Methods of Dating the Earth Part 1: Relative Dating

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.