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Chapter 4: Geologic Time: A Story in the Rocks Chapter Contents

Book Title: Earth

Printed By: Kevin Murray ([email protected])

© 2015, 2011 Cengage Learning, Cengage Learning

4

Geologic Time: A Story in the Rocks

Chapter Introduction

4-1 Earth Rocks, Earth History, and Mass Extinctions

4-1a Extraterrestrial Impacts

4-1b Volcanic Eruptions

4-1c Supercontinents and Earth’s Carbon Dioxide Budget

4-2 Geologic Time

4-3 Relative Geologic Time

4-3a Interpreting Geologic History From Fossils

4-4 Unconformities and Correlation

4-4a Unconformities

4-4b Correlation

4-5 Absolute Geologic Time

4-5a Radioactivity and Half-Life

4-5b Radiometric Dating

4-6 The Geologic Time Scale

4-6a The Earliest Eons of Geologic Time: Precambrian Time

4-6b The Phanerozoic Eon

Chapter Review

Key Terms

Chapter Review

Review Questions

Chapter 4: Geologic Time: A Story in the Rocks Chapter Introduction

Book Title: Earth



Chapter Introduction



View of the Grand Canyon, looking North. Over one mile of vertical topographic

relief exists between the tree-covered canyon rim on the skyline and the Colorado

River in the foreground. The unlayered rock exposed directly above the river is

Precambrian gneiss and schist. This rock, which formed deep within the crust, was

uplifted and eroded prior to Cambrian time. After more than a billion years of

erosion, the Cambrian Tapeats sandstone was deposited. The Tapeats is the

prominent cliff-forming layer sitting directly over the Precambrian metamorphic

rocks. The red- and tan-colored layers in the distance are younger Paleozoic

sedimentary rocks.

MARTIN M303/ SHUTTERSTOCK.COM

Rocks are records of events that took place at the time they formed. They are books.

They have a different vocabulary, a different alphabet, but you learn how to read them.

John McPhee

http://shutterstock.com/



Chapter 4: Geologic Time: A Story in the Rocks: 4-1 Earth Rocks, Earth History, and Mass Extinctions

Book Title: Earth



4-1 Earth Rocks, Earth History, and Mass Extinctions

In many locations, th-century geologists found thick sequences of rock layers with

abundant fossils that we now know represent millions, or hundreds of millions, of years of

Earth history. The geologists also found layers of rock containing few or no fossils, lying

above the fossil-rich rocks. And even higher in the sequence of rock layers, they found

abundant fossils again, but of organisms very different from those in the older rocks. Most

surprising, many of the most abundant fossilized organisms in lower layers were never

seen again in younger rocks. Those organisms simply had disappeared from the face of

Earth forever.

This fossil record suggests that, more than once, a sudden, catastrophic event has abruptly

decimated life on Earth and that new life-forms have emerged following these mass

extinction (A sudden, catastrophic event during which a significant percentage of all life-

forms on Earth become extinct.) . However, scientists did not immediately accept the idea

that catastrophic events had caused sudden mass extinctions. Instead, they suggested that

the rock record might not be complete. Maybe some rocks had been destroyed by erosion,

or perhaps sedimentary rocks had not formed for a long period of time. According to this

reasoning, the part of the record that was lost could have contained evidence for the

gradual decline of the species that became extinct and the slow emergence of new species.

Following the conventional wisdom of the time, scientists concluded that the extinctions of

old life-forms and the emergence of new ones occurred slowly, as a result of gradually

changing conditions.

But if pieces of the rock record were missing in some locations, perhaps they existed in

other places on Earth. Geologists searched for fossiliferous rocks of the appropriate ages to

fill in the gaps and provide evidence for gradual extinctions—but these rocks were never

found. Eventually, the evidence for near-instantaneous extinctions became compelling.

From studies of this type, scientists learned that at least six times in Earth history,

catastrophic extinctions have decimated life on Earth (Figure 4.1).

Figure 4.1

The red line shows that the number of families of organisms, indicated by the

thickness of the blue-shaded areas, has varied throughout Earth history. Sudden

leftward shifts of the red line indicate mass extinctions. The numbers in the left

column indicate millions of years ago.



Cengage Learning 2015; TRILOBITES: CHANTAL DE BRUIJNE/ SHUTTERSTOCK.COM

The most dramatic extinction occurred about million years ago, at the end of the

Permian Period. At that time, percent of all species in the oceans suddenly died out. On

land, about two-thirds of reptile and amphibian species and percent of insect species also

vanished. In his book describing this extinction, author Douglas Erwin exclaims that it was




“the closest life has come to complete extermination since its origin.”

The death of most life-forms at the end of the Permian Period left huge ecological voids in

the biosphere. Ocean ecosystems changed as new organisms emerged in an environment

relatively free of predators and competition. On land, dinosaurs and many other terrestrial

animals and plants appeared and proliferated.

About million years later, at the end of the Cretaceous Period, another catastrophic

extinction wiped out up to percent of Earth’s genera, including the dinosaurs. This mass

extinction occurred million years ago. Small mammals survived this disaster, facing a

new world free of the efficient predators that had hunted them. The number of new

mammal species increased rapidly after this extinction event, eventually leading to the

evolution of humans.

What kinds of catastrophic events caused these sudden extinctions? Why would species that

had survived for tens of millions of years disappear so rapidly? We consider three

hypotheses for mass extinctions. Each of these hypotheses involves large-scale systems

interactions among the biosphere, the geosphere, the atmosphere, and the hydrosphere. In

the first hypothesis, described next, a fifth (extraterrestrial) system is seen as the trigger

that initiated radical changes in all four Earth systems.

Chapter 4: Geologic Time: A Story in the Rocks: 4-1a Extraterrestrial Impacts

Book Title: Earth



4-1a Extraterrestrial Impacts

In the late 1970s, the father-and-son team of Walter and Luis Alvarez were studying

sedimentary rocks deposited when the dinosaurs and so many other life-forms were

becoming extinct. In one layer, they found abundant dinosaur fossils. Just above it, they

found very few fossils of any kind and no dinosaur fossils. Between these two rock layers

they found a thin, sooty layer of clay. They brought samples of the clay to the laboratory

and found that it contained high concentrations of the element iridium. This discovery was

surprising because iridium is rare in Earth rocks. Where did it come from?

Although rare in Earth’s crust, iridium is more abundant in meteorites. In a landmark

paper published in 1980, Walter and Luis Alvarez suggested that million years ago, a

meteorite roughly kilometers in diameter hit the Earth with explosive energy many

thousands of times greater than that of today’s entire global nuclear arsenal. The

collision vaporized both the meteorite and Earth’s crust at the point of impact, forming a

plume of hot dust and gas that rose into the upper atmosphere and circled the globe. This

thick, dark cloud blotted out the Sun, significantly reducing photosynthesis for up to several

years. Particularly hard hit were microscopic algae and other tiny photosynthesizing

organisms in the oceans that provide the base of the food chain there. The widespread

elimination of this food base almost certainly contributed to the extinction of larger marine

organisms such as ammonites. Also killed off were all the dinosaurs and the flying and

swimming reptiles. Analysis of the fossil record has suggested that nothing with a body

mass exceeding kilograms (about the size of a large dog) survived.



The now-famous 1980 paper by the Alvarez team created a considerable stir among

geologists, in part because the paper cited compelling evidence for the impact but did not

identify the actual impact site. At about the same time, geophysicist Glen Penfield was

analyzing magnetic surveys of the Yucatan Peninsula for the Mexican oil company Pemex,

when he discovered the presence of a large, symmetrical semicircle in the Gulf of Mexico on

the northwest side of the peninsula. Obtaining data from an older gravity survey of the

peninsula, Penfield noticed that a second semicircle was present onshore and that the two

semicircles together formed a large, circular structure about kilometers across.

Although Penfield and his coworkers suspected that the circular structure might be the

“missing” impact site, they were not able to secure any rock samples from the area to

examine for evidence of an impact, and Pemex did not allow them to publish the

geophysical data behind their discovery. Instead, they published a short abstract and

presented a brief talk in 1981, announcing the discovery of a large igneous body in the

Yucatan Peninsula. It was not until 1991— years after the Alvarez paper—that a team of

scientists (among them Penfield) had amassed sufficient data, including rock samples from

old Pemex oil wells in the area, to publish the first paper describing the site as an impact

structure. Since then, many analyses of the impact site and surrounding areas have

suggested that it occurred million years ago, coinciding with the Cretaceous extinction

event.

Although much evidence in support of the Yucatan site as a major impact structure has

been published since 1991, its relation to the mass extinction at the end of the Cretaceous

Period is still the subject of vigorous debate. Careful analyses of fossils preserved in sediment

cores recovered from the crater have indicated that the impact actually occurred about

years prior to the extinction event. In addition, other possible large impact sites of

Late Cretaceous age have been described in the North Sea, the Ukraine, and India; at the

latter location, an impact crater more than kilometers in diameter—the biggest on

Earth—has been reported (Figure 4.2).

Figure 4.2

Strong evidence suggests that several large meteorites struck Earth about

million years ago. Many scientists think the impact site in the Yucatan Peninsula of

Mexico contributed to the extinction event at the end of the Cretaceous Period,

although impact sites in the Ukraine and India likely also played a role.



© Cengage Learning

Although the Yucatan impact site was hailed for many years as the “smoking gun” that

caused the Cretaceous terminal extinction event, these more recent findings cast doubt on

that simple explanation. Some scientists have suggested that widespread volcanism in India,

combined with severe greenhouse conditions at the end of the Cretaceous, caused

significant environmental stress that reduced the population sizes of many species (or

eliminated them entirely) prior to the Yucatan impact. These scientists have suggested that

the impact itself did not catastrophically kill a healthy, thriving biosphere, but rather

pushed an already very-stressed biosphere over the threshold that led to a major extinction.

Although the extraterrestrial impact hypothesis still is very popular overall, reports of other

major impact sites of Late Cretaceous age and uncertainty about the role that factors such

as volcanism may have played will keep scientific interest fixed for the foreseeable future on

the cause(s) of the Cretaceous extinction event.

Chapter 4: Geologic Time: A Story in the Rocks: 4-1b Volcanic Eruptions

Book Title: Earth



4-1b Volcanic Eruptions

A volcanic eruption ejects gas and fine ash into the atmosphere. The volcanic ash serves like

an umbrella that reflects sunlight back into space and thus causes climatic cooling. One

volcanic gas, sulfur dioxide, forms small particles called aerosols that also reflect sunlight

and cool Earth. Carbon dioxide is another gas emitted by volcanoes. It is a greenhouse gas

that can cause warming of Earth’s atmosphere. Thus, volcanoes erupt materials capable of

causing both atmospheric cooling and warming. In many eruptions, the cooling

overwhelms the warming effects. This cooling can occur rapidly—within a few weeks or

months of the eruption.

Scientists have noted that some mass extinctions coincided with unusually high rates of

volcanic activity. For example, massive flood basalts erupted onto Earth’s surface in Siberia

roughly million years ago—at the same time that the Permian extinction occurred.

According to one hypothesis, explosive volcanic eruptions blasted massive amounts of



volcanic ash and sulfur aerosols into the air. The ash and aerosols spread like a pall

throughout the upper atmosphere, blocking out the Sun. Earth’s atmospheric and oceanic

temperatures plummeted, plants withered, and animals starved or froze to death. If this

hypothesis is correct, the Permian extinction (and perhaps others) resulted from a classic

Earth systems interaction, in which a volcanic eruption occurring within the geosphere

altered the composition of the atmosphere, with profound consequences for atmospheric

and marine temperatures, and for life.

Chapter 4: Geologic Time: A Story in the Rocks: 4-1c Supercontinents and Earth’s Carbon Dioxide Budget

Book Title: Earth



4-1c Supercontinents and Earth’s Carbon Dioxide Budget

In modern oceans, cold polar seawater sinks to the seafloor and flows as a deep-ocean

current toward the equator. This current transports oxygen to the deep-ocean basins and

forces the deep water to rise near the equator, where it is warmed. The current thus mixes

both heat and gases throughout the oceans and the atmosphere.

Surface marine organisms that photosynthesize absorb atmospheric carbon dioxide and

use it to produce organic tissue. These organisms then die and settle to the seafloor, thereby

removing carbon from the atmosphere and transporting it to the seafloor. Some of the

carbon converts back to carbon dioxide as organisms living in the deeper ocean water

consume the settling organic matter, and some of this carbon dioxide is returned to the

atmosphere as modern ocean currents mix the deep and shallow seawater.

Ocean currents are partially controlled by the positions of the continents. Recall from

Chapter 1 that the continents slowly drift around the globe. Several times in Earth history,

all the continents have joined together to form one giant supercontinent. One such

supercontinent, called Pangea, assembled during Permian time (Figure 4.3). The growth of

Pangea also caused a single global ocean to be formed.

Figure 4.3

Computer models of circulation in the ocean suggest that during Permian time,

assembly of all continents into the supercontinent Pangea prevented vertical

mixing of ocean water. The lack of circulation caused carbon dioxide removed

from the atmosphere to accumulate in the deep ocean as calcium carbonate,

causing Earth’s climate to cool.



© Cengage Learning

Computer calculations indicate that the assembly of all continents into a single landmass

and the development of a single global ocean largely prevented mixing of the surface water

and the ocean depths during Permian times. As a result, carbon accumulated in ever-

increasing concentrations in the deep oceans as carbon dioxide was removed from the

atmosphere and stored as carbon in the deep oceans (Figure 4.4A). Removal of carbon

dioxide from the atmosphere caused a decrease in greenhouse warming and resulted in

global cooling. Some scientists have suggested that this global cooling led directly to the

formation of continental ice sheets.

When water cools, it becomes denser. The Late Permian global cooling eventually reached a

threshold at which polar surface water became denser than the deep water. At this point

the cool, dense surface water sank, displacing the carbon dioxide–rich deep water and

causing it to rise towards the sea surface, where massive amounts of carbon dioxide were

released into the atmosphere (Figure 4.4B). According to this hypothesis, the high levels of

atmospheric carbon dioxide asphyxiated life both in the seas and on the continents and

caused the Permian extinction event. Later, new organisms evolved when the carbon

dioxide levels fell.

Figure 4.4

(A) Global cooling in the Late Permian Period. (B) The Permian extinction.



CLOUDS HILL IMAGING LTD./CORBIS © Cengage Learning © Cengage Learning © Cengage Learning

Chapter 4: Geologic Time: A Story in the Rocks: 4-2 Geologic Time

Book Title: Earth



4-2 Geologic Time

While most of us think of time in terms of days or years, Earth scientists commonly refer to

events that happened millions or billions of years ago. In Chapter 1 you learned that Earth

is approximately billion years old. Yet humans and our humanlike ancestors have

existed for only to million years, and recorded history is only a few thousand years old.

How do scientists measure the ages of rocks and events that occurred millions or billions of



years ago, long before recorded history or even human existence?

Scientists measure geologic time in two ways. Relative age (An approach to determining

the timing of geologic events based on the order in which they occurred, rather than on the

absolute number of years ago in which they occurred.) determination refers only to the

order in which events occurred and is based on a few simple principles. For example, in

order for an event to affect a rock, the rock must exist first. Thus, the rock must be older

than the event. This principle seems obvious, yet it is the basis of much geologic work. As

you learned in Chapter 3, sediment normally accumulates in horizontal layers. However,

the sedimentary rocks in Figure 4.5 are folded and tilted. We deduce that the folding and

tilting occurred after the sediment accumulated. The order in which rocks and geologic

features formed can usually be interpreted by such observation and logic.

Figure 4.5

The limestone forming this cliff in Montana was deposited about million years

ago as horizontal layers of sediment in a shallow sea. About million years

later, tectonic forces uplifted and folded the layers.

COPYRIGHT AND PHOTOGRAPH BY MARC S. HENDRIX

Absolute age (The age of a geologic event based on the absolute number of years ago it

occurred.) is age in numeric years. For example, the dinosaurs became extinct about

million years ago. The Permian extinction occurred million years ago. The Yellowstone

Volcano first erupted million years ago. Absolute age tells us both the order in which

events occurred and the amount of time that has passed since they occurred.

Chapter 4: Geologic Time: A Story in the Rocks: 4-3 Relative Geologic Time

Book Title: Earth



4-3 Relative Geologic Time



Absolute age measurements have become common only since the second half of the th

century. Prior to that time, geologists used field observations to determine relative ages.

Even with today’s sophisticated laboratory instrumentation, most field geologists routinely

make relative age determinations based on the following simple principles:

The principle of original horizontality (The principle that most sediment is deposited as

nearly horizontal beds, and therefore most sedimentary rocks started out with nearly

horizontal layering.) is based on the observation that sediment usually accumulates in

horizontal layers (Figure 4.6A). If sedimentary rocks lie at an angle, as in Figure 4.6B, or if

they are folded as you saw in Figure 4.5, we can infer that tectonic forces tilted or folded

them after they formed.

Figure 4.6

(A) The principle of original horizontality tells us that most sedimentary rocks were

originally deposited in horizontal layers, like these in western North Dakota. (B)

When we see tilted sedimentary rocks such as these in central Utah, we infer that

they were tilted after the layers were deposited.



COPYRIGHT AND PHOTOGRAPH BY MARC S. HENDRIX COPYRIGHT AND PHOTOGRAPH BY MARC S. HENDRIX

The principle of superposition (The principle that in undisturbed layers of sediment or

sedimentary rock, the age becomes progressively younger from bottom to top; younger

layers always accumulate on top of older layers.) states that in an undeformed sequence of

layered sedimentary rocks, the oldest layers are at the bottom and the youngest layers are

at the top. For example, in Figure 4.7, the sedimentary layers become progressively

younger in the order and . Two notable exceptions to this principle occur if

tectonic forces have turned the layers of sedimentary rock upside down or have caused an

older layer to be shoved up and over a younger layer.

Figure 4.7

The principle of superposition. In a sequence of sedimentary beds, the oldest bed

(labeled 1) is the lowest, and the youngest (labeled 5) is on top.



© Cengage Learning

The principle of crosscutting relationships (The obvious principle that a rock or feature

must first exist before anything can happen to it; thus, if a dike of basalt cuts across a layer

of sandstone, the basalt is younger.) is based on the obvious fact that a rock must first exist

before anything can happen to it. Figure 4.8 shows an igneous intrusion—called a dike—

cutting across conglomerate. Clearly, the dike must be younger than the conglomerate.

Figure 4.9 shows sedimentary rocks intruded by three granite dikes. Dike cuts dike , and

dike cuts dike , so is younger than , and is the youngest. The sedimentary rocks

must be older than all the dikes.

Figure 4.8

An igneous dike cutting across conglomerate in northwestern Wyoming. By the

principle of crosscutting relationships, the dike must be younger than the

conglomerate.


Figure 4.9

Three granite dikes cutting sedimentary rocks. The dikes become younger in the

order . The sedimentary rocks must be older than all three dikes.



© Cengage Learning

A similarly obvious fact is that a rock must first exist before pieces of it can be broken off

and incorporated into another geologic unit. Hence, the principle of included fragments

(The principle that a rock unit must first exist before pieces of it can be broken off and

incorporated into another rock unit.) requires that a conglomerate that contains clasts of

granite must be younger than the age of the granite. Similarly, pieces of sandstone

incorporated into a basalt flow must be older than the flow.

The theory of evolution (The biological theory that life-forms have changed in their

physical and genetic characteristics over time.) states that life-forms have changed

throughout geologic time. Species emerge, persist for awhile, and then become extinct.

Recall from Chapter 3 that fossils consist of the remains and other traces of prehistoric life

preserved in sedimentary rocks. Fossils are useful in determining relative ages of rocks,

because different animals and plants lived at different times in Earth history. For example,

trilobites (Figure 4.10) lived from about million to about million years ago, and the

first dinosaurs appeared about million years ago.

Figure 4.10

Trilobites were marine creatures that thrived in Earth’s ancient seas before the

time of the dinosaurs.



CHANTAL DE BRUIJNE/SHUTTERSTOCK.COM

The principle of superposition tells us that sedimentary rock layers become younger from

bottom to top. If the rocks formed over a long time, different fossils appear and then vanish

from bottom to top, in the same order in which the organisms evolved and then became

extinct. Rocks containing dinosaur bones must be younger than those containing trilobite

remains.

The principle of faunal succession (The principle that species succeeded one another

through time in a definite order, so the relative ages of fossiliferous sedimentary rocks can

be determined by their fossil content.) states that species succeeded one another through

time in a definite and recognizable order and that the relative ages of sedimentary rocks

can therefore be recognized from their fossils.

Chapter 4: Geologic Time: A Story in the Rocks: 4-3a Interpreting Geologic History From Fossils

Book Title: Earth



4-3a Interpreting Geologic History From Fossils

Paleontologists study fossils to understand the history of life and evolution. The oldest-

known fossils are traces of bacteria-like organisms that lived about billion years ago. A

much younger fossil consists of the frozen and mummified remains of a Bronze Age man,

recently found in a glacier near the Italian-Austrian border.

Fossils also allow geologists to interpret ancient environments, because most organisms

thrive in specific environments and do not survive in others. For example, all modern

species of coral live in marine environments. Corals do not survive in fresh or even

brackish (slightly salty) water. Yet today, fossil corals are abundant in the rocks of many



mountain environments, including the Canadian Rockies and the Tian Shan (translated

‘Heavenly Mountains’) of western China. Both of these mountain ranges are hundreds of

kilometers or more away from the nearest ocean. The fossil corals tell us that the rocks

exposed in these mountains were once submerged beneath marine waters. Today,

geologists understand that tectonic processes raised the marine rocks and their fossils to

their present altitudes.

Chapter 4: Geologic Time: A Story in the Rocks: 4-4 Unconformities and Correlation

Book Title: Earth



4-4 Unconformities and Correlation

The -kilometer-high walls of the Grand Canyon are composed of sedimentary rocks lying

on older igneous and metamorphic rocks (Figure 4.11). Their ages range from about

million years to nearly billion years. The principle of superposition tells us that the deepest

sedimentary rocks are the oldest, and the rocks become younger as we climb up the canyon

walls. However, no principle assures us that the rocks formed continuously from billion to

million years ago. The rock record may be incomplete. Suppose that no sediment were

deposited for a period of time, or that erosion removed some sedimentary layers before

younger layers accumulated. In either case, a gap would exist in the rock record. We know

that in an undeformed layered sequence of sedimentary rock, any one layer is younger

than the layer below it, but without more information we do not know how much younger.

Figure 4.11

The -kilometer-high walls of the Grand Canyon are composed of sedimentary

rocks lying on older igneous and metamorphic rocks. The Precambrian schist in

the lower part of the photo is about billion years old and lies beneath the

horizontally bedded Tapeats Sandstone, which is about million years old. The

dotted white line between these two units is a nonconformity. The Redwall

limestone, deposited about million years ago, forms the skyline.



TOM GRUNDY/SHUTTERSTOCK.COM

Chapter 4: Geologic Time: A Story in the Rocks: 4-4a Unconformities

Book Title: Earth



4-4a Unconformities

Layers of sedimentary rocks are conformable (A term describing sedimentary layers that

were deposited continuously, without detectable interruption.) if they were deposited

without detectable interruption. An unconformity (A physical surface between layers of

sediment or sedimentary rock that represents an interruption in the deposition of sediment,

including an interruption that involves partial erosion of the older layer. Also the physical

surface between eroded igneous and overlying sedimentary layers deposited following

erosion. Types of unconformities include disconformity, angular unconformity, and

nonconformity.) is a bounding surface that exists between two rock units and represents an

interruption in deposition, usually of long duration. Some rock may have been eroded

during the time frame represented by the unconformity, but eventually deposition of

sediment resumed and buried the unconformity. Thus, an unconformity is a physical

feature (a bounding surface) that represents a long interval of time for which no geologic

record exists in that place. The missing record may involve billions of years.

Several types of unconformities exist. In a disconformity (A type of unconformity in which

the sedimentary layers above and below the unconformity are parallel.) , the sedimentary

layers above and below the unconformity are parallel (Figures 4.12 and 4.13). A

disconformity may be difficult to recognize unless an obvious soil layer or erosional surface

developed. However, geologists can identify disconformities through careful analysis of the

rock texture, composition, sedimentary structures, and fossil content. In some cases,



disconformities can be identified by determining the ages of rocks, using methods based on

fossils and absolute dating, described below.

Figure 4.12

A disconformity is created when there is a pause in the deposition of sediment,

sometimes accompanied by minor erosion. For example, when a river channel

shifts its position, the sandbar formed at its mouth no longer receives sediment and

begins to subside. Commonly, organisms will burrow into this stable surface

before the river channel shifts back and deposition resumes on the bar.



© Cengage Learning

Figure 4.13

The horizontal bedding plane behind the yellow field book is a disconformity

formed on a river-mouth bar such as that illustrated in Figure 4.12. Small holes in

the rock (red arrows) below the disconformity were formed by burrowing shrimp

during the break in deposition. Eventually, deposition resumed and buried the

disconformity.


In an angular unconformity (An unconformity in which younger sediment or

sedimentary rocks rest on the eroded surface of tilted or folded older rocks.) , tectonic

activity tilted older sedimentary rock layers and erosion planed them off before the younger

sediment accumulated (Figures 4.14 and 4.15).

A nonconformity (A type of unconformity in which layered sedimentary rocks lie on an

erosion surface cut into igneous or metamorphic rocks.) is an unconformity in which



sedimentary rocks lie on igneous or metamorphic rocks. The nonconformity shown in

Figure 4.11 represents a time gap of about billion years.

Figure 4.14

An angular unconformity develops when older sedimentary rocks are folded and

partially eroded before younger sediment accumulates.

© Cengage Learning

Figure 4.15



In this classic angular unconformity in the Grand Canyon, flat-lying Cambrian

sedimentary rocks rest atop tilted and partially eroded Precambrian sedimentary

rocks.

PHOTO AND COPYRIGHT BY MARLI MILLER

Chapter 4: Geologic Time: A Story in the Rocks: 4-4b Correlation

Book Title: Earth



4-4b Correlation

Ideally, geologists would like to develop a continuous history for each region of Earth by

interpreting rocks that formed in that place throughout geologic time. However, there is no

single place on Earth where rocks were continuously formed and preserved. Consequently,

in any one place, the rock record (The rocks that currently exist on Earth and contain the

record of its history.) —that is, the rocks that currently exist on Earth and contain the

record of its history—is full of unconformities, or historical gaps. To assemble as complete

and continuous a record as possible, geologists gather geological evidence from many

localities and match rocks of the same age from different localities in a process called

correlation (The process of establishing the age relationship of rocks or geologic features

from different locations on Earth; can be done by comparing sedimentary characteristics of

the layers or the types of fossils found in them. There are two types of correlation: time

correlation (age equivalence) and lithologic correlation (physical continuity of the rock unit).)

(Figure 4.16). But how do we correlate rocks of the same age over great distances?

Figure 4.16

Correlation of sedimentary rocks from three different locations in Utah and

Arizona allows geologists to develop a more complete picture of geologic history in

the region, despite the fact that each location contains only rocks of limited ages.

The rock units, such as the Tapeats Sandstone and the Moenkopi Formation, are

called formations. A formation is a formally-defined, mappable rock unit that



occurs in a specific region.

© Cengage Learning

If you follow a single sedimentary bed from one place to another, then it is clearly the same

layer in both places. But this approach is impractical over long distances or where rocks are

not exposed. Another problem arises when time correlation is based on physically tracing

out a single sedimentary layer from one place to another. Suppose, for example, you are

attempting to trace a layer of sandstone deposited by a beach. If the relative sea level rose

and slowly inundated a continental interior, the beach would also migrate inland. However,

those beach deposits closer to the edge of the continent would be older than the beach

deposits further inland. Thus, although the entire layer would be physically continuous, it

would not be the same age everywhere and therefore would not correlate in time (Figure

4.17). Over a few hundred meters, the age difference within the sandstone layer may be

unimportant, but over hundreds of kilometers it could vary by millions of years.

Figure 4.17

As relative sea level rises slowly through time, a beach migrates inland, forming a

single sand layer that is of different ages at different localities.



© Cengage Learning 2015

So there are two kinds of correlation: time correlation refers to age equivalence, and

lithologic correlation refers to the continuity of a rock unit. The two are not always the

same, because some rock units, such as the sandstone, were deposited at different times in

different places. To construct a record of Earth history and a geologic time scale, Earth

scientists use several types of evidence to correlate rocks of the same age.

Index fossil (A fossil that dates the layers where it is found because it came from an

organism that is abundantly preserved in rocks, was widespread geographically, and

existed as a species or genus for only a relatively short time.) are a very useful tool for

correlation, because they can precisely indicate the ages of sedimentary rocks. The best

index fossils are abundantly preserved in rocks, are geographically widespread, existed as a

species or genus for only a relatively short time, and are easily identified in the field.

Floating or swimming tiny marine organisms make some of the best index fossils because



they spread rapidly and widely throughout the seas and they evolved quickly. The shorter

the time that a species existed, the more precisely the index fossil reflects the age of a rock.

In many cases, the presence of a single type of index fossil is sufficient to establish the age of

a rock. More commonly, an assemblage of several fossils is used to date and correlate rocks

(Figure 4.18). For example, the exact position of the Cretaceous-Tertiary boundary in the

sediment core from the Yucatan meteor impact site, described above, was determined by

using tiny floating marine organisms as index fossils.

Figure 4.18

Index fossils demonstrate age equivalency of sedimentary rocks from widely

separated localities. Sedimentary beds containing the same index fossils are

interpreted to be of the same age. In this figure, the fossils show that at locality B,

layer 3 is missing because layer 4 lies directly on top of layer 2. Either layer 3 was

deposited and then eroded away before was deposited at locality B, or layer 3 was

never deposited there. At locality C, all layers above are missing, either because

of erosion or because they were never deposited.

© Cengage Learning

Another correlation tool is the key bed (A thin, widespread, easily recognized sedimentary

layer that can be used for correlation because it was deposited rapidly and simultaneously

over a wide area.) —a thin, widespread, easily recognized sedimentary layer that was

deposited rapidly and simultaneously over a wide area. Many volcanic eruptions eject great

volumes of fine, glassy volcanic ash into the atmosphere. Wind carries the ash over great

distances before it settles. Some historic ash clouds have encircled the globe. When the ash

settles, it forms a depositional layer, and the glassy fragments within the ash layer typically

crystallize to form clay. Because such volcanic eruptions occur at a precise point in time,

the ash deposit is the same age everywhere. Thus, the thin, sooty, iridium-rich clay layer

deposited roughly million years ago from debris of a giant meteorite impact (described

at the beginning of this chapter) is a classic example of a key bed.

Chapter 4: Geologic Time: A Story in the Rocks: 4-5 Absolute Geologic Time

Book Title: Earth





4-5 Absolute Geologic Time

Geologists have a challenging task—they are attempting to measure the absolute age of

events that occurred before history was recorded. Measuring absolute age relies on

1. a process that occurs at a constant rate, such as Earth rotating once every hours,

and

2. some way to keep a cumulative record of that process, such as marking a calendar

each time the Sun rises.

Measurement of time with a calendar, a clock, an hourglass, or any other device depends

on these two factors.

Fortunately, scientists have found a natural process that occurs at a constant rate and

accumulates its own record: it is the radioactive decay of elements that are present in many

rocks. Thus, many rocks have built-in calendars.

To begin to understand radioactivity, we need a basic understanding of the atom. An atom

consists of a small, dense nucleus surrounded by a cloud of electrons. A nucleus consists of

positively charged protons and neutral particles called neutrons. All atoms of any given

element have the same number of protons in the nucleus. However, the number of

neutrons may vary. Isotopes (Atoms of the same element that have different numbers of

neutrons.) are atoms of the same element with different numbers of neutrons. For

example, all isotopes of carbon contain protons, but only also has neutrons.

has neutrons, and has neutrons.

Many isotopes are stable, meaning that they do not change with time. is one

such isotope; if you studied a sample of for a million years, all of the atoms

would remain unchanged. However, other isotopes are unstable or radioactive. Given time,

their nuclei spontaneously decay, forming a different isotope and commonly a different

element. For example, , the one unstable isotope of carbon, spontaneously decays

to form .

A radioactive isotope such as is known as a parent isotope. An isotope created by

radioactivity, such as , is called a daughter isotope.

Many common elements, including carbon and potassium, consist of a mixture of

radioactive and nonradioactive isotopes. With time, the radioactive isotopes decay, but the

nonradioactive ones do not. Some elements, such as uranium, consist only of radioactive

isotopes. The amount of uranium on Earth slowly decreases as it decays to other elements,

such as lead.

Chapter 4: Geologic Time: A Story in the Rocks: 4-5a Radioactivity and Half-Life

Book Title: Earth



4-5a Radioactivity and Half-Life



If you watch a single atom of , when will it decompose? This question cannot be

answered, because any particular atom may or may not decompose at any time.

Each atom has a certain probability of decaying at any time. Averaged over time, half of

the atoms in any sample of will decompose in years. The half-life (The

time it takes for half of the atoms of a radioactive isotope in a sample to decay.) is the time

it takes for half of the atoms in a radioactive sample to decompose. The half-life of

is years. Therefore, if gram of were placed in a container,

gram would remain after years, gram after years, and so on. Each

radioactive isotope has its own half-life; some half-lives are fractions of a second, and others

are measured in billions of years.

Chapter 4: Geologic Time: A Story in the Rocks: 4-5b Radiometric Dating

Book Title: Earth



4-5b Radiometric Dating

Two properties of radioactivity are essential to the calendars in rocks. First, the half-life of a

radioactive isotope is constant. It can be measured in the laboratory and is unaffected by

geologic or chemical processes, so radioactive decay occurs at a known, constant rate.

Second, as a parent isotope decays, its daughter accumulates in the rock. The longer the

rock exists, the more the daughter isotope accumulates. Radiometric dating (The process

of measuring the absolute age of rocks, minerals, and fossils by measuring the

concentrations of radioactive isotopes and their decay products.) is the process of

determining the absolute ages of rocks, minerals, and fossils by measuring the relative

amounts of parent and daughter isotopes.

Figure 4.19 shows the relationships between age and relative amounts of parent and

daughter isotopes. At the end of one half-life, percent of the parent atoms have decayed

to daughter. At the end of two half-lives, the mixture is percent parent and percent

daughter. To determine the age of a rock, a geologist measures the proportions of parent

and daughter isotopes in a sample and compares the ratio to a similar graph. Consider a

hypothetical parent-daughter pair having a half-life of million years. If we determine that

a rock contains a mixture of percent parent isotope and percent daughter, Figure

4.19 shows that the age is two half-lives, or in this case million years.

Figure 4.19

As a radioactive parent isotope decays to its daughter isotope, the proportion of

parent decreases (blue line) and the amount of daughter increases (red line). The

half-life is the amount of time required for half of the parent to decay to daughter.

At time zero, when the radiometric calendar starts, a sample is percent parent.

At the end of one half-life, percent of the parent has converted to daughter. At

the end of two half-lives, percent of the sample is parent and percent is

daughter. Thus, by measuring the proportions of parent and daughter in a rock, the

rock’s age in half-lives can be obtained. Because the half-lives of all radioactive

isotopes are well known, it is simple to convert age in half-lives to age in years.



© Cengage Learning

If the half-life of a radioactive isotope is short, that isotope gives accurate ages for young

materials. For example, the half-life of years for dating allows age

determinations for materials younger than about years. It is useless for older

materials, because by years, the amount of that remains in the sample is

too small to measure accurately.

The opposite limitation exists with isotopes with long half-lives. Such isotopes provide

accurate ages for old rocks, but not enough daughter accumulates in young rocks to be

measured. For example, has a half-life of billion years. In a geologically

short period of time— million years or less—so little of its daughter has accumulated that

it is impossible to measure accurately. Therefore, is not useful for rocks

younger than about million years. The radioactive isotopes that are most commonly

used for dating are summarized in Table 4.1.

Table 4.1

The Most Commonly Used Isotopes in Radiometric Age Dating

Isotope

(Parent) (Daughter) Half-Life of Effective Minerals and Other



Parent

(Years)

Dating Range

(Years)

Materials That Can

Be Dated with This

Isotope

± Anything that was

once alive: wood,

other plant matter,

bone, flesh, or shells;

also, carbon in carbon

dioxide dissolved in

groundwater, deep

layers of the ocean,

or glacial ice

billion

billion

Muscovite

Biotite

Hornblende

Whole volcanic rock

billion

million

billion

million–

billion

Zircon

Uraninite and

pitchblende

billion million–

billion

Muscovite

Biotite

Potassium feldspar

Whole metamorphic

or igneous rock

© Cengage Learning

Chapter 4: Geologic Time: A Story in the Rocks: 4-6 The Geologic Time Scale

Book Title: Earth



4-6 The Geologic Time Scale

As mentioned earlier, no single locality exists on Earth where a complete sequence of rocks

formed continuously throughout geologic time. However, geologists have analyzed and



correlated rocks from many localities around the world to create the geologic time scale

(A chronological arrangement of geologic time subdivided into eons, eras, periods, epochs,

and ages.) , a formal subdivision of Earth time based on numerical dates (Figure 4.20). The

geologic time scale is frequently revised as more rock units are dated radiometrically and

more fossil assemblages are described around the world.

Figure 4.20

The geologic time scale is a record of geologic time divided into eons, eras, periods,

and epochs. Time is given in millions of years (for example, stands for

million, which is billion) and is not drawn to scale here. Little is known about

events that occurred during the early part of Earth’s history. Therefore, the first

billion years are given relatively little space on this chart, while the more recent

Phanerozoic Eon, which spans only million years, receives proportionally

more space.



© Cengage Learning

Just as a year is subdivided into months, months into weeks, and weeks into days, geologic

time is split into smaller intervals. The units are named, just as months and days are. The

largest time units are eons (The largest unit of geologic time. The most recent eon, the

Phanerozoic Eon, is further subdivided into eras.) , which are divided into eras (A geologic

time unit. Eons are divided into eras and, in turn, eras are subdivided into periods.) . Eras

are subdivided, in turn, into periods (A geologic time unit longer than an epoch and

shorter than an era.) , which are further subdivided into epochs (A geologic time unit

longer than an age and shorter than a period.) and, even further, into ages (The shortest

period of geologic time. Epochs are divided into ages.) .

The geologic time scale was originally constructed on the basis of relative age



determinations. When geologists developed radiometric dating, they added absolute ages to

the time scale. Geologists commonly use the time scale to date rocks in the field. Imagine

that you are studying sedimentary rocks in the walls of the Grand Canyon. If you find an

index fossil or a key bed that has already been radiometrically dated by other scientists, you

know the age of the rock you are studying and you do not need to send the sample to a

laboratory for radiometric dating.

Chapter 4: Geologic Time: A Story in the Rocks: 4-6a The Earliest Eons of Geologic Time: Precambrian Time

Book Title: Earth



4-6a The Earliest Eons of Geologic Time: Precambrian Time

The earliest eons—the Hadean, Archean, and Proterozoic Eons—often are not subdivided at

all, even though together they constitute a time interval of billion years—nearly

percent of Earth’s history. These early eons are commonly referred to with the informal

term Precambrian (A term referring to all of geologic time before the Paleozoic Era,

encompassing approximately the first billion years, or roughly percent, of Earth’s

history. Also refers to all rocks formed during that time.) , because they preceded the

Cambrian Period, when fossil remains became very abundant.

The Hadean Eon (The earliest time in Earth’s history, ranging from billion years ago to

billion years ago.) (Greek for “beneath the Earth”) is the earliest time in Earth history

and ranges from the planet’s origin billion years ago to billion years ago. Only a few

Earth rocks are known that formed during the Hadean Eon, and no fossils of Hadean age

are known, making it impossible to subdivide the Hadean Eon based on fossils.

There are few fossils among the rocks of the Archean Eon (A division of geologic time

to billion years ago. The oldest-known rocks formed at the beginning of, or just prior to,

the start of the Archean Eon.) (Greek for “ancient”), and they are not preserved well

enough to allow for finely tuned subdivision of this eon that spanned from to billion

years ago. The fossil record does indicate that life began on Earth during the Early Archean,

and geologists recently have described fossilized algal structures, called stromatolites, in

rocks as old as billion years in western Australia.

More-diverse groups of fossils have been found in sedimentary rocks of the Proterozoic

Eon (The portion of geologic time occurring during a period from billion to million

years ago.) (Greek for “earlier life”), billion to million years ago. The most complex

are multicellular and have different kinds of cells arranged into tissues and organs. A few

types of Proterozoic shell-bearing organisms have been identified, but shelled organisms did

not become abundant until the Paleozoic Era.

Chapter 4: Geologic Time: A Story in the Rocks: 4-6b The Phanerozoic Eon

Book Title: Earth



4-6b The Phanerozoic Eon



Sedimentary rocks of the Phanerozoic Eon (The most recent million years of geologic

time, including the present, represented by rocks that contain evident and abundant fossils.)

, which covers the most recent million years of geologic time, contain abundant fossils.

Four changes occurred at the beginning of Phanerozoic time that greatly improved the

fossil record:

1. The number of species with shells and skeletons dramatically increased.

2. The total number of individual organisms preserved as fossils increased greatly.

3. The total number of species preserved as fossils increased greatly.

4. The average sizes of individual organisms increased.

Shells and skeletons are much more easily preserved than plant remains and soft body

tissues. Thus, in rocks of earliest Phanerozoic time and younger, the most abundant fossils

are the hard, tough shells and skeletons.

Subdivision of the Phanerozoic Eon into three eras is based on the most common types of

life during each era. Sedimentary rocks that formed during the Paleozoic Era (The part of

geologic time occurring during a period from to million years ago. During this era

invertebrates, fishes, amphibians, reptiles, ferns, and cone-bearing trees were dominant.)

(Greek for “old life”) contain fossils of early life-forms, such as invertebrates, fishes,

amphibians, reptiles, ferns, and cone-bearing trees (Figure 4.21). The Paleozoic Era ended

abruptly about million years ago when the terminal Permian mass extinction wiped

out percent of all marine species, two-thirds of reptile and amphibian species, and

percent of insect species.

Figure 4.21

In Paleozoic time, the land was inhabited by reptiles and amphibians and covered

by ferns, ginkgoes, and conifers.



WARD’S NATURAL SCIENCE ESTABLISHMENT

The Mesozoic Era (The part of geologic time roughly from to million years ago.

Dinosaurs rose to prominence and became extinct during this era.) is most famous for the

dinosaurs that roamed the land. Mammals and flowering plants also evolved during this

era. The Mesozoic Era ended million years ago with another mass extinction that wiped

out the dinosaurs.

During the Cenozoic Era (The latest of the three Phanerozoic eras, million years ago to

the present.) (Greek for “recent life”), mammals and grasses became abundant. Humans

have evolved and lived entirely in the Cenozoic Era.

The eras of Phanerozoic time are subdivided into periods, the time unit most commonly

used by geologists. Some of the periods are named after special characteristics of the rocks

formed during those periods. For example, the Cretaceous Period is named from the Latin

word for chalk (creta), after chalk beds of this age in Africa, North America, and Europe.

Other periods are named for the geographic localities where rocks of that age were first

described. For example, the Jurassic Period is named for the Jura Mountains of France and

Switzerland. The Cambrian Period is named for Cambria, the Roman name for Wales,

where rocks of this age were first studied.

In addition to the abundance of fossils, another reason that details of Phanerozoic time are



better known than those of Precambrian time is that many of the older rocks have been

deformed, metamorphosed, and eroded. It is simple probability that the older a rock is, the

greater the chance that metamorphism or erosion has destroyed the rock or the features

that record Earth’s history.

Virtual Field Trip

Geologic Time

COPYRIGHT DR. PARVINDER S. SETHI

Ready to Go!

On this virtual field trip we will visit both Grand Canyon National Park in Arizona

and Capitol Reef National Park in Utah. The scenery is spectacular, and the rocks of

both parks display evidence of the great length of geologic time. The rocks also

illustrate the techniques that geologists use to determine the relative ages of rocks

and geologic events. These techniques include the principles of superposition,

original horizontality, crosscutting relationships, and faunal succession described in

Section 4-3 (“Relative Geologic Time”) of this chapter. Although early geologists

dignified these concepts first by calling them “laws” and, more recently,

“principles,” they really are simple applications of common sense to an

understanding of the ways in which rocks form. For example, Figure 4.8 shows an

igneous dike cutting across conglomerate. Common sense tells us that the

conglomerate must have been present before the dike intruded into it. Therefore,

the dike must be younger than the conglomerate, an application of the principle of

crosscutting relationships. You may also wish to refer to Section 3-3 (“Sedimentary

Rocks”) in Chapter 3 for more information regarding the processes that formed the

rocks of these wonderful public parks.




Goals of the Trip

Some of the goals of this trip are to understand that:

1. Earth is about billion years old.

2. Much of Earth history, including environments and geologic events, is

recorded in rocks such as those in Grand Canyon and Capitol Reef

National Parks.

3. You can apply simple geologic tools and concepts, such as the principles

listed previously, to understand how the rocks formed, the

environmental conditions under which they formed, and the geologic

events that affected the rocks after they formed.

4. You can also apply these principles to interpret the relative ages of rocks

and geologic events.

Follow-Up Questions

1. The principle of superposition tells us that the Bright Angel Shale in the

Grand Canyon walls must be younger than the underlying Tapeats

Sandstone. Refer to Section 4-5 (“Absolute Geologic Time”) in this chapter

and consider how a geologist might measure the ages of the two

formations in years rather than in relative time.

2. The narrative for this virtual field trip explains that the Tapeats

Sandstone is mainly a shallow marine sand deposited as sea level rose

during Cambrian time, causing the slow inundation of western North



America. Refer to Section 4-4 (“Unconformities and Correlation”) in this

chapter and consider how, although it is laterally continuous across the

Grand Canyon, the same physical sandstone layer is younger by up to

several million years in other parts of the United States.

3. Refer to the sections titled “Unconformities and Correlation” and

“Relative Geologic Time” in this chapter and consider how a geologist

might use fossils to identify an unconformity.

What to See When You Go

The photos and videos in this field trip display the great beauty and awesome scale

of the rocks and topographic features of Grand Canyon and Capitol Reef National

Parks. The narrative explains the geologic principles and concepts that are

illustrated by the rocks of the parks and are described in the section “Relative

Geologic Time” in this chapter and under “Sedimentary Structures” in Chapter 3.

Notice especially the layered nature of sedimentary rocks and changes in lithology

from layer to layer. Think about why the layers are laterally continuous for many

miles, and in some cases for hundreds of miles. Consider what the environment of

deposition was like when each sediment layer was deposited and how the loose

sediment became hard rock.





Consider the angular unconformity (dotted white line in picture below) from the

Grand Canyon. Review the section titled “Unconformities and Correlation” in this

chapter and consider the sequence of events that formed the unconformity.


Fossils such as those pictured below are abundant in some of the sedimentary rocks

of Grand Canyon and in many other sedimentary rocks around the world. Consider

how a geologist might use fossils to correlate sedimentary layers over distances of

hundreds of miles.




Chapter 4: Geologic Time: A Story in the Rocks Chapter Review

Book Title: Earth



Chapter Review

Key Terms

absolute age (The age of a geologic event based on the absolute number of years ago

it occurred.)

ages (The shortest period of geologic time. Epochs are divided into ages.)

angular unconformity (An unconformity in which younger sediment or

sedimentary rocks rest on the eroded surface of tilted or folded older rocks.)

Archean Eon (A division of geologic time to billion years ago. The oldest-

known rocks formed at the beginning of, or just prior to, the start of the Archean

Eon.)

Cenozoic Era (The latest of the three Phanerozoic eras, million years ago to the

present.)



conformable (A term describing sedimentary layers that were deposited

continuously, without detectable interruption.)

correlation (The process of establishing the age relationship of rocks or geologic

features from different locations on Earth; can be done by comparing sedimentary

characteristics of the layers or the types of fossils found in them. There are two types

of correlation: time correlation (age equivalence) and lithologic correlation (physical

continuity of the rock unit).)

disconformity (A type of unconformity in which the sedimentary layers above and

below the unconformity are parallel.)

eons (The largest unit of geologic time. The most recent eon, the Phanerozoic Eon, is

further subdivided into eras.)

epochs (A geologic time unit longer than an age and shorter than a period.)

eras (A geologic time unit. Eons are divided into eras and, in turn, eras are subdivided

into periods.)

evolution (The biological theory that life-forms have changed in their physical and

genetic characteristics over time.)

geologic time scale (A chronological arrangement of geologic time subdivided into

eons, eras, periods, epochs, and ages.)

Hadean Eon (The earliest time in Earth’s history, ranging from billion years ago

to billion years ago.)

half-life (The time it takes for half of the atoms of a radioactive isotope in a sample to

decay.)

index fossil (A fossil that dates the layers where it is found because it came from an

organism that is abundantly preserved in rocks, was widespread geographically, and

existed as a species or genus for only a relatively short time.)

isotopes (Atoms of the same element that have different numbers of neutrons.)

key bed (A thin, widespread, easily recognized sedimentary layer that can be used for

correlation because it was deposited rapidly and simultaneously over a wide area.)

mass extinction (A sudden, catastrophic event during which a significant percentage

of all life-forms on Earth become extinct.)

Mesozoic Era (The part of geologic time roughly from to million years ago.

Dinosaurs rose to prominence and became extinct during this era.)

nonconformity (A type of unconformity in which layered sedimentary rocks lie on

an erosion surface cut into igneous or metamorphic rocks.)



Paleozoic Era (The part of geologic time occurring during a period from to

million years ago. During this era invertebrates, fishes, amphibians, reptiles, ferns,

and cone-bearing trees were dominant.)

periods (A geologic time unit longer than an epoch and shorter than an era.)

Phanerozoic Eon (The most recent million years of geologic time, including the

present, represented by rocks that contain evident and abundant fossils.)

precambrian (A term referring to all of geologic time before the Paleozoic Era,

encompassing approximately the first billion years, or roughly percent, of

Earth’s history. Also refers to all rocks formed during that time.)

principle of crosscutting relationships (The obvious principle that a rock or

feature must first exist before anything can happen to it; thus, if a dike of basalt cuts

across a layer of sandstone, the basalt is younger.)

principle of faunal succession (The principle that species succeeded one another

through time in a definite order, so the relative ages of fossiliferous sedimentary

rocks can be determined by their fossil content.)

principle of included fragments (The principle that a rock unit must first exist

before pieces of it can be broken off and incorporated into another rock unit.)

principle of original horizontality (The principle that most sediment is deposited as

nearly horizontal beds, and therefore most sedimentary rocks started out with nearly

horizontal layering.)

principle of superposition (The principle that in undisturbed layers of sediment or

sedimentary rock, the age becomes progressively younger from bottom to top;

younger layers always accumulate on top of older layers.)

Proterozoic Eon (The portion of geologic time occurring during a period from

billion to million years ago.)

radiometric dating (The process of measuring the absolute age of rocks, minerals,

and fossils by measuring the concentrations of radioactive isotopes and their decay

products.)

relative age (An approach to determining the timing of geologic events based on the

order in which they occurred, rather than on the absolute number of years ago in

which they occurred.)

rock record (The rocks that currently exist on Earth and contain the record of its

history.)

unconformity (A physical surface between layers of sediment or sedimentary rock

that represents an interruption in the deposition of sediment, including an

interruption that involves partial erosion of the older layer. Also the physical surface



between eroded igneous and overlying sedimentary layers deposited following

erosion. Types of unconformities include disconformity, angular unconformity, and

nonconformity.)

Chapter 4: Geologic Time: A Story in the Rocks Chapter Review

Book Title: Earth



Chapter Review

Chapter Review

4-1

Earth Rocks, Earth History, and Mass Extinctions

At least six catastrophic mass extinctions have occurred in geologic history.

The greatest, 248 million years ago, annihilated percent of all marine

species, two-thirds of reptile and amphibian species, and percent of insect

species.

4-2

Geologic Time

Scientists measure geologic time in two ways. Relative age measurement

refers only to the order in which events occurred. Absolute age is measured in

years.

Figure 4.11

The -kilometer-high walls of the Grand Canyon are composed of

sedimentary rocks lying on older igneous and metamorphic rocks. The

Precambrian schist in the lower part of the photo is about billion

years old and lies beneath the horizontally bedded Tapeats Sandstone,

which is about million years old. The dotted white line between

these two units is a nonconformity. The Redwall limestone, deposited

about million years ago, forms the skyline.



TOM GRUNDY/ SHUTTERSTOCK.COM

4-3

Relative Geologic Time

Determinations of relative time are based on geologic relationships among

rocks and the evolution of life-forms through time. The criteria for relative

dating are summarized in a few simple principles: the principle of original

horizontality, the principle of superposition, the principle of crosscutting

relationships, and the principle of faunal succession.

4-4

Unconformities and Correlation

Layers of sedimentary rock are conformable if they were deposited without

major interruptions. An unconformity represents a major interruption of

deposition and a significant time gap between formation of successive layers

of rock. In a disconformity, layers of sedimentary rock immediately above

and below the unconformity are parallel. An angular unconformity forms

when lower layers of rock are tilted and partially eroded prior to deposition of

the upper beds. In a nonconformity, sedimentary layers lie on top of an

erosion surface developed on igneous or metamorphic rocks. Correlation is

the process of establishing the age relationship of rocks from different

locations on Earth by comparing characteristics of the layers or the fossils

found in those layers. Index fossils and key beds are important tools in time

correlation—the demonstration that sedimentary rocks found in different

geographic localities formed at the same time.




4-5

Absolute Geologic Time

Absolute time is measured by radiometric dating, which relies on the fact that

radioactive parent isotopes decay to form daughter isotopes at a fixed known

rate as expressed by the half-life of the isotope. The cumulative effects of the

radioactive decay process can be determined by the accumulation of daughter

isotopes in rocks and minerals.

Figure 4.18

Index fossils demonstrate age equivalency of sedimentary rocks from

widely separated localities. Sedimentary beds containing the same

index fossils are interpreted to be of the same age. In this figure, the

fossils show that at locality B, layer 3 is missing because layer 4 lies

directly on top of layer 2. Either layer 3 was deposited and then eroded

away before was deposited at locality B, or layer 3 was never

deposited there. At locality C, all layers above are missing, either

because of erosion or because they were never deposited.

© Cengage Learning

4-6

The Geologic Time Scale

Worldwide correlation of rocks of all ages has resulted in the geologic

column, a composite record of rocks formed throughout the history of Earth.

The major units of the geologic time scale are eons, eras, periods, and epochs.

The Phanerozoic Eon, the most recent 543 million years of geologic time, is

finely and accurately subdivided because sedimentary rocks deposited at this



time are often well preserved and they contain abundant well-preserved

fossils. In contrast, Precambrian rocks and time are only coarsely subdivided

because fossils are scarce and poorly preserved and the rocks are often

altered.

Chapter 4: Geologic Time: A Story in the Rocks Review Questions

Book Title: Earth



Chapter Review

Review Questions

1. Describe the two ways of measuring geologic time. How do they differ?

2. Give an example of how the principle of original horizontality might be used

to determine the order of events affecting a sequence of folded sedimentary

rocks.

3. Explain a conformable relationship in sedimentary rocks.

Chapter 4: Geologic Time: A Story in the Rocks Review Questions

Book Title: Earth



© 2014 Cengage Learning Inc. All rights reserved. No part of this work may by reproduced or used in any form or by any means -

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Geologic Time: A Story in the Rocks - Poudre …staffweb.psdschools.org/bkatz/Geosp/Chapter 4-...

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Transcript of Geologic Time: A Story in the Rocks - Poudre …staffweb.psdschools.org/bkatz/Geosp/Chapter 4-...