Читать книгу Geology For Dummies - Alecia M. Spooner - Страница 23

Scientists use two approaches to determine the age of rocks and rock layers: relative dating and absolute dating.

Relative dating provides ages of rock layers in relation to one another — for example, stating that one layer is older or younger than another is. The study of rock layers, or strata, is called stratigraphy. In methods of relative dating, geologists apply principles of stratigraphy such as these:

 Rock layers below are generally older than rock layers above.

 All sedimentary rock layers are originally formed in a horizontal position.

 When a different rock is cutting through layers of rock, the cross-cutting rock is younger than the layers it cuts through.

These principles and a few others that I describe in Chapter 16 guide geologists called stratigraphers in interpreting the order of rock layers so that they can form a relative order of events in Earth’s history.

However, sometimes simply knowing that something is older than — or younger than — something else is not enough to answer the question being asked. Absolute dating methods use radioactive atoms called isotopes to determine the age in numerical years of some rocks and rock layers. Absolute dating methods may determine, for example, that certain rocks are 2.6 million years old. These methods are based on the knowledge, learned from laboratory experiments, that some atoms transform into different atoms at a set rate over time. By measuring these rates of change in a lab, scientists can then measure the amount of the different atoms in a rock and provide a fairly accurate age for its formation.

If the process of obtaining absolute dates from isotopes seems very complex, don’t worry: In Chapter 16, I explain in much more detail how absolute dates are calculated and how they are combined with relative dates to construct the geologic timescale: a sequence of Earth’s geological history separated into different spans of time (such as periods, epochs, and eons).