Geology is the study of the earth's history (i.e. changes, and processes) and the life that existed, and still does exist, on the earth today. Geology can be divided into several smaller topics. Petrology, for instance, describes different stones and studies their origination. Geomorphology studies the changes of the earth's surface. Structural and tectonic geology studies the transformation of stones and the origination of mountains. Seismology studies earthquakes and volcanology studies volcanoes. The study of geology developed significantly during the 20th century, resulting in many of these smaller and more specific sciences.
Historic geology attempts to describe and define the earth's development. Fossils are one of many ways to reach this goal. The earth's development can be ranked on a timescale. There are several methods to study the developments. The first method is stratification, which is the study of the presence of fossils in the earth's layers. Another method is radiometric age determination. This method is necessary because some layers do not contain fossils. Stones from these layers are from a period when there was no life on earth. The study of the radioactivity of stones is another method by which their ages can estimated. This method is very useful for stones greater than 570 million years old.