Quick questions on Fossils and geological time: HSC Earth and Environmental Science Module 5

Relative dating orders events without giving a number, using a set of geological principles. The principle of superposition states that in an undisturbed sequence the oldest layer lies at the bottom. The principle of original horizontality states that sediments are deposited in flat layers, so tilted beds were disturbed after deposition. Cross-cutting relationships state that a feature, such as a fault or igneous intrusion, is younger than the rock it cuts.

An index fossil is most useful for correlation when the organism was widespread, abundant, easy to identify and lived for only a short span of geological time. Such fossils let geologists match rock layers between distant locations and assign them to the same time interval. Trilobites and graptolites are classic index fossils for the Palaeozoic.

Absolute dating uses radioactive decay to give a numerical age. A radioactive parent isotope decays to a stable daughter isotope at a constant rate measured by its half-life, the time for half the parent atoms to decay. By measuring the ratio of parent to daughter in a sample, geologists calculate elapsed time. Carbon-14, with a half-life of about 5,730 years, dates organic material up to roughly 50,000 years old.

A granite intrusion cuts across three sedimentary layers. Explain how you would determine whether the granite is older or younger than the layers. [3 marks]

Explain why carbon-14 cannot be used to date a 200-million-year-old fossil, and name a method that could. [3 marks]