Page 48 - Fundamentals of Geomorphology

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THE GEOMORPHIC SYSTEM

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. Two-
thirds 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 atmo-
sphere 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.