Page 108 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 91

from the underlying bedrock, which is a production of of human activity in geomorphic processes, and supplied
lithospheric processes. many illustrations of the quantities of material involved
Gravity, water, and wind transport unconsolidated in mining, construction, and urban development. Recent
weathered material in the regolith across hillslopes and work confirms the potency of mining and construc-
down river valleys. Local accumulations form stores of tion activities in Earth surface change. In Britain, such
sediment. Sediment stored on slopes is talus, colluvium, processes as direct excavation, urban development, and
and talluvium. Talus is made of large rock fragments, waste dumping are driving landscape change: humans
colluvium of finer material, and talluvium of a fine deliberately shift some 688 to 972 million tonnes of
and coarse material mix. Sediment stored in valleys is Earth-surface materials each year; the precise figure
alluvium. It occurs in alluvial fans and in floodplains. depends on whether or not the replacement of overbur-
All these slope and valley stores, except for talus, are den in opencast mining is taken into account. British
fluvial deposits (transported by flowing water). rivers export only 10 million tonnes of solid sediment
and 40 million tonnes of solutes to the surrounding
seas. The astonishing fact is that the deliberate human
HUMANS AS GEOMORPHIC AGENTS transfers move nearly fourteen times more material than
natural processes. The British land surface is changing
Humans have become increasingly adept at ploughing faster than at any time since the last ice age, and per-
land and at excavating and moving materials in con- haps faster than at any time in the last 60 million years
struction and mining activities. Indeed, humans are (Douglas and Lawson 2001).
so efficient at unintentionally and deliberately moving Every year humans move about 57 billion tonnes of
soils and sediments that they have become the lead- material through mineral extraction processes. Rivers
ing geomorphic agent of erosion (e.g. Hooke 2000). transport around 22 billion tonnes of sediment to the
Placing human-induced erosion in a geological per- oceans annually, so the human cargo of sediment exceeds
spective demonstrates the point (Wilkinson 2005). The the river load by a factor of nearly three. Table 3.5 gives
weathered debris stored in continental and oceanic a breakdown of the figures. The data suggest that, in
sedimentary rocks suggest that, on average, continen- excavating and filling portions of the Earth’s surface,
tal surfaces have lowered through natural denudation humans are at present the most efficient geomorphic
at a rate of a few tens of metres per million years. By agent on the planet. Even where rivers, such as the
contrast, construction, mining, and agricultural activi- Mekong the Ganges, and the Yangtze, bear the sedi-
ties presently transport sediment and rock, and lower all ment from accelerated erosion within their catchments,
ice-free continental surfaces by a few hundred metres per they still discharge a smaller mass of materials than the
million years. Therefore, the human species is now more global production of an individual mineral commodity
important at moving sediment than all other geomorphic in a single year. Moreover, fluvial sediment discharges
processes put together by an order of magnitude. to the oceans from the continents are either similar in
The key areas of human influence on sediment fluxes magnitude to, or smaller than, the total movement of
are through mining and construction, agriculture, and materials for minerals production on those continents.
dam building.
Soil erosion
Mining and construction
In transporting sediment to the oceans, rivers main-
Locally and regionally, humans transfer solid materials tain a vital leg of the rock cycle and a key compo-
between the natural environment and the urban and nent of the global denudation system. The amount
industrial built environment. Robert Lionel Sherlock, in of sediment carried down rivers is a measure of
his book Man as a Geological Agent: An Account of His land degradation and the related reduction in the
Action on Inanimate Nature (1922), recognized the role global soil resource. Many factors influence fluxes of