It is important to note that most soil material is mineral and all minerals are derived from rocks. The rocks of the earth are very varied in composition, depending on the way they were formed and the time they were formed. There are three groups of rocks namely, igneous, sedimentary and metamorphic
Igneous rocks have solidified from molten rocks, either in volcanoes or inside the
crust.
• Sedimentary rocks are laid down
(deposited) under water as a sediment. They are made from fragments worn away
from older rocks, or from the remains of living organisms. They can appear on
the surface after being forced upwards by movement of the Earth’s plates.
• Metamorphic rocks are rocks that
have been changed by heat or pressure, or both. They could originally have been
either igneous or sedimentary rocks.
IGNEOUS ROCKS
Sometimes enough heat is generated in the Earth’s crust and upper mantle
to melt rocks. This came directly from molten rock called magma which was a hot
liquid solution of mineral matter. This molten magma, when it cooled and
solidified, it did so in single uniform masses. Granite and basalt are examples
of igneous rocks. The magma, from which igneous rocks are formed after cooling,
contains silicon dioxide, aluminium oxide, oxides of iron, magnesium oxide,
calcium oxide, sodium oxide and potassium oxide. All these compounds are
released to the soil as nutrients during weathering. There are two main types
of igneous rock: intrusive and extrusive.
• Intrusive igneous rock forms when the magma crystallises while
it is still underground. Granite is a typical intrusive igneous rock. It is
formed from high-viscosity magma that has cooled slowly at depth to give large
crystals of light-coloured minerals.
• Extrusive igneous rock forms when the rising molten rock breaks
through to the surface as lava. The lava cools and crystallises at the Earth’s
surface. Basalt is an example of this type of rock. It is formed from
low-viscosity magma that has cooled rapidly to give small crystals of mainly
dark-coloured minerals.
Both types of igneous rock are crystalline. This is particularly obvious
in granite, where the interlocking crystals can be seen by eye. The deeper in
the Earth’s crust that the magma cooled, the longer the crystals in the granite
took to form. The slower the crystallisation process, the larger the crystals
in the rock. Intrusive rock can be found as sills and dykes formed within the
crust. Lava flows form alternating layers of solidified lava and erupted rock
and ash.
(a) The crystals in granite can be seen by eye and are interlocking, (b) The crystals in basalt are very small and must be viewed through a microscope. |
Intrusive and extrusive igneous rocks |
SEDIMENTARY ROCKS
Sedimentary rocks are made up of fragments of older rock or the remains
of living organisms. This rock was laid down as sediment on the Earth surface,
particle by particle in layers, when the region was under water. Examples of
the sedimentary rocks are sandstone (which consists of quartz, silica, iron
oxide, calcium carbonate, etc.); and limestone or chalk and phosphate rocks.
Generally, sedimentary rocks are named after the size
of fragments from which they are made:
•
a rock made from pebbles is called conglomerate,
•
a rock made from sand is simply called sandstone,
•
a rock made from fine mud is called a mudstone, though
if it is flaky and breaks easily it is called shale,
•
a rock made from the shells and skeletons of organisms
that lived in water is called limestone.
All rocks exposed on the Earth’s surface are worn away by weathering and
erosion. This material is transported by gravity, wind, ice, rivers and seas. In
the geological past, these sediments were forced close together and became
compacted into rock as more and more material was deposited. The pressure of
the material above compressed the sediment into rock. Hence the grains are held
together by natural chemicals, as other minerals seeped between the fragments
and solidified, holding (or cementing) the deposit together like a ‘glue’,
precipitated from the water. These sedimentary rocks formed layers or strata.
Sedimentary rocks are got from already existing metamorphic or sedimentary or
igneous rocks.
METAMORPHIC ROCKS
Metamorphic rocks are formed when rocks that originated beneath the
Earth’s surface are altered by the action of great heat and pressure. These are
rocks which once happen to be either igneous or sedimentary rock, and changed
their character after being subjected to heat, pressure or simply chemical
action. Such conditions occur at subduction zones or where plates collide. Examples
of this type rocks include marble and slate. Marble is a metamorphic rock formed by this type of action on
limestone. Slate is metamorphosed
mudstone or shale. Any fossils that may have been present in the sedimentary
rock are obliterated. Many precious and semi-precious stones and minerals are
found in metamorphic rock.
THE ROCK CYCLE
The different types of rock undergo changes that occur over a long timescale. Rocks are slowly transformed (changed) in type by weathering and sedimentation or by conditions of intense heat and pressure. These changes by which rocks are recycled from one form to another are known as the rock cycle.
The rock cycle |
In this cycle the rocks that are exposed on the surface are weathered.
The particles are carried away by erosion and deposited as sediment. Eventually
these deposits become sedimentary rock, which may then be brought back to the
surface by movement of the Earth’s crust. Alternatively, that sedimentary rock
may be crushed and heated to form metamorphic rock. In turn, this rock may be
melted deep in the crust to form magma, which is then squeezed to the surface.
Here, on cooling, it forms igneous rock, and the cycle may begin again.
The different characteristics of the rocks can be summarised in the
following table
Table 3.1: Features of the different rock types
Rock type |
Features |
Igneous rocks |
·
have no fossils ·
have an interlocked crystal
structure are usually hard |
Sedimentary Rocks |
·
may have layers visible ·
have separate grains ·
may be quite soft, and
disintegrate on rubbing ·
could contain fossils ·
if calcium carbonate is
present in the rock, will fizz when added to acid |
Metamorphic rocks |
·
may be hard, though may be
split along a cleavage plane ·
may be banded or streaked ·
may contain mica flakes in
streaks or layers ·
may have a crystalline
appearance ·
contain no fossils ·
if marble, will fizz with
dilute acid |
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