Geological processes are constantly occurring all over the world. As we speak new rocks are slowly being formed and old ones are weathering away, faults are moving, and portions of tectonic plates are being created and destroyed. Coal-mining in Yorkshire, which depended on very detailed knowledge of local geology, is almost extinct but obtaining natural gas by ‘fracking’ is constantly being discussed. To put matters into perspective if the life of the earth is seen as 24 hours then geological processes, and the rocks and minerals they create, have continued throughout the day. Humankind, and the archaeology it produces, came on to the scene at one second to midnight! No complete rocks older than 3600 MYrs have been discovered on earth but the planet is believed to have formed 4600 MY ago and the oldest dated mineral is a zircon at 4400 MY which suggests an early forming crust. The oldest living things, colonies of bacteria, are dated at 3400 MY.
Rocks can be classified into three major classes: igneous, sedimentary and metamorphic. Igneous rocks are formed by the crystallisation of magma (or the accumulation of ejecta from volcanoes). Igneous rock on the surface, like a granite mountain, is sooner or later, ‘weathered’ to form sediments. Sediments and sedimentary rocks are common in Yorkshire and limestone and the coal measures are rocks of this type. Exposure to heat and/or pressure produces metamorphic rock. The original rock could be igneous, sedimentary, or even metamorphic. Metamorphic rocks can be foliated or non-foliated. Foliation is the parallel alignment of minerals under pressure, slate being a good example. The pictures show: Flamborough Head (sedimentary: chalk), the Whin Sill in Northumbria (igneous: dolerite) and an overgrown quarry at Harden Moor near Bradford (sedimentary: coal measure sandstone).