FAQ - Radioactive Age-Dating
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 how do scientists use relative dating to study the geologic history of other planets other events in Earth history? Prior to the late 17th century, geologic time was thought to be the same as historical time. Archbishop James Ussher of Armagh, Ireland,added up generations from the Old Testament and determined that Earth formed on October 23, BCE.
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. 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. The principle of Uniformitarianism how do scientists use relative dating to study the geologic history of other planets 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.
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 online dating and chatting sites 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 previously.
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 how do scientists use relative dating to study the geologic history of other planets 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. 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. 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. 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. 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 dating in nl or evolved throughout Earth history.