThat which is included is _________

(older? Younger?)

Principle of inclusions

6. Principle of

fossil succession

• Proposed by Georges Cuvier (1769-1832)• Dominated European geologic thinking!

– The physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area

• Six major catastrophes occurred, corresponding to the six days of biblical creation.The last one was the biblical flood

• (also relatively modern, and built on Usher’s Biblical age of the Earth)

Catastrophism

• Principle of uniformitarianism– Present-day processes have operated throughout geologic

time. This includes the physical, chemical and biological processes

– Developed by James Hutton, advocated by Charles Lyell (1797-1875)

• Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point Scotland

• We now call what he observed an unconformity but he properly interpreted its formation

• Term uniformitarianism was coined by William Whewell in 1832

Uniformitarianism

Unconformity at Siccar Pointthe tilted, lower rocks resulted from severe upheavals that formed mountainsThe mountains were then worn away and covered by younger flat-lying rocksthe erosional surface represents a gap in the rock record

• Hutton viewed Earth history as cyclical

Uniformitarianism

deposition uplift

• Old Earth: geologic processes operate over a vast amount of time

• Modern view of uniformitarianism– Today, geologists assume that the principles or laws

of nature are constant but the rates and intensities of change have varied through time

erosion

Sequence of Events

Key to Rock Types

Unconformities: 3 Types

1 2 3

• Understanding absolute dating requires knowledge of atoms and isotopes

• The nucleus of an atom is composed of – protons – particles with a positive electrical charge– neutrons – electrically neutral particles– electrons – the negatively charged particles – encircling the

nucleus• atomic number

– Equal to the number of protons– helps determine the atom’s chemical properties and the

element to which it belongs

Using Radioactive Decay to obtain numerical age

• Atomic mass number = number of protons + number of neutrons– The different forms of an element’s atoms with varying

numbers of neutrons are called isotopes • Different isotopes of the same element have different atomic mass

numbers but behave the same chemically

• Most isotopes are stable, but some are unstable

• Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

Isotopes

• Radioactive decay -the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element

• Three types of radioactive decay:– In alpha decay, two protons and two neutrons (alpha

particle) are emitted from the nucleus.

Radioactive Decay

– In beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton.

Radioactive Decay

– In electron capture decay, a proton captures an electron and converts to a neutron.

• Some isotopes undergo only one decay step before they become stable.– Examples:

• rubidium 87 decays to strontium 87 by a single beta emission• potassium 40 decays to argon 40 by a single electron capture

• But other isotopes undergo several decay steps– Examples:

• uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps

• uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps

Radioactive Decay

Uranium 238 decay

• The half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope

• The half-life of a specific radioactive isotope is constant and can be precisely measured

• Can vary from less than 1/billionth of a second to 49 billion years

• Is geometric not linear, so has a curved graph

Half-Lives

• In this example of uniform linear change, water is dripping into a glass at a constant rate

Uniform Linear Change

– In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2

Geometric Radioactive Decay

• For example: – If a rock has a parent/daughter ratio of 1:3

= a parent proportion of 25%, – and the half-live is 57 million years,

Determining Age

– 25% means it is 2 half-lives old.

– the rock is 57 x 2 =114 million years old.

• Most radiometric dates are obtained from igneous rocks

• As magma cools and crystallizes, – radioactive parent atoms separate from previously

formed daughter atoms– Some radioactive parents are included in the crystal

structure of certain mineralsDating of sedimentary rocks RARE: dating the mineral glauconite, because it forms in certain marine environments as a reaction with clay during the formation of the sedimentary rock

What Materials Can Be Dated?

• Crystallization of magma separates parent atoms – from previously formed daughters

• This resets the radiometric clock to zero.• Then the parents gradually decay.

Igneous Crystallization

• In glauconite, potassium 40 decays to argon 40– because argon is a gas, it can easily escape from a

mineral• A closed system is needed for an accurate date

– that is, neither parent nor daughter atoms can have been added or removed from the sample since crystallization

• If leakage of daughters has occurred – it partially resets the radiometric clock and the age will be

too young• If parents escape, the date will be too old.• The most reliable dates use multiple methods.

Sources of Uncertainty

• During metamorphism, some of the daughter atoms may escape– leading to a date that is too young.– However, if all of the daughters are forced out during

metamorphism, then the date obtained would be the time of metamorphism—a useful piece of information.

• Dating techniques are always improving.– Presently measurement error is typically <0.5% of the age,

and even better than 0.1%– A date of 540 million might have an error of ±2.7 million

years or as low as ±0.54 million

Sources of Uncertainty

a. A mineral has just crystallized from magma.

Dating Metamorphism

b. As time passes, parent atoms decay to daughters.

c. Metamorphism drives the daughters out of the mineral as it recrystallizes.

d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time.

Dating the whole rock yields a date of 700 million years = time of crystallization.

• The isotopes used in radiometric dating – need to be sufficiently long-lived so the amount of parent

material left is measurable

• Such isotopes include:Parents Daughters Half-Life (years)

Long-Lived Radioactive Isotope Pairs Used in Dating

Uranium 238 Lead 206 4.5 billionUranium 235 Lead 207 704 millionThorium 232 Lead 208 14 billionRubidium 87 Strontium 87 48.8

billionPotassium 40 Argon 40 1.3 billion

• Uranium in a crystal will damage the crystal structure as it decays

• The damage can be seen as fission tracks under a microscope after etching the mineral

Fission Track Dating

• The age of the sample is related to – the number of

fission tracks – the amount of

uranium

• Carbon is found in all life• It has 3 isotopes

– carbon 12 and 13 are stable but carbon 14 is not– Carbon 14 has a half-life of 5730 years– Carbon 14 dating uses the carbon 14/carbon 12 ratio of

material that was once living

• The short half-life of carbon 14 – makes it suitable for dating material < 70,000 years old

• It is not useful for most rocks, – but is useful for archaeology – and young geologic materials

Radiocarbon Dating Method

• Carbon 14 is constantly forming in the upper atmosphere

• When a high-energy neutrona type of cosmic ray strikes a nitrogen 14 atomit may be absorbed by the nucleus and eject a proton changing it to carbon 14

• The 14C formation rate – is fairly constant – has been calibrated against tree rings

Carbon 14

• The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organisms

• While the organism lives it continues to take in carbon 14 but when it dies the carbon 14 begins to decay– without being replenished

• Thus, carbon 14 dating – measures the time of death

Carbon 14

• The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk)

• The width of the rings are related to climate can be correlated from tree to tree– a procedure called cross-dating

• The tree-ring time scale now extends back 14,000 years

Tree-Ring Dating Method

• In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

Tree-Ring Dating Method

Summary

• James Hutton viewed Earth history as cyclical and very long– His observations were instrumental in establishing the

principle of uniformitarianism• Charles Lyell articulated uniformitarianism in a way

that soon made it the guiding doctrine of geology• Uniformitarianism holds that

– the laws of nature have been constant through time and that the same processes operating today have operated in the past, although not necessarily at the same rates