A pool in a 3.5 billion year old landscape near Marble Bar
Geology: The rock record and the appearance of life
Time is measured by geologists in terms of millions and billions of years. Imagining the age of the Pilbara - around 3.5 billion years - is hard to grasp. Geological Survey of Western Australia scientist Dr Kath Grey explains it this way: Stretch out an arm. Imagine 3.5 billion years ago as being your shoulder. Now imagine filing your finger tips. You have just filed away the entire history of human civilisation. Another analogy that is used is a clock, and if an hour represents the entire history of the Earth since its formation 4.6 billion years ago, then human civilisation is represented only in the last minute of the hour.
Scientists use the rock record as a sort of book that begins with pages of fragments of information, buried in the deep time of billions of years ago. Later in the book, more and more is known - there are fossil animals and plants that chart a history leading to our present. Rocks are composed of minerals. There are three basic types of rock:
Igneous rock - these form when molten rock cools and hardens, whether below ground (making granite) or above ground (making basalt)
Sedimentary rock - these form by chemical precipitation or settling of various kinds of materials such as bits of former rock or shells. As more material is laid over it, the loose material becomes rock - in the process often enclosing the remains of past life, leaving them in the rock record.
Metamorphic rock - originates with one of the two other basic rock types and is formed when their properties are changed by high temperature and/or pressure. Common metamorphic rocks are marble and gneiss. Metamorphic rock on Mars is thought to be rare to non-existent due to the lack of tectonic activity.
Geologic time is measured on a relative or absolute basis. Relative refers to the stratigraphy or layering of rock sequences and 'index fossils' - species that lasted only for a specific period. The absolute scale is measured by radiometric dating - which uses the natural radioactive decay of certain radioactive elements to other elements.
Geological time explained
Exploring geological time
Basic rock identification