Exploration geophysics. An introduction / Разведочная геофизика. Введение

The exact age of the earth is not known, but it is thought to be at least 4.5 billion years old Rocks and fossils (the remains of plants and animals preserved in the rocks) can be dated by measuring the decay rate of radioactive material that they contain. The number of radioactive particles given off by a substance during a certain time period provides a surprisingly accurate estimate of the age of the substance. The geologic past is measured by means of a geologic time chart. Each interval of time has been given a name so that a particular time in the past can be referred to more easily. Periods in history are referred to in terms such as “the ice age” “the iron age” and “the atomic age”. These time periods are measured in centuries or millennia at most. Intervals of geologic time, by contrast are measured in millions of years. For example, the dinosaurs became extinct about 70 million years ago. Another way to express it is “dinosaurs died at the end of the Cretaceous period.” <...>