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Geologic time vs. absolute time
Posted November 20, 2013 by Sarah Werning in Background, Geology, Paleontology
Today, I offer some background information on the geologic time scale and why it is so hard to figure out how old rocks are.
Earth’s history is divided into different chunks of geologic time, going all the way back to the formation of our planet. Unlike calendars or clocks, which divide time into units of equal length (e.g., days or seconds), the divisions of geologic time aren’t the same length. For example, the Mesozoic Era was ~186 million years long, whereas the preceding Paleozoic Era lasted ~289 million years. At 2.6 million years, the Pleistocene Epoch was much shorter than the Miocene Epoch (20.4 million years long). These divisions may seem arbitrary at first, but they’re not; geologic time is based on the succession of rock layers. Geologic time was the first method scientists used to understand the sequence of events in Earth’s history. More recently, we’ve used other methods to associate actual dates with different rock layers, thus linking geologic time (a relative method) with absolute time (= numbers of years old). This merger of geologic time and absolute time is the geologic time scale. (Don’t have a geologic time scale handy? Get one here for free!)
Geologic time is hard to sort out. The first step requires understanding the relative order of the rock layers. For almost 1000 years, we’ve recognized that rock layers are organized in the order in which they were deposited. 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. Horizontality is important because today’s rocks are not always in their original orientation.
https://blogs.plos.org/paleo/2013/11/20/geologic-time-vs-absolute-time/