These exceptional lengths of time seem unbelievable, but they are exactly the spans of times that scientists use to describe the Earth. Have places like the Grand Canyon and the Mississippi River been around for all of those years, or were they formed more recently? When did the giant Rocky Mountains form and when did dinosaurs walk the Earth? To answer these questions, you have to think about times that were millions or billions of years ago.
Historical geologists are scientists who study the Earth's past. They study clues left on the Earth to learn two main things: For example, they have learned that the Mississippi River formed many millions of years after the Grand Canyon began forming. They have also concluded that dinosaurs lived on the Earth for about million years. Some examples of events listed on the geologic time scale include the first appearance of plant life on Earth, the first appearance of animals on Earth, the formation of Earth's mountains, and the extinction of the dinosaurs.
You will learn about some of the scientific principles that historical geologists use to describe Earth's past. You will also learn some of the clues that scientists use to learn about the past and shows you what the geologic time scale looks like. Before you work through this lesson, think about the following questions. Be sure that you can answer each one.
They will help you better understand this lesson. The first principle you need to understand about geologic time is that the laws of nature are always the same. This means that the laws describing how things work are the same today as they were billions of years ago. This law has always been true and always will be true. Knowing the natural laws helps you think about Earth's past, because it gives you clues about how things happened very long ago.
It means that we can use present-day processes to interpret the past. Imagine you find fossils of sea animals in a rock. The laws of nature say that sea animals must live in the sea. That law has never changed, so the rock must have formed near the sea. The rock may be millions of years old, but the fossils in it are a clue for us today about how it formed.
Now imagine that you find that same rock with fossils of a sea animal in a place that is very dry and nowhere near the sea. How could that be? Remember that the laws of nature never change. Therefore, the fossil means that the rock definitely formed by the sea. This tells you that even though the area is now dry, it must have once been underwater. Clues like this have helped scientists learn that Earth's surface features have changed many times.
Spots that were once covered by warm seas may now be cool and dry. Places that now have tall mountains may have once been low, flat ground. These kinds of changes take place over many millions of years, but they are still slowly going on today. The place where you live right now may look very different in the far future. 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. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating , archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique.
Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The regular order of occurrence of fossils in rock layers was discovered around by William Smith. While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor , he found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England.
Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Methods for relative dating were developed when geology first emerged as a natural science in the 18th century.
Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. The principle of intrusive relationships concerns crosscutting intrusions.
In geology, when an igneous intrusion cuts across a formation of sedimentary rock , it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths , batholiths , sills and dikes. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. The principle of inclusions and components states that, with sedimentary rocks, if inclusions or clasts are found in a formation, then the inclusions must be older than the formation that contains them.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
FAQ - Radioactive Age-Dating
Determining the age of surfaces on Mars
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