Geologic Time Chapter
Prepared by Iggy Isiorho for
Dr. Isiorho
Time and Geology

The Key to the Past




Uniformitarianism – Principle that geologic processes operating at
present are the same processes that operated in
the past. The principle is stated more succinctly
as “The present is the key to the past.”
Actualism – The principle that the same processes and natural laws
that operated in the past are those we can actually
observe or infer from observations as operating at
present. Under present usage, uniformitarianism has the
same meaning as actualism for most geologists.
Numerical age – Age given in years or some other unit of time.
Relative time – The sequence in which events took place (not
measured in time units).


Relative Time

The geology of an area may seem, at first glance, to be hopelessly
complex. A non-geologist might think it impossible to decipher the
sequence of events that created such a geologic pattern. However, a
geologist has learned to approach seemingly formidable problems by
breaking them down to a number of simple problems. As an example,
the geology of Grand Canyon can be analyzed in four parts: (1)
horizontal layers of rock; (2) inclined layers; (3) rock underlying the
inclined layers (plutonic and metamorphic rock); and (4) the canyon
itself, care into these rocks.


Principles Used to Determine
Relative Age




Original Horizontality
Superposition
Lateral continuity
Cross-cutting relationships


Original Horizontality

Original Horizontality – The deposition
of
most water-laid sediment in
horizontal or near-horizontal
layers that are essentially
parallel to the earth’s surface.
Back
Superposition

Superposition – A principle or law stating
that within a sequence of
undisturbed sedimentary rocks, the
oldest layers are on the bottom, the
youngest on the top.
Back
Lateral Continuity

Lateral Continuity – Principle that states
that an original sedimentary layer
extends laterally until it tapers or
thins at its edge.
Back
Cross-cutting Relationships

Cross-cutting Relationships – A principle
or law stating that a disrupted
pattern is older than the cause
of disruption.
Back
Other Time Relationships

Inclusions – A fragment of rock that is
distinct from the body of
igneous rock in which it is
enclosed.


Correlation

In geology correlation usually means determining
time equivalency of rock units. Rock units may be
correlated within a region, a continent, and even
between continents. Various methods of correlation
are described below along with examples of how
the principles we described earlier in this chapter
are used to determine whether rocks in one area are
older or younger than rocks in another area.


Physical Continuity

Finding physical continuity – that is, being able to
trace physically the course of a rock unit – is one way
to correlate rocks between two different places. In Fig.
8.13, the prominent white layer of cliff-forming rock,
the Coconino Sandstone, exposed along the upper part
of Grand Canyon can be seen all the way across the
photograph. You can physically follow this unit for
several tens of kilometers, thus verifying that,
wherever it is exposed in Grand Canyon, it is the same
rock unit.


Similarity of Rock Types



Under some circumstances, correlation between two regions can be
made by assuming that similar rock types in two regions formed at
the same time. This method must be used with extreme caution,
especially if the rocks being correlated are common ones.
Correlation by similarity of rock types is more reliable if a very
unusual sequence of rocks is involved. If you find in one area a layer
of green shale on top of a red sandstone that, in turn, overlies basalt
of a former lava flow, and then find the same sequence in another
area, you probably would be correct in concluding that the two
sequences formed at essentially the same time.
When the hypothesis of continental drift was first proposed (see
Chapter 1) important evidence was provided by correlating a
sequence of rocks (Fig. 8.16) consisting of glacially deposited
sedimentary rock.


Correlation by Fossils



Faunal succession – A principle or law stating that fossil species
succeed one another in a definite and
recognizable order; in general, fossils in
progressively older rock show increasingly
greater differences from species living at
present.
Index fossil – A fossil from a very short-lived species known to
have existed during a specific period of geologic
time.
Fossil assemblage – Various different species of fossils in a
rock.


The Standard Geologic Time Scale


Standard Geologic Time Scale – A
worldwide relative scale of geologic time
divisions.
The geologic time scale, representing an extensive
fossil record, consists of three eras, which are
subdivided into periods, which are, in turn divided
into epochs. (Remember that this is a relative time
scale.)


Unconformity

Unconformity – A surface that represents a break in the
geologic record, with the rock unit
immediately above it being considerably
younger than the rock beneath.
3 types of unconformity:



Disconformities
Angular unconformities
Nonconformities


Disconformities

Disconformities – A surface that represents
missing rock strata but beds above
and below that surface are parallel
to one another.
Back
Angular unconformities

Angular unconformities – An unconformity
in which younger strata overlie
an erosion surface on tilted or
folded layered rock.
Back
Nonconformities

Nonconformities – An unconformity in which an
erosion surface on plutonic or
metamorphic rock has been covered
by younger sedimentary or volcanic
rock.
Back
Numerical Age

Counting annual growth rings in a tree trunk will tell you
how old a tree is. Similarly, layers of sediment deposited
annually in glacial lakes can be counted to determine how
long those lakes existed. But only within the few decades
since the discovery of radioactivity have scientists been
able to determine numerical ages of rock units. We have
subsequently been able to assign numerical values to the
geologic time scale and determine how many years ago the
various eras, periods, and epochs began and ended.


Isotopic Dating


Radioactivity provides a “clock” that begins working when
radioactive elements are sealed into newly crystallized
minerals. The rates at which radioactive elements decay
can be measured and duplicated in many different
laboratories. Therefore, if we can determine the ratio of a
particular radioactive element and its decay products in a
mineral, we can calculate how long ago that mineral
crystallized.
Determining the age of a rock through its radioactive
elements is known as isotopic dating.


Isotopes and Radioactive Decay

Radioactive decay – The spontaneous nuclear
disintegration of certain isotopes.

Daughter product – The isotope produced by radioactive
decay.

Half-life – The time it takes for a given amount of a
radioactive isotope to be reduced by one-half.


Radiocarbon Dating


Because of its short half-life of 5,730 years,
radiocarbon dating is useful only in dating things
and events accurately back to about 40,000 years –
about seven half-lives.
The technique is most useful in archaeological
dating and for very young geologic events. It has
even been used to date historical artifacts, such as
the Shroud of Turin.


Use of Isotopic Dating

In order for an isotopic age determination to be accurate,
several conditions must be met. To ensure that the isotopic
system has remained closed, the rock collected must show
no signs of weathering or hydrothermal alteration. Second,
one should be able to infer there were no daughter isotopes
in the system at the time of closure or make corrections for
probably amounts of daughter isotopes present before the
“clock” was set. Third, there must be sufficient parent and
daughter atoms to be measurable by the mass spectrometer
being used.


How Reliable is Isotopic Dating?

Half-lives of radioactive isotopes, whether shortlived, such as used in medicine, or long-lived, such
as used in isotopic dating, have been found not to
vary beyond statistical expectations. The half-life of
each of the isotopes we use for dating rocks has not
changed with physical conditions or chemical
activity nor could the rates have been different in
the distant past. It would violate laws of physics for
decay rates (half-lives) to have been different in the
past.

Back to the Beginning