Time and the earth: relative time, radiometric dating, absolute time, the age of the earth. Early attempts to work out age of earth: charles darwin (19th c, recognised that fossils in the rock record represent the evolutionary sequence of life forms. So evolution requires an enormous interval of time. A relative geological time scale: developed by the end of the 19th century, based on, superposition relationships between different rock successions with different fossil assemblages, evolutionary succession of life forms, major extinction events. Igneous and sedimentary rocks that are heated and subject to pressure during major tectonic deformational events chemical reactions cause growth of new metamorphic minerals. Significance of dating: age of new mineral formation = age of change in rock, not age of original rock. Applications of radiometric dating: dating of lavas associated with fossil bearing sedimentary rocks, = absolute ages to boundaries of geological time periods on the relative time scale.