VisualizingGeologyFirst Edition

Chapter 3How Old Is Old?

The Rock Record and Deep Geologic Time

Copyright © 2008 by John Wiley & Sons, Inc.

Barbara W. MurckBrian J. SkinnerDana Mackenzie

Jurassic

Jura mountains in Switzerland contains millions of fossil ammonites.

Ammonites. No tyrannosaurs or velociraptors lived in the Jurassic, they lived in the Cretaceous period.

Relative age – younger vs. older

Numerical age – The age of a rock or geological feature in years before the present.

Relative age – The age of a rock, fossil, or other geologic feature relative to another feature.

Stratigraphy – The science of rock layers and the process by which strata are formed.

Strata – Bands of rocks, and individual band is called stratum.

Horizontal strata (Badlands)

All these strata were originally deposited as horizontal layers.

All these strata were originally deposited as horizontal layers.

Four key principles of stratigraphy:Principle of original horizontality Principle of stratigraphic superpositionPrinciple of cross-cutting relationshipPrinciple of lateral continuity

Unconformity – A substantial gap in a stratigraphic sequence that marks the absence of part of the rock.

Four key principles of stratigraphy:

Principle of original horizontality – All sediments were originally laid horizontally.

Principle of stratigraphic superposition – In an undisturbed sequence of strata, younger sediments lay on top of older sediments.

Principle of cross-cutting relationship – Stratum must be always older than any feature that disrupts or cuts the sequence.

Principle of lateral continuity – Sediments substantial gap in a stratigraphic sequence that marks the absence of part of the rock.

Principle of crosscutting relationships

Principle of crosscutting relationships

Fossils and correlation

Paleontology – The study of fossils and the record of ancient life on Earth; the use of fossils for the determination of relative ages.

Fossils – usually consist of hard parts like shells , bones, or wood whose forms have been preserved in sedimentary rocks.

Correlation – A method of equating the ages of strata that come from two or more different places.

Pachypleosaurus a marine reptile that lived 230 mya

Burrow fossils of marine creatures

Impression of soft tissue of a fish called Eusthenopteron that lived 380 mya

Lebachia a conifer 250 million years old

A modern conifer

Fossil adaptation

Fossil adaptation

Correlation – A method of equation the ages of strata that come from two or more different places.

What are the four principles on which stratigraphy is based?

How can fossils provide information about the relative ages of rocks?

How can the principles of stratigraphy be used to determine the relative ages of strata?

What is the difference between conformable and uncomformable strata?

The Geologic Column

Through worldwide correlation, nineteenth-century geologists assembled the Geologic Column, or stratigraphic time scale, a composite diagram showing the succession of all known strata, fitted together in chronological order, on the basis of their fossils and other evidence of relative age.

Geologic Column – The succession of all known strata, fitted together in relative chronological order.

The period is the basic unit of geological time in which a single type of rock system is formed. Two or more periods comprise a geological Era. Two or more Eras form an Eon, the largest division of geologic time. Some periods are divided into epochs.

Four units of geological time:

Eons –

Eras –

Periods –

Epochs –

Four units of geological time:

Eons – The vast majority of Earth’s history is divided into three eons in which fossils are extremely rare or nonexistent. The Hadean (beneath the Earth), Archean (ancient), and Proterozoic (early life). In the Phanerozoic (visible life), the fossil record suddenly becomes much more detailed. Eras – The Phanerozoic Eon is divided into three shorter units called eras, the Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). These eras were separated by major extinction events, when more of 70% of the species perished.Periods – The eras are then divided into periods. The earliest period of the Paleozoic era, the Cambrian period is noteworthy, because of the “explosion” of unparalleled diversity of life. Epochs – Periods are divided into smaller units called epochs. The Tertiary and Quaternary periods are somewhat familiar because of the emergence of humans and their ancestors.

Two different analogies can be used to describe DEEP TIME

Imagine that the age of the earth is contained in 1 calendar year (12 months). January 1 is 4.6 billion years ago. Precambrian time goes through about October 25, reptiles evolved on Dec. 7, K-T extinction on Christmas day, Homo sapiens do not occur until 11:00 p.m. on December 31, and the last glacier receded at 11:58:45 p.m.  Think of the age of the earth as the length of your arm, origin of the earth at your shoulder and present day at the tip of the fingernail on your middle finger. A single stroke of a nail file can erase human existence.

Markers of geologic time – Rocks anywhere in the world that contain the same species of ammonite can be reliably dated to the Jurassic Period.

Event: Date in Calendar Year:

Origin of the Earth January 1

Oxygen appears July 25

First mammals December 15

Atlantic Ocean appears December 19

 On December 31st:

 Homo sapiens appears 11:49 PM

Last ice leaves Ohio 11:58:17 PM

Sumerian civilization 11:59:19 PM

 Moses 11:59:38 PM

 Birth of Christianity 11:59:47 PM

 Fall of Rome 11:59:49 PM

 Birth of Islam 11:59:51 PM

 Norman conquest 11:59:54 PM

 Black Death 11:59:56 PM

 Galileo 11:59:57 PM

 Columbus 11:59:57 PM

 WWI 11:59:59.4 PM

 Hiroshima 11:59:59.6 PM

 End of Cold War 11:59:59.93 PM

 Turn of the millennium 11:59:59.9993 PM

Cenozoic era – Birds and mammals flourished. In this scene from 15 million years ago in the Tertiary Period, we can see some possible ancestors of primates.

Mesozoic Era – This era saw the rise of dinosaurs, which were the dominant vertebrates on land for many million of years. The first flowering plants and the first mammals.

Paleozoic era – The evolution of life progressed from marine invertebrates to fish, amphibians, and reptiles. This scene is from 350 mya during the Carboniferous period.

What are the major subdivisions of the geological scale?

Why does the geologic time scale apply to rocks everywhere on Earth, not just those in one locality?

What major biological event distinguishes the Phanerozoic Eon from the previous (Precambrian) eons?

Numerical ageEarly attempts

How old is the Earth?4.6 billion years (4,600,000,000 years)Radiometric dating (Uranium, Thorium). Mass spectrometer.

Numerical age

Early ideas of the age of the Earth:

1654 Archbishop Usher (Ireland), genealogy in Bible - Earth was created October 26, 4004 BC, 9:00 am - Earth was 6000 years old.

1754 Edmund Halley suggested that we could measure the salt content that is added to the sea by a river input.

Numerical ageEarly attempts

1899 John Joly estimated that the earth was 90 million years old. Joly and Halley did not see the sea as an open system where salt is removed by evaporation and other chemical reactions.

Charles Darwin estimated the Earth to be about 300 million years old.

Numerical ageEarly attempts

Lord Kelvin rejected Darwin’s estimate and using thermodynamics he calculated that the Earth was 20 million years old. Kelvin did not know about radioactivity.

Radioactivity – A process in which an element spontaneously transforms into another isotope of the same element, or into a different element.

Radioactivity and numerical ages –

We have stated before that most chemical elements have two or more isotopes that have the same number of protons per atom but a different number of neutrons per atom.

Most naturally occurring isotopes have stable nuclei. However a number of them – such as carbon 14 and potassium 40 – are unstable.

Radioactivity and numerical ages –

Any isotope that spontaneously undergoes such a nuclear change is said to be radioactive and the process of change is referred to as radioactive decay.

Radioactive decay involves the nucleus, and it is not a chemical reaction.

Radioactivity and numerical ages –

Half life – the time needed for half of the parent atoms of a radioactive substance to decay into daughter atoms.

Radiometric dating – The use of naturally occurring radioactive isotopes to determine the numerical age of minerals, rocks, or fossils.

Radioactive parent isotopes and their stable daughter products

Radioactive Parent Stable Daughter

Potassium 40 Argon 40

Rubidium 87 Strontium 87

Thorium 232 Lead 208

Uranium 235 Lead 207

Uranium 238 Lead 206

Carbon 14 Nitrogen 14

Half Lives for Radioactive Elements

Radioactive Parent Stable Daughter Half life

Potassium 40 Argon 40 1.25 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207704 million

years

Uranium 238 Lead 2064.47 billion

years

Carbon 14 Nitrogen 14 5730 years

Complete the following exercise on the web for virtual dating

at

http://www.sciencecourseware.org/VirtualDating/

Magnetic Polarity DatingPaleomagnetism – The study of rock magnetism in order to determine the intensity and direction of Earth’s magnetic field in the geological past.

Time is central to the study of Earth that geologist are always seeking new ways to estimate ages.

A new method developed in the 1960’s involves paleomagnetism, the study of Earth’s past magnetic field.

Magnetic Polarity Dating

Earth’s magnetic field reverses its polarity at regular intervals, but on average, once every half million years.

Magnetic reversal – A period of time in which Earth’s magnetic polarity reverses itself.

The Earth’s northern magnetic pole moves to the south pole, but note that the geographic north and south poles are the same.

Dating human ancestors – The Haddar region of northern Ethiopia

Dating human ancestors – The Haddar region of northern Ethiopia

Dating human ancestors – The Haddar region of northern Ethiopia

The Age of Earth

The oldest rocks on Earth are not necessarily the oldest because most of the rocks on Earth have been subject to a lot of geologic activity (being melted and exposed to the surface again and again).

The oldest know Precambrian rock is about 4.0 billion years old. Although individual mineral grains from a sedimentary rock in Australia has been dated to be 4.4 billion years.

Earth’s oldest rock – The Acasta gneiss in northern Canada was formed 4.0 billion years ago.

There is strong evidence that the Earth formed at the same time than the Moon and meteorites. Moon dust and meteorites (they have remained virtually unaltered since the formation of the solar system) have been dated to be about 4.56 billion years old. Planetary scientist believe that indeed the Sun’s entire planetary system, formed at that time.

The universe is calculated to be about 14 billion years old.

The Allende meteorite, which fell in Mexico is slightly older than 4.6 billion years.

What is the oldest age that has been obtained from material found on Earth? Does this match the presumed age on Earth? Why or why not?

How have meteorites and rocks from the Moon helped geologists to determine the age of Earth?

Explain why the oldest rocks are not necessarily the same age as the planet.

What were some of the early attempts to calculate the age of the Earth? Why were they inaccurate?

Why is radioactive decay useful as a “geologic clock”?

What geologic materials are the most useful for obtaining numerical ages through radiometric techniques? Which are the least useful, and why?

Explain why scientists currently believe Earth is about 4.56 billion years old.

How can magnetic polarity reversals contribute useful information about rock fossil ages?

The Grand Canyon contains 2 billion years of Earth’s history.

Trilobite’s tracks found on the Tapeats Sandstone (600 million years old)

What happened here?

What is the difference between relative age and numerical age?

What is the principle of crosscutting relationships?

What is the principle of original horizontality?

What is the law of stratigraphic superposition?

What is the principle of lateral continuity?

What is an unconformity?

How do we use fossils to correlate strata?

What are the eras that make up the Phanerozoic?

The most distinctive changes in the fossil record occur across the boundaries between (periods, eras, epochs)

Potassium-40 is a naturally occurring radioisotope that decays to Argon-40. The half life of Potassium-40 is 1.3 billion years. What would be the age of a sample that contained a 3:1 ratio of Potassium-40 to Argon-40?

Why we cannot find on Earth the oldest rocks in the solar system?

Label the following

The dinosaurs were dominant in which era?

Which is the alpha emission and which is the beta decay?

How old is the layer marked by xx?

When we see sedimentary rock units that are twisted or tilted, we know that some force must have disturbed the strata after they were deposited. The principle upon which we base this inference is the principle of _____.A) stratigraphic superpositionB) cross-cutting relationshipsC) original horizontality.D) stratigraphic correlation

The principle of stratigraphic superposition tells us that _____.A) a rock unit is older than any geologic feature that disturbs or cuts it.B) the oldest rock strata in any undisturbed sedimentary sequence are on the bottom, and the youngest are on the top.C) sedimentary rock strata are deposited in flat-lying layers, one on top of another.D) All of the above are true.

For the block diagram below, what type of unconformity is labeled as 2?

A) disconformityB) nonconformityC) angular unconformityD) None of the above, the contact is conformable.

For the block diagram below, what type of unconformity is labeled as 1?

A) disconformityB) nonconformityC) angular unconformityD) None of the above, the contact is conformable.

For the block diagram below, what type of unconformity is labeled as 3?A) disconformityB) nonconformityC) angular unconformityD) None of the above, the contact is conformable.

For the stratigraphic section of the Grand Canyon depicted in the photograph below, which rock unit is older?A) Muav LimestoneB) Kaibab LimestoneC) Coconino SandstoneD) Tapeats Sandstone

What principle allowed you to determine the relative ages of different parts of the stratigraphy of this section of the Grand Canyon?

A) principle of superpositionB) principle of faunal succession C) principle of original horizontalityD) principle of cross-cutting relationships

For the illustration below, which decay sequence represents alpha decay?

A) 1B) 2

For the illustration below, which decay sequence represents beta decay?

A) 1B) 2

Our best estimate for the age of the Earth is _____.A) 4.4 billion yearsB) 4.56 billion yearsC) 4.4 million yearsD) 4.56 million years

Describe the process of alpha decay. How does the daughter element compare to the parent with respect to atomic number and mass?

Why is it difficult to determine the numerical age of Earth?