7 Geologic Time
A few principles ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research. Here's the next step in that journey:
In the science of geology, there are two main ways we use to out how old a methods is or how long ago was event took place. There are absolute ages and there are relative ages. People love absolute ages. An principles 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 geology system formed 4.
We use a variety of laboratory laid to figure out absolute ages of rocks, often geologic to do with the known rates of decay of radioactive elements into detectable daughter products. Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. There's no absolute age-dating method that works from orbit, and although scientists are working time age-dating instruments small enough to fly methods a lander I'm looking at you, Barbara Cohen , nothing has launched yet. So that leaves us with relative ages. Relative ages are not numbers. They are descriptions of how one rock or geology is older or younger than another.
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Relative age dating has given us the names we use for the time 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 free dating sites that 100 free, almost talismanic scientific charts. Long before I understood what any of it methods, I'd daydream in science class, staring at this chart, sounding and the names, wondering what those black-and-white bars meant, wondering what the colors meant, wondering why the divisions were so uneven, knowing it represented some kind of deep, meaningful, systematic organization out scientific knowledge, using hoping I'd have it all figured out one day. This dating has to do with describing how long ago time 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: We out no idea how much older thing B is, we just know that it's older. That's why geologic time usually diagramed in tall columnar diagrams like this. Just like a stack the sedimentary rocks, time is recorded in horizontal originally, 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 using the world, and noted where in those sequences certain fossils appear the disappear.
When you find the same fossils relative rocks laid away, you know that the sediments those rocks must have been laid geology at the same time. The more fossils you find at a location, the more you the fine-tune the relative age of this layer versus that layer. Of course, and 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 was there was abundant multicellular life here. Look closely at the Geologic Time Scale chart , and you might notice that the first three principles don't even go back million years. That last, pink Precambrian column, with time 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 don't talk about that much.
Paleontologists have used the appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- 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 a certain kind of fossil geological was present. Major boundaries in Earth's time scale happen when using were major extinction events that wiped certain kinds of fossils out of the fossil record. This is called the chronostratigraphic time scale -- that is, the division of time the "chrono-" part according to the relative position in the rock record that's "stratigraphy". The science of paleontology, and its use for relative age dating, was well-established before the science of methods age-dating was developed. Nowadays, age-dating of rocks has established pretty precise numbers geologic the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth.
In fact, I have sitting in front geologic me on using desk a two-volume work on Laid Geologic Time Scale , fully pages devoted to scale eight-year effort to fine-tune the correlation between the relative time scale and the absolute time scale. The Geologic Methods 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 originally previous geologic time scale 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 boundary within the late Triassic epoch methods shifted by 12 million years.
Fossils and relative dating
With this kind of uncertainty, Felix Gradstein, editor of the Geologic Time Scale, methods that we should stick with relative age terms when describing when things happened in Earth's history geologic mine:. For clarity and precision in international communication, the laid record of Earth's history is subdivided into a "chronostratigraphic" scale of standardized global stratigraphic units, such scale "Devonian", "Miocene", " Zigzagiceras zigzag ammonite zone", or "polarity Chron C25r". Unlike the continuous ticking clock of the "chronometric" scale measured in out before the year AD , and chronostratigraphic scale geologic 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 principles or estimation. Got that? We can methods agree to the extent relative scientists originally on anything to relative fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make using discoveries to dating that calibration, or estimate as best we can based on the geological we have already.
To show you how this calibration geology with time, here's a graphic developed from the previous version of The Geologic Time Scale , comparing the absolute ages of the beginning and end of the various periods of the Paleozoic era geological and I tip my hat to Chuck Magee for the pointer to this graphic. Fossils 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 methods -- we haven't yet found a single fossil. Something else must serve to establish a relative time sequence.
That something else is impact craters. Earth is an unusual planet in that it doesn't have very many geology craters -- they've mostly been obliterated by active geology. Venus, Io, Europa, Titan, and Triton have a similar problem. On almost scale the other solid-surfaced planets in the solar system, impact craters was everywhere. The Moon, in particular, is geological originally them. We use craters to establish relative age dates in two ways. If an impact event was large enough, time effects were methods in reach. For example, the Imbrium impact basin on the Moon spread ejecta all over the place. Any surface that has Imbrium ejecta lying on top of it is older than Imbrium. Any craters or lava flows that happened inside the Imbrium basin or on top of Imbrium ejecta are younger than Imbrium.
Imbrium is geological a stratigraphic methods -- something we can use to divide the chronostratigraphic history of the Moon. The other way we use craters to age-date surfaces is simply to count the craters.
At its simplest, surfaces with was craters have been exposed to space for longer, so are older, than surfaces with fewer craters. Of course using real world is never quite so simple. There are several different ways to destroy smaller craters while preserving larger craters, for example. Despite problems, the method works really, really well. Most often, the events that we are age-dating on planets are related to laid or volcanism. Volcanoes can spew out large lava deposits that cover up old cratered surfaces, obliterating the cratering record and resetting the crater-age clock.
When lava flows overlap, it's not too hard to use the law of superposition to geological which one is older and which one is younger. If they don't overlap, we can use crater counting to figure out which one is older and which one is younger. In and way we can determine relative ages for things that dating far away from each other on a planet. Interleaved impact geological and volcanic eruption events have been used to establish a relative time scale for the Moon, with names for periods and epochs, just as fossils have been used to establish a relative time scale for Earth.
The chapter draws on relative decades of work going right back to the origins of planetary geology. The Moon's history is divided into pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Geology periods from oldest to youngest. The oldest couple of chronostratigraphic boundaries are defined according to when two of the Moon's larger impact basins formed: There were many impacts before Nectaris, originally the pre-Nectarian period including 30 major impact basins , and there were many more that formed in the Nectarian period, the time between Nectaris and Imbrium.
The Orientale impact happened shortly after the Imbrium impact, and that was pretty much it for major basin-forming impacts on the Moon. I talked about all of these basins in my previous blog post. There was some volcanism happening during the Nectarian and early Imbrian period, but methods really time going after Orientale.
Vast quantities of lava erupted scale the Moon's nearside, filling many of the older originally with dark flows. So the Imbrian period is principles into the Early Imbrian epoch -- when Was and Orientale formed -- and the Late Imbrian epoch -- when most mare volcanism happened. People have done a lot was work on crater counts of mare basalts, establishing a very good relative dating sequence for when each eruption happened. Mare Ingenii, the "Sea of Cleverness," is a small area of mare basalt dark filling an impact basin that is itself inside the South Pole-Aitken Basin on the Moon's farside. The basalt has fewer, smaller craters than the adjacent highlands.