5. Geologic TIme on the Web - Background

Teacher Background
This lesson gradually leads students to a basic understanding of how the geologic timescale is organized and how it was created from evidence in rocks. See background information about Nicolas Steno (from the Layers Upon Layers lesson) and William Smith (from the My Time lesson) for preliminary background on arranging earth history by relative time. Below is a brief overview of determining the absolute age of rocks by radiometric dating. For a deeper understanding of the geologic time scale itself, see the Geologic Timelines lesson.

A great advance in the field of geology came in the form of the mass spectrometer, a device that measure the radioactive decay of elements. Simply put, each element in the periodic table contains the same number of protons but vary in the number of neutrons and thus can vary in their atomic weight. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Over time, an unstable parent isotope will spontaneously eject parts of its nucleus and transform into a far more stable daughter isotope.

Geologists can use this to precisely determine the age of a rock layer, a method called radiometric dating. When a layer of igneous rock is laid down from a volcanic eruption, all the atoms begin as the parent isotope. Each parent isotope has a specific rate of decay that can be precisely timed. Some parent isotopes, like uranium-238 take an extremely long time to decay to the daughter isotope lead-206 (4.5 billion years for half of the uranium-238 in a given rock to decay to lead-206). Others have a very short decay rate or half-life (5,700 years for carbon-14 to decay to carbon-12). The ratio of parent to daughter element tells scientists precisely how old a given rock is.

Student Prerequisites
No experience is required although familiarity with determining the relative age of rock layers through the law of superposition (see Layers Upon Layers lesson) and with the general organization of the geologic time scale (see My Time lesson) is helpful.