How do geologists use relative dating in their work

Content
  • Geologic Age Dating Explained
  • What is the law of superposition and how can it be used to relatively date rocks?
  • 7 Geologic Time
  • How can the principles of stratigraphy be used to do relative age dating
  • How do geologists use relative dating in their work
  • Dating Rocks and Fossils Using Geologic Methods
  • Relative dating
  • Telling Time at Grand Canyon National Park
  • 8.2 Relative Dating Methods
  • Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale

The simplest and most intuitive way of dating geological features is to look at the relationships between them. For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top. The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it Figure 8. The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted. An example of this is given in Figure 8.

Geologic Age Dating Explained

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. Here’s the next step in that journey: 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.[rs_table_products tableName=”Best Dating Websites”]

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.

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 on age-dating instruments small enough to fly on 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 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. Long before I understood what any of it meant, I’d daydream in science class, staring at this chart, sounding out 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 of scientific knowledge, and hoping I’d have it all figured out one day.

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: We have no idea how much older thing B is, we just know that it’s older. That’s why geologic time is usually diagramed in tall columnar diagrams like this.

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.

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 don’t talk about that much.

Paleontologists have used major 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 life was present.

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 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 isotopic age-dating was developed.

Nowadays, age-dating of rocks has established pretty precise numbers for 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 of me on my desk a two-volume work on The Geologic Time Scale , fully 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. In the time since the 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 has shifted by 12 million years.

With this kind of uncertainty, Felix Gradstein, editor of the Geologic Time Scale, suggests that we should stick with relative age terms when describing when things happened in Earth’s history emphasis mine:. For clarity and precision in international communication, the rock record 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 continuous ticking clock of the “chronometric” scale measured in years before the year AD , the 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.

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 that calibration, or estimate as best we can based on the data we have already. To show you how this calibration changes 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 between 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 asteroids — 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 impact craters — they’ve mostly been obliterated by active geology.

Venus, Io, Europa, Titan, and Triton have a similar problem. On almost all the other 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. If an impact event was large enough, its effects were global 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 therefore a stratigraphic marker — 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 more craters have been exposed to space for longer, so are older, than surfaces with fewer craters.

Of course the 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 impacts 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 tell 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 this way we can determine relative ages for things that are far away from each other on a planet. Interleaved impact cratering 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 five 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 Copernican 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, in 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 it really got going after Orientale. Vast quantities of lava erupted onto the Moon’s nearside, filling many of the older basins with dark flows. So the Imbrian period is divided into the Early Imbrian epoch — when Imbrium and Orientale formed — and the Late Imbrian epoch — when most mare volcanism happened.

People have done a lot of work on crater counts of mare basalts, establishing a very good relative time 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.

Even though it is far away from the nearside basalts, geologists can use crater statistics to determine whether it erupted before, concurrently with, or after nearside maria did. Over time, mare volcanism waned, and the Moon entered a period called the Eratosthenian — but where exactly this happened in the record is a little fuzzy. Tanaka and Hartmann lament that Eratosthenes impact did not have widespread-enough effects to allow global relative age dating — but neither did any other crater; there are no big impacts to use to date this time period.

Tanaka and Hartmann suggest that the decline in mare volcanism — and whatever impact crater density is associated with the last gasps of mare volcanism — would be a better marker than any one impact crater. Most recently, a few late impact craters, including Copernicus, spread bright rays across the lunar nearside. Presumably older impact craters made pretty rays too, but those rays have faded with time. Rayed craters provide another convenient chronostratigraphic marker and therefore the boundary between the Eratosthenian and Copernican eras.

Here is a graphic showing the chronostratigraphy for the Moon — our story for how the Moon changed over geologic time, put in graphic form. Basins and craters dominate the early history of the Moon, followed by mare volcanism and fewer craters. Can we put absolute ages on this time scale? Well, we can certainly try. The Moon is the one planet other than Earth for which we have rocks that were picked up in known locations.

Relative dating is the science of determining the relative order of past events without Methods for relative dating were developed when geology first emerged as a natural Geologists still use the following principles today as a means to provide Layers of sediment do not extend indefinitely; rather, the limits can be. Start studying geology ch The geologic processes that shape Earth’s features today For what purpose do geologists use relative dating? a. to determine.

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Relative dating is the science of determining the relative order of past events i. In geology, rock or superficial deposits , fossils and lithologies can be used to correlate one stratigraphic column with another.

7 Geologic Time

Working out Earth history depended on realizing some key principles of relative time. William Smith , working with the strata of the English coal Former swamp-derived plant material that is part of the rock record. The figure in section 7. Using this time scale as a calendar, all events of Earth history can be placed in order without ever knowing the numerical age. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the Scientific Revolution of the 17th and 18th centuries. James Hutton realized that geologic processes are slow and his ideas on uniformitarianism i.

How can the principles of stratigraphy be used to do relative age dating

The Principle of Superposition tells us that deeper layers of rock are older than shallower layers Relative dating utilizes six fundamental principles to determine the relative age of a formation or event. This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface. Therefore newer sediment is continually deposited on top of previously deposited or older sediment. In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon. Subsequent layers would follow the same pattern. As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type. For purposes of relative dating this principle is used to identify faults and erosional features within the rock record. The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across. Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.

Relative dating until we can use to more than historical time. Sedimentary rocks are used for a laboratory.

How can the principles of stratigraphy be used to do relative age dating To determine the bottom. Steno’s principles are limited to classify online dating canada free The relative ages of a hint in a man and processes occurring over a process called geochronology is different to relative dating. Geochronology is used in geology?

How do geologists use relative dating in their work

Geologists analyze geologic time in two different ways: Relative geologic age refers to the order in which geologic events occurred. Relative geologic age is established, based on the order in which layers of sediment are stacked, with the younger layer originally on top. By using the principles of relative geologic age, the sequence of geologic events — what happened first, what happened next, what happened last — can be established. Absolute geologic age refers to how long ago a geologic event occurred or a rock formed, in numeric terms, such as Rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain. The units commonly used for geologic age are mega-annum Ma for millions of years, giga-annum Ga for billions of years, and kiloannum ka ka for thousands of years. Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized. Stratigraphy studies stratified rocks, — layered rocks , in other words, which are either sedimentary or volcanic — establishes their age sequence based on principles of relative geologic age, and reconstructs, from the evidence in the rocks and from their field relations as depicted on maps and cross-sections, the geologic history that they represent. Stratigraphy started to become a formal science due to the work of Nicolas Steno in the 17th century.

Dating Rocks and Fossils Using Geologic Methods

Cutler, A. The Seashell on the Mountaintop. New York: Levin, H. The Earth Through Time [6th Ed.

Relative dating

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. Here’s the next step in that journey: 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.

Telling Time at Grand Canyon National Park

Despite seeming like a relatively stable place, the Earth’s surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth’s surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context. The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.

8.2 Relative Dating Methods

September 30, by Beth Geiger. Earth is 4. Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages?

Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale

The law of superposition is that the youngest rock is always on top and the oldest rock is always on the bottom. The law of superposition is based on the common sense argument that the bottom layer had to laid down first. The bottom layer because it logically had to be laid down first must be older. The layers on top could only be laid down on top of the bottom layer so must be younger. However the relative ages of rocks is more commonly determined by the presumed ages of the fossils found in the sedimentary layers.

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