Geology Reference
In-Depth Information
geologic processes were instrumental in establishing the
principle of uniformitarianism
(see Chapter 1), the concept
that the same processes seen today have operated over vast
amounts of time. Because Hutton relied on known processes
to account for Earth history, he concluded that Earth must
be very old and wrote that “we fi nd no vestige of a begin-
ning, and no prospect of an end.”
Unfortunately, Hutton was not a particularly good
writer, so his ideas were not widely disseminated or accepted.
In 1830, Charles Lyell published a landmark book,
Principles
of Geology
, in which he championed Hutton's concept of
uniformitarianism. Instead of relying on catastrophic events
to explain various Earth features, Lyell recognized that im-
perceptible changes brought about by present-day processes
could, over long periods of time, have tremendous cumu-
lative effects. Through his writings, Lyell fi rmly established
uniformitarianism as the guiding principle of geology. Fur-
thermore, the recognition of virtually limitless amounts of
time was also necessary for, and instrumental in, the accep-
tance of Darwin's 1859 theory of evolution (see Chapter 18).
After establishing that present-day processes have oper-
ated over vast periods of time, geologists were nevertheless
nearly forced to accept a very young age for Earth when a
highly respected English physicist, Lord Kelvin (1824-1907),
claimed, in a paper written in 1866, to have destroyed the
uniformitarian foundation of geology. Starting with the
generally accepted belief that Earth was originally molten,
Kelvin assumed that it has gradually been losing heat and
that, by measuring this heat loss, he could determine its age.
Kelvin knew from deep mines in Europe that Earth's
temperature increases with depth, and he reasoned that
Earth is losing heat from its interior. By knowing the size of
Earth, the melting temperatures of rocks, and the rate of heat
loss, Kelvin calculated the age at which Earth was entirely
molten. From these calculations, he concluded that Earth
could not be older than 400 million years nor younger than
20 million years. This wide discrepancy in age refl ected un-
certainties in average temperature increases with depth and
the various melting points of Earth's constituent materials.
After establishing that Earth was very old and that present-
day processes operating over long periods of time account for
geologic features, geologists were in a quandary. If they accepted
Kelvin's dates, they would have to abandon the concept of seem-
ingly limitless time that was the underpinning of uniformitar-
ian geology and one of the foundations of Darwinian evolution
and squeeze events into a shorter time frame.
Kelvin's reasoning and calculations were sound, but his
basic premises were false, thereby invalidating his conclu-
sions. Kelvin was unaware that Earth has an internal heat
source, radioactivity, that has allowed it to maintain a fairly
constant temperature through time.* His 40-year campaign
for a young Earth ended with the discovery of radioactivity
near the end of the 19th century, and the insight in 1905 that
natural radioactive decay can be used in many cases to date
how long ago a rock formed. His calculations were no longer
valid and his proof for a geologically young Earth collapsed.
Although the discovery of radioactivity destroyed
Kelvin's arguments, it provided geologists with a clock that
could measure Earth's age and validate what geologists had
long thought—namely, that Earth was indeed very old!
Before the development of radiometric dating techniques,
geologists had no reliable means of absolute dating and
therefore depended solely on relative dating methods. Re-
call that relative dating places events in sequential order, but
does not tell us how long ago an event took place. Although
the principles of relative dating may now seem self-evident,
their discovery was an important scientific achievement
because they provided geologists with a means to interpret
geologic history and develop a relative geologic time scale.
The 17th century was an important time in the development
of geology as a science because of the widely circulated writ-
ings of the Danish anatomist Nicolas Steno (1638-1686).
Steno observed that when streams flood, they spread out
across their fl oodplains and deposit layers of sediment that
bury organisms dwelling on the floodplain. Subsequent
fl oods produce new layers of sediments that are deposited or
superposed over previous deposits.
When lithified, these layers of sediment become sedi-
mentary rock. Thus, in an undisturbed succession of sedi-
mentary rock layers, the oldest layer is at the bottom and the
youngest layer is at the top. This
principle of superposition
is the basis for relative-age determinations of strata and their
contained fossils (
◗
Figure 17.2a).
Steno also observed that, because sedimentary particles
settle from water under the infl uence of gravity, sediment is
deposited in essentially horizontal layers, thus illustrating the
principle of original horizontality
(Figure 17.2a). Therefore,
a sequence of sedimentary rock layers that is steeply inclined
from the horizontal must have been tilted after deposition
and lithifi cation (Figure 17.2b).
Steno's third principle, the
principle of lateral continuity
,
states that sediment extends laterally in all directions until it
thins and pinches out or terminates against the edge of the
depositional basin (Figure 17.2a).
James Hutton is credited with discovering the
principle
of cross-cutting relationships.
Based on his detailed studies
and observations of rock exposures in Scotland, Hutton rec-
ognized that an igneous intrusion or fault must be younger
than the rocks it intrudes or displaces (
Figure 17.3).
Although this principle illustrates that an intrusive igne-
ous structure is younger than the rocks it intrudes, the associa-
tion of sedimentary and igneous rocks may cause problems in
relative dating. Buried lava fl ows and sills look very similar in a
sequence of strata (
◗
*Actually, Earth's temperature has decreased through time because the
original amount of radioactive materials has been decreasing and thus is
not supplying as much heat. However, the temperature is decreasing at a
rate considerably slower than would be required to lend any credence to
Kelvin's calculations.
Figure 17.4). A buried lava fl ow, however,
is older than the rocks above it (principle of superposition),
◗
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