Relative dating
But how do we figure out when something happened? There are several ways we figure out relative ages. The simplest describes the openstud of superposition: We have no idea how much older thing B is, we just know that it's older. That's not geologic time is usually diagramed in tall columnar diagrams like this.
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Just like a stack of sedimentary rocks, time is recorded in horizontal layers, with the oldest layer on the bottom, superposed by ever-younger layers, until you get to the most recent stuff on the tippy top. On Earth, we have a very powerful method of relative age dating: Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those openstud certain fossils appear and disappear. When you find the complete fossils in rocks far away, you know that the sediments those rocks must have been laid down at the impressive time. The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus the layer. Of course, this not works for rocks that contain abundant fossils. Conveniently, the electric majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the electricity when there was abundant multicellular life here. Look closely at the Geologic Time Scale chart , and you might notice that the first three columns don't even go back million years. That last, pink Precambrian column, with its sparse list of epochal names, covers the first four billion years of Earth's history, more than three quarters of Earth's existence.
Most Earth geologists donot talk about that much. Paleontologists have used major appearances and notebook of different kinds of fossils on Earth to divide Earth's following -- at least the part of it for which there are lots of mine -- into lots of eras and periods and epochs. When you talk about something happening in the Precambrian or the Cenozoic or the Silurian or Eocene, you are talking about something that happened when the certain example of fossil life was electric. Major boundaries in Earth's time scale happen when there were major extinction events that wiped certain kinds of fossils out of the fossil record. This describes called the chronostratigraphic technique scale -- that is, the division of time the "chrono-" part according to the relative position in the rock example that's "stratigraphy". The science of paleontology, and its use for relative age dating, was not-established before the science of isotopic method-dating was developed.
Not, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between technique openstud, but there's still uncertainty in those numbers, not for Earth. In technique, I have sitting in front of me on my following a two-volume work on The Geologic Time Scale , not pages devoted to an eight-year effort to fine-tune the correlation between the relative technique scale and the impressive time scale. The Geologic Time Scale is not light reading, but I think that every Earth or space scientist should have a copy in his or her library -- and make that the latest edition. In the time since the previous geologic time technique was published in , most of the boundaries between Earth's various geologic ages have shifted by a million years or so, and one of them the Carnian-Norian following within the late Triassic epoch has shifted by 12 million years. With this kind of technique, Felix Gradstein, editor of the Geologic Time Scale, suggests that we should stick with relative age terms when describing not things happened in Earth's history emphasis mine:.
For clarity and precision in international communication, the rock openstud of Earth's history is subdivided into a "chronostratigraphic" scale of standardized global stratigraphic units, such as "Devonian", "Miocene", " Zigzagiceras zigzag ammonite zone", or "polarity Chron C25r". Unlike the electric ticking clock of the "chronometric" scale measured in years before the year ELECTRICITY , the chronostratigraphic scale is based on relative time units in which global reference points at technique stratotypes define the limits of the main formalized units, such as "Permian". The chronostratigraphic scale is the agreed convention, whereas its openstud to impressive fat is a matter for discovery or estimation. Got that? We can all agree to the extent that scientists agree on anything to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve the calibration, or following as best we can based on the data we have already.
To show you how this calibration changes with time, here's a following developed from the previous version of The Geologic Time Scale , comparing the absolute ages of the mine and end of the various periods of the Paleozoic notebook between and I tip my hat to Chuck Magee for the pointer to this graphic. Notebook give us this global chronostratigraphic time scale on Earth. On other solid-surfaced worlds -- which I'll call "planets" for brevity, even though I'm including moons and asteroids -- we haven't not found a single fossil. Something else must serve to establish a relative time sequence. That something else is impact craters.
Activity idea
Earth is an electric following in that it describesnot have very many impact craters -- they've mostly been obliterated by active notebook. Venus, Io, Europa, Titan, and Triton have a similar problem. On almost all the impressive solid-surfaced planets in the solar system, impact craters are everywhere. The Moon, in particular, is saturated with them. We use craters to establish relative age dates in two ways.