2 THE GEOMORPHIC SYSTEM (p. 31-32)

The Earth’s topography results from the interplay of many processes, some originating inside the Earth, some outside it, and some on it. This chapter covers:

grand cycles of water and rock
the wearing away and the building up of the land surface
tectonics, erosion, and climate

The Earth’s surface in action: mountain uplift and global cooling

Over the last 40 million years, the uplift of mountains has been a very active process. During that time, the Tibetan Plateau has risen by up to 4,000 m, with at least 2,000 m in the last 10 million years. Twothirds of the uplift of the Sierra Nevada in the USA has occurred in the past 10 million years. Similar changes have taken place (and are still taking place) in other mountainous areas of the North American west, in the Bolivian Andes, and in the New Zealand Alps. This period of active mountain building seems to be linked to global climatic change, in part through airflow modification and in part through weathering. Young mountains weather and erode quickly. Weathering processes remove carbon dioxide from the atmosphere by converting it to soluble carbonates. The carbonates are carried to the oceans, where they are deposited and buried. It is possible that the growth of the Himalaya scrubbed enough carbon dioxide from the atmosphere to cause a global climatic cooling that culminated in the Quaternary ice ages (Raymo and Ruddiman 1992, Ruddiman 1997). This shows how important the geomorphic system can be to environmental change.

ROCK AND WATER CYCLES
The Earth’s surface – the toposphere – sits at the interfaces of the solid lithosphere, the gaseous atmosphere, and the watery hydrosphere. It is also the dwelling-place of many living things. Gases, liquids, and solids are exchanged between these spheres in three grand cycles, two of which – the water or hydrological cycle and the rock cycle – are crucial to understanding landform evolution. The third grand cycle – the biogeochemical cycle – is the circulation of chemical elements (carbon, oxygen, sodium, calcium, and so on) through the upper mantle, crust, and ecosphere, but is less significant to landform development, although some biogeochemical cycles regulate the composition of the atmosphere, which in turn can affect weathering.

Water cycle

The hydrosphere – the surface and near-surface waters of the Earth – is made of meteoric water.The water cycle is the circulation of meteoric water through the hydrosphere, atmosphere, and upper parts of the crust. It is linked to the circulation of deep-seated juvenile water associated with magma production and the rock cycle. Juvenile water ascends from deep rock layers through volcanoes, where it issues into the meteoric zone for the first time. On the other hand, meteoric water held in hydrous minerals and pore spaces in sediments, known as connate water, may be removed from the meteoric cycle at subduction sites, where it is carried deep inside the Earth.

The land phase of the water cycle is of special interest to geomorphologists. It sees water transferred from the atmosphere to the land and then from the land back to the atmosphere and to the sea. It includes a surface drainage system and a subsurface drainage system. Water flowing within these drainage systems tends to be organized within drainage basins, which are also called watersheds in the USA and catchments in the UK. The basin water system may be viewed as a set of water stores that receive inputs from the atmosphere and deep inflow from deep groundwater storage, that lose outputs through evaporation and streamflow and deep outflow, and that are linked by internal flows. In summary, the