Evidence Used By Scientists to Determine the Age of the Earth and Sequence
of Ages in the Rocks
During the middle
1800s, Charles Lyell (seen in the image to your left) was presiding over the
British Museum of Natural History. A top geologist, and renowned scientific
thinker and writer, Lyell has made some wonderful discoveries of interesting
fossils embedded in the rocks, and was intrigued by sedimentation patterns of
those same rocks. Lyell was convinced that the Earth was very old, far older
than traditional methods of Biblical patriarch and Israelite king lists led
religious leaders of the day to estimate a value a little over 6000 years. Lyell
noticed the aging patterns of rocks and concluded that much time is required
to transform pristine rock into aged rock that was observed. Sedimentary rock
could not have developed in a few thousand years. It must have taken millions
of years or even longer. Key to Lyell's interpretation was a pair of doctrines:
1) The doctrine of gradualism stated that geologic processes happen a slow
rates, and that over long periods of time, gradual changes can accumulate into
large-scale change. A fine example for USA students is the Grand Canyon. The
Colorado River has been running through the Arizona desert for a very long time,
and slowly that river has gouged out a canyon in the relatively weak sandstone
and limestone of the area. Over a very long time, the gradual process of gouging
resulted in the deep canyons observed today.
2) The doctrine of uniformitarianism states that the rate of geologic processes
in the past is the same as the rate observed in the present. By thus measuring
the present rate of a geologic process and the accumulation of the results of
this process, it becomes possible to determine the total time that passed in
the development of the final appearance.
Lyell's work was
heavily influenced by the study of James Hutton (seen in the image to the left).
It was Hutton in 1795 who presented these theorized patterns of uniform rates
and significant age of the Earth. Hutton's ideas were new to the geology community
and an affront tothe religuous community. It did not help his cause that he
was a poor writer. As a result, his ideas did not gain acceptance until Lyell
published his "Principes of Geology" in 1830.
Completing the shift in thinking of the Earth in terms of millions of years
or more was a French geologist named George Cuvier (seen below and to your left).
In 1812, Cuvier proposed an explanation for the different layers of rock in
a local gorge and offered an explanation for the presenceof different fossilized
organisms in different stata. Essentially, Cuvier was the father of a new geologic
field ... Paleontology.
Cuvier and his
contemporaries had discovered a geological world characterized by discontinuous
and abrupt transitions between layers of sedimentary rock. Many of the layers
were thick and relatively uniform with respect to their physical characteristics
and the types of fossils contained therein. They apparently represented extended
and rather tranquil episodes. However, these layers were commonly overlain by
strikingly different layers that contained totally different sets of fossils.
In many cases, these layers would alternate between marine and terrestrial/freshwater
sediments. Moreover, the transition from one layer to the next was typically
addition to each layer (or stratum) having different sets of fossils, there
was an apparent directionality in the type of fossils present, at least with
respect to quadrupeds (f3). The fossil viviparous quadrupeds (mammals) in surficial
deposits (e.g., mastodons and mammoths) were similar to but different from modern
forms. Go deeper and the quadrupeds become less and less similar to modern forms.
For instance, Curvier and his collaborator Alexandre Brogniart (1770-1840) recovered
mammalian fossils from a gypsum stratum near Paris that were strikingly different
from modern species (f4). Go even deeper and you'll only find oviparous quadrupeds
The sharply delineated and often alternating marine and terrestrial strata
combined with the apparent directionality in quadruped fossils led Cuvier to
conclude that the earth had been subjected to repeated "revolutions",
some of which were severe or catastrophic enough to lead to the extinction of
whole fossil faunas. In other words, the history of life was characterized by
periods of relative tranquility terminated by widespread and possibly global
Living organisms without number have been the victims of the catastrophes.
Some were destroyed by deluges, others were left dry when the seabed was suddenly
raised; their races are even finished forever, and all they leave in the world
is some debris that is hardly recognizable to the naturalist.
Unlike uniformitarians such as James Hutton (f6), Cuvier could not accept the
idea that these revolutions were caused by current geological processes such
as erosion and volcanoes. To him, they simply couldn't explain the massive and
abrupt changes he found in geologic strata. "The thread of operations is
broken; nature has changed course, and none of the agents she employs today
would have been sufficient to produce her former works."
Cuvier also saw evidence for extraordinary processes in the extinctions of
whole faunas. In contrast with his contemporary Lamarck, he was committed to
the idea that species were immutable (f7). Cuvier regarded animals to be highly
integrated, functional machines. This concept, which led to his famous "correlation
of parts" (f8), precluded the possibility of transformation (or evolution).
Every organized being forms a whole, a unique and closed system, in which all
the parts correspond mutually, and contribute to the same definitive action
by a reciprocal reaction. None of its parts can change without the others changing
too; and consequently each of them, taken separately, indicates and gives all
Erosion, floods, landslides and volcanic eruptions simply couldn't explain
the disappearance of a successful, resilient and highly functional species,
much less to the disappearance entire faunas. Something extraordinary, something
beyond human experience, must have caused the extinctions.
Thus, Hutton proposed the Earth is very old, Lyell believed that processes
in the geologic record took a long time and gradually, while Cuvier proposed
that the fossil record in the rock strata could be identified by representative
fossils. Cuvier noticed abrupt changes in the rock strata that seemed to indicate
cataclysmic changes occurring repeatedly throughout Earth's past, and the possibility
of mass extinctions was brought to the fore. You will learn about these extinction
level events later in the course when studying Asteroid
Impacts. From a biological standpoint, Darwin believed in slow and gradual
changes in evolution that would lead to differences in the morphological appearance
of related organisms, but the fossil record does not support gradual changes.
Stephan Jay Gould proposed a theory called Punctuated Equilibrium to explain
the gaps in the fossil record, very similar to what Cuvier proposed over 180
years before, but this is material for my AP Biology course.
Conclusion: Geologists can determine
different ages in the rock strata by looking at the fossils embedded in this
strata. Geologists can determine that deeper sediments will contain older
fossils, and upper sediments will contain younger fossils. Now it is only a
matter of determining the age of the rocks and geologists can assignment times
to the fossil record. A more detailed commentary, far better than mine, can
be found at the UCMP
site from which I get so much of my information:)
Dating the Rock
use a technique called Potassium-Argon dating to determine the age of very old
rocks. A radioactive isotope of Potassium degrades into an inert atom of Argon
over a very slow time period. Potassium is a solid and Argon is a gas. When
rock is in a melted state, the Argon gas simply escapes while Potassium remains
behind. When the molten rock solidifies, any Argon that results from the decay
of radioactive Potassium will be trapped in tiny gas vesicles within the rock
sample. Over a long period of time, one can measure the ratio of this radioactive
Potassium to accumulated Argon and get a pretty good idea of the age of a rock.
In this fashion, rocks all over the world can be dated. This is technique that
is used to determine the age of the granite, schist, and gneiss formations that
make up much of the igneous and metamorphic rock of the continents. The rock
of the Canadian shield is believed to be 4.1 billion years old. A short trip
down HWY 19 toward Marshall, Minnesota leads you into a town called Morton.
Walk behind a local hotel and you will be standing on an old quarry where "Morton
Gneiss" was cut out for decorative facades on buildings in downtown Minneapolis.
You stand there and can feel the thrill of being atop rock that is among the
oldest stuff in the western hemisphere! I was so over-joyed with this sensation
that I chopped some pieces with my geologist hammer and later collected some
larger samples for our family fireplpace
The decay half-life is shown in the graph to your left.
Radioactive isotopes don't tell much about the age of sedimentary rocks (or
fossils). The radioactive minerals in sedimentary rocks are derived from the
weathering of igneous rocks. If the sedimentary rock were dated, the age date
would be the time of cooling of the magma that formed the igneous rock. The
date would not tell anything about when the sedimentary rock formed.
A technique that is readily employed is to look at the layers of sedimentary
rock and try to determine when those layers were laid down and cemented. Later,
intrusions of igneous rock from beneath the sedimentary rock may push up into
the upper layers. By determining the age of the intrusive rock and comparing
this timing to the place where the intrusion took place, it becomes possible
to determine the age of the when the sediments were deposited. Equally valuable
is to look at the representative fossils in the strata and get the age of the
fossils. It is apparent that the fossil was formed by the sediments that covered
the dead organism. If you can determine the date of the fossil, you can derive
the date of the stratum in which the fossil lies.
Geologists have applied these methods to give a history of the Earth and have
subsequently named the Ages of the Earth after representative fossil finds in
particular layers of sediments and the place where those sediments were first
discovered. A detailed look at the Ages of the Earth is found at the UCMP
Site (complete with special links to each Era and Period, but a more simple
look at the Paleo-Record of Earth's History is found below:
To date a sedimentary rock, it is necessary to isolate a few unusual minerals
(if present) which formed on the seafloor as the rock was cemented. Glauconite
is a good example. Glauconite contains potassium, so it can be dated using the
To get a nice perspective on time, please move to
Time Scale Model.
After you have studied the Time Scale of the Earth's History, it is time to
do a few activities that will help you better understand how geologists determine
the age of rocks. An excellent exercise that also teaches the methods used to
date rocks is found within the pages of a University of Michigan course entitled
Global Change. The page, Clocks
in Rocks, was moved to this website, with the hotlink
to the exercise at the bottom of the page. It is a great lesson. Please
move ahead to either or both of the activities listed below: