Visualizing
Geology
First Edition
Barbara W. Murck
Brian J. Skinner
Dana Mackenzie
Chapter 3
How Old Is Old?
The Rock Record and Deep Geologic Time
Copyright © 2008 by John Wiley & Sons, Inc.
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 superposition
Principle of cross-cutting relationship
Principle 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 age
Early 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 age
Early 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 age
Early 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 207
704 million years
Uranium 238
Lead 206
4.47 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 Dating
Paleomagnetism – 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 Argon40?
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 superposition
B) cross-cutting relationships
C) 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) disconformity
B) nonconformity
C) angular unconformity
D) None of the above, the contact is conformable.
For the block diagram below, what type of unconformity is
labeled as 1?
A) disconformity
B) nonconformity
C) angular unconformity
D) None of the above, the contact is conformable.
For the block diagram below, what type of unconformity is
labeled as 3?
A) disconformity
B) nonconformity
C) angular unconformity
D) 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 Limestone
B) Kaibab Limestone
C) Coconino Sandstone
D) 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 superposition
B) principle of faunal succession
C) principle of original horizontality
D) principle of cross-cutting relationships
For the illustration below, which decay sequence represents
alpha decay?
A) 1
B) 2
For the illustration below, which decay sequence represents
beta decay?
A) 1
B) 2
Our best estimate for the age of the Earth is _____.
A) 4.4 billion years
B) 4.56 billion years
C) 4.4 million years
D) 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?