ABSTRACT
Rock material forms Earth’s largest constituent by far and has undergone continuous transformation since it condensed from a collapsing interstellar gas and dust cloud 4·6 billion years (4·6 Ba) ago. The rock volume of 1·083 × 1012 km3 and mass of 5·977 × 1024 kg compares with 1·4 × 109 km3 and 1·40 × 1021 kg of global water and 5·13 × 1018 kg of atmospheric gases. The relatively small size of our planet, just 40,000 km in circumference, is highlighted by passenger jets capable of circumnavigation in forty-eight hours and orbiting satellites in ninety minutes. The average radius of 6,371 km from surface to centre is equivalent to the distance from London to Chicago. Yet the mass, character and age of Earth’s rocks can be hard to comprehend. So, too, are its origins in astro-geological processes which formed our solar system, through the gravity concentration of matter from a supernova explosion c. 6 Ba ago. What interest, then, should geographers have in planetary processes dominated by imponderable origins, astronomic time scales, tiny geothermal energy flows compared with solar irradiation of the atmosphere and vast but almost entirely concealed material reserves? How far do they influence human habitat and lives at Earth’s surface? This chapter explains long-term, large-scale geological processes which form Earth’s dynamic foundations. Plate tectonics provides its unifying theme, linking Earth’s early evolution with the geologically recent neotectonic emplacement of its principal global landforms. Subsequent reshaping by surface geomorphic processes is the subject of later chapters. Table 10.1 shows their far-reaching effects and time scales of operation.
