1.1 The Earth's crust
On the scale of a human lifetime, these rocks and the landscapes of which they are part can seem static and immutable, at least in a country such as Britain where there are no active volcanoes, and earthquakes are infrequent and fairly small. None the less, coastal erosion and the biological degradation of rocks to form soils are reminders that geological processes are occurring ‘in our own backyard’ and that the Earth's surface is continuously being reshaped.
Over hundreds of millions of years, the surface of the Earth is rearranged by the slow grind of geological processes. In particular, the rigid outer layer of the Earth (some 100 km thick and known as the lithosphere) is divided into about a dozen plates, which change shape, size and position as their margins experience growth, destruction or collision (see Figure 1). So, although the processes can be very slow, given enough time the results include the generation and destruction of mountain ranges that stretch across continents, and the opening and closing of ocean basins. All these geological processes leave their mark by forming new rocks. And those rocks, whether sandstone formed from sand in a desert, or volcanic lava flows, hold clues to the processes that formed them, and therefore the conditions at the time of their formation. In some cases they contain information on past climatic conditions, and some rocks contain fossils that reveal the history of life and the story of evolution. Rocks can provide a narrative of the Earth's history, but we need to learn how to read their tales.
The aims of this section are to demonstrate how many of the features shown by rocks are inherited from the processes that formed the rock, to introduce you to the skills of sketching features seen in rock exposures, and to show how those features can be interpreted in a simple sequence of events through geological time.