Earth Sciences / 27The extreme conditions produced by the folding and shearing of rock can alter its basic structure by
causing some of its minerals to recrystallize, sometimes in new ways. This process, called
‘metamorphism’, also happens when rock of any type comes into contact with molten rock, during
the intrusion of magmatic material for example. Marble is limestone or dolomite (dolostone) that
has been subjected to metamorphism at high temperature. Such shells as it contained are completely
destroyed as the calcium carbonate recrystallizes as the mineral calcite. If quartz or clay particles are
present, new minerals may form, such as garnet and serpentine. Hard limestone containing fossils is
often called marble, but there are no fossils in true marble.
Slate is also a metamorphic rock, derived from mudstone or shale, in which the parallel align-ment
of the grains, due to the way the rock formed, allows the rock to cleave along flat planes (HOLMES,
1965, pp. 168–170). It may contain fossils, although they are uncommon and usually greatly deformed,
because slate forms when the parent sedimentary rock is squeezed tightly between two bodies of
harder rock that are moving in parallel but opposite directions, so its particles, and fossils, are dragged
out. It is this that gives slate its property of ‘slaty cleavage’ which, with the impermeable surface
imparted at the same time, makes it an ideal roofing and weatherproofing material. Metamorphic
rocks are widely distributed and with practice you can learn to recognize at least some of them.^7
All the landscapes we see about us and the mineral grains that are the starting material for the soils
which form over their surfaces are produced by these processes. The intrusion or extrusion of igneous
rock supplies raw material. This weathers to provide the mineral grains which become soil when
they are mixed with organic matter, or is transported to a place where it is deposited as sediment.
Pressure converts sediments into sedimentary rocks, which may then be exposed by crustal movements,
so that erosion can recommence. Metamorphic rocks, produced when other rocks are subjected to
high pressures and/or temperatures, are similarly subject to weathering. It is the cycling of rocks,
from the mantle and eventually back to it through subduction, that produces the physical and chemical
substrate from which living organisms can find subsistence.
8. Weathering
No sooner has a rock formed than it becomes vulnerable to attack by weathering. The word
‘weathering’ is slightly misleading. We associate it with wind, water, freezing, and thawing. These
are important agents of weathering, but they are not the only ones. Weathering can be chemical as
well as physical and it often begins below ground, completely isolated from the weather.
Beneath the surface, natural pores and fissures in rocks are penetrated by air, containing oxygen and
carbon dioxide, and by water into which a wide variety of compounds have dissolved to make an
acid solution. Depending on their chemical composition, rock minerals may dissolve or be affected
by oxidation, hydration, or hydrolysis (HOLMES, 1965, pp. 393–400). Oxidation is a reaction in
which atoms bond with oxygen or lose electrons (and other atoms gain them, and are said to be
‘reduced’). Hydration is the bonding of water to another molecule to produce a hydrated compound;
for example, the mineral gypsum (CaSO
4
.2H
2
O) results from the hydration of anhy-drite (CaSO
4).
Hydrolysis (lysis, from the Greek lusis, ‘loosening’) is a reaction in which some parts of a molecule
react with hydrogen ions and other parts with hydroxyl (OH) ions, both derived from water, and this
splits the molecule into two or more parts.
The result of chemical weathering can be seen in the limestone pavements found in several parts of
England, Wales, and Ireland.^8 South Devon, England, is famous for its red sandstones, well exposed
in the coastal cliffs of the Torbay area. These date from the Devonian Period, some 400 million years