The Earth formed from a ball of molten rock minerals. The least
dense rocks floated on the top and as they cooled the surface layer of
granite, with basalt just below, solidified to form the Earth’s crust. The
Earth’s surface has had a long and turbulent history during which it
has frequently fractured, crumpled, lifted and fallen, with more molten
material being pushed up from below through the breaks in the crust and
From the moment that rocks are exposed to the atmosphere, they are
subject to weathering. This breakdown of the rocks is brought about by
the effect of chemical, physical and biological factors.
is mainly brought about by the action of carbonic
that is produced wherever carbon dioxide and water mix, as in
rainfall. Some rock minerals dissolve and are washed away. Others are
altered by various chemical reactions, most of which occur when the rock
surface is exposed to the atmosphere. All but the inert parts of rock are
eventually decomposed and the rock crumbles as new minerals are formed
and soluble material is released. Oxidation
is particularly important in
the formation of iron oxides, which give soils their red and yellow (when
aerobic), or blue and grey colours (in anaerobic conditions).
Physical or mechanical weathering
processes break the rock into
smaller and smaller particles without any change in the chemical
character of the minerals. This occurs on exposed rock surfaces along
with chemical weathering but, in contrast, has little effect on rocks
protected by layers of soil.
The main agents of physical weathering are frost, heat, water, wind
and ice. In temperate zones, frost is a major weathering agent. Water percolates into cracks in the rock and expands on freezing. The pressures
created shatter the rock and, as the water melts, a new surface is exposed
to weathering. In hot climates the rock surface can become very much
hotter than the underlying layers. The strains created by the different
amounts of expansion and the alternate expansion and contraction cause
fragments of rock to flake off the surface; this is sometimes known as
the ‘onion skin’ effect. Moving water or wind carries fragments of rock
that rub against other rocks and rock fragments, wearing them down.
Where there are glaciers the rock is worn away by the ‘scrubbing brush’
effect of a huge mass of ice loaded with stones and boulders bearing
down on the underlying rock.
is attributable to organisms such as mosses, ferns
and flowering plants which fragment rock by both chemical and physical
means, e.g. they produce carbon dioxide which, in conjunction with
water, forms carbonic acid; roots penetrate cracks in the rock and, as
they grow thicker, they exert pressure which further opens up the cracks.
|Figure 17.3 Rocks. Granite: pink (left)
(top) sandstone (right) slate
are those formed from the molten material of the Earth’s
crust. All other rock types, as well as soil, are ultimately derived from
them. When examined closely, most igneous
rocks can be seen to be a mixture of crystals. Granite
is one of the commonest and contains
crystals of quartz, white and shiny, felspars
that are grey or pink, and micas, which are
shiny black (see Figure 17.3). Many of these
crystalline materials have a limited use in
landscaping as formal structures rather than
in the construction of rock gardens; more
commonly they are used in monuments and
As granite is weathered (‘rotted’) the felspars are
converted to kaolinite (one of the many forms
of clay) and soluble potassium, a plant nutrient.
Similarly, the mica present is chemically
changed to form clay and yield soluble minerals.
Whilst the many types of clay retain much of
the potassium, sodium, calcium, etc., the soluble
material is carried by water to the sea making the
sea ‘salty’. The inert quartz grains are released
and form sand grains.
is derived from accumulated
fragments of rock. Most have been formed in
the sea or lakes to which agents of erosion
carry weathered rock. Organisms in the seas
with shells die and accumulate on the bottom
of the sea. Layers of sediment build up and,
under pressure and slow chemical change, eventually become rock strata such as shale, chalk or limestone.
In subsequent earth movements much of it has been raised up above
sea level and weathered again. Similarly, the sand grains that
accumulate to great depths in desert areas eventually become sandstones
|Figure 17.4 Limestone
Moving water and winds are able to carry rock particles and are thus
important agents of erosion. As their velocity increases the ‘load’
are able to carry increases substantially. The fast-moving water in
streams is able to carry large particles, but in the slower-moving rivers
some of the load is dropped. The particles settle out in order of size
(see settling velocities). This leads to the sorting
of rock fragments,
i.e. material is moved and deposited according to particle size. By the
time the rivers have reached the sea or lakes only the finest sands, silts
and clays are in the water. As the river slows on meeting the sea or lake
all but clay is dropped. The clay eventually settles slowly in the quieter
waters of the sea or lake. Moving ice is also an agent of erosion, but
the load dropped on melting consists of unsorted particles known as boulder clay
The type of sedimentary rock formed depends on the nature of its
ingredients. Sandstones, siltstones and mudstones are examples of
sedimentary rocks derived from sorted particles in which characteristic
layers are readily seen. Limestones are formed from the accumulation
of shells (see Figure 17.4) or the precipitation of materials from solution
mixed with varying amounts of deposited mud. Chalk is a particularly
pure form derived from the calcium carbonate remains of minute
organisms that lived in seas in former times. Many of these are attractive
materials for use in hard landscaping, where care should be taken to
align the strata (layers) for a natural effect.
is formed from igneous or sedimentary rocks.
The extreme pressures and temperatures associated with movements
and fracturing in the Earth’s crust or the effect of huge depths of rock
on underlying strata over very long periods of time has altered them.
Slate is formed from shale, quartzite from sandstone, and marble from
limestone. Metamorphic rock tends to be more resistant to weathering
than the original rock.