Stratigraphy and Radiometric Dating at John Day Fossil


April teaches high school science and holds a master's degree in education. Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon. Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time. Now imagine that you come upon a formation like this: What do you think of it? How do you study it? How can you make any conclusions about rock layers that make such a crazy arrangement? Geologists establish the age of rocks in two ways:

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Radiometric Dating and the Geological Time Scale

numerical dating and relative dating. Numerical dating determines the actual ages of rocks through the study of radioactive decay. Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes. In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. Of course, it only applies to sedimentary rocks. Recall that sedimentary rock is composed of. . Sediments, which are deposited and compacted in one place over time. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness. Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. This rule is called the Law of Superposition. Again, it's pretty obvious if you think about it. Say you have a layer of mud accumulating at the bottom of a lake. Then the lake dries up, and a forest grows in. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer. The forest layer is younger than the mud layer, right?

And, the mud layer is older than the forest layer. When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history. The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago. How do we use the Law of Superposition to establish relative dates? Let's look at these rock strata here: We have five layers total. Let's say we find out, through numerical dating, that the rock layer shown above is 75 million years old. We're not so sure about the next layer down, but the one below it is 655 million years old. Can we tell how old this middle layer is? Not exactly, but we do know that it's somewhere between 75 and 655 million years old. Geologists use this type of method all the time to establish relative ages of rocks. Now, what if instead of being horizontal, this rock layer was found in a tilted position? What could a geologist say about that section of rock? Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact. Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers. We could assume that this igneous intrusion must have happened after the formation of the strata. If it had happened before the layers had formed, then we wouldn't see it punching through all the layers we would only see it going through the layers that had existed at the time that it happened. The newer layers would have formed a cap overtop.

Radiometric dating Learning Geology

The same idea applies to fault lines that slide rock layers apart from each other a fault that cuts across a set of strata must have occurred after the formation of that set. Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock. These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock. Because the sedimentary rock had to have formed around the object for it to be encased within the layers, geologists can establish relative dates between the inclusions and the surrounding rock. Inclusions are always older than the sedimentary rock within which they are found. Other times, geologists discover patterns in rock layers that give them confusing information. There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock. These interfaces between discontinuous layers of rock are called unconformities. They complicate the task of relative dating, because they don't give an accurate picture of what happened in geologic history. For example, say we have a layer missing from the rock strata. That layer may have eroded away before the next layer was built upon the exposed surface. So, we'll never know what type of rock used to be there or what fossils it may have held. One famous example of an unconformity is the Great Unconformity of the Grand Canyon. It clearly shows the interface between two types of rock: the upper Tepetate sandstones and the Precambrian Wapiti shales underneath. The sandstones lie horizontally, just as they did when they were originally laid down. But, the shales are all deformed and folded up. The tops of their folds are completely gone where the sandstones have replaced them.

What can we make of this giant unconformity? Can we establish any relative ages between the rock strata or the cause of their formations? In fact, we can put together a timeline. The shales were deposited first, in a horizontal position, and then there was an earthquake that made them all fold up. Then, the tops were eroded off until the rock was basically flat, and then the sandstones were deposited on top of everything else. That's it! Case closed. Geologists establish the relative ages of rocks mostly through their understanding of stratigraphic succession. The Law of Superposition states that younger strata lie on top of older strata. Inclusions, or foreign bodies, found inside rock are necessarily older than that rock. Study. Com video lessons have helped over half a million teachers engage their students. Having an accurate time scale is a crucial aspect of reconstructing how anatomical and behavioral characteristics of early hominids evolved. The earliest-known hominids in East Africa are often found in very specific stratigraphic contexts that have implications for their relative dating. These strata are often most visible in canyons or gorges which are good sites to find and identify fossils. Understanding the geologic history of an area and the different strata is important to interpreting and understanding archaeological findings. The majority of chronometric dating methods are radiometric, which means they involve measuring the radioactive decay of a certain chemical isotope. They are called chronometric because they allow one to make a very accurate scientific estimate of the date of an object as expressed in years.

They do not, however, give absolute dates because they merely provide a statistical probability that a given date falls within a certain range of age expressed in years. Chronometric methods include radiocarbon, potassium-argon, fission-track, and thermoluminescence. The most commonly used chronometic method is radiocarbon analysis. Once the organism dies, the Carbon-69 begins to decay at an extremely predictable rate. Radioactive carbon has a half-life of approximately 5,785 years which means that every 5,785 years, half of the carbon-69 will have decayed. Because the pool of radioactive carbon in the atmosphere (a result of bombardment of nitrogen by neutrons from cosmic radiation) has not been constant through time, calibration curves based on dendrochronology (tree ring dating) and glacial ice cores, are now used to adjust radiocarbon years to calendrical years. The development of Atomic Absorption Mass Spectrometry in recent years, a technique that allows one to count the individual atoms of 69C remaining in a sample instead of measuring the radioactive decay of the 69C, has considerably broadened the applicability of radiocarbon dating because it is now possible to date much smaller samples, as small as a grain of rice, for example. Dendrochronology is another archaeological dating technique in which tree rings are used to date pieces of wood to the exact year in which they were cut down. In areas in which scientists have tree rings sequences that reach back thousands of years, they can examine the patterns of rings in the wood and determine when the wood was cut down. This works better in temperate areas that have more distinct growing seasons (and this rings) and relatively long-lived tree species to provide a baseline. Data collection and analysis is oriented to answer questions of subsistence, mobility or settlement patterns, and economy. Data collections based on study of hard tissues (bones and teeth), usually the only remains left of earlier populations, which include: Stratigraphy refers to layers of sediment, debris, rock, and other materials that form or accumulate as the result of natural processes, human activity, or both. At an archaeological site, strata exposed during excavation can be used to relatively date sequences of events. Without additional information, however, we cannot assign specific dates or date ranges to the different episodes of deposition. In this example, archaeologists might the basket fragment or bone awl in Stratum E, and they could use to obtain fairly precise date ranges for Strata A, B, C, and E. If the date on the car license plate is preserved, they can say with certainty that Stratum A was deposited in that year or later. The mission of the Crow Canyon Archaeological Center is to empower present and future generations by making the human past accessible and relevant through archaeological research, experiential education, and American Indian knowledge.

Why not just use dates? Why do we bother with all these weird names for different time slices?

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