We study our Earth for many reasons: to find water to drink, oil to run our cars, coal to heat our homes, to know where earthquakes will occur, to predict landslides or floods, and to understand our natural surroundings. Geology may be divided into physical geology and historical geology. Physical geology deals with the materials of the Earth, crust movements, the structure of the Earth, and various processes and agencies, such as weather, wind, streams, glaciers, the sea, organisms, volcanoes, earthquakes, and subterranean water. Historical geology is the branch of geologic science that relates to the past history of the Earth, as Earth is not stable and nothing on its surface is permanent. To understand the world we live in, we study Earth's history by examining the record of past events preserved in rocks, the layers of the rocks being the pages in our history book.
Rock layers are also called strata, and the science that treats these layered rocks is stratigraphy. To tell the age of most layered rocks, scientists study the fossils these rocks contain. Fossils are the remains of once-living organisms. The science that studies fossil remains is paleontology, and fossils help scientists compare life in different environments such as oceans and land. Scientists must study fossils from a variety of environments to build a complete picture of the animals and plants that lived in the past.
Today the animals and plants that live in the ocean are very different from those that live on land, and the animals and plants that live in one part of the ocean or on one part of the land are very different from those in other parts. Similarly, fossils, animals, and plants from different environments are different. Scientists must study fossils from a variety of environments to build a complete picture of the animals and plants that lived in the past. The science that studies fossil remains is paleontology. This study, together with stratigraphy, helps us understand Earth's history.