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