17 interpreting earth history
TRANSCRIPT
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CHAPTER 17
INTERPRETING GEOLOGIC HISTORY: WHAT HAPPENED AND WHEN DID IT HAPPEN?
PURPOSE
To learn how to determine the relative ages of rocks and geologic processes and use these methods tointerpret complex geological histories.
To learn how numeric (absolute) ages of rocks are calculated and apply them to dating geologic
materials and events.
To see how geologists piece together Earth history from widely separated areas.
MATERIALS NEEDED
Pen, pencils, and a calculator
17.1 INTRODUCTION
Youve learned to identify minerals, use mineralogy and texture to interpret the origin of rocks,
deduce which agents of erosion have affected a given area, and recognize evidence for tectonic events.
With these skills you can construct a three-dimensional picture of Earth, using topography and surface
map pattern to infer underground relationships.
This chapter adds the fourth dimension time: the ages of rocks and processes. Geologists ask
two different questions about age: Is a rock or process older or younger than another? (their relative
ages) and Exactly how many years old are they? (their numericor absoluteages). We look first how
relative ages are determined, then at methods for calculating numeric age, and finally combine them to
decipher geologic histories of varying complexity.
17.2 PHYSICAL CRITERIA FOR DETERMINING RELATIVE AGE
Common sense is the most important resource for determining relative ages. Most reasoning
used in relative age dating is intuitive and the basic principles were used for hundreds of years before we
could measure numeric ages. Geologists use two types of information to determine relative age:physical
methodsbased on features in rocks and relationships between them, andbiological methodsthat use
fossils. We will focus first on the physical methods and return to fossils later.
2009 Allan Ludman and Stephen MarshW.W. Norton & Company
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Figure 17.2 Cross-bedded sandstones, Zion National Park, Utah
Congratulations! Your common sense got that right too. ThisPrinciple of Cross-cutting
Relationshipsis useful for many geologic materials and processes, such as (see Figure 17.3):
Figure 17.3 Applying the Principle of Cross-cutting Relationships
a.Dikes intruding granite of the Sierra Nevada b. Vertical fault offsetting volcanic ash, c. Folded sedimentary rocks,Batholith, Yosemite National Park, California deposits Kingman, Arizona. Utah
An intrusive igneous rock that cuts across other rock units (Figure 17.3a) must be ________than
the units it cuts across. Using this principle, label the order of intrusion of the rocks in Figure 17.3a
A fault (Figure 17.3b) must be ______ than the rocks it offsets. While youre at it, use the Principle ofOriginal Horizontality to label the oldest and youngest (horizontal) volcanic ash layers that the fault cuts.
The process that folded the rocks in Figure 17.3c must be _______________ than the rocks. Why cant
you use the Original Horizontality to determine the relative ages of the sedimentary layers in this photograph?
Interpret cross-cutting broadly and use itnot only for features that physically cut others, but
also for processes that have affectedmaterialsas in the folding in Figure 17.3c). Contact
Fault
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metamorphism is an example of a process that affects rocks without physically cutting across them.
Lava bakes the rock or sediment it flows across. A sill intruded between two sedimentary beds doesnt
cut across them but its contact metamorphism changes their mineralogy and texture. In both cases, the
metamorphism, and therefore the igneous rock that caused it, must be younger than the rocks it affects.
This principle can help solve the following geologic problem: A horizontal layer of basalt has been found
between two horizontal beds of sandstone.
Using your knowledge of the cross-cutting nature of contact metamorphism, how could you tell
whether the basalt was a lava flow or a sill?
17.2.3: Principle of Inclusions: The reasoning here applies any material found within another rock or
sediment, whether the rock or inclusion is igneous, sedimentary, or metamorphic. For example, examine
Figure 17.4a, a photograph of a granitic intrusive containing inclusions of medium-grained gabbro.
Which is older, the granite or the fragments? Explain.
Examine the conglomerate in Figure 17.4b. The clasts in this conglomerate are fragments of
rhyolitic tuff eroded from an Ordovician island arc. Clasts in sedimentary rock must be __________
than the sedimentary rock in which they are included.
Figure 17.4 Inclusions in igneous and sedimentary rocks
a. Gabbro inclusions in the Baring Granite, Maine b. Fragments of rhyolite tuff in conglomerate,
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17.2.4 Sedimentary structures: Some sedimentary features indicate where the top or bottom of a bed
was at the time the rock formed. These top-and-bottom structures are extremely helpful when
sedimentary rocks have been deformed so that the Principle of Superposition can no longer be used.
Figure 17.5 shows how some of these features help determine the relative ages of sedimentary rocks.
Figure 17.5 Sedimentary top-and-bottom indicators of relative age
a. Mud cracks: Mud cracks are widest at the top and narrow downward. The diagram at right istherefore right-side up, indicating that the bottom-most bed is older than the blue-gray bed above it.
a.c.Symmetrical ripple marks: The sharp points of the ripple marks point toward the top of the bed.
b. Graded beds: The coarsest grains settle fastest and lie at the bottom of the bed. They are
followed by progressively smaller grains, producing the size gradation. Arrows in the diagram
at the right show how a geologist would interpret the upright nature of the graded beds.
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Sketch diagrams showing how these three sedimentary structures would appear in beds that had been
turned upside-down.
i. Mud cracks ii. Graded beds iii. Symmetrical ripple marks
d. Impressions: Dinosaur footprints (left) and raindrops (right). These features form when something (here
dinosaur and raindrops) sank into soft sediment. The sedimentary features in one of these photographs are
right-side-up but those in the other are overturned. Which is which? Explain your reasoning.
EXERCISE 17.1: USING SEDIMENTARY STRUCTURES TO UNRAVEL HISTORY
Figure 17.6 shows the value of sedimentary structures in relative age dating. A cliff face exposing
horizontal rock (left side of Figure 17.6) might be incorrectly interpreted as undeformed horizontal strata without
the top-and-bottom information that prove that the layers have been folded and that some must have been
overturned. The right side of Figure 17.6 shows what the geologist might have seen had the adjacent rocks not
been eroded away or covered by glacial deposits.
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Figure 17.6 Reversals of top-and-bottom features reveal folding
Upright symmetrical ripple marks Upright graded bedding
Number the layers, with 1 representing the oldest rock. Which is the youngest?
17.3. UNCONFORMITIES: EVIDENCE FOR A GAP IN THE GEOLOGIC RECORD
When rocks are deposited continuously in a basin without interruption by tectonic activity, uplift,
or erosion, the result is a stack of parallel. Beds in a sequence are conformablebecause the each has the
same (conforms to)shape and orientation of the others, as in Figure 17.1.
Tilting, folding, and uplift leading to erosion interrupts this simple history and breaks the
continuity of deposition. In these cases, younger layers are not parallel to the older folded or tilted beds
and erosion may remove large parts of the rock record leaving a gap in an areas history. We may
recognize that deposition was interrupted but cant tell how long the interruption lasted or what
happened during it. A contact indicating a gap in the geologic record is called an unconformity(Figure
17.X) because the layer above it is not conformable with those below.
There are three kinds of unconformity. Adisconformityseparates parallel beds but some older
rocks below the disconformity were removed by erosion (Figure 17.xa). Anonconformityis an erosion
surface separating older igneous rocks from sedimentary rocks deposited after the pluton was eroded
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(Figure 17.7c). When rocks above an unconformity cut across folded or tilted rocks below it, the contact
is called anangular unconformity(Figure 17.7b).
Figure 17.7: Origin of unconformities
a. Disconformity b. Angular unconformity c. Nonconformity
Figure 17.8 shows an angular unconformity in the Grand Canyon that cuts across vertically
layered Precambrian metamorphic rocks and light-colored dikes that intrude them. Rocks above the
unconformity are nearly horizontal, unmetamorphosed Cambrian sedimentary rocks. This angular
unconformity separates rocks that had very different geologic histories and the gap that it represents
marks a period of history for which the rock record in this area is missing. Without numerical age
dating, we could not know how much geologic history is missing.
In this case, nearly 500,000,000 years (half a billion!) elapsed between Precambrian
metamorphism and dike intrusion and Cambrian deposition. To put this in perspective, consider that in
the 500,000,000 years since the Cambrian rocks were deposited: a supercontinent broke into several
plates separated by ocean basins; the ocean basins were subducted, forming a new supercontinent and
creating huge mountains; the mountains were worn down by millions of years of erosion; the second
supercontinent also broke into several plates separated by todays oceans and continents.
Erosion
Intrusive
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Figure 17.8 Angular unconformity in the Grand Canyon
The red line marks a profound unconformity separating vertical Precambrian metamorphic rocks
(below) and unmetamorphosed, nearly horizontal Cambrian sedimentary rocks (above).
EXERCISE 17.2 APPLYING PHYSICAL PRINCIPLES OF RELATIVE AGE DATING
Now apply the principles youve just learned to interpreting geologic histories of various degrees
of complexity. Figures, 17.9, 17.10, 17.11 and 17.12 simulate four Grand Canyon-scale cliff exposures in which
rock units have been labeled for easy identification. The labels have no meaning with respect to relative age.
Arrange them from oldest to youngest and explain your reasoning based on the principles outlined above. There
will be no doubt about some relative ages, but others will not be clear-cut. Indicate the uncertainties and why they
exist. Note that rock units and some contacts are labeled, but the geologic eventsthat produced the relationships
are not.Include those events in your histories.
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Figure 17.9 Geologic cross-section #1
Figure 17.10 Geologic cross-section #2
A
F
C
B
D
G
E
H
I
J
K
Youngest
Oldest
A
D
C
A
C
EE
F F
G
H
I I
J
K
Youngest
Oldest
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Figure 17.11 Geologic cross-section #3
Figure 17.12 Geologic cross-section #4
A
B
C
E
F
G
H
I
K
J
D
Youngest
Oldest
A
C
F
BE
H
D
G
I
J
Youngest
Oldest
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17.4 BIOLOGICAL METHODS FOR RELATIVE AGE DATING AND CORRELATION
In 1793, British canal builder and amateur naturalist William Smith noticed different fossils in
the layers he was excavating. He found wherever he worked that some fossils were always found in
rocks that lay above others, and suggested that fossils could be used to tell the relative ages of the rocks.
17.4.1 Principle Of Faunal And Floral Succession: Geologists confirmed his hypothesis by showing
that fossil animals (fauna) and plants (flora) throughout the world record an increasing complexity of
life forms from old rocks to young ones. This is thePrinciple of Faunal and Floral Succession. Not all
fossils can be used to date rocks because some, like blue-green algae, have existed over most of geologic
time and are not specific to a narrow span.
Index fossilsare remains of plants or animals that lived throughout the world but existed for only
a short span of geologic time before becoming extinct. When we find an index fossil, we know that the
rock in which it is found dates from that unique segment of time. This is like knowing that a Ford Edsel
could only have been made in the three years between 1957 and 1960, or the Model A between 1903 and
1931. Tyrannosaurus rex, for example lived only in the span of geologic time known as the Cretaceous
Period; it should not have been in a Jurassic (an earlier Period) park.
17.4.2 THE GEOLOGIC TIME CHART
Combine the sequence of rock units determined by physical methods with the ability of index
fossils to place those units in their correct position in time and the result is the Geologic Time Chart
(Figure 17.13). The chart divides geologic time into progressively smaller segments called Eons, Eras,
Periods, and Epochs (not shown). Era names reveal the complexity of their life: Paleozoic = ancientlife;
Mesozoic = middlelife; and Cenozoic = recentlife. The end of an era is marked by a major change in
life forms, like an extinction in which most life forms disappear and others fill their ecologic niches.
Nearly 90% of fossil genera became extinct at the end of the Paleozoic Era, making room for the
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dinosaurs and the extinction of the dinosaurs at the end of the Mesozoic Era made room for us
mammals. Several Period names come from areas where rocks of that particular age were best
documented: Devonian fromDevonshire in England; Permian from the Permbasin in Russia; and the
Mississippian and Pennsylvanian from ..
The Geologic Time Chart was originally based entirely on relative age dating. We knew that
Ordovician rocks and fossils were older than Silurian rocks and fossils, but had no way to tell how much
older. The ability to calculate the numerical ages needed to do came nearly 100 years after the original
time chart. Section 17.5 will explore methods of numerical age dating.
17.4.3 Fossil Age Ranges:
Some index fossils ages are very specific. The trilobitesElrathiaandRedlichialived only during
the Middle and Early Cambrian, respectively. Others lived over a longer span, like Cybele(Ordovician
and Silurian) and the brachiopodLeptaena(Middle Ordovician to Mississippian). But even index fossils
with broad ranges can yield specific information if they occur with other index fossils whose overlap in
time limits the possible age of the rock. This can be seen even in the broad fossil groups shown in Figure
17.13. Trilobite, fish, and reptile fossils each span several periods of geologic time but if specimens of
all three are found together, the rock that contains them could only have been formed during the
Pennsylvanian or Permian.
EXERCISE 17.3: DATING ROCKS BY COMBINING FOSSIL RANGES TO
Figure 17.14 shows selected Paleozoic brachiopod species (a) and graphs their ranges within the
geologic record (b).
a. Based on the overlaps in their ranges:i. What fossil assemblage would indicate a Permian age?ii. A Silurian age?iii. An Ordovician age?
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Figure 17.13 The Geologic Time Chart
ERANumerical
age (Ma)Period Age ranges of selected fossil groups
CENOZO
IC
Quaternary
Tertiary
MESOZOIC
Cretaceous
Jurassic
Triassic
PALEOZOIC
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Proterozoic
Eon
Several
Eras
Archaean
Eon
Several
Eras
Hade
an
Eon
1.8
65.5
251
200
145
542
488
444
299
318
359
416
2,500 = 2.5 billion
3,800 = 3.8 billion
Am
hibia
ns
Retiles
Mammals
Shelledanimals
Flowering
plants
Dinosaurs
Fish
Trilobites
Humans
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Figure 17.14 Selected brachiopods and their age ranges in the fossil record
b.Apply overlapping ranges to the cross-sections in Exercise 17.2 In Figure 17.9:i.IfNeospirifer is found in Unit D, Platystrophiain F, and Strophomenain A: suggest an age for C.
Explain your reasoning.
ii.What is the extent of the gap in the geologic record represented by the angular unconformity (E)?
a.Selected brachiopod species and their ranges in geologic time
Platystrophia Schizophoria Cranaena Strophomena Leptaena Chonetes
Atrypa Mucrospirifer Neospirifer Petrocrania Cleiothyridium Lingula
Triassic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Leptaena
Schizophoria
Mucrospirifer
Neospirifer
Petrocrania
Platystrophia
Strophomena
b. Age ranges of brachiopod species
Atrypa
Chonetes
Cranaena
Cleiothyridium
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In Figure 17.10, Strophomenais found in E, Platystrophiaand Petrocraniain F, and Petrocrania,
Chonetes, andNeospiriferin C.
i.When were units E,A,F, and I tilted? Explain.ii.What is the extent of the gap in the geologic record represented by the contact between D and the
tilted rocks beneath it? Explain.
iii.When in geologic time did the fault cutting units K, C, I, F, A, and E occur? Explain.
17.5 CORRELATION: FITTING PIECES OF THE PUZZLE TOGETHER
There is no place where the entire 4.6 billion years of Earth history is revealed not even in the
Grand Canyon. To work out a history (and time chart) for the entire planet, we must decipher the records
of local areas and combine them to build the big picture. But it is hard to know which sandstone in
Kansas was deposited at the same time as sandstones in Pennsylvania much less in Japan, Kenya, or
France especially if none have index fossils. Geologists use physical and biological methods to
correlateunits from different areas showing that they formed at the same time.
EXERCISE 17.4 LITHOLOGIC CORRELATION:
Five years ago, geologists determined the relative ages of rocks in two parts of the midcontinent but thesequences were not the same (Figure 17.15). Unfortunately, similar rocks appear at several places in each section
making it difficult to know exactly which limestone, for example, in the western section correlates with a
particular limestone in the east. It is better to compare sequences of units based on similar sequencesof
depositional environments rather than similarities in single rock units.
a. Examine the two stratigraphic columnsbelow for similar sequences of rock types andenvironments. Were environments indicated by the sedimentary rocks the same in both places? Was deposition
continuous (conformable) in both areas? If not, explain what might have caused the differences and
unconformities.
b. Suggest correlations between units of the two sequences by connecting the tops and bottoms of units inthe western section to those of what you infer are their correlatives in the eastern section. Explain your reasoning.
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During field mapping last year, trilobites were found in limestone units in both areas (Figure 17.16).
Paleontologists reported that these were identical index fossils from a narrow span of Middle Cambrian time.
Using this additional information:
a)Draw lines indicating correlative units in Figure 17.16.
b)Why does the presence of index fossils make correlation more accurate than correlation based on
lithologic and sequence similarities alone?
Limestone Siltstone ConglomerateSandstoneShale
11
15
13
16
18
1
2
3
4
5
7
6
19
9
8
14
12
10
17
20
Western sequence
B
C
D
E
F
I
J
H
G
Eastern sequence250 miles
Figure 17.15 Correlation of lithologic sequences
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17.5 NUMERICAL AGE DATING
The Geologic Time Chart (Figure 17.13) was used for relative age dating for about 150 years
before numerical ages could be added. Numerical age dating is based on the fact that nuclei of some
atoms of elements found in minerals (parent elements) decay to form atoms of new elements (daughter
elements) at a fixed rate regardless of conditions.
The decay clock begins when a mineral containing the parent element crystallizes during
igneous and metamorphic processes. With time, the amount of the parent decreases and the amount of
the daughter increases (Figure 17.17). The amount of time the process takes depends on the decay rates
Limestone Siltstone ConglomerateSandstoneShale
11
15
13
16
18
1
2
3
4
5
7
6
19
9
8
14
12
10
17
20
Western sequence
B
C
D
E
F
I
J
H
G
Eastern sequence250 miles
Figure 17.16 Correlation assisted by index fossils
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of the different isotopes. Thehalf-lifeis the amount of time it takes for half the parent atoms in a
mineral sample to decay to an equal number of daughter atoms (the center hourglass below).
Figure 17.17 Parent:daughter ratios during radioactive decay
Table 17.1 lists the isotopes used commonly in numerical age dating, their half-lives, and the
minerals used in dating. In general, an isotope can date ages as old as ten of its half-lives; any older than
that and there wouldnt be enough parent atoms to measure accurately. They are best used for relatively
recent ages. Conversely, isotopes with very long half-lives, like147
Samarium decay so slowly that they
can only be used to date very old rocks.
Table 17.1: Geologically important radioactive decay schemes
Parent IsotopeDaughter decay
productHalf-life (years)
Useful dating
Range (yrs)Dateable materials
147Samarium 143Neodimium 106 billion
> 10,000,000
Garnets, micas
87Rubidium
87Strontium 48.8 billion
Potassium-bearing minerals (mica,
feldspar, hornblende)
238Uranium 206Lead 4.5 billionUranium-bearing minerals(zircon,
apatite, uraninite)
235Uranium 207Lead 713 millionUranium-bearing minerals(zircon,
apatite, uraninite)
40Potassium 40Argon 1.3 billion > 10,000Potassium-bearing minerals (mica,
feldspar, hornblende)
14Carbon 14Nitrogen 5,370 100 to 70,000 Organic materials
To calculate the numerical age of a rock, geologists crush it to separate minerals containing the
desired isotope. A mass spectrometer determines the parent:daughter ratio and we then solve a
logarithmic equation to calculate the age. Your calculation is easier: once you know the parent:daughter
ratio, you can use Table 17.2 to calculate age by a simple multiplication.
% Parent 100 75 50 25 0
% Daughter 0 25 50 75 100
Parent:daughter
ratio----- 3:1 1:1 1:3 ---
Parent
Daughter
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Table 17.2 Calculating the numeric age of a rock from decay scheme half-lives
% ofparentatoms
remaining
Parent:
Daughter
ratio
# of
half-
lives
elapsed
Multiply
half-life by
___ to
determine
age
% ofparentatoms
remaining
Parent:
Daughter
ratio
# of
half-
lives
elapsed
Multiply
half-life by
___ to
determine
age
100 ----- 0 0 35.4 0.547 1 1.500
98.9 89.90 1/64 0.016 25 0.333 2 2.000
97.9 46.62 1/32 0.031 12.5 0.143 3 3.000
95.8 22.81 1/16 0.062 6.2 0.066 4 4.000
91.7 11.05 1/8 0.125
84.1 5.289 0.250
70.7 2.413 0.500 0.05 11Dont
bother!There are too
few parent
atoms to
measure
accuratelyenough
50 1.000 1 1.000 0.025 12
EXERCISE 17.5 COMBINE NUMERICAL AND RELATIVE AGE DATING
a. First, get some practice calculating ages using Tables 17.1 and 17.2. How old is a rock if it contains:i. a 235Uranium:207Lead ratio of 46.62? _____________yearsii. a 87Rubidium:87Strontium ratio of 89.9? _____________yearsiii. 6.2% of its original 14C? _____________yearsiv. 97.9% of its original 14K? _____________years
b. Now add more detail to the cross-sections in Exercise 2.i. In Figure 17.11: Dike E zircon [ 235U:207Pb=11.05]
Dike A zircon[238U:206Pb=22.81];
Pluton E hornblende parent 40K=84.1%]
How old is E? ______________; A?________________; F?___________________
How old (in years and using Period names) are layers B,H,K, and J?
When were these layers folded?
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When did the unconformity separating I from the underlying rocks form?
How old are layers I and D?
ii. In Figure 17.12: H has a thin volcanic ash bed at its base with zircons [235U:207Pb = 46.62]D has zircons with [235U:207Pb= 2.413]
F has hornblendes with 50% of its original 40K
How old is H? ______________; D?________________; F?___________________
In years and using Period names,
a. When were layers C, G, and J folded?b. How large a gap in the geologic record is represented by the unconformity below Layer B?c. How old is layer I?d. If Layer I contains the brachiopodsNeospirifer, Chonetes, and Schizophoria,how does this
help narrow its possible age?
e. In that case, how large a gap in the geologic record is represented by the unconformity belowLayer H?