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Chapter 21 Fossils & the Rock Records
Produced by
S. Koziol
11-4-2014
21.1 The Geological Time Scale
Objectives • Describe the geological
time scale
• Distinguish among the
following geological
time scale divisions:
eons, era, period, and
epochs
Geological time scale
The geological
time scale divides
Earth’s history
into units from its
origin to the
present.
Remember scale issues….
Geologic time
Geologic time in order from shortest to longest
 epoch, period, era, eon
Geologic time – divisions?
Periods are defined by the abundance or
extinction of life-forms.
Geological time (continued)
On the geologic time scale, the smallest unit of
time is called an epoch.
We are currently in the
Holocene epoch of the
Quaternary period.
Greater collection of fossils
has been found and
paleontologists have more
detailed knowledge of the
events that occurred
during those times.
Geological Time – most recent
The most recent period is the Quaternary
The Anthropocene is an
informal geologic
chronological term for
the proposed epoch
that began when
human activities had a
significant global
impact on the Earth's
ecosystems.
Geological Time - longer
The Archean and Proterozoic are examples of
eons.
Eras vs Periods
Both are units of time on the geologic time scale.
Eras
Eras are longer spans of time,
measured in hundreds of
millions to billions of years.
They are defined by
differences in life-forms found
in rocks.
Periods
Periods are usually measured
in terms of tens of millions of
years to hundreds of millions
of years. They are defined by
the life-forms that were
abundant or became extinct
during the time in which
specific rocks were deposited.
Geological
Time Scale
(Blank)
21.2 Relative-age Dating of Rocks
Objectives • Apply the principle for determining
relative age to interpret rock
sequences.
• Describe an unconformity and how
it is formed within the rock record.
Laws of Superposition
Student guided , remember the foam boards
Undisturbed rock sequence
The oldest rock layer in an undisturbed rock
sequence occurs at the bottom of the sequence.
Which rock layer was laid down 1st?
Which book was laid down 1st?
The principle of superposition
The principle of
superposition
states that, in an
undisturbed
sequence, the
oldest rocks are at
the bottom of the
sequence and
successive layers
are younger than
those below
them.
Cross-cutting relationship
You can use the
principle of
cross-cutting
relationships to
infer that a
fault or an
intrusion is
younger than
the rock it cuts
across.
Correlation
The matching of rock layers from one geographic
area with those of another area is known as
correlation.
Uniformitarianism
The principle of uniformitarianism states that
the processes occurring today have been
occurring on Earth since it formed. However, the
rate, intensity, and scale with which these
processes occur have changed.
Unconformity
When part of the rock record is destroyed, the
erosional gap that forms is an unconformity.
Angular unconformity
The gap in the rock
record that occurs
between folded or
uplifted rock layers
and a sedimentary
rock layer on top
of them is called
an angular
unconformity.
Nonconformity
A buried erosional surface between a nonsedimentary rock and a sedimentary rock is called
a nonconformity.
A nonconformity in the Wind River
gorge of Wyoming.
21.3 Absolute-age Dating of Rocks
Objectives • Explain the several different methods used
by scientist to determine absolute age.
• Describe how objects are dated by the use
of selected radioactive elements.
• Explain how annual tree rings and glacial
varves are used to date geological events.
Dating techniques
Relative-age dating
Relative-age dating places the
ages of rocks and the events
that formed them in order,
without exact dates. This is
done by comparing one
event with another or one
rock with another rock.
Absolute-age dating
In contrast, absolute-age
dating determines the
actual age of a rock, a fossil,
or an object. This is done
through radiometric dating,
a process that determines
the ratio of parent material
to daughter product in a
given sample of rock or
fossil.
Half-life
The amount of time it takes for one-half of the
original amount of an isotope to decay is
known as its half-life.
Radiometric dating
Radiometric dating is used to determine the absolute
age of a rock. This is done using the idea of half-life.
Parent
Daughter
t1/2
Useful Range
238U
206Pb
4.5 b.y
235U
207Pb
710 m.y
232Th
208Pb
14 b.y
40K
40Ar &
40Ca
1.3 b.y
>10,000 years
87Rb
87Sr
47 b.y
>10 million years
14C
14N
5,730 y
100 - 70,000 years
Type of Material
>10 million years
Igneous Rocks and Minerals
Organic Material
Key Bed
A key bed contains distinctive material that
geologists can easily recognize in the rock
record and use as a time marker.
Dendochronology
The science of dendochronology uses the annual growth of tree
rings to date events and environmental changes.
Permineralization
In the process of permineralization, pore spaces
within an organism’s shell are filled in with
mineral substances.
21.4 Remains of organisms in the rock records
Objectives
• Define fossil.
• Explain several methods by which fossils
can be preserved.
• Describe the characteristics of an index
fossil.
• Discuss how fossils can be used to
interpret Earth’s past physical and
environmental history.
Amber
Fossil insects can be found imbedded in amber,
the hardened sap of prehistoric trees.
Mold
When the original
parts of an
organism in a
sedimentary rock
are weathered and
eroded, a
hollowed-out
impression called
a mold forms.
Casts
Natural casts can be formed when minerals are
deposited within the mold.
Fossils
Molds, casts, coprolites, and petrified wood are
all example of fossils
Original Preservation (rare)
Mummified animals found in dry caves can be
examples of fossils with original preservation.
Discovered in 1977, this six-to-eight-month-old woolly mammoth
baby named Dima is pictured in situ near Kirgiljach River in
northeast Siberia.
5,300 year old Iceman discovered in
Austrian Alps, 1991
Fossils
Fossils are the remains or evidence of once-living plants or animals.
They provide clues about Earth’s past environmental conditions
and evolutionary changes in organisms over time. They also help to
correlate rock layers from one area to another.
Index fossils
Geologists use index fossils to correlate rock layers over large
geographic areas or to date a particular rock layer. To be useful,
an index fossil must be easily recognized, abundant, and widely
distributed geographically. It must also have lived over a short
period of time.
Index fossils (continued)
Index fossils are useful to geologists if the fossils
have lived over a short period of time
Evolution
The adaptation of life-forms to changes in the
environment is known as evolution.
Evolution (continued)
Evolution is the process by which organisms
adapt to changes in their environments.
End of the Precambrian
The end of the Precambrian is marked by the
appearance of organisms with hard parts.
Remember this is speaking to
the end of the pre-Cambrian