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Relative dating is used to determine the relative order of past events by comparing the age of one object to another. This determines where in a timescale the object fits without finding its specific age; for example you could say you're older than your sister which tells us the order of your birth but we don't know what age either of you are. There are a few methods of relative dating, one of these methods is by studying the stratigraphy. Stratigraphy is the study of the order of the layers of rocks and where they fit in the geological timescale.


Geologic Time. From the beginning of this course, we have stated that the Earth is about 4. How do we know this and how do we know the ages of other events in Earth history?

Prior to the late 17th century, geologic time was thought to be the same as historical time. The goal of this lecture is come to come to a scientific understanding of geologic time and the age of the Earth. In order to do so we will have to understand the following:. In order to understand how scientists deal with time we first need to understand the concepts of relative age and numeric age. To better understand these concepts, let's look at an archeological example: Imagine we are a group of archeologists studying two different trash pits recently discovered on the Tulane University campus and at the Audubon Zoo where they all aksed for you.

By carefully digging, we have found that each trash pit shows a sequence of layers. Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits. Notice that at this point we do not know exactly how old any layer really is. Thus we do not know the numeric age of any given layer. In geology, we use similar principles to determine relative ages, correlations, and numeric ages. Relative ages - Principles of Stratigraphy Correlations - Fossils, key beds, lithologic similarity Numeric ages - Radiometric dating.

Stratigraphy is the study of strata sedimentary layers in the Earth's crust.

Geologist in the s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks. Once these age relations were worked out, another principle fell into place - the principle of fossil succession. We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.

Overview of relative and absolute dating

The principle of Uniformitarianism was postulated by James Hutton who examined rocks in Scotland and noted that features like mudcracks, ripple marks, graded bedding, etc. He concluded that process that are currently operating on the Earth must be the same processes that operated in the past. This principle is often stated as "the present is the key to the past". A more modern way of stating the same principle is that the laws of nature as outlined by the laws of chemistry and physics have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.

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Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers. Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layer will be on the bottom and the youngest layer on top. This is the same principle used to determine relative age in the trash pits discussed ly.

In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins. Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface. Sediment deposited on steep slopes will be washed away before it is buried and lithified to become sedimentary rock, but sediment deposited in nearly horizontal layers can be buried and lithified.

Thus rocks that we now see inclined or folded have been disturbed since their original deposition. If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance. Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon. Look at the many photographs of the Grand Canyon in your textbook.

Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon. The Grand Canyon is particularly good for this because different sedimentary rocks have different colors. Principle of Cross-cutting Relations. Younger features truncate cut across older features. Faults, dikes, erosion, etc. Similarly, the rhyolite dike cuts only the mudstone and the sandstone, but does not cut across the shale.

How do geologists know how old a rock is?

Thus, we can deduce that the mudstone and shale are older than the rhyolite dike. But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike. Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion. If the enclosing rock is an igneous rock, the inclusions are called xenoliths.

In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock. Similarly, the overlying rhyolite flow contains inclusions of the basalt, so we know that the basalt is older than the rhyolite.

Dating rocks and fossils using geologic methods

This principle is often useful for distinguishing between a lava flow and a sill. Recall that a sill is intruded between existing layers. In the case shown here, we know that the basalt is a sill because it contains inclusions of both the underlying rhyolite and the overlying sandstone. This also tells us that the sill is younger than the both the rhyolite and the sandstone. When magma comes in contact with soil or cold rock, it may cause the soil or rock to heat up resulting in a baked zone in the surrounding rock near the contacts with the igneous rock.

Such margins indicate that the igneous rock is younger that the soil or rock that was baked. Application of the Principles of Stratigraphy. Figure Although we will go over this in lecture, you should study the methods and reasoning used so that you could determine the geologic history of any sequence of rocks. Once geologists had worked the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age.

It was soon recognized that some fossils of once living organisms only occurred in very old rocks and others only occurred in younger rocks. Furthermore, some fossils were only found within a limited range of strata and these fossils, because they were so characteristic of relative age were termed index fossils.

With this new information, in combination with the other principles of stratigraphy, geologists we able to recognize how life had changed or evolved throughout Earth history. This recognition led them to the principle of fossil succession, which basically says that there is a succession of fossils that relate to the age of the rock.

Because the Earth's crust is continually changing, i. When sediment is not being deposited, or when erosion is removing ly deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks. We call such a break in the stratigraphic record a hiatus a hiatus was identified in our trash pit example by the non-occurrence of the Ceramic Cups layer at the Zoo site.

When we find evidence of a hiatus in the stratigraphic record we call it an unconformity. An unconformity is a surface of erosion or non-deposition. Three types of unconformities are recognized. Angular Unconformity. Because of the Principles of Stratigraphy, if we see a cross section like this in a road cut or canyon wall where we can recognize an angular unconformity, then we know the geologic sequence of events that must have occurred in the area to produce the angular unconformity.

7 geologic time

Angular unconformities are easy to recognize in the field because of the angular relationship of layers that were originally deposited horizontally. Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface.

The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion. Disconformities are much harder to recognize in the field, because often there is no angular relationship between sets of layers. Disconformity are usually recognized by correlating from one area to another and finding that some strata is missing in one of the areas. The unconformity recognized in the Zoo trash pit is a disconformity.

How old are rocks?

Disconformities can also be recognized if features that indicate a pause in deposition, like paleosols ancient soil horizonsor erosion, like stream channels are present. A Formation is a a rock or group of rocks that differ from rocks that occur above or below and have distinctive characteristics and fossils such that the rocks can be recognized over wide areas.

Formations are given a formal name, normally a geographic locality. If it is a group of rocks, for example, interbedded sandstones and shales, then it might be called something like the Toroweap Formation. If it is a single rock type, then only the rock name is specified in the formation name, for example the Kaibab Limestone.

If several formations can be grouped together as a distinctive set of formations, this called a Group. For example the Supai Group. Geologist often make a graphic to display stratigraphic information in an understandable way.

Such a graphic, as shown above is a called a stratigraphic column. The column shows the relative thicknesses of each Formation or Group, the Formation Name, and gives an approximate idea of whether the rocks are hard- cliff forming units or softer more easily erodable units.

People often say that rocks exposed in the Grand Canyon offer a complete record of geologic history, however this is incorrect. Note that there are several unconformities in the Grand Canyon Stratigraphic Column that represent gaps in the record.

For example the Nonconformity near the bottom represent a gap of about 1. Nowhere on Earth is there a complete section that shows strata deposited over the entire history of the Earth. In the past, some areas were above sea level and being eroded and other areas were below sea level where deposition was occurring. Thus, in order to develop a complete record, correlations must be undertaken in order to see how everything fits together.

Over the past years detailed studies of rocks throughout the world based on stratigraphic correlation have allowed geologists to correlate rock units and break them into time units. The result is the geologic column on nextwhich breaks relative geologic time into units of known relative age. Note that the geologic column was established and fairly well known before geologists had a means of determining numeric ages.

Thus, in the geologic column shown below, the numeric ages in the far right-hand column were not known until recently. The Eons are divided into Eras only Phanerozoic Eras are shown in the chart. These include, from oldest to youngest:. The Eras are divided into Periods. The Periods are often named after specific localities. Further subdivisions of Periods are called Epochs. Only Epochs of the Cenozoic Era are shown in the Chart. Note that for this course, you need to know the Eons, Eras, and Periods in age order. You will not be asked about the Epochs at least for now.

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Adapted by Sean W.


Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate.


Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.