Today, I offer some background information on the geologic time scale and why it is so hard to figure out how old rocks are. Unlike calendars or clocks, which divide time into units of equal length e. This merger of geologic time and absolute time is the geologic time scale. Get one here for free! Geologic time is hard to sort out. The first step requires understanding the relative order of the rock layers. This idea was first put forth by the Persian polymath Avicenna , and later presented more formally by the geologist and Catholic bishop Nicholas Steno.
They proposed that within a vertical column of rock layers, the oldest ones are on the bottom, and the youngest are on top. Today, we call this the principle of superposition. Related to superposition is the principle of original horizontality , which just means that rock layers are more or less horizontal when they are first deposited.
In this image, layer I is the oldest layer, and layer A is the youngest. Of course, the real world is more complicated than the above image. After rock layers form, their position can change through faulting or deformation. For example, the entire column can become tilted, in which case the rock layers get older or younger as you move horizontally along the ground, rather than vertically up a cliff: The rocks were originally deposited as a stack of horizontal layers but were later tilted through geologic processes.
As you move from left to right in this picture, the rock layers go from oldest to youngest. Also, rock layers erode at different rates in different places. For example, if there is a river running between two hills, the old rock layers will erode faster there than at the top of the hills. New rock layers are more likely to form in the riverbed and adjacent floodplains than on the hilltops. Different rates of erosion, deposition, and tectonic activity mean that the relative order of rock layers can be difficult to sort out in some places.
To illustrate, look at the first image of rock layers above, with layers A through I. Also, there is no trace of layer A in this whole region, although it may be preserved somewhere else.
But we can reconstruct the relative sequence of some events. For example, Layer 1 is older than layers 2, 3, and 4, which lie on top of it thanks, superposition! Once you have the correct order of your rock layers sorted out, you can begin to associate the rock columns in your area with columns in other regions. You might then hypothesize that the red layers are the same layer, based on their position relative to the grey layer.
More often than not, aligning rock columns is more complicated than this. The appearance of rock layers depends on the local chemical and environmental conditions when they formed. Layers forming under the deep ocean will look very different than those forming in a coral reef, riverbed, desert, or swamp. Even more confusing, if environmental conditions are very similar, rocks deposited at two different points in time might look similar.
One line of evidence for simultaneous deposition is when two layers contain the same fossil species. This is much easier to do for oceanic rock layers, because some ocean species e.
The types and sequence of these fossils can be helpful in lining up distant rock columns. In this picture, fossils are white, blue layers were deposited underwater, and the other colors formed in terrestrial environments. Columns 1 and 3 are mostly terrestrial, but there are three times when the ocean invaded the land — times of high sea level or inland seas.
Because the oceanic fossil species are identical, these are probably the same layers seen in column 2; if so, these layers represent the same points in time in all three columns. The sequence of the oceanic fossil species is important, too: Using the sequence of fossil species to correlate rock layers across big distances is called biostratigraphy , and it was extremely important for understanding the basic succession of rock layers on a global scale.
Biostratigraphy is still important today for oil exploration, and is also used to align bores when drilling tunnels. Once scientists had the rock layers aligned and their basic relative sequence sorted out, they noticed large-scale patterns in the succession of the fossils at a global scale.
Globally, blastoids , trilobites , and acanthodians were common in oceanic rocks up until a certain point, after which those types of fossils were never found again. Above that point, totally different types of fossils were found, such as plesiosaurs. Similar patterns were found in oceanic fossils as in terrestrial animal and plant fossils.
Smaller shifts between species rather than faunas could be used to bracket smaller subregions of the rock column. Image modified from Mazza et al. If I find an essay I wrote for Mrs. To find out how old something is in terms of years, you need a different metric, one that can determine absolute time. One common way to do this is radiometric dating. Radioactive isotopes are unstable. Each radioactive isotope decays at a specific rate and results in specific stable isotopes — these are just basic properties of the atoms themselves.
To estimate when that process started, you first take a substance and figure out its ratio between the radioactive isotopes and the stable post-decay isotopes.
We use carbon dating to determine the ages of once-living things because it is present in plant and animal tissues.
This is a long time compared to the High School Era, but really short compared to the age of most rocks. So, we use other isotopes to date rocks; ones that decay at a slower rate. These isotopes have much slower rates of decay i. Using radiometric dating methods, we can link absolute time to geologic time. These methods have already been used to date the rock layers containing the oceanic fossils that define and bracket the divisions of geologic time.
So we now know how long each major division of geologic time lasted. Radiometric methods also have been used to date some terrestrial rock layers. Rather than roughly correlating terrestrial layers based on their associations with oceanic rock layers, we can now compare dates of terrestrial layers directly.
As more and more rock layers are tagged with absolute dates, the geologic time scale is getting finer and finer resolution.