Geologic Time

Geologic Time
Interest in extremely long periods of time sets geology and
astronomy apart from other sciences. Geologists think in terms
of billions of years for the age of the earth and its oldest rocksnumbers that, like the national debt, are not easily comprehended. Nevertheless, the time scales of geologic activity are
important for environmental geologists because they provide a
way to measure human impacts on the natural world. For
example, we would like to know the rate of natural soil from
solid rock to determine whether topsoil erosion from agriculture is too great or not. Likewise, understanding how climate
has changed over millions of years is vital to properly assess
current global warming trends. Clues to past environmental
change an~ well-preserved in many different kinds of rocks.
Geologists evaluate the age of rocks and geologic events
using two different approaches. Relative age dating is the
technique of determining a sequence of geological events,
based upon the structural relations of rocks. Absolute age
dating provides the actual ages for rocks in years before
the present. Relative age is determined by applying geologic
laws bJscd upon the structural relations of rocks. For
instance, the Law of Superposition tells us that in a stack of
undeformed sedimentary rocks, the stratum (layer) at the
top is the roungest. The Law of Cross-cutting Relationships
tells us that a fault is younger than the youngest rocks it displaces or cuts. Similarly, we know that a pluton is younger
than the rocb it intrudes (• Figure 2.24).
Using these laws, geologists arranged a great thickness of
sedimentary rocks and their contained fossils representing
an immense span of geologic time. The geologic age of a particular sequence of rock was then determined by applying
the Law of Fossil Succession, the observed chronologie
sequence of life-forms through geologic time. This allows
fossiliferous rocks from two widely separated areas to be correlated by matching key fossils or groups of fossils found in
the rocks of the two areas (Figure 2.24). Using such indicator
fossils and radioactive dating methods, geologists have developed the geologic time scale to chronicle the documented
events of earth history (• Figure 2.25}. Note that the scale is
divided into units of time during which rocks were
deposited, li fe evolved, and significant geologic events such
as mountain building occurred. Eons are the longest time
intervals, followed, respectively, by eras, periods, and epochs.
The Phanerozoic ("revealed life") Eon began 570 million
years ago with the Cambrian Period, the rocks of which contain the first extensive fossils of organisms with hard skeletons. Because of the significance of the Cambrian Period,
the informal term Precambrian is widely used to denote the
time before it, which extends back to the format ion of the
earth 4.6 billion yea rs ago. Note that the Precambrian is
34
Chapter Two
Getting Aro und in Geo logy
Fossil correlation
and succession
• FIGURE 2.24 Geolog1c cross sections
1llustratmg the laws of SuperpoSitiOn,
Cross-curtmg Relat ionships {intrus1o n
and fault ing), and Fossi l Succession. The
l1mestone beds a re correlative because
t hey contain t he same fossils. The num erals ind1cate relat ive ages, with 1 being the
youngest.
Fault (younger than 1)
O
Ill
sandstone
~ Shale
Schist
Plutonic rocks
~ Limestone/dolomite
divided into the Archea n and Pro terozoic Eo ns, with the
Archea n Eon extending back to the fo rmatio n of the o ldest
kn ow n in- place roc ks about 3.9 billio n yea rs ago.
Precambri an tim e accounts fo r 88 percent of geologic time,
and the Phanerozoic fo r a mere 12 percent. The eras of geologic time correspo nd to the relative complexity of life fo rms:
Paleozoic (oldest life), Meso7oic (middle life), and Cenozoic
(most recent life). Enviro nmental geologists are most interested in the even ts of the past few million yea rs, a mere heartbeat in th e histo r y of the ea rth .
Absolute age-dating requires some kind of natura l clock.
The ticks of the clock may be the an nual growth rings of trees
or established rates of d isintegration o f radioactive elements
to form o ther elements. At th e turn of the twent ieth century,
America n chemist and physicist Bertram Bo rden Boltwood
( 1870- 1927) discovered that the ratio of lead to ura nium in
ura nium -bea ring rocks increases as the rocks' ages increase.
l ie deve lo ped a process fo r determ ining the age o f ancient
geologic events that is unaffected by heat o r p ressure radiometric dating. The "ticks" of the radioactive clocks are
radioactive decay processes-spo ntaneous disintegratio ns of
the nuclei of heavier elements such as uranium a nd tho rium
to lead. A rad ioactive element may decay to ano ther element,
o r to an isotope of the same element. This decay occurs at a
precise rate that can be determ ined experimenta lly. The most
comm o n emissio ns are alpha particles Cia), which are
helium ato ms, and beta particles (fs-), wh ich are nuclea r
electron s. New radiogenic "daughter" elements o r isoto pes
result fro m this alpha decay and beta decay (• Figure 2.26).
For example, of the three isotopes of carbo n, 12C, 13C, and
11
C, o nly 14C is radioactive, and this radioactivity can be used
to date events between a few hundred and a few tens-oftho usa nds of years ago. Carbo n- 14 is fo rmed continually in
the upper atmosphere by neut ron bom bard ment of nitrogen,
and it exists in a ftxed ratio to the commo n isotope, 12C. All
plants and animals contain radioactive 14C in equilibrium
with the atmospheric abundance until they die, at which time
14
C begins to decrease in abundance and , alo ng with it, the
object's radioactivity. T hus by measur ing the radioactivity of
an ancient parchment, log, o r piece of charcoal and comparing
the measurement with the activity of a modern standard, the
age of archaeological materials and geological even ts can be
determ ined (+ Figure 2.27). Ca rbon- 14 is fo rmed by the collision of cosmic neutrons with 14 N in the atmosphere, and then
it decays back to 14 N by emitting a nuclea r electron (B ).
iN + neutro n
--)>
t4c
b
>-
1
•tc + proton
jN + B-
1
Bo th ca rbon- the radioactive "parent" element-and
nitrogen- the radiogenic "daughter" -have 14 atom ic mass
units. However, one of 14C's neutro ns is converted to a proton by the emission of a beta particle, and the carbon
changes (or transmutes) to nit rogen. T his process proceeds at
a set rate that can be expressed as a half-life, the time
required fo r half of a popula tion of rad ioactive atoms to
decay. For 14 C this is about 5,730 years.
Radioactive elements decay expo nentially; that is, in two
half-lives o ne-fo urth o f the o riginal number of ato ms remain,
in th ree half- lives o ne-eighth remain, and so on (• Figure
2.28). So few parent atoms remain after seven o r eight halflives (less than I percent ) that experimental uncertainty creates li mits fo r the vario us radiometric dating methods.
Whereas the practical age limit fo r dating carbonbea ring materials such as wood, paper, and cloth is about
40,000 to 50,000 years, 238 U disi ntegrates to 206 Pb and has a
Age of the Earth
Eon
l Era
Penod
I
Epoch
Quaternary
Recent or
Holocene
Millions of Years Ago
Ql
c
0
Ql
5
Ice Age ends
16
Ice Age begins
Earliest humans
Pliocene
53
Ol
0
'"c0
Q)
Q)
z
~
u
C1l
e
Q)
c
Q)
M1ocene
23.7
Oligocene
366
Ql
1-
Ol
0
Q)
~
Eocene
57 8
(\i
c..
Paleocene
-
66
0
0N
e
Q)
c
ca
0
Cretaceous
g
JuraSSIC
5N
144
Tnassic
c.
c..
245
286
Carbon1ferous
5N
g
(\i
0..
Pennsylvan1an
----
~
Perm1an
0
----..____
208
Q)
::!:
Major Geologic and Biologic Events
001
Ple1stocene
35
-
~
•
320
Formation of Himalayas
Formation of Alps
Extinction of dinosaurs
Formation of Rocky Mountains
First birds
Formation of Sierra Nevada
First mammals
Breakup of Pangaea
First dinosaurs
Formation of Pangaea
Formation of Appalachian Mountatns
Abundant coal-formtng swamps
First reptiles
MISSISs1pp1an
360
First amphibians
Devon1an
408
Silurian
438
First land plants
505
First fish
OrdOVICian
Cambrian
I
Il
570
Earliest shelled animals
ProteroZOIC Eon
2,500
Archean Eon
-----------------------
Earliest fossil record of life
3.800
4 600
• FIGURE 2.25
Geolog1c time sca le. Adapted !Tom A
R. Palmer, Geology, 1983,504
half-life of 4.5 X I09 years. Uranium-238 emits alpha particles (helium atoms). Since both alpha and beta disintegrations can be measured with a Geiger counter, we have a
means of determining the half-lives of a geological or archaeological ~ample.
Table 2.6 shows radioactive parents,
daughters, and half-lives commonly used in age-dating.
Age of the Earth
Before the adYent of radiometric dating, determining the age
of the earth was a source of controversy between established
religious interpretations and early scientists. Archbishop
Ussher ( I 585-1656), the Archbishop of Armagh and a professor at Trinity College in Dublin, declared that the earth was
formed in the year 4004 B.C. Ussher provided not only the
year but also the day, October 23, and the time, 9:00 A.M .
Ussher has many detractors, but Stephen J. Gould of Harvard
University, though not proposing acceptance of Ussher's date,
was not one of them. Gould pointed out that Ussher's work
was good scholarship for its time, because other religious
scholars had extrapolated from Greek and Hebrew scriptures
that the earth was formed in 5500 B.C. and 376 I B.(., respectively. Ussher based his age determination on the verse in the
Bible that says "one day is with the Lord as a thousand years"
(2 Peter 3:8). Because the Bible also says that God created
heaven and earth in 6 days, Ussher arrived at 4004 yea rs
36
Chapter Two
Getting Aroun d in Geo logy
+ FIGURE 2.26 (a) Alpha emission,
whereby a heavy nucleus spontaneously emits a helium atom and is
reduced 4 atomic mass units and 2
atomic numbers. (b) Emission of a
nuclear electron (I?. particle), which
changes a neutron to a proton and
thereby forms a new element without
a change of mass.
Parent
nucleus
Daughter
nucleus
Alpha
particle
Change 1n
atomic number
Change in
atomic
mass number
-2
-4
+1
0
Alpha decay
(a)
Parent
nucleus
Daughter
nucleus
Beta
particle
Beta decay
(b)
•
Proton
Q Neutron
B.C.- th e extra fo ur years because he believed Christ's birth
yea r was wrong by that amount of time. Ussher's age fo r the
earth was accepted by many as "gospel" for almost 200 yea rs.
By the late J800s geologists believed that the earth was
on the order of 100 m illion years old. They reached their esti mates by dividing the total thickness of sedimentary rocks
(tens of kilometers) by an assumed annual rate of deposition
(mm/year). Evolution ists such as Charles Darwin thought
that geologic time must be almost limitless in order that
minute changes in organism s co uld eventually produce the
Isotopes
P11ret1t
Dlluglrter
Uranlum-238
-
__-•,....
Uranlum-235
present diversity of species. Both geologists and evolutionists
were emba rrassed when the British physicist William
Thomson (later Lord Kelvin) demonstrated with elegant
m athematics how the ea rth could be no older than 400 million yea rs, and maybe as young as 20 million years. Thomson
based this on the rate of cooling of an initially molten earth
and the assumption that the material composing the earth
was incapable o f crea ting new heat through time. He did not
know about radioactivity, wh ich adds heat to rocks in the
cru st and ma ntle.
Dating Range,
Years
Years
10 m1llion to 4.6 b1ll1on
L
704 million
14 billion
Strontium-87
Electron
Half-Life,
4.5 billion
Thonum-232
Rubidlum-87
e
48.8 billion
-•-
!it _..Z1rcon
-~
_
Uranin1re
---
10 mill1on ro 4.6 billion
Mustovite
BIOtite
Orthoclase
Potasslum-40
Carbon-14
--5,730 years
100,000 ro 4.6 billion
--..._-
...
less than 100,000
"two
....
Whole metamorphic or igneous roc~
Glauconite
Hornblende
•
Muscovite
100
~1
................ 1
M1neral at t1me
· ••••••••••••• ••• · of crystallization
• Atoms of parent element
• Atoms of daughter element
I
COSrl'IC
1................ 1 M1neral
after
•••••••••••••••• one half-life
' rad1at1on
Nlt'ogen-14
~
Neutron capture •
'
~ Carbon- 14
0
(I)
Cl
~
25
(I)
· 'C s c.ib )rbcd
;. '!long wtth 12C and
C 1nto the t1ssue
of living organ1sms
in a fatrly constant
e
cT.
•••••••••••••••• after three
half-lives
0
I
,,,0
1.......... ...... 1 M1neral
12.5
6.25
3125
2
3
4
5
Time units
'
When an organism dies, 14 C converts
back to 14 N by beta decay.
!
Carbon-14
Nltrogen-14
• Proton
• Neutron
+ FIGURE 2 .27 Carbon-14 is fonmed from nitrogen-14 byneu·
tron capture and subsequent proton em1ss1on. Carbon-14 decays
back to nltrogen-14 by emission of a nuclear electron (g ).
Bertram Boltwood postulated that older uraniumbearing minerals should carry a higher proportion of lead
than vounger samples. He analyzed a number of specimens
of known relative age, and the absolute ages he came up with
r.mged from 410 million to 2.2 billion years old. These ages
put Lord Kelvin's dates based on cooling rates to rest and
ushered in the new radiometric dating technique. By extrapolating backward to the time when no radiogenic lead had
been produced on earth, we arrive at an age of 4.6 billion
v~ars for the earth. This corresponds to the dates obtained
from meteorites and lunar rocks, which are part of our solar
Sf•tem. The oldest known intact terrestrial rocks are found in
the \casta Gneiss of the Slave geological province of
Canada's orthwest Territories. Analyses of lead to uranium
ratios on the gneiss's zircon minerals indicate that the rocks
are 3.96 billion years old. However, older detrital zircons on
the order of 4.0-4.3 billion years old have been found in
• FIGURE 2 . 28 Decay of a radioactive parent element with
time. Each time unit is one half-life. Note that a fter two half·
lives one-fourth of the parent element remains, and t hat after
three half-lives one-etghth remains.
wc~tcrn Australia, indicating that some stable continental
crust was present as early as 4.3 billion years ago ( Table
2.7). Suffice it to say that the earth is very old and that there
has been abundant time to produce the featu res we see today
(• Figure 2.29).
Environmental geology deals mostly with the present
and the recent past-the time interval known as the
Holoce11e Epoch ( I0,000 yea rs ago to the present) of the
Quntemary Period. However, because the Great lee Age
advances of the Pleistocene Epoch of the Quaternary Period
(which preceded our own Holocene Epoch) had such a
tremendous impact on the landscape of today, we will investigate the probable causes of the "ice ages" in order to speculate a bit abou t what the future might bring. In addition,
Pleistocene glacial deposits are valuable sources of underground water, and they have economic value as sources of
building materials. We will see that "ice ages" have occurred
several times throughout geologic history (see Cha pter 11 ).
What is most impressive about geologic time is how
short the period of human life on earth has been. If we could
*TABLE
2.7
Earth's Oldest l<nown Materials
Age, Billions
Material
Location
of western Australia
Rock
Z1rcon minerals 1n the Acasta
Gne1ss, N.W. Terr , Canada
Sed1menrary
rock .,.
lsua Greenstone Bek
Greenland
~ Fossils
Algae and bactena
of Years
38
Chapt er Two
Getting Around in Geology
EARTH YEAR
Proto
Month ::::::==~~=====~~:=~~~======~=~==~==~==~=~~==~=~~~~~:~~~~=~
Billion years '---'::..._--'---'--"---'----'-------'-.::....:-__..J'----.L.::..-"---'----'----'--'---
~Geologic eras Precambnan
-
First
Days f Million years 650
I
10
I
Paleozoic
NOVEMBER
F1Sh
15
20
500
Mesozo1c
ICenozo1c
DECEMBER
Amph1b1ans
25
400
Repldes
5
10
15
Mammals
20
25
100
200
300
DECEMBER31 1201 AM
F1rs! h0m1n1ds
Ice Ages beg n
Mammals develop
Hours r-~~~~3-~~-~6-~~-~~~-~12-'-~~-~~~~~18_~~~~~-;
Million years ~,____;1-=2~0-'------=9~0___;_ _ _ _ _ ___:6;.:.:0::...__ _ _ _ _ ____:3:.._0::..__ _ _1_6=-------'
DECEMBER 31 11 OOFM
Homo Sap1ens
ICE' Ages end
Minutes l - - - -1...:.0_ _ _2....;0_ _ _3;.,.0_ _~
40
_ _~5-9_th_r'1_,n-'l
Thousand years .____ _ _ _ ___:300..;;.:;.._ _ _ _ _ _1..;;.00.:..___ _~
+ FI GURE 2 . 29 The past 4.6 billion yea rs of
geologic t ime compressed into 12 months. Note
that the first h ard-shelled marine organ1sm does
not appear until ab out November 1, a nd that
humans as we know them have been around on ly
smce the last hour before t h e New Year.
Seconds
tee leaves
North Amenca
10
DECEMBER31 1159FM
Sealevel
stabilizes
40
30
20
BCIAD
5C
Years~~~.:..__----~~
~=--------4000~_ _ _ _ _2000_..:;__ _ _~
compress the 4.6 billion years since the earth form ed into
one calendar year, Homo sapiens would appear about 30 minutes before midnight on December 31 (see Figure 2.29). The
last ice-age glaciers would begin wasting away a bit more
than 2 minutes before midnight, and written history would
exist for o nly the last 30 seconds of the year. Perhaps we
should keep this calendar in mind when we hear that the
dinosa urs were an unsuccessful group of reptiles. After all,
they endured 50 times longer th an ho minids have existed on
the planet to date. At the present rate of population growth,
there is so me doubt that humankind as we know it will be
able to survive anywhere nea r that long.
The ea rth we see to d ay is the product of earth processes
acting o n earth materials over the immensity of geologic
time. The respo nse of the materials to the processes has been
the formation of mountain ranges, plateaus, and the multitude of o ther physical features fo und o n the planet. Perhaps
the greatest of these processes has been the formation and
movement of platelike segm ents of the earth's surface, whi ch
influence the distribution of earthquakes and volcanic eruptions, global geography, mineral deposits, mountain ranges,
and even climate. These plate tectonic processes arc the subject of the next chapter.