Geologic Time: Concepts and Principles

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Geologic Time: Concepts and Principles

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Title: Geologic Time: Concepts and Principles

1
Chapter 2
Geologic TimeConcepts and Principles
2
THE GRAND CANYON
Horizontally bedded sedimentary strata as seen
from the North Rim of the Grand Canyon
illustrating the immensity of geologic time. It
took hundreds of millions of years for these
strata to be deposited as layers of sediment that
were eventually converted into rock. The
geologic history of the Grand Canyon region can
be read from these sedimentary layers. (photo by
E.L. Crisp, May 2002)
3
Grand Canyon

More than 2 billion years of Earth history are
preserved,
like pages of a book,
in the rock layers of the Grand Canyon
Reading this rock book we learn
that the area underwent episodes of
mountain building
advancing and retreating shallow seas, etc.
We know these things by
applying the principles of relative dating to the
rocks
and recognizing that present-day processes
have operated throughout Earth history

4
Concept of Geologic Time

Geologists use two different frames of reference
when discussing geologic time
Relative dating involves placing geologic events
in a sequential order as determined
from their position in the geologic record
It does not tell us how long ago
a particular event occurred,
only that one event preceded another
For over 200 hundred years geologists
have been using relative dating
to establish a relative geologic time scale

5
BRIEF HISTORY OF GEOLOGY AND DISCUSSION OF
GEOLOGIC TIME CONCEPTS

There was very little advancement in geology
until the middle of the eighteenth century. This
dark time (prior to mid-1700's) for all
scientific and original thought was mostly due to
a strict interpretation of the Book of Genesis in
the Bible. Geologic time was considered to be
but a few thousand years (and some people today
still adhere to a young Earth based on a literal
interpretation of the Bible).
Fossils were regarded as creatures engulfed by
the Biblical Flood, freaks of nature, inventions
of the devil, or figured stones.

6
BRIEF HISTORY OF GEOLOGY (cont.)

In 1650, James Ussher (1581-1665),
Archbishop of Armagh, Ireland, calculated, using
genealogies described in Genesis, that Earth was
created on October 22, 4004 B. C. Thus, Earth is
only about 6000 years old. (INTERESTING NOTE
Leonardi da Vinci (1452-1519) estimated that it
took 200,000 years just to deposit the sediments
in the Po River Valley in Italy.)
During the late 1700s and into the early 1800s,
many naturalists believed that Earth history
consisted of a series of catastrophic upheavals
that had shaped the geologic features of the
earth. Those who believed in this concept of
catastrophic earth history became known as
CATASTROPHISTS. Baron Georges Cuvier (1769-1832)
is credited as the first to propose this concept
to explain the rock record. Cuvier proposed that
the physical and biological history of Earth is
explained by a series of sudden widespread
catastrophes. Each catastrophe killed life forms
in a portion of the area affected, new life forms
were created (by Divine Power) or migrated in
from elsewhere.

7
HISTORY OF GEOLOGY (cont.)

JAMES HUTTON (1727-1797), a Scottish medical
doctor (and often referred to as the FATHER OF
GEOLOGY), proposed a concept in the late 1700s
now referred to as UNIFORMITARIANISM. Hutton
never practiced medicine, but was very interested
in the processes which formed and shaped the
earth.
By careful observations, he proposed that the
physical, chemical, and biological laws of nature
operated the same way in the past as they do
today thus, the present is the key to the
past and we can interpret the rock record as
resulting from the same laws of nature that
operate today.
This is the concept of uniformitarianism.

8
MODERN GEOLOGIC PHILOSOPHY

One of Hutton's greatest contributions to geology
was his concept of UNIFORMITARIANISM.
This concept, meaning "the present is the key to
the past", states that by studying geologic
processes in operation today we can safely assume
that such processes operated in the past and thus
we can interpret rocks as a response to geologic
processes.
With modification, this concept is still the
basis for modern geologic thought.
We now realize that, although the processes
themselves probably have not changed with time,
the rates of some geologic processes may have
varied drastically from time to time.
However, the basics laws of nature are still the
same today as they were in the past.
So, by using this principle and others we have
constructed a relative time scale.

9
Relative Geologic Time Scale

The relative geologic time scale has a sequence
of
eons
eras
periods
epochs

10
Concept of Geologic Time

The second frame of reference for geologic time
is absolute dating
Absolute dating results in specific dates
for rock units or events
expressed in years before the present
It tells us how long ago a particular event
occurred
giving us numerical information about time
Radiometric dating is the most common method
of obtaining absolute ages
Such dates are calculated
from the natural rates of decay
of various natural radioactive elements
present in trace amounts in some rocks

11
Geologic Time Scale

The discovery of radioactivity
near the end of the 19th century
allowed absolute ages
to be accurately applied
to the relative geologic time scale
The modern geologic time scale is a dual scale
a relative scale
and an absolute scale

12
Changes in the Concept of Geologic Time

During the 1700s and 1800s Earths age
was estimated scientifically
Georges Louis de Buffon (1707-1788)
calculated how long Earth took to cool gradually
from a molten beginning
using melted iron balls of various diameters.
Extrapolating their cooling rate
to an Earth-sized ball,
he estimated Earth was 75,000 years old

13
Changes in the Concept of Geologic Time

Others used different techniques
Scholars using rates of deposition of various
sediments
and total thickness of sedimentary rock in the
crust
produced estimates of lt1 million
to more than 2 billion years.
John Joly used the amount of salt carried
by rivers to the ocean
and the salinity of seawater
and obtained a minimum age of 90 million years

14
Relative-Dating Principles

Six fundamental geologic principles are used in
relative dating
Principle of superposition (Steno, 1669)
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
This method is used for determining the relative
age
of rock layers (strata) and the fossils they
contain

15
THE GRAND CANYON
Kaibab Limestone
Toroweap Formation
Coconino Sandstone
Hermit Shale
Supai Group

Horizontally bedded sedimentary strata as seen
from the North Rim of the Grand Canyon
illustrating the immensity of geologic time. It
took hundreds of millions of years for these
strata to be deposited as layers of sediment that
were eventually converted into rock. The
geologic history of the Grand Canyon region can
be read from these sedimentary layers. (photo by
E.L. Crisp, May 2002)

16
Relative-Dating Principles

Principle of original horizontality
Nicolas Steno (1669)
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

17
THE GRAND CANYON
Kaibab Limestone
Toroweap Formation
Coconino Sandstone
Hermit Shale
Supai Group

Horizontally bedded sedimentary strata as seen
from the North Rim of the Grand Canyon
illustrating the immensity of geologic time. It
took hundreds of millions of years for these
strata to be deposited as layers of sediment that
were eventually converted into rock. The
geologic history of the Grand Canyon region can
be read from these sedimentary layers. (photo by
E.L. Crisp, May 2002)

18
THE MORRISON FORMATION
Horizontal beds of the Morrison Formation near
Cleveland, Utah.
19
THE MORRISON FORMATION(again)

The Morrison Formation at Dinosaur National
Monument, Utah. Note that the beds are strongly
dipping here.

20
DEFORMATION OF ONCE HORIZONTAL SEDIMENTARY STRATA
Sidling Hill Syncline on I-68 near Cumberland,
Maryland (Photo by E. L. Crisp, August, 2005)
21
Relative-Dating Principles

Principle of lateral continuity
Nicolas Steno (1669)
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
must be younger than the rocks
it intrudes or displaces

22
THE GRAND CANYON
Kaibab Limestone
Toroweap Formation
Coconino Sandstone
Hermit Shale
Supai Group

Horizontally bedded sedimentary strata as seen
from the North Rim of the Grand Canyon
illustrating the immensity of geologic time. It
took hundreds of millions of years for these
strata to be deposited as layers of sediment that
were eventually converted into rock. The
geologic history of the Grand Canyon region can
be read from these sedimentary layers. (photo by
E.L. Crisp, May 2002)

23
CROSS-CUTTING RELATIONSHIPS
An basalt dike cutting through granite. The
basalt dike is younger than the granite. (Photo
taken on Cadillac Mountain, Bar Harbor, Maine by
E. L. Crisp, August, 2005).
24
Cross-cutting Relationships

This is a small reverse fault in Allegheny Group
rocks along Schultz Road in Pleasants County,
West Virginia. A dashed line represents the
fault that is probably associated with the
formation of the Burning Springs Anticline.
Light colored mudstones are adjacent to the
asphalt road. The Lower Freeport continuous coal
is near the center of the photo with the Upper
Freeport sandstone at the top.

25
Relative-Dating Principles

Other principles of relative dating
Principle of inclusions
Principle of fossil succession

26
PRINCIPLE OF INCLUSIONS

The PRINCIPLE OF INCLUSIONS states that
inclusions of one kind of rock in another are
always representative of the older rock
material. For example, if a granitic magma has
intruded into a sandstone and chunks of sandstone
have been incorporated into the rising magma, as
cooling occurs there will be inclusions of
sandstone in the granite and the inclusions will
represent the older rock.

27
PRINCIPLE OF INCLUSIONS
28
CORRELATION

Correlation is the matching up of rocks in one
area to those in another area.
There are two types of correlation of rock units.
Physical Correlation correlation of rock units
based on physical characteristics of the rocks or
position in a sequence of rocks. Assumes that
the rock units were once continuous.
Time-rock Correlation correlation of rock units
that are time equivalent (rock units in different
areas that are of the same age).

29
PHYSICAL CORRELATION
30
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31
CORRELATION USING FOSSILS

What are fossils?
Any remains or evidence of activity of a once
living organism (usually restricted to
prehistoric time).
Scientists who study fossils are called
paleontologists (not archeologists!!!).
Two major types of fossils Body fossils and
trace fossils.

32
Fossils evidence of past life

Types of fossils
Indirect evidence includes Trace Fossils
Tracks
Burrows
Coprolites fossil dung and stomach contents
Gastroliths stomach stones used to grind food
by some extinct reptiles

33
A dinosaur footprint
34
The Formation of Body Fossils

The usual prerequisites for fossilization to
form body fossils is the possession of hard parts
(bones, teeth, mineralized exoskeleton, etc.) and
the rapid burial of the hard parts by sediment
(this reduces the amount of oxygen present to
very low levels and slows decomposition of the
hard parts). Usually soft parts of an organism
rot rapidly. Only rarely are soft parts
preserved (such as skin impressions for
dinosaurs), but under some conditions they are
preserved and give paleontologists valuable
information that is usually not present in the
rock record. After burial some sort of
mineralization typically occurs. Unaltered
remains are very rare.

35
Altered Remains

Permineralization Mineral matter from
percolating ground waters is added to pores and
cavities in bones, shell, teeth, etc. In this
type of preseravation the original material is
still present with new mineral matter added to
the void spaces. Many dinosaur bones are
preserved by this method.
Replacement Sometimes original hard parts (bone
in the case of dinosaurs) is replaced (sometimes
referred to as petrified, which means turned to
stone) with new mineral matter of a different
composition than the original mineral matter
(often at a molecular level, so the
microstructure of the original mineral matter is
preserved). Silica (as microcrystalline quartz,
SiO2), iron oxide (hematite, Fe2O3), and calcium
carbonate (calcite, CaCO3) are common replacement
minerals (they are also common permineralizing
agents). Many dinosaur bones are both
permineralized and partially replaced.

36
Altered Remains

Recrystallization The recrystalliztion of
fossils is another common type of preservation in
which the original mineral present simply
recrystallizes (the original crystals grow larger
and fill most of the void space). This is more
common in invertebrate fossils (such as bivalves
clams, brachiopods, gastropods, etc.) than in
vertebrate fossils. This form of preservation
usually destroys or partially obscures the
original microstructure of the skeletal
material. An example would be the
recrystallization of a clam shell originally
composed of the mineral aragonite (a metastable
form of calcium carbonate) to calcite (the more
stable form of calcium carbonate at low
temperatures).

37
Altered Remains

Carbonization Sometimes soft parts and/or hard
parts of the body of an organism are compressed
by burial before decomposition is complete such
that the volatile substances (such as oxygen,
nitrogen, carbon dioxide, water, etc.) are
squeezed out leaving behind a film of fairly pure
carbon. This is particulary common in the
preservation plant fossils (such as ferns and
leaves Look at the fossil leaves and insects
from the Green River Formation of Utah that are
present in the Geology Lab at WVUP, these are
preserved by carbonization) and some
invertebrates, but also occurs sometimes for
vertebrates (for example, fifty million year old
fossil fish of the Eocene Green River Formation
of Wyoming, Colorado, and Utah).
Molds and Casts Sometimes the hard parts (bone
or other material) (and sometimes even soft
tissue) of organisms are buried by sediment and
even may remain until the sediment is lithified
(by compaction and cementation), but are later
dissolved by acidic ground waters percolating
through the pores of the rock (or decomposed by
other processes). This will leave an impression
of the external morphology of the original
material that was buried. This is called an
external mold. If later the mold is filled in
with mineral matter or sediment, a cast is formed
which mimics the external morphology of the
original material. Sometimes internal cavities
of skeletons (from both invertebrates and
vertebrates) may be filled with sediment or
mineral matter resulting in a mold of the
internal morphology of the cavity that was
filled, this is called an internal mold. Internal
molds are quite common for some invertebrates
(such as for clams and gastropods).

38
Natural casts of shelled invertebrates
39
PRINCIPLE OF FOSSIL SUCCESSION

Although rocks may be correlated based on
physical correlation and superposition, this can
only be done in a limited area where beds can be
traced from one area to another. Also if we are
correlating over a large area (from region to
region, or continent to continent), it is
unlikely that we can use physical correlation
because rock types will change.
To correlate over large regions and to correlate
age-equivalent strata, geologists must use
fossils. The use of fossils to correlate
sedimentary strata is based on the work of
William Smith (1812), the first to accurately
state and use the Principle of Fossil Succession.
The Principle of Fossil Succession states the
assemblages of fossils succeed themselves in a
definite and determinable order and the age of
sedimentary strata can be determined by their
contained fossils.
To use the Principle of Fossil Succession,
geologists and paleontologists use Index Fossils
(Guide Fossils).

40
PRINCIPLE OF FOSSIL SUCCESSION

The Principle of Fossil Succession is based on
the following
Life has varied through time. Of course this
implies that evolutionary change has occurred
over time.
Because biologic diversity has varied over time,
fossil assemblages are different in successivly
younger strata.
The relative ages of fossil assemblages can be
determined by superposition.

41
PRINCIPLE OF FOSSIL SUCCESSION
42
INDEX FOSSILS

Index fossils are used to correlate
age-equivalent strata via the Principle of Fossil
Succession.
Index fossils have the following characteristics
Short geologic time range.
Wide geographic distribution
Abundant
Easily recognizable

43
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44
CONCURRENT RANGE ZONES
45
Some More History of Geologic Time
46
Neptunism

Neptunism
All rocks, including granite and basalt,
were precipitated in an orderly sequence
from a primeval, worldwide ocean.
proposed in 1787 by Abraham Werner (1749-1817)
Werner was an excellent mineralogist,
but is best remembered
for his incorrect interpretation of Earth history

47
Neptunism

Werners geologic column was widely accepted
Alluvial rocks
unconsolidated sediments, youngest
Secondary rocks
rocks such as sandstones, limestones, coal,
basalt
Transition rocks
chemical and detrital rocks, some fossiliferous
rocks
Primitive rocks
oldest including igneous and metamorphic

48
Catastrophism

Catastrophism
concept 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 deluge

49
Neptunism and Catastrophism

These hypotheses were abandoned because
they were not supported by field evidence
Basalt was shown to be of igneous origin
Volcanic rocks interbedded with sedimentary
and primitive rocks showed that igneous activity
had occurred throughout geologic time
More than 6 catastrophes were needed
to explain field observations
The principle of uniformitarianism
became the guiding philosophy of geology

50
Uniformitarianism

Principle of uniformitarianism
Present-day processes have operated throughout
geologic time.
Developed by James Hutton (1726-1797), advocated
by Charles Lyell (1797-1875)
William Whewell coined the term
uniformitarianism in 1832
Hutton applied the principle of uniformitarianism
when interpreting rocks at Siccar Point, Scotland
We now call what Hutton observed an unconformity,
but he properly interpreted its formation

51
SICCAR POINT, SCOTLAND
52
Unconformity at Siccar Point

Hutton explained that
the tilted, lower rocks

resulted from severe upheavals that formed
mountains
these were then worn away
and covered by younger flat-lying rocks

the erosional surface

represents a gap in the rock record

53
Uniformitarianism
erosion
erosion

Hutton viewed Earth history as cyclical

deposition
uplift

He also understood
that 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
Some geologists prefer the term actualism

54
Crisis in Geology

Lord Kelvin (1824-1907)
knew about high temperatures inside of deep mines
and reasoned that Earth
was losing heat from its interior
Assuming Earth was once molten, he used
the melting temperature of rocks
the size of Earth
and the rate of heat loss
to calculate the age of Earth as
between 400 and 20 million years

55
Crisis in Geology

This age was too young
for the geologic processes envisioned
by other geologists at that time,
leading to a crisis in geology
Kelvin did not know about radioactivity
as a heat source within the Earth

56
Absolute-Dating Methods

The discovery of radioactivity
destroyed Kelvins argument for the age of Earth
and provided a clock to measure Earths age
Radioactivity is the spontaneous decay
of an atoms nucleus to a more stable form
The heat from radioactivity
helps explain why the Earth is still warm inside
Radioactivity provides geologists
with a powerful tool to measure
absolute ages of rocks and past geologic events

57
Atoms A Review

Understanding absolute dating requires
knowledge of atoms and isotopes
All matter is made up of atoms
The nucleus of an atom is composed of
protons particles with a positive electrical
charge
neutrons electrically neutral particles
with electrons negatively charged particles
outside the nucleus
The number of protons ( the atomic number)
helps determine the atoms chemical properties
and the element to which it belongs

58
Isotopes A Review

Atomic mass number
number of protons number of neutrons
The different forms of an elements 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

59
Radioactive Decay

Radioactive decay is the process whereby
an unstable atomic nucleus spontaneously
transforms
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.

60
Radioactive Decay

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

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

61
Uranium 238 decay
62
Half-Lives

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

63
Half-Lives

The length of half-lives for different isotopes
of different elements
can vary from
less than one billionth of a second
to 49 billion years!
Radioactive decay
is geometric (or exponential), NOT linear,
and produces a curved graph

64
Uniform Linear Change

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

65
Geometric Radioactive Decay

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

66
Determining Age

By measuring the parent/daughter ratio
and knowing the half-life of the parent
which has been determined in the laboratory
geologists can calculate the age of a sample
containing the radioactive element
The parent/daughter ratio
is usually determined by a mass spectrometer
an instrument that measures the proportions
of atoms with different masses

67
Determining Age

Example
If a rock has a parent/daughter ratio of 13
or a ratio of (parent)/(parent daughter) 14
or 25,
and the half-life is 57 million years,
how old is the rock?

25 means it is 2 half-lives old.

the rock is 57my x 2 114 million years old.

68
What Materials Can Be Dated?

Most radiometric dates are obtained
from igneous rocks
As magma cools and crystallizes,
radioactive parent atoms separate
from previously formed daughter atoms
Because they are the right size
some radioactive parents
are included in the crystal structure of cooling
minerals

69
What Materials Can Be Dated?

The daughter atoms are different elements
with different sizes
and, therefore, do not generally fit
into the same minerals as the parents
Geologists can use the crystals containing
the parent atoms
to date the time of crystallization

70
Igneous Crystallization

Crystallization of magma separates parent atoms
from previously formed daughters
This resets the radiometric clock to zero.
Then the parents gradually decay.

71
Sedimentary Rocks

Generally, sedimentary rocks can NOT be
radiometrically dated
The date obtained would correspond to the time of
crystallization of the mineral,
when it formed in an igneous or metamorphic rock,
and NOT the time that it was deposited as a
sedimentary particle
Exception The mineral glauconite can be dated
because it forms in certain marine environments
as a reaction with clay minerals
during the formation of the sedimentary rock

72
Sources of Uncertainty

During metamorphism, some of the daughter or
parent atoms may escape
leading to a date that is inaccurate.
However, if all of the daughters are forced out
during metamorphism,
then the date obtained would be the time of
metamorphisma useful piece of information.
Dating techniques are always improving.
Presently measurement error is typically lt0.5
of the age, and in some cases, 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

73
Long-Lived Radioactive Isotope Pairs Used in
Dating

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)

Most of these are useful for dating older rocks
Uranium 238 Lead 206 4.5 billion Uranium
234 Lead 207 704 million Thorium 232
Lead 208 14 billion Rubidium 87 Strontium
87 48.8 billion Potassium 40 Argon 40 1.3
billion
74
Radiocarbon Dating Method