TYPES OF ROCKS AND ROCK CYCLE

INTRODUCTION • The earth is composed of various kinds of elements. These elements

are in solid form in the outer layer of the earth and in hot and molten form in the interior.

• About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium and the rest is constituted by titanium, hydrogen, phosphorous, manganese, Sulphur, carbon, nickel and others

INTRODUCTION • Before moving on to the classification of rocks, let us get a clear

picture of the elements present in the Earth’s crust. It is described in the following table.

Important Elements of the Earth’s Crust

Element Percentage

Oxygen 46.6

Silicon 27.7

Aluminum 8.1

Iron 5.0

Calcium 3.6

Sodium 2.8

INTRODUCTION • More than one clement of the earth's crust are organized to form

compounds which are known as minerals and minerals are organized to form rocks. The important mineral groups are silicates, carbonates, sulphides, metal oxides etc.

INTRODUCTION 1. The silicate minerals are very important rock making minerals. The most outstanding rock-forming silicate mineral groups are quartz and feldspar.

• Feldspar: Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminum etc. are found in specific feldspar variety. Half of the earth’s crust is composed of feldspar. It has light cream to salmon pink colour. It is used in ceramics and glass making.

INTRODUCTION • Quartz: It is one of the most important components of sand and

granite. It consists of silica. It is a hard mineral virtually insoluble in water. It is white or colorless and used in radio and radar among other electronic equipments. It is one of the most important components of granite.

• Mica: It comprises of potassium, aluminum, magnesium, iron, silica etc. It forms 4 per cent of the earth’s crust. It is commonly found in igneous and metamorphic rocks. It is used in electrical instruments.

INTRODUCTION 2. Carbonate group of minerals is very much susceptible to chemical weathering and erosion in humid areas. Calcite is the most important mineral of this group.

3. Sulphide minerals include pyrites, iron sulphides etc. When these minerals come in contact with water or air, these form ferric hydroxides and sulfuric acids.

INTRODUCTION 4. Metallic elements like iron, aluminum etc. after reacting with atmospheric oxygen form metal oxides which are commercially very important. They are sub-divided into following categories:

a. Precious metals: gold, silver, platinum etc.

b. Ferrous metals: iron and other metals often mixed with iron to form various kinds of steel.

c. Non-ferrous metals: include metals like copper, lead, zinc, tin, aluminum etc. Non-ferrous metals

ROCKS • The earth’s crust is composed of rocks. Rocks are sold huge

aggregation of minerals. Rock may be hard or soft depending on their constituent minerals. Also rocks can be found in varied colors for the same reason. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents.

ROCKS • An elementary study of rocks is very important for us to study the

lithosphere and related landforms and processes associated with it. Feldspar and quartz are the most common minerals found in rocks. Study of rocks will also help further in understanding of soil formation process.

ROCKS • Petrology is science of rocks. A petrologist studies rocks in all their

aspects viz., mineral composition, texture, structure, origin, occurrence, alteration and relationship with other rocks. There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation.

TYPES OF ROCKS 1. Igneous Rocks: solidified from magma and lava;

2. Sedimentary Rocks: formed through the lithification and compression and cementation of the sediments;

3. Metamorphic Rocks: formed due to change either in the form or composition of either igneous or sedimentary rocks.

TYPES OF ROCKS Igneous Rocks

• As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.

• Igneous rocks are also called as primary rocks because these were originated first of all the rocks during the formation of upper crust of the earth.

IGNEOUS ROCKS • The igneous rocks are formed when magma cools and solidifies.

When magma in its upward movement cools and turns into solid form it is called igneous rock.

• All the subsequent rocks were formed, whether directly or indirectly, from the igneous rocks in one way or the other. This is why igneous rocks are also called as parent rocks.

IGNEOUS ROCKS Types of Igneous Rocks

• The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth. The igneous rocks are more commonly classified on the basis of the mode of occurrence into two major groups viz.

• Intrusive igneous rocks

• Extrusive igneous rocks.

IGNEOUS ROCKS Intrusive Igneous Rocks:

When the rising magmas during a volcanic activity do not reach the earth's surface rather they are cooled and solidified below the surface of the earth, the resultant igneous rocks are called intrusive igneous rocks. Cooling and solidification inside the surface of the Earth is slow as heat is trapped inside for a longer duration of time. Therefore, crystals of intrusive rocks are large. Example: Granite, gabbro, etc.

IGNEOUS ROCKS Extrusive Igneous Rocks:

The igneous rocks formed due to cooling and solidification of hot and molten lavas at the earth's surface are called extrusive igneous rocks. Sudden cooling at the surface results in small and smooth grains. There is sudden cooling because heat of magma is directly transferred to the air. These rocks are also called as volcanic rocks. Example: Basalt.

IGNEOUS ROCKS Characteristics of Igneous Rocks

• Igneous rocks do not have strata like sedimentary rocks.

• Igneous rocks do not contain fossils because

• When the ancient igneous rocks were formed due to cooling and solidification of molten rock materials at the time of the origin of the earth, there was no life on newly born earth and

• Since the igneous rocks are formed due to cooling and solidification of very hot and molten materials and hence any remains of plants or animals (fossils) are destroyed because of very high temperature.

IGNEOUS ROCKS Characteristics of Igneous Rocks

• Igneous Rocks are rich in metallic minerals like iron, cobalt, etc.

• Igneous rocks are mostly associated with the volcanic activities and thus they are also called as volcanic rocks. Igneous rocks are generally found in the volcanic zones.

• Since water does not penetrate the rocks easily and hence igneous rocks are less affected by chemical weathering

SEDIMENTARY ROCKS • The word ‘sedimentary’ is derived from the Latin word ‘sedimentum’,

which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments.

• Such fragments are transported by different exogenous agencies and deposited.

• These deposits through compaction turn into rocks. This process is called lithification.

• Thus, sedimentary rocks are formed by the process of sedimentation, compaction, cementation and solidification.

SEDIMENTARY ROCKS • Compaction: Process of removing water from sediments deposited on

the surface under pressure which is a result of increasing thickness of sediments over a period of time. With compaction, particles come close to each other.

• Cementation: It is aggregation of sediments.

• Solidification: With cementation, unconsolidated sediments are changed into solidified and consolidated blocks.

SEDIMENTARY ROCKS Types of Sedimentary Rocks

• Depending upon the mode of formation, sedimentary rocks are classified into three major groups:

• Mechanically formed: Previously formed rocks are subjected to mechanical or physical disintegration and thus the rocks are broken into fragments of different sizes. These materials are obtained, transported and deposited at suitable places by different exogenous processes (geological agents) like running water (rivers), wind, glaciers, and sea waves. Sandstone, conglomerate, limestone, shale, loess etc. are examples;

SEDIMENTARY ROCKS Types of Sedimentary Rocks

• Organically formed: The sediments derived from the disintegration or decomposition of plants and animals are called organic sediments. Geyserite, peat, coral reefs, chalk, limestone, coal etc. are some examples;

SEDIMENTARY ROCKS Types of Sedimentary Rocks

• Chemically formed: Running water contains chemical materials in suspension. These chemical materials after being settled down and compacted and cemented form chemical sedimentary rocks on the beds of lakes, ponds, rivers, sea, oceans, etc. Gypsum, salt rocks chert, limestone, halite, potash etc. are some examples.

SEDIMENTARY ROCKS Characteristics of Sedimentary Rocks

• Sedimentary rocks contain several layers or strata but these are seldom crystalline rocks. Hence, they are stratified rocks.

• Sedimentary rocks are found over the largest surface area of the globe. It is believed that about 75 per cent of the surface area of the globe is covered by sedimentary rocks. Hence these rocks are the most important source of soil.

SEDIMENTARY ROCKS Characteristics of Sedimentary Rocks

• Sedimentary rocks are formed of sediments derived from the older rocks, plant and animal remains and thus these rocks contain fossils of plants and animals.

• Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous and impermeable

METAMORPHIC ROCKS • Metamorphic rocks, as the word 'metamorphism' implies, are formed

due to changes in the forms of other rocks. These rocks form under the action of pressure, volume and temperature (PVT) changes.

• Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.

• Gneiss, granite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.

METAMORPHIC ROCKS • The change in the form of the rocks during the process of

metamorphism takes place in two ways viz.

(i) physical metamorphism pertaining to changes in textural composition of the rocks and

(ii) chemical metamorphism, leading to changes in the chemical composition of the rocks. Sometimes, both the processes of metamorphism become operative together.

METAMORPHIC ROCKS • Sometimes, the form of the rocks is so changed due to intense

metamorphism that it becomes difficult to find out the original form of the rocks. Some rocks, after metamorphism, become harder than their original forms, such as marble from limestone and quartzite from sandstone.

METAMORPHIC ROCKS • Foliation, defined as streaking or parallel arrangement of the

constituent crystals (of metamorphic rocks) is the most common characteristic feature of metamorphic rocks.

• The coarse grained metamorphic rocks are imperfectly foliated (e.g. gneisses from granites) while fine-grained metamorphic rocks are perfectly foliated (e.g. schist from shale).

METAMORPHIC ROCKS Types of Metamorphic Rocks

• The agents and factors of metamorphism sometimes operate separately and sometimes work together. The processes of metamorphism may be classified on the bases of

(i) the nature of the agents of metamorphism and

(ii) place and area involved in metamorphism.

METAMORPHIC ROCKS On the basis of the nature of agents:

• Thermal metamorphism (due to heat): The materials of rocks chemically alter and recrystallise due to thermal metamorphism.

• Dynamic metamorphism (due to pressure): Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.

SEDIMENTARY ROCKS On the basis of place or area

• Contact metamorphism (localized in area): In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.

• Regional metamorphism (involving larger area): In regional metamorphism, rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both.

SEDIMENTARY ROCKS Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.

Original Rock Metamorphic Rock

Basalt Greenstone

Clay Slate

Coal Anthracite, Graphite, Diamond

Limestone Marble

Sandstone Quartzite

Granite Gneiss

ROCK CYCLE • Rocks do not remain in their original form for long but may undergo

transformation.

• Rock cycle is a continuous process through which old rocks are transformed into new ones. Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks. Igneous rocks can be changed into metamorphic rocks.

• The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks.

ROCK CYCLE • The crustal rocks (igneous, metamorphic and sedimentary) once

formed may be carried down into the mantle (interior of the earth) through subduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence) and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.

ROCK CYCLE