All the way at the beginning of this geology series, we talked about Earth’s history and
the geological timescale spanning 4.5 billion years of Earthly events,
from the Hadean eon to the Phanerozoic eon we are still living in today. But how is it
that geologists determine the age of the Earth and its different rock formations?
There are two methods of dating rocks. These are relative dating, which qualitatively
compares the age of formations based on their stratigraphic sequence, and absolute dating,
or radiometric dating, which uses the decay of radioisotopes to calculate a rock’s precise age.
Before the advent of radiometric dating in the early 1900s, geologists assigned relative dates
to different layers in an outcrop based on the inferred sequence of their deposition,
or emplacement, which they would then compare to other outcrops using distinctive formations called
marker beds. In fact, most sedimentary rocks are dated using relative dating since they do
not meet the criteria for radiometric dating, which we will discuss in the next tutorial.
Let’s now discuss some of the principles that geologists use when determining relative ages. The
principle of original horizontality states that sediments accumulate in horizontal layers called
beds, though some sand layers that are deposited as dunes may be inclined as much as 35 degrees.
This is called crossbedding. In cross bedded sedimentary rocks,
the sediment is deposited in sets at the angle of repose on the lee side of dunes and ripples,
which explains their inclination. In addition, only sand-sized sediment can form cross beds.
Say for example that you find an outcrop with a horizontally layered siltstone on top of a
vertically layered siltstone. When you apply the principle of original horizontality, you realize
that there was a large time gap between deposition of the horizontal and vertical formations.
The principle of superposition states that, unless tectonic forces have overturned the
outcrop, beds on the bottom are usually older than beds on top.
The principle of cross-cutting relationships states that any geological feature which cuts
across a rock must be younger than the feature it disrupts. So, if an igneous intrusion is found
cutting through a sandstone, the intrusion must be younger than the sandstone it cuts through.
The principle of inclusions states that any rock fragments that are a part of a larger
formation must be older than the formation they are a part of. For example, lithic fragments,
which are pieces of a preexisting rock, are common types of grains in sedimentary rocks; so if you
find a clast of schist in a sandstone, the schist must be older than the sandstone it is a part of.
The principle of faunal succession states that there is a historical order in which organisms
evolved over time, and that certain specific fossils can be used to determine a rock’s age.
Organisms that only existed for a short period of Earth’s history are most useful for this,
and their fossils are called index fossils. For example, if you find a trilobite fossil
in a bed, then it must have been deposited between the Cambrian and Permian Periods.
Another useful concept for qualitatively dating rocks is that of “missing time”, or gaps in the
rock record called unconformities. There are four types of unconformities: a nonconformity,
an angular unconformity, a disconformity, and a paraconformity, all of which represent missing
time. But what exactly is meant by missing time? Recall the Wilson cycle from a previous tutorial,
and the ways that geologic environments change over time. For example, during one period an area
may be a part of a sedimentary basin, but then get uplifted 100 million years later during an
orogeny, transforming the once sedimentary environment into an erosional environment,
and then, after another 100 million years, it could once again become a sedimentary basin.
Let’s consider what the rock record would look like here. Sedimentary rocks would be deposited
during the first period, which would later get eroded during uplift, removing some amount
of the sedimentary record, which would later be capped by sediments from the last period.
The rocks that were eroded during uplift represent missing time,
and the gap between the two sedimentary layers is called an unconformity.
Let’s rigorously define the types of unconformities. A disconformity is an erosional
boundary between two beds of sedimentary rock, as in the example we just discussed. A paraconformity
is also a boundary between two beds of sedimentary rocks, but is not erosional and simply represents
a period of nondeposition. They represent less missing time than a disconformity. A nonconformity
is a boundary between an older non-sedimentary rock, like an igneous or metamorphic rock,
and younger sedimentary rock layered on top. And an angular unconformity is a boundary between
tectonically tilted layers of sedimentary rocks and overlying horizontal strata.
So, that covers the principles that geologists use to assign relative dates to Earth’s rock layers,
and some of the challenges that this method poses. As we mentioned,
prior to the development of radiometric dating, this was all that geologists were able to do.
But radiometric dating is a powerful technique, so let’s move forward and learn about that next.