Our Environment - Mining and the Environment: From Ore to Metal - page 297

Geology

The science of geology is concerned with the history, behaviour, and nature of the Earth. It
deals with the physical and chemical composition, distribution and structure of rocks, soils,
and their mineral constituents; and with the processes of magmatic differentiation, crystal-
lization, metamorphism, chemical alteration and precipitation, weathering, sedimentation
and induration by which soils and rocks are formed and subsequently decomposed. It also
deals with movements of tectonic plates which form the Earth's crust; with the associated
stresses (tectonic forces) which lead to earthquakes (sometimes causing tsunamis); with
ruptures of the Earth (faults) caused by these stresses; and with volcanos and geysers that
periodically erupt ejecting large volumes of solids and/or liquids, commonly accompanied
by toxic gases.
Economic geology is the branch of geology that deals with the genesis and nature of
ores, and the range of techniques used to discover and evaluate these ores.
Rocks are classified into three categories, based on their origin:
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Igneous rocks are those which solidify by cooling of magma, either within the Earth's
crust (plutonic rocks) or extruded on to the Earth's surface (volcanic rocks). These
rocks are categorized and classified by their chemical and mineralogical composition
and texture. Common examples include granite which is a plutonic rock of acidic com-
position, and basalt which is a volcanic rock of basic (alkaline) composition.

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Metamorphic rocks result from the transformation of pre-existing rocks under condi-
tions of high temperature and/or high pressure, which change the mineral assemblages
and also cause textural changes, particularly the development of mineral banding and
foliation. Classification of metamorphic rocks is based partly on the characteristics of
the pre-existing rock, as well as the type and grade of metamorphism as reflected in
the mineral assemblage and the texture. Examples are slate (which results from low
grade metamorphism of shale), schist which results from higher grade metamorphism,
and marble, which results from re-crystallization of limestone at high temperature.

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Sedimentary rocks which form by deposition and accumulation of particles from weath-
ering of other rocks; particles ejected into the atmosphere by volcanic eruptions; the
solid remains of organisms; and crystals and colloids precipitating from surface waters.
Deep burial, consolidation over time and in some cases inter-granular reactions cause
the deposited sediments to lithify, i.e. become rock. Classification of sedimentary rocks
is based mainly on composition, and grain size. The most common sedimentary rocks
are sandstone, shale, and limestone. Peat and coal are also classified as sedimentary.

Soils are derived from rocks initially by chemical weathering, which involves oxida-
tion and leaching, preferentially dissolving some mineral components. This weakens the
rock fabric, eventually leading to physical disintegration under the forces of temperature
change, freezing and thawing, and erosion by wind and water.
Residual soils are those which form from the parent rock in situ . The development of soil
from parent rock is a slow process. In some situations, any material weakened by weather-
ing, is removed by erosion almost immediately, so that little or no residual soil accumulates.
Elsewhere, in the absence of significant erosion, soil profiles may reach depths of 50 m or
more. Most older residual soils exhibit profile development, in which several more or less
distinct horizons can be distinguished, each with a different texture and colour.
Transported soils are more or less unconsolidated sediments removed from the parent
material and subsequently re-deposited elsewhere. They include:
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Fluvial soils deposited in rivers;

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Alluvium - soils deposited by moving water in flood plains and deltas;