ROCKS AND THE ROCK CYCLE

 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.

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.

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