Granite : a planetary response to liquid water
Inaugural lecture delivered at Stellenbosch University on 7 October 2008.
Granites are coarse-grained igneous rocks, rich in quartz and feldspars and containing one or more hydrous minerals, such as micas and amphiboles. They have crystallised from silica-rich magmas that contained significant amounts of dissolved H2O. Most such magmas are created when the pressures and temperatures, in hydrated rocks deep in the planet’s crust, exceed those of the solidus, producing melt and crystalline residue. During this process H2O need not be present in a free fluid, but the planet’s near-surface environments do need to have abundant liquid water to produce weathered and hydrated rocks that ultimately melt to make the magmas. Liquid water in sufficient amounts (oceans) to trigger the chain of processes that leads to the formation of granites occurs on only one terrestrial planet, namely Earth. This explains why only Earth of all the planets in the solar system has plate tectonics, granites, continents and terrestrial life.