|
Relative Dating
Stratigraphy and the Principles of Relative Dating
Relative dating falls under the sub-discipline of geology known
as stratigraphy. Stratigraphy is the science of rock strata, or
layers. Layering occurs in sedimentary rocks as they accumulate
through time, so rock layers hold the key to deciphering the succession
of historical events in Earth’s past.
The fundamental principles of stratigraphy are deceptively simple
and easy to understand, but applying them to real rocks and fossils
can be quite challenging. Here are the four fundamental principles
of stratigraphy that form the foundation of our understanding of
Earth’s history:
- The Principle of Original Horizontality:
When sediments are laid down on Earth’s surface, they form
horizontal or nearly horizontal layers. This means that non-horizontal
rock layers were tilted or folded after they were originally deposited.
- The Principle of Lateral Continuity:
Rock layers extend for some distance over Earth’s surface—from
a few meters to hundreds of kilometers, depending on the conditions
of deposition. The point is that scientists can relate layers
at one location to layers at another. This is critical for stratigraphic
correlation (see below).
- The Principle of Superposition:
As layers accumulate through time, older layers are buried beneath
younger layers. If geologists can determine which way was originally
“up” in a stack of layers, they can put those strata
in the correct historical order. (Rarely, after a sequence of
layers has been deposited and compressed to form rock, it may
be literally overturned by thrusting of the Earth’s crust
as continental plates collide. In these rare places the youngest
rocks in a sequence are on the bottom, but such overturned sequences
can be identified by the extensive faulting and breaking of rocks,
and because the same original sequence of rocks is frequently
present elsewhere in undisturbed order.)
- The Principle of Faunal Succession:
This principle is attributed to William Smith, an English
engineer in the late 1700s. Smith noticed that the kinds of fossils
he found changed through a vertical succession of rock layers,
and furthermore, that the same vertical changes in fossils occurred
in different places. Using the fossils collected from rocks in
one part of England, Smith could predict the succession of rocks
and fossils in other parts of England. The observation that fossils
change in a consistent manner through stratigraphic successions
can be extended to the entire world. Smith’s discovery formed
a key line of evidence for evolution (it predates the birth of
Charles Darwin in 1809), but it is an observed property of the
rock record and is independent of natural selection, Darwin’s
proposed mechanism of evolution.
Relative dating of rocks and fossils
from an area is based on the Principle of Superposition, which enables
scientists to put historical events in order. Relating the succession
of events in one region to those in another requires that the two
areas be stratigraphically correlated. Correlations can be made by
tracing rock strata from one area to another by using the Principle
of Lateral Continuity or by relating the fossils of the two areas
using the Principle of Faunal Succession.
The Grand Canyon as an Example of the Principles of Stratigraphy
The Grand Canyon spectacularly exposes rocks spanning hundreds of
millions of years of Earth’s history.
Many of the rock layers exposed in the walls of the Grand Canyon have
not been disturbed by mountain building or other forms of deformation
since they were originally laid down on Earth’s surface. This
is an example of original horizontality. Some older layers, however,
have been tilted; the surface where these tilted layers are overlain
by undeformed strata is called an angular unconformity.
Many of the undisturbed formations can be traced from one end of the
Grand Canyon to the other, a distance greater than 435 kilometers
(270 miles). This is an example of lateral continuity. Some of the
same formations are also exposed hundreds of miles farther away in
other parts of the Southwest.
The oldest rocks in the Grand Canyon are exposed at the base of the
gorge and are late Proterozoic. These rocks are overlain by younger
Paleozoic-age rocks. This is an example of superposition: In a pile
of sediment, the oldest deposits are at the bottom of the pile, underneath
younger deposits.
Each major layer of sedimentary rock in the Grand Canyon contains
different types of fossils. The succession of fossils in the Grand
Canyon is consistent everywhere in the canyon and is also similar
to the succession of fossils in other parts of North America and on
other continents. This is an example of how the principle of faunal
succession has been used to recognize that the Grand Canyon includes
rocks from the Cambrian, Devonian, Mississippian, Pennsylvanian, Permian,
and other geologic periods (each characterized by different fossils).
Biostratigraphy
Correlation based on fossils is the focus of biostratigraphy. Most
species live only for a few million or tens of millions of years before
they become extinct or evolve into new species. This fact implies
that rocks containing fossils of the same species were probably formed
within a few million years of one another. The more species that can
be matched in this way, the more precise the estimate of the relative
age of a rock layer. Organisms with high turnover rates (meaning that
new species appear very rapidly and last for a short period of time)
and a high likelihood of being preserved as fossils are ideal for
biostratigraphy. Graptolites and conodonts are good examples from
the Paleozoic, and mammals, foraminifera, and pollen are often used
for biostratigraphy in Cenozoic rocks.
By using the four principles of relative dating, geologists can compile
a detailed sequence of events based on relative time. The process
is not always straightforward, however, because the geologic record
is often discontinuous. Rock layers representing a particular time
may be missing from an area because no sediment was deposited there
at that time, or because sediments that were deposited were eroded.
Furthermore, fossils may be absent or poorly preserved, and interpretations
of evolutionary relationships within fossil groups may be incorrect.
If sediments were deposited in different environments (such as land
and oceans) comparison is difficult because most organisms are adapted
to a relatively narrow range of environmental conditions.
Until the advent of radiometric dating, there was no independent way
to test the accuracy of relative dating of sedimentary sequences.
In stratigraphic sections around the world radiometric dating techniques
have verified the relative ages of sedimentary rocks that had been
determined long before from the fossils they contained.
Rocks whose ages have been determined by absolute dating can be incorporated
into a succession of strata determined by relative dating. Then geologists
can use correlation to infer the ages of rocks and fossils that cannot be
directly dated. |
