Rocks are assemblages of minerals of similar or different types, and there are three main kinds of rocks which all soils are formed from.
2.1.1 Types of rocks geological rocks
Rock is a solid material that occurs naturally, and it composed of one or more minerals. Rocks are assemblages of mineral of similarly or different types and there are three main kinds of rocks; igneous, sedimentary and metamorphic rocks.
Igneous rocks
The word igneous rock is derived from the Latin word ignis which means fire, or magmatic rocks. This is because igneous rock is formed through cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either earth’s mantle or crust. The melting is primarily caused by three processes which are;
- Increase in temperature
- Decrease in pressure
- Change in composition
Figure 2.1: Different types of igneous rocks
Igneous rocks can be divided into two main categories which are intrusive and extrusive rocks. These igneous rocks are classified according to their texture, colour and composition. These properties are determined from the deposition and chemistry of magmas.
Properties of intrusive rocks or plutonic rocks also known as hypabyssal
The intrusive igneous rocks are formed by cooling of magma in deep interior of the earth which much slower than the cooling process that occurs outside the earth’s crust. Therefore, since the magma cools slowly it results to coarse-grained or large size crystals. These rocks are classified according to the shape and size of the intrusive body and its relation to other formations into which it intrudes. These intrusive formations are batholiths, stocks, laccoliths, sills and dikes. During the slow cooling process, the magma solidifies within the crust and form coarse textured rocks such as dolerite, granite and gabbro. These intrusive rocks usually may occupy huge areas of the earth when exposed by erosion. As a result, there intrusive igneous rocks can be identified with the naked eye.
When the intrusive igneous rocks form deeper within the earth crust are known are plutonic (or abyssal) rocks and are normally coarse-grained. On the other hand, when intrusive igneous rocks are formed near the surface are termed subvolcanic or hypabyssal rocks and usually are medium-grained. Normally, plutonic rocks are more common than hypabyssal rocks and often form dikes, sills, laccoliths and lopoliths.
Figure 2.2: Intrusive igneous rocks
Extrusive igneous rocks or volcanic rocks
The rocks are formed by crystallization of the magma that occurs on the earth surface. The magma which is brought to the earth surface through volcanic eruptions, solidifies at a faster rate than in intrusive igneous rocks. Hence the such rocks are smooth, crystalline and fine-grained. Basalt is a common extrusive igneous rocks and forms lava flows, lava sheets and lava plateaus.
Figure 2.3: Extrusive igneous rocks
Identification of the types of igneous rocks
Igneous rocks are classified according to their texture, colour and composition. The difference of these parameters depends on environment of deposition and chemistry of magmas. These properties explained below:
Texture
When magma cools slowly it forms large crystals, however, when magma cools fast then small crystals are formed and at times a glassy texture can be formed this way. Through these textural differences igneous rocks can therefore be classified into either intrusive and extrusive rocks. As explained on the above subsections, intrusive igneous rocks are formed by solidifying of magma before reaching soil surface resulting to coarse grained texture, on the other hand extrusive are formed from a magma that solidify on the soil surface resulting to fined grained rocks.
Colour
The rate in which the magma cools influences the colour that the rock will resemble. Intrusive igneous rocks such as granite have a lighter coloured mineral while the extrusive rocks such as basalt has a majorly dark mineral form.
Composition
Chemical composition is also used to classify igneous rocks. This is based on silica content that is within the rock. Feldspars consists of silica minerals content that is above 75%, minerals while mafic minerals form when silica minerals are less than 75%.
Sedimentary rocks
Earth’s crust is continuously eroded by water runoffs, rivers, glaciations and wind. These eroded soil materials are carried along by water, wind and glaciers are eventually deposited as layers of sediment (bedding). Sedimentary rocks are formed by the deposition of material at the earth’s surface and/or within water. The process of the deposition and accumulation of these eroded particles to form sedimentary rocks is known as sedimentation. These rocks cover approximately 66% of the earth crust while the remaining 34% is igneous and metamorphic rocks. Examples of sedimentary rocks are sandstone, shale, limestone, dolomite, rock salt, loess, coal, amber, pumice, banded ironstone and rock gypsum to mention a few.
The importance of sedimentary rocks
- They are a source of limestone deposits
- They are a source of coal and oil
- They are important in the interpretation of earth’s history
Table 2.1: The description of the type of sedimentary rocks
Metamorphic rocks
Metamorphic rocks are formed when sedimentary and igneous rocks have undergone a physical and chemical transformation due to high temperature and pressure below the zone of diagenesis. The original rock, prior metamorphism is referred to protolith. The metamorphic rocks can still retain the original texture of the protolith when they have experienced a low degree of alteration. However, as the degree of metamorphism increases the original texture is replaced with metamorphic textures and other properties such as bulk chemical composition. Examples of these types of rocks are slate (which turns to shale), marble (which turns to limestone), and quartzite which turns to sandstone.
The agents (temperature and pressure) of metamorphism are explained as follows:
Temperature
Temperature plays an important role in several metamorphism processes as outlined below;
- Increase in temperatures leads to higher energy resulting to chemical reactions which breaks chemical bonds and reform resulting to changed rock’s chemistry during metamorphism.
- Escalated temperature results in the growth of crystals.
- Increased temperatures during the process of metamorphism causes small crystals to unite and form a coarser grained rock.
- Individual minerals within a rock are only stable to a certain temperature range, however, when temperature increases it cause the minerals to be unstable and thereby reacting with each other forming new minerals.
Pressure
Pressure influences metamorphism in two forms mainly lithostatic pressure and directed pressure.
Lithostatic pressure: This is when the rock is experiencing pressure from the weight of an overlying rocks and the pressure is experienced uniformly by the metamorphic rock. The pressure comes from all directions therefore there is no preferred orientation or rearrangement by crystals within a metamorphic rock.
Figure 2.4: Lithostatic pressure
Direct pressure: Unlike the lithostatic pressure, this pressure is of motion and action. Minerals is stable over a range of pressure, when the pressure conditions change during metamorphism and exceed a mineral’s stability range the mineral will transform to a new phase.
Classification: Classification of metamorphic rocks is determined by texture and the degree of metamorphism. Normally, three types of classification criteria are used and are explained as follows:
- Mineralogical: The most abundant minerals are used as a prefix to a textural term. For example, schist which consists of biotite, garnet, quartz and feldspar, would be called a biotite-garnet schist.
- Texture: Metamorphic rocks normally involve foliation which are mainly the distinct and repetitive layers. These layers are caused by different arrangement of silicate sheets within the rock.
- Chemical: A mineral assemblage can be used to determine a general chemical composition. For instance, a schist contains large amounts of quartz and feldspar and some garnet and muscovite would be called a garnet-muscovite quartz-feldspathic schist.
Figure 2.5: Metamorphic rocks in transformation