Over time an infant develops and changes into an adult. Photographs and growth charts from pediatrician visits can provide evidence that document the transformations of an infant into an adult. Photo albums or baby books are also a great way to show the sequence of events in a person’s life. The Earth has been through some major changes too. The landscape, atmosphere, types of animals and plants are different from those that existed millions of years ago. Geologists study the history and structure of the Earth. What methods are used by geologists to learn about the history of the Earth? What is a geologic time scale? In this lesson, you will learn about how scientists have developed and organized a record of Earth’s history.

In the past, a massive landmass of all the continents called Pangaea existed. The fossil remains of the same plants and animals in Africa and South America suggest that these two continents were once joined together. This suggests that these animals lived during different time periods. Scientists use the principles of relative-age dating to determine the relative age of rocks. These principles are original horizontality, superposition, cross cutting relationships, and inclusions. The principle of superposition states that older rocks are located beneath the layers of younger rocks. Radioactive isotopes are used to determine the numerical age of rocks. Using the information of relative-age dating, absolute-age dating, and fossil records, geologists were able to develop a geologic time scale to divide Earth’s history into time intervals.

Relative Age Dating

Some geologists believe the Earth is about 4.6 billion years old based on observations of rocks. In 1869, geologist John Wesley Powell led his expedition down the Colorado River. He noticed the different colors of the sedimentary rocks that lined the Grand Canyon. The different layers of sedimentary rocks provided a rock record of Earth’s past. A rock record provides evidence of geologic events and processes. The same processes that are affecting the Earth now have also occurred in the past. Geologists hypothesized that the Colorado River began to flow through the Grand Canyon millions of years ago. The same movement of water that takes place now along the Colorado River also happened millions of years ago. Please watch the Extreme Erosion video to learn more about John Wesley Powell’s expedition into the Grand Canyon and the geologic processes that created the Grand Canyon. The link to the Extreme Erosion video is located on the sidebar

Before Powell made his observations in the Grand Canyon, a Scottish physician, James Hutton, was recording the geological transformations taking place on his farm in the 1700s. Hutton’s principle of uniformitarianism states that the geologic processes that are happening now also occurred in the past. Hutton believed that the same forces and processes that were changing the landscape on his farm had also changed the surface of the Earth in the past.   Geologists modified Hutton’s principle of uniformitarianism by emphasizing that the geological processes occurred at various rates over time. Therefore the geological processes in the past and present may be the same but the rate at which they occur may be different.

Scientists can determine the order in which geologic events occurred by using a method called relative-age dating. It involves studying and examining the layers of rocks. Relative age dating is sequential. Geologists use relative age dating to identify when rocks formed based on the location of their layers. Therefore they can put when the rocks formed in chronological order. Geologists can determine the sequential order of geologic events; but not exactly when the events happened. There are several principles used for relative dating. They are the law of superposition, the principle of original horizontality, the principle of cross-cutting relationships, and the principle of inclusion.

The law of superposition states that the oldest layers of sedimentary rock are found at the bottom or below the layers of successive younger layers of rocks. Sedimentary rocks are used to study relative age dating because they form boundaries between each rock layer. The picture to the left is an example of the law of superposition. According to the picture, the darkest rock layer is older than the rocks above it.

The principle of original horizontality states that sedimentary rocks are deposited in horizontal layers. The layers of the rocks will appear flat in their original horizontal position if they are undisturbed. Movement due to plate tectonics can deform or alter these horizontal layers. The age of the rocks can be difficult to determine because the layers have been disturbed by faults or intrusions. A fault is a break in the rocks that make up the Earth’s crust, usually along transform plate boundaries. Earthquakes that occur along the San Andreas Fault in southern California are examples of how rocks can change their original position because of their location along a fault. An intrusion forms when magma cools and solidifies into igneous rocks. During the process, new igneous rock cuts through layers of existing rocks. The principle of cross-cutting relationships occurs when faults or igneous rocks from magma cut through other rocks that are younger.

The principle of inclusion states that fragments of a rock are older than the rock layer that contains the fragments. Inclusions are fragments of rocks that are located in another type of rock. For example, pieces of igneous rock can be found in sedimentary rock. The sedimentary rock would be younger than the igneous rock. Weathering and erosion contributed to the formation of sedimentary rock layers with inclusions of igneous rock.

Gaps in the geologic record can occur when time is missing or out of sequence. A discrepancy in the geologic rock record is called an unconformity. Unconformities are caused by changes in the Earth’s surface due to factors such as weathering, erosion, earthquakes, and volcanoes.  The changes can not only remove layers of rock but also disrupt the sequential order of the layers of rocks.  The three major types of unconformities are nonconformity, angular unconformity, and disconformity.

The picture above is an example of angular unconformity. This type of unconformity is associated with mountain building. Angular unconformity occurs when a layer of horizontal sedimentary rock develops over a layer of eroded and tilted sedimentary rock. The eroded and tilted layer between the two horizontal rock layers is called an angular unconformity. Disconformity is very similar to angular unconformity in the manner in which layers of sedimentary rocks form above each other.  Unlike angular unconformity, the eroded layer of rock does not tilt. The very difficult to identify erosional layer between the two horizontal layers of sedimentary rocks is called a disconformity.  Nonconformity is the eroded surface that separates younger sedimentary rock from either metamorphic or igneous rock.  The eroded surface is easy to identify.

 

 

 

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