Transcript SGES 1302 Lecture9

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 9: Principles of Relative Dating
Lecture 9:
GEOLOGIC TIME SCALE:
Principles of relative dating
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Geologic Time
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Time is an important element in all geological processes.
Geologists in 19th century recognised that Earth had experience many
episodes of mountain building and erosion, which required great spans of
geologic time.
Earth is very old. But how old?
Before the advancement of radiometric dating, geologic events are based
on relative dating principles.
The age of the Earth and its inhabitants are now determined through two
complementary lines of evidence: relative dating (stratigraphic age) and
absolute dating (radiometric age).
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Relative vs Absolute dating
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Relative dating places fossils in a temporal sequence
by noting their positions in layers of rocks, known as
strata. As shown in the diagram, fossils found in
lower strata were typically deposited first and are
deemed to be older.
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By studying and comparing strata from all over the
world we can learn which came first and which came
next, but we need further evidence to ascertain the
specific, or numerical, ages of fossils.
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Absolute dating relies on the decay of radioactive
elements that gives the actual number of years that
have passed since an event occurred. By dating
volcanic ash layers both above and below a fossilbearing layer, as shown in the diagram, you can
determine “older than X, but younger than Y” dates
for the fossils.
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Geologists have assembled a geological time scale
on the basis of numerical dating of rocks from around
the world.
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Geologic Time Scale
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Interpreting the history of the Earth is a prime goal of earth science.
The complex history of the earth can be revealed by studying the clues
preserved in the rocks, especially sedimentary rocks.
Geological events by themselves (e.g. Age of Dinasour) have little meaning
until they are put into a time perspective.
The development of the GEOLOGIC TIME SCALE has change the way
people think about time. Earth is much older than anyone had previously
thought.
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Geological Principles
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17th & 18th century: Doctrine of CATASTROPHISM: Earth’s landscape had
been formed primarily by great catastrophes. Mountains and canyons were
explained as being produced by sudden and often worldwide disasters
triggered by unknown causes that no longer operate.
Late 1700s: Principle of UNIFORMITARIANISM by James Hutton. It is the
pillar of modern geology. “the physical, chemical, and biological laws that
operate today have also operated in the geologic past” This idea is
commonly expressed as “The present is the key to the past”. Hutton
demonstrated that geological processes occur over extremely long period of
time. Weak, slow-acting processes could over a long period of time produce
effects just as great as those resulting from major catatrophic events.
Although the principle of uniformitarianism is still relevant today, some
geologic processes may not be equally important or operate in the same
rate.
Earth is ever-changing through the geologic time. “…that we find no vestige
of a beginning – no prospect of an end” Hutton, 1788.
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Principles of Relative Dating
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Placing rocks in their proper sequence of formation (which ones formed
first) but cannot tell us how long ago the event took place.
Relative dating is still widely used today. It is not replaced by absolute
dating, but supplemented it.
Law of Superposition
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Most basic principle of relative dating.
 In an undeformed sequence of sedimentary rock, each bed is older than the one
above it and younger than the one below
 Also applicable to other surface
deposited materials such as
lava flows, volcanic ash etc.
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Principles of Relative Dating
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Principle of Original Horizontality
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Layers of sediment are generally deposited in a horizontal position.
 Rock layers that are flat means that they have not been disturbed and thus still
have their original horizontality.
 If they are inclined or folded, they must have been moved into that position by
crustal disturbances after their deposition.
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Principles of Relative Dating
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Principle of Cross-cutting Relationships
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When a sequence of sedimentary rocks is cut by faults or intruded by igneous
rocks, the faults and igneous rocks must be younger and occurred after the
sedimentary layers were deposited.
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Principles of Relative Dating
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Inclusions
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Inclusions are pieces of rock that are
contained within another.
 The rock mass adjacent to the one containing
the inclusions must have been formed first in
order to provide the rock fragments. The rock
containing the inclusions is the younger of
the two.
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Principles of Relative Dating
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Unconformities
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Conformable: when layers of rocks were
deposited without interuption.
No place on earth has a complete set of
conformable sedimentary rock layers.
Interuption of deposition – break in the rock
record is call unconformity
Unconformity represents a long period
during which deposition has ceased,
erosion removed previously formed rocks
and then deposition resumed.
There are 3 types of unconformities:
Angular unconformity, disconformity &
nonconformity
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Angular Unconformity
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Tilted or folded sedimentary rocks that are
overlain by younger, more flat lying strata.
 There is a pause in deposition, period of
deformation and erosion.
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Disconformity
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Not easily identified because both sides
are near parallel.
 Easier to identify if the older rocks are
deeply cut by erosion.
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Nonconformity
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A break separating the older igneous or
metamorphic rocks from the younger
sedimentary layers.
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Principle of faunal succession
“Fossil organisms succeed one another in a definite and determinable
order, and therefore any time period can be recognised by its fossil
content”
 Fossils are arranged according to their age and do not present
randomly or haphazardly
 Basic principle of historical geology
 Index fossil
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Correlation of Rock Layers
To develop a stratigraphic time scale for the entire
Earth, rocks of similar age in different regions must
be matched up by CORRELATION.
To correlate rocks over great distances, fossil
records are required.
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Succession in geologic
development
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Geologic Time Scale
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Most of the original geologic time scale
was based on studies of strata in
Europe in mid 1900s
Major units are generally named after a
geographic area where they are well
exposed
The rock units are separated by major
changes in rock types, unconformities
or fossil groups
Rocks in other parts of the world that
contain the same fossil groups are
considered as having the same age.
Absolute age for the standard geologic
column is added in the 20th century with
data from radiometric dating.
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