We often express time in hours or days, and 10 or 20 years certainly feels like a long time. Imagine if you needed to think about one million, million, or even several billion years.
The Geologic Time Scale A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research. In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place.
There are absolute ages and there are relative ages. People love absolute ages. An absolute age is a number. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4.
We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.
So that leaves us with relative ages.
Relative ages are not numbers. Earth history relative time and absolute are descriptions of how one rock or event is older or younger than another. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets.
Relative-age time periods are what make up the Geologic Time Scale. The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts. This all has to do with describing how long ago something happened.
But how do we figure out when something happened? There are several ways we figure out relative ages. The simplest is the law of superposition: 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 sequences certain fossils appear and disappear. When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer. Of course, this only works for rocks that contain abundant fossils.
Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here. 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 a certain kind of fossil life was present.
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed. In fact, I have sitting in front of me on my desk a two-volume work on The Geologic Time Scalefully pages devoted to an eight-year effort to fine-tune the correlation between the relative time scale and the absolute 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. Unlike the continuous ticking clock of the "chronometric" scale measured in years before the year ADthe chronostratigraphic scale is based on relative time units in which global reference points at boundary stratotypes define the limits of the main formalized units, such as "Permian".
The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation.
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 that calibration, or estimate as best we can based on the data we have already.Many authors choose to present the history of a complex subject by breaking it up into major threads and following the history of each thread separately.
How many years is a "long time"? We often express time in hours or days, and 10 or 20 years certainly feels like a long time. Imagine if you needed to think about one million, million, or even several billion years.
The Earth is billion years old.
Have places like the Grand Canyon and the. Earth is the third planet from the Sun and the only astronomical object known to harbor srmvision.coming to radiometric dating and other sources of evidence, Earth formed over billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural srmvision.com revolves around the Sun in days, a period known as an Earth year.
21st Century Earth History. Introduction. When Gene Roddenberry conceived Star Trek in the s, there was unanimous enthusiasm about space travel and other evolving technologies, as well as almost undisputed optimism concerning future achievements.
Both relative and absolute time are important ways we describe events in Earth's history. Absolute time, also called chronometric time, gives us distinct measurements and points of reference. Figure 2. How relative dating of events and radiometric (numeric) dates are combined to produce a calibrated geological time scale.
In this example, the data demonstrates that "fossil B time" was somewhere between and million years ago, and that "fossil A time.