Geologic dating principles
Rocks are composed of particles ranging from microscopic grains to full sized crystals and crystal grains of different kinds of minerals, and containing many different identifiable physical characteristics.It is conceptually important that each rock has an origin in concepts of place, time, and physical and chemical conditions. These changes may be rapid (such as a volcanic explosion) or gradual, taking place over millions or billions of years, and involving movement over great distances, both at the surface or to deep within the Earth's crust below us.Some of the most useful fossils for dating purposes are very small ones.For example, microscopic dinoflagellates have been studied and dated in great detail around the world.This chapter presents a mix of information that is essential (fundamental) to all following chapters.This chapter is an introduction to rocks and minerals, and the rock cycle.
Stone is another common term used to describe rock. Figure 2-2 shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions.Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.The method of reading the order is called stratigraphy (layers of rock are called strata).Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.
However, most rocks we see around us form very slowly in settings that are not visible on the land surface.