Introduction
How do scientists begin to decipher Earth history? Scientists start by asking questions
such as “Which geologic process came first?” or “What is the age of the materials that
make up the earth?” Once various geological processes are chronologically arranged
and ages of rocks are determined, scientists can put together a more accurate story of
earth history which can be represented by a geologic time scale. The geologic time
scale represents various time divisions of earth history revealed by a combination of
relative and absolute dating methods. In this lab, students will learn how to apply the
principles of relative and absolute dating methods to understand the development of the
geologic time scale.
Stratigraphy represents the study of sedimentary layers or rock strata. Individual layers
are called stratum. A geologist’s initial challenge in the field is to subdivide the earth’s
stratigraphy into mapped units that can be traced throughout the field area or correlated
from one site to the next. Such mapped units are called formations. Formation
subdivisions are called members. The boundaries between formations or members are
referred to as contacts. Explaining and deciphering earth history requires the use of
geologic dating techniques and stratigraphic terminology.
Lab Objectives
1. Students will learn and apply various terms that apply to stratigraphic
sections.
2. Students will apply various relative dating principles and unravel earth history
using block diagrams.
3. Students will understand the concept of radioactive decay and apply absolute
dating techniques.
4. Students will grasp the basic concept behind the geologic time scale.
Relative Time Dating
Using various geologic principles proposed by the Danish naturalist, Nicholas Steno
(1669), a geologist can “relatively” date a sequence of geologic events. In other words,
which geologic event occurred first? The relative dating technique does not assign a
numerical value to rocks and geologic events, only the chronological sequencing --“Which came first?” Nicholas Steno proposed five relative dating principles that are
used today when establishing the chronological order of geologic events. Listed below
are the principles of relative dating.
The Principle of Original Horizontality
Sedimentary rock layers, under the influence of gravity, are originally deposited as
horizontal layers. If strata are no longer horizontal, in other words folded, tilted, or
faulted, then tectonic activity from the earth’s crust has more than likely taken place.
Layers of sedimentary rocks
are deposited in horizontal
layering under the influence
of gravity.
LS
SH
SS
Granite
Horizontal stratigraphy is
considered undisturbed
layers.
The Principle of Superposition
In any undisturbed sequence of strata, the oldest stratum is at the bottom and overlying
strata progressively become younger toward the top of the stratigraphic section.
Any sedimentary layer
on the bottom is the oldest
deposit.
A
Youngest layer
B
C
D
E
Oldest layer
The Principle of Inclusions
Any piece of rock (clast) that becomes included in another rock body must be older than
the rock or sediment into which it has been incorporated.
A
B
In Diagram A, the granite
body is older due to
“pieces” of granite
incorporated into the
overlying sandstone layer.
Older
Younger
In Diagram B, the granite
body is younger due to
“pieces” of sandstone
incorporated into the granite.
The Principle of Cross-Cutting Relations
Any geologic feature that cuts across a rock or sediment must be younger than the rock
or sediment it cuts across. Cross-cutting features may include igneous dikes, granitic
intrusions, and/or faults.
Fault F is cutting across
intrusion B; therefore, fault
F must be younger.
Fault-F
Intrusion
dike-D
Intrusive dike D cuts across
intrusion B, folded strata,
and layers C and E;
therefore, dike D must be
younger.
The Principle of Unconformities
An unconformity is a rock surface that represents a gap in the geologic record or missing
time (but does time really stop?). An unconformity is analogous to a page missing from a
book. An unconformity can represent a period of non-deposition or a period of
erosion. There are three major types of unconformities. The following diagrams
illustrate the 3 major types of unconformities.
Unconformity (represented by the “squiggly” line)
Disconformity
Angular Unconformity
Non-conformity
Disconformity:
a period of erosion or non-deposition within sedimentary layers that has not been
disturbed
Angular Unconformity:
a period of erosion or non-deposition where strata are angled against overlying
horizontal layers
Non-Conformity:
a period of erosion or non-deposition where sedimentary strata overlie crystalline
rocks (igneous or metamorphic rocks)
Applying the Principles to Unravel Geologic History
Observe the block diagram below, and use the above relative dating principles to
unravel the geologic history. In other words, which came first? Start with the oldest
event, and record your observation on the bottom line as you work your way up to the
last event. When observing sedimentary or folded rock units, use the term “deposition
of.” When observing igneous or metamorphic rock units, use the term “emplacement of.”
Study the block diagram below, and develop the geologic history using the various
principles outlined above.
Deposition of B,J,F
Unconformity
Emplacement of intrusion dike R
Deposition of P,K,M,S
youngest
oldest
In the diagram below, use the previous example above to decipher the geologic history,
starting with the oldest event to the youngest. Use the rock unit symbols at the back of
the lab to denote igneous, metamorphic, and sedimentary rock types. Do this diagram
before attempting diagrams in part B of the lab. Make sure you fully understand how to
chronologically list the geologic events before moving to part B.
Youngest
oldest
Absolute Time Dating
Absolute dating or radiometric age dating allows scientists to measure an element’s
radioactive content to determine an actual “number” of years or geologic age. Elements
that are radioactive and considered unstable are known as isotopes. Unlike deductive
reasoning (which geologic event occurred first) applied to relative dating techniques,
absolute dating indicates that geologic processes are slow and a rock’s age can be
measured in the millions or billions of years. Radiometric dating of rocks began around
the end of the 19th century and is based on the fact that isotopes decay from unstable
elements to stable elements at predictable, constant, measurable rates. For example,
the isotope uranium (U238- unstable) decays into lead (Pb206-stable). The unstable
isotope is known as the parent element and decays into the daughter element, which
represents the stable element. So, U238 is the parent element, and P206 represents the
daughter element.
Applying an actual number to geologic formations, using radiometric dating, scientists
measure the half-life which represents the time required for half of the parent element’s
atoms to change into the decayed daughter product. Therefore, a half-life represents
only half of the remaining radioactive atoms left from the parent atom. Below is an
example of counting half-lives.
100 atoms of parent
50 atoms of parent
25 atoms of parent
12.5 atoms of parent
6.25 atoms of parent
0 atoms of daughter
50 atoms of daughter
75 atoms of daughter
87.5 atoms of daughter
93.75 atoms of daughter
0 half-life
1 half-life
2 half-lives
3 half-lives
4 half-lives
Given the example above, can the parent material ever reach 0 atoms? As the parent
material decays at a constant rate, the time it takes to reach the decay of half the atoms
is measureable. For example, it takes 4.6 billion years to measure 50 atoms of U238
(parent) and 50 atoms of Pb206, which is one half-life. Table 1 shows the half-lives for
various isotopes. Because Carbon-14 has a very short half-life, the Carbon-14 method is
useful in dating organic material only as old as about 70,000 years. Using very precise
Geiger counter-like instruments establishes a relationship between the “specific activity” of
an organic substance and its age. Specific activity is a measure of the amount of remaining
Carbon-14, expressed in counts/min/gm of material (ct/min/gm). This relationship is given
in the graph below.
Table-1
Parent Isotope
Stable Daughter
1 Half-Life
Uranium-238
Lead-206
4.5 billion yr
Uranium-235
Lead-207
704 million yr
Thorium-232
Lead-208
14.0 billion yr
Rubidium-87
Strontium-87
48.8 billion yr
Potassium-40
Argon-40
1.25 billion yr
Samarium-147
Neodymium-148
106 billion yr
Carbon-14
Nitrogen-14
5,730 yrs
Introduction to the Geologic Time Scale
The modern geologic time scale was developed by the end of the 19th century and is
based on stratigraphic and fossil studies in Northern Europe and the United States. The
scale provides a standard reference for dating rocks throughout the world. Currently, the
geologic time scale represents age relationships compiled from both relative and
absolute dating techniques.
The geologic time scale attempts to chronologically
organize Earth history by subdividing the Earth’s development into Eras, Periods and
Epochs. Three eras represent the longest division of geologic time. The Paleozoic Era
refers to “ancient life,” the Mesozoic Era to “medieval life,” and the Cenozoic Era to
“modern life.”
Rocks considered older than the Paleozoic Era are known as
Precambrian Era, which includes about 80% of Earth history. Eras are further
subdivided into geologic periods. Geologic periods are divided into epochs. Below is a
standard illustration of the modern geologic time scale.
Part A - Definitions
geologic time scale
relative dating
absolute dating
isotope
parent material
daughter material
half-life
stratigraphy
formations
member
contact
principle of Original Horizontality
principle of Superposition
principle of Inclusions
principle of Cross-Cutting Relations
unconformity
disconformity
angular unconformity
non-conformity
PART B- Unraveling Earth History Using Block Diagrams
Part C- Absolute Dating
Examine the block diagram below. Five samples have been collected from various rocks
and sediments in this area.
Sample A is a contact metamorphic rock with datable zircon grains that can be used to
establish the timing of metamorphism.
Sample B contains datable biotite from an igneous dike.
Sample C contains biotite from a separate dike.
Sample D is a fragment of wood from ancient sediments now preserved on top of a
mesa.
Sample E is wood from sediments along the shore of a modern lake.
a. Use information in Table 1 and the Carbon-14 graph to determine the age for each
sample listed above.
Sample A --U238 = 75%, Pb206 = 25%
Age = ___________
Sample B – K40 = 50%, Ar40 = 50%
Age = ___________
Sample C – Rb87 = 98.4%, St87 = 1.5%
Age = ____________
Sample D --1 ct/min/gm
Age = ____________
Sample E – 8 ct/min/gm
Age = ____________
b. Use your knowledge of relative dating to determine the correct sequence
of igneous, metamorphic, and sedimentary rocks in the diagram above. Do the
absolute ages you calculated in part a (above) agree with the relative ages of
the five samples?
Part D- Critical Thinking Questions and the Geologic Time Scale
1. Explain the difference between relative and absolute dating methods.
2. If the oldest rocks on Earth are dated at approximately 3.8 billion years old, how
do scientists predict various half-lives of Rb87 – St87 at 48.8 billion years?
Introduction to the Geologic Time Scale
1.
Memorize the periods and their associated eras. Also, know the epochs related
to both the Quaternary and Tertiary Periods. Believe it or not, this is common
knowledge for non-science majors!
2.
Using absolute dates from the geologic time scale, record the absolute age
between each era. How long are the Paleozoic, Mesozoic, and Cenozoic Eras?
3.
What era and periods are associated with the presence of dinosaurs?
4.
Using absolute dates, from the geologic time scale construct a time-scaled
version of the geologic time scale showing the eras, periods, and epochs.
a. Which part of the geologic time scale is dominant (time-wise)? Why?
b. Why is the upper part of the geologic time scale subdivided in more detail
than the lower part of the scale?
c. What part of the scale represents the existence of humans?
d. Is the existence of dinosaurs longer or shorter than that of man? How
much shorter or longer?
5.
Write down your mnemonic phrase for memorizing the periods of the geologic
time scale (NO NAUGHTY PHRASES!)
Use the geologic rock symbols below to decide if stratigraphic layers in each relative
dating block diagram represent igneous, sedimentary, or metamorphic rocks. If layers
are igneous or metamorphic, use the term emplacement. If layers are sedimentary, use
the term deposit. Use these terms when writing down your geologic history for each
block diagram.