Igneous rocks are crystalline solids which form directly from the cooling of magma.

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151PTE321 Engineering Geology

Lecture 3

Dr. Seyed Mehdi Alizadeh

Rock Types

Rock Types: Igneous Rock

 Igneous rocks are crystalline solids which form directly from the cooling of magma.

 This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state.

 Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are).

 Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled.

 In general, the slower the cooling, the larger the crystals in the final rock.

Rock Types: Igneous Rock

 Example of Igneous Rocks: Granite and Basalt.

Granite Basalt

Igneous Rocks Classification

• Igneous rocks are classified by their content of essential minerals i.e. those that make up the bulk of the rock – this is known as the mode of the rock. Minor amounts of accessory minerals are not considered.

• As we will see later, this is fairly straightforward for coarse grained rocks but can be very difficult for very fine grained ones – and impossible for glassy rocks.

Igneous Rocks Classification – Cont’d

• Two terms used in connection with grain size are:

• phaneritic: individual crystals can be distinguished with the naked eye.

• aphanitic: most of the individual crystals cannot be distinguished with the naked eye.

• Igneous rocks are divided into two main groups on the basis of their field relations or on their grain size.

• PLUTONIC – crystallized at depth. Phaneritic. Average crystal or grain size > 5mm (coarse); 1–5mm

• (medium); 0.5–1mm (fine grained).

• VOLCANIC – extruded at the surface of the Earth. Aphanitic. Grain size < 0.5mm (very fine grained).

Plutonic Rocks

• Plutonic rocks are classified according to their modal mineral content i.e. actual mineral content in volume %. The classification (from 1976) is known as the “Streckeisen” system.

• For the naming of plutonic rocks, five groups of minerals are used. These are:

• Q quartz

• A alkali feldspar – orthoclase, microcline, perthite, anorthoclase and

• albite (An00–05)

• P plagioclase (An05–100)

• F feldspathoid minerals or FOIDS – nepheline, leucite, sodalite etc.

• M mafic minerals. “Mafic” stands for Mg-Fe minerals and includes olivine, pyroxenes,

• amphiboles, micas, garnets, oxide and sulphide minerals etc.

Volcanic Rocks

• Volcanic rocks have, by definition, an aphanitic groundmass but their classification is more complicated than that of plutonic rocks. As we have seen, the latter are classified according to the minerals present. This is often difficult or impossible to determine for volcanic rocks.

• in many cases (e.g. when a rock is extremely fine grained) a chemical analysis is required and the so-called TAS classification is used (TA = total alkalies Na2O + K2O; S = silica).

• Many volcanic rocks are formed by explosive activity, resulting in the formation of fragmentary rocks – the so-called pyroclastic rocks (pyro = heat; clast = fragment). This is also taken into consideration in the naming of volcanic rocks.

TAS system

• Volcanic rocks show a great range of variation in chemical compositions. This variation is, however, far from random and shows a series of rational patterns.

• Two of the most important and useful chemical parameters in volcanic rocks are SiO2 and Na2O + K2O (total alkalies). The vast majority of volcanic rocks have between 35 and 75% SiO2 and 1–16% total alkalies.

• The TAS system is widely used for the classification of volcanic rocks when a mineralogical classification is difficult to apply.

• Terms which are used in the volcanic rock names are based solely on the SiO2-content of the rock. This is the total amount of the chemical component silicon dioxide (silica) and is NOT the amount of quartz that is a mineral with the composition SiO2.

TAS system –Cont’d

• The TAS (Total Alkalies vs. Silica) classification for volcanic rocks. This classification is based on chemical analysis of the rock for SiO2, K2O and Na2O.

Mineral Assemblages

• Most igneous rocks are classified according to their mineralogy. Rocks that consist entirely of glass cannot, of course, be classified on a mineralogical basis

• Apart from ultramafic rocks, plutonic and volcanic rocks only differ in terms of grain size and texture; their mineralogical variation is essentially similar.

• The relationship between mineral assemblage (i.e. all the important minerals in a rock) and rock composition is illustrated in Fig. 5.5.

Mineral Assemblages

• Variations in mineral assemblages in plutonic and volcanic rocks.

Rock Types: Sedimentary Rock

 Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks.

 This accumulation get compacted and cemented together.

 There are three main types of sedimentary rocks: clastic, chemical, and organic sedimentary rocks.


• There are two types of process that cause the alteration of rocks exposed on the surface of the Earth –physical and chemical.

• Physical or mechanical alteration results in rocks breaking into smaller fragments; these rock fragments are called detritus.

• Different grain sizes of detrital fragments are named as shown in Table 6.1.


• Table 6.1. Grain size of unconsolidated sediments


• Boulders, cobbles and pebbles are coarse-grained, sand is medium-grained, and silt and clay are fine- grained.

• There are a variety of processes that result in physical weathering. Rocks buried in the crust are under pressure because of the weight of the overlying deposits and hotter than at the surface (the increase in temperature with depth is usually in the range 20–40°C/km).

• Joints form when pressure is reduced as a result of uplift and erosion.

Weathering – joints

• Development of joints in granites and sediments as a result of uplift and erosion.

• Granites (homogeneous, massive rocks) commonly form onion- like sheets, whereas sediments develop

vertical fractures.

Weathering • Physical and chemical weathering operate together.

Mechanical processes give more cracks (fractures) and thereby a greater surface area. Chemical processes, which operate on surfaces, are therefore enhanced by extensive fracturing

Weathering • Minerals become altered at different rates. Quartz is a very resistant

mineral and is often the only one to survive chemical weathering; all the others alter to clay (see below).

• Sandy beaches are usually dominated by quartz.

• Minerals alter at different rates. Feldspar and dark minerals in granite alter relatively rapidly to clay minerals whereas quartz is very resistant. The clay fraction is easily removed, leaving quartz sand as the final product of weathering.

 Clastic sedimentary rock:

 Clastic sedimentary rocks are accumulations of clasts – little pieces of broken up rock which have compacted and cemented.

 Example of clastic sedimentary rocks are sandstone, shale, siltstone and conglomerate.

Rock Types: Sedimentary Rock

Sandstone Shale

Siltstone Conglomerate

Rock Types: Sedimentary Rock

 Chemical sedimentary rock:

 Many of these rock form when standing water evaporates, leaving dissolved minerals behind. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long period of time .

 Example of chemical sedimentary rocks are gypsum and dolomite.

Gypsum Dolomite

Rock Types: Sedimentary Rock

 Organic sedimentary rock:

 Any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an “organic” sedimentary rock.

 Example of organic sedimentary rocks are coal and coquina.

Coal Coquina

What is Limestone? • Limestone is a sedimentary rock.

• Composed primarily of calcium carbonate (CaCO3).

• It most commonly forms in shallow marine waters.

• It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, and algal.

• It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.

Moccasin Limestone Lower Ordovician

Marl-Limestone Upper Jurassic-Oxfordian

Rock Types: Metamorphic Rock

 The metamorphic get their name from “meta” (change) and “morph” (form).

 Any rock can become a metamorphic rock. All that is required for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions.

 In most cases, this involves burial which leads to a rise in temperature and pressure.

 Common metamorphic rocks include schist, gneiss, and marble.

Rock Types: Metamorphic Rock

Slate Schist

Marble Gneiss