17 interpreting earth history

8/13/2019 17 Interpreting Earth History

1/21

1

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


2/21


3/21

3

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


4/21

4

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,


5/21

5

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.


6/21

6

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.


7/21

7

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


8/21

8

(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


9/21

9

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.


10/21

10

Figure 17.9 Geologic cross-section #1


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


11/21

11



A

B

C

E

F

G

H

I

K

J

D

Youngest

Oldest

A

C

F

BE

H

D

G

I

J

Youngest

Oldest


12/21

12

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


13/21

13

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?


14/21

14

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


15/21

15

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


16/21

16

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.


17/21

17

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


18/21

18

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


19/21

19

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


20/21

20

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?


21/21

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?

17 interpreting earth history

Documents