CHAPTER 13: GEOLOGIC

TIME

Geology is the “Science of Time”

Oldest known rocks on Earth: gneiss of the Nuvvuagittuq greenstone belt, Québec, formed ~4.28 bya

What are some of the geological events that had to take place before this rock in Sego Canyon Utah was available for painting?

Imagine the amount of time required first for these rocks to form, then for

uplift to take place and finally for this deep canyon to be eroded.

Age of the Earth

• In 1788, before the Royal Society of Edinburgh, James Hutton theorized that the Earth was so old that ...

"The result ... of this physical enquiry is that we find no vestige of a beginning, no prospect of an end."

This statement was very insightful, but it is no longer strictly true. Why not?

How do geologists measure time?

• Relative dating is a system of reasoning that is used to determine the chronological sequence or order of a series of geologic events

• Radiometric (or absolute) dating uses the natural phenomenon of decay of radioactive isotopes within minerals to calculate their chronological age

Radiometric techniques are not the only way to acquire absolute geological dates. Can you think of any others?

Why is it important to study geologic time?

Some important events in the Earth’s history

Early Estimate

• Early estimates of Earth’s age were based on religious dogma

• In the mid 1600s, Archbishop James Ussher use the bible to “determine” the exact date of the creation of the Earth as ...

“… nightfall preceding 23 October 4004 BC”

The Age of the Earth

• Scientific analysis of Earth’s age, based on thickness and assumed depositional rates of sedimentary rocks, and of calculations of the time it has taken for the Earth to cool down, led to estimates of hundreds of millions of years.

• Beginning in the late 1700s, philosophers and naturalists, and geologists using Relative Dating techniques, proposed theories of "Deep Time".

• In the 1900s, geologists using Radiometric Dating estimated the age of the Earth to be ...

• ~4.55 - ~4.6 byWilliam Smith’s 1815 geological map of England and Wales

Geologic Events Used to Determine Age

• Five types of geologic events can be used to determine relative ages• Intrusion• Erosion• Deposition• Faulting• Rock deformation

• Intrusion and erosion in this case

Which of these rock types is the oldest?

Deposition, faulting & erosion

Erosion & deposition

Deposition, faulting and erosion

Deposition, rock deformation and erosion

Seven stratigraphic principles

• Seven stratigraphic principles are used in relative dating

Principle of superposition Principle oforiginal horizontality

What is the basis of the principle of original horizontality?

Principle oforiginal lateral continuity

Principle ofcross-cutting relationships

Principle of inclusions Principle of unconformities

Which rock type is youngest in each of these two situations?

The principle of faunal succession (evolution)

How could you apply the principle of faunal succession to date rocks?

Relative dating determines the order of geologic events

(Follows tilting and erosion)

Rise and fall of relative sea levelproduces certain rock sequences

North America during the Cretaceous

The interior seaways of the Cretaceous are not just a result of a high sea level. What other geological process is responsible?

Radioactive Decay

• Radiometric dating is based on constant and known rates of radioactive decay to estimate the age of geologic samples

The concepts of radioactive decay and half-life

What are the relative proportions of the parent and daughter elements at 2 half-lives and at 4?

Samples

• Geologists must select an appropriate radioisotope when dating a sample… appropriate to the type of material being studied and to its expected age.

Which isotope pairs might be useful to date something that is expected to be around 1 million years old?

Accurate dating requires understanding numerous sources of uncertainty

• What is the relationship between the sample and the event it represents?

• What is the approximate age of the sample?

• What radioisotope is most appropriate?

• What are the potential sources of uncertainty in the analytical process?

Above is a gneiss intruded by a granitic dike. If you dated the gneiss, what would that tell you about the age of it’s parent rock, and what about the age of the dike?

238U is an appropriate radioisotope to date Earth's age. Why?

Uranium-238 Decay Chain

Radon in buildings is the second leading cause of lung cancer

Potassium-Argon

• Potassium is abundant in the crust in minerals like feldspar, mica and amphibole

• 40K decays to 40Ca and 40Ar• Used to date igneous rocks• Original atmospheric argon is a factor• Potassium-Argon is an extremely important isotopic clock for older igneous

and metamorphic rocks.

Carbon dating is ideal for relatively young carbon-rich material

Earth’s age is measured using severalindependent observations

• "Absolute" ages of primordial crust, Moon, and meteorites• Abundance of lead isotopes• Consistency between these observations

Oldest radiometrically dated mineral ~4.4 bya

The sample shown here is sandstone from the Narryer Gneiss of Western Australia. The dates are from tiny grains of the mineral zircon (not visible here). Are they representative of the age of the sandstone? If not, why not?

Oldest radiometrically dated rocks

This garnet-bearing gneiss of the Nuvvuagittuq greenstone belt, on the Québec shore of Hudson Bay, has been dated at ~4.28 bya. Why are such old rocks so rare on earth?

?

Most meteorites formed ~4.4 - ~4.7 bya

The oldest moon rocks formed ~4.3 - ~4.6 bya. There is relatively little rock on the moon that is younger than a few bya. Why not?

COPYRIGHT

Copyright © 2014 John Wiley & Sons Canada, Ltd. All rights reserved. Reproduction or translation of this work beyond that permitted by Access Copyright (The Canadian Copyright Licensing Agency) is unlawful. Requests for further information should be addressed to the Permissions Department, John Wiley & Sons Canada, Ltd. The purchaser may make back-up copies for his or her own use only and not for distribution or resale. The author and the publisher assume no responsibility for errors, omissions, or damages caused by the use of these programs or from the use of the information contained herein.