3 Rocks-Types, Rock cycle

Meenal Mishra

ROCKS -TYPES, ROCK CYCLE

1. Objectives

1. Define rocks and classify them based on their origin
2. Distinguish the three principal rock types
3. Explain the concept of rock cycle
4. Discuss the importance of study of rock

3. Description

3.1 What are Rocks?

Rocks occur on the Earth’s surface either as broken chunks (pebbles, cobbles or boulders) that have moved down along slope or by being transported in ice, water or wind or as bedrock that is still attached to the Earth’s crust. Geologists refer to an exposure of bedrock as an outcrop that may appear in the field as ridge or cliff, along road/railway cuttings or nala or stream cutting (Figure 1).

Rock is a naturally occurring solid aggregate of minerals. They provide a historical record of geologic events which give insight into interactions among components (crust and mantle) and spheres (lithosphere, atmosphere, hydrosphere and biosphere) of the Earth System. Rocks occur in a range of colours and textures.

Petrology is the branch of geology that studies rocks and the conditions under which they are formed. It deals with the origin, occurrence, structure, composition and history of rocks. The term ‘petrology’is derived from the two Greek words pétros and logos meaning “rock” and ‘discourse or explanation’ respectively.

Rock is a coherent, naturally occurring solid, consisting of an aggregate of minerals. Let us analyse the different aspects of this definition.

Coherent: Minerals are held in a rock together and can be separated. However, a pile of unattached mineral grains, e.g. loose sand does not constitute a rock.

Naturally occurring: Materials like brick do not qualify to be a rock. Rock has to be naturally occurring.

Aggregate of minerals: You have read that rocks consist of aggregate of mineral grains grown or stuck together.

Let us understand the definition of rock with the help of an example. Look carefully at granite with a hand lens (Figure 2). Granite rock comprises primarily of quartz, K-feldspar, plagioclase feldspar, biotite and mica or hornblende. There are some translucent portions in granite which are constituted of mineral quartz. Flesh pink mineral with tabular habit and pearly luster is K-feldspar (potash feldspar). White mineral with tabular habit and striations, shows properties of plagioclase feldspar. While the brown mineral occurring in thin sheets or flakes and showing pearly luster is mineral biotite mica. You may also find dark coloured mineral with stubby crystals which is probably hornblende. Other minerals like zircon and sphene can occur in granite, but they are not as common as those mentioned above.

Each of the constituent mineral retains its properties in the aggregate of minerals that comprise a rock. A few rocks are composed of non-mineral matters. Coal is considered as rock as it  often occurs in layered structure although it consists of organic material. Obsidian and pumice are considered as volcanic rocks even though they are made of glassy material.

3.2 Classification of Rocks

It took nearly a century to unravel the origin of rock. Neptunists, led by Abraham Werner (1749– 1817) believed that all rocks were derived by the processes of chemical precipitation from ocean. Plutonist group championed by James Hutton (1726–1797) were of the belief and claimed by demonstrating the presence of igneous rocks that they were the evidence of Earth’s internal heat. James Hutton, a Scottish gentleman farmer and doctor, with many others fostered the idea of the genetic – classification of rocks. Hutton is revered as the “Father of Geology”., After much struggle by the end of the eighteenth century most geologists had accepted the genetic scheme of classifying rocks. Based on their origin the rocks are classified

into three groups or types of rocks. They are igneous rocks, sedimentary rocks, and metamorphic rocks (Figure 3).

According to the estimation made by Clarke and Washington, the lithosphere consists of 95% igneous rocks, 5% sedimentary rocks, including shale, sandstone and limestone (the metamorphic rocks being the altered equivalent of one or other of these rocks).

1. Igneous rocks are solidified from molten or partly molten material called magma. They also referred as primary rocks, e.g. granite, basalt, dolerite.
2. Sedimentary rocks form by the cementing together of loose clasts or sediments (fragments or grains) that had been produced by physical, chemical or biological weathering of pre-existing rocks (igneous, metamorphic or even earlier formed sedimentary rocks) when they are subjected to geological agents like wind, water, glacier, etc. They may also result from chemical precipitation from a solution, practically, at a normal temperature and pressure in sea or ocean. They are also termed secondary rocks because they are derived from pre-existing rocks, e.g. sandstone, shale, limestone.
3. Metamorphic rocks are derived from pre-existing igneous, sedimentary or metamorphic rocks when they suffer pressure and temperature and undergo mineralogical, chemical and/or structural changes, e.g. phyllite, quartzite, marble.

Figure 3. Sketches showing different physical appearance of (a) igneous, (b) sedimentary and (c) metamorphic rocks

We can now summarise the general characteristics of igneous, sedimentary and metamorphic rocks in Table 1.

Table 1 General characteristics of three major groups of rocks

3.3 Igneous Rocks

Igneous rocks (word derived from the Latin ignis, meaning “fire” or “to ignite”) are formed by cooling and crystallisation of hot molten material called magma, which rises up from the mantle inside Earth and cools. This cooling may happen either below or above the surface of the Earth. Thus depending upon whether the cooling and crystallization of igneous rocks took place beneath or above the surface of the Earth. They may be grouped into intrusive or extrusive igneous rocks respectively.

1. Intrusive igneous rocks are formed by the cooling and crystallization of magma at depth. Intrusive also known as plutonic rocks crystallise when magma cools in the magma chamber or intrudes the country rocks or the rock bodies enclosing an intrusive mass of igneous rock. The magma beneath the surface of the Earth undergoes slow cooling resulting in the formation of large crystals giving rise to coarse grained rocks, recognized by their large, interlocking crystals visible in hand specimen, e.g. granite, granodiorite, gabbro, and diorite.

2. Extrusive igneous rocks are also known as volcanic rocks. They are formed when the hot molten material erupts at Earth’s surface, spreads out as lava flow and undergoes rapid cooling in the contact with air and water. They may have air cavities or vesicles, indicating that gas has escaped from the site on release of pressure. If the overlying rock has fractures, then the pressure may be released and a sizeable volume of molten rock will extrude to the surface. Extrusive igneous rocks, such as basalt, rhyolite, trachyte are easily recognised by their fine grained or glassy texture.

The igneous rocks based on their mode of occurrence can be classified into three types (Figure 4) based on their texture:

Plutonic rocks: The term plutonic is derived from Pluto the Roman God of the underworld. These rocks undergo cooling and consolidation beneath the surface of the Earth or with in the magma chamber such as granite, gabbro. Plutonic rocks occur as intrusive bodies like batholith, e.g. Mount Abu, Ladakh batholith.

Volcanic rocks: These rocks undergo cooling and consolidation at the surface of the Earth in contact with air or water such as basalt, rhyolite. They occur as extrusive bodies like lava flow, e.g. Deccan basalts, Malani rhyolite, Jodhpur.

Hypabyssal rocks: These rocks undergo cooling and consolidation at the shallow level/ near the surface of the Earth such as dolerite. They are medium grained and occur often as dykes or sills.

The most of the minerals present in igneous rocks are silicates, partly because -silica is – abundant in Earth’s crust and partly because many silicate minerals melt at the high temperatures and pressures reached in deeper parts of the crust and in the mantle. The silicate minerals most commonly found in igneous rocks include quartz, feldspars, micas, pyroxenes, amphiboles and olivine.

3.4 Sedimentary Rocks

The sedimentary rock is formed at or near the surface of the Earth in one of several ways discussed in section 3.2. We can compare the layers of sedimentary rocks to the pages of

book that record stories of earlier events and environments of our dynamic planet Earth.

Sedimentologists are a specific group of geologists who study sedimentary rocks.

Based on their origin sedimentary rocks can be classified into clastic and nonclastic rocks (Figure 5).

1. Clastic rocks comprise siliciclastic sediments which are made up of physically deposited particles such as grains of quartz and feldspar derived from weathered pre-existing rocks (the term ‘clastic’ is derived from the greek word klastos, meaning “broken”). These sediments are laid down by geological agents like water, wind and ice. The most abundant silicate minerals in siliciclastics sedimentary rocks are quartz, feldspar and clay minerals. Clay minerals are formed by weathering and alteration of pre-existing silicate minerals, such as feldspar. Some dark minerals like pyroxene and amphiboles, micas and garnet may also be present. Sediments are the precursors of sedimentary rocks that are found at Earth’s surface as layers of loose particles, such as sand, silt, and the shells of organisms. These particles originate in the processes of weathering and erosion. The loose grains of sediment transform into sedimentary rock by following five steps:

Weathering refers to the entire chemical, physical and biological processes that break up and decay rocks into fragments and dissolved substances of various sizes. These particles are then transported by erosion, the set of processes that loosen soil and rock – and move them downhill or down stream to a place where they are deposited as layers of sediments.

Erosion refers to the combination of processes that separate rock or regolith such as abrasion, plucking caused by moving air, water or ice.

Transportation can occur by gravity, wind, water or ice. They can carry sediments. The ability of a medium to carry sediment depends on its viscosity and velocity.

Deposition is the process by which sediments (a) settles out of transporting medium due to decrease in velocity or (b) precipitate from a solution due to saturation or change in temperature/pressure, the medium is no longer sediment carry.

Lithification is the transformation of the loose sediment into solid rock. During lithification the sediments accumulate in layers, compress under their own weight and/or what buries them and form a hardened mass.

2. Nonclastic rocks consist of the biological and chemical group of sediments that form by the growth of shell masses or cementing together of shells and shell fragments; by the accumulation and subsequent alteration of organic matter from living organisms; or by the precipitation of minerals from water solutions. Calcite is precipitated by marine organisms to form shells or skeletons which form biological sediments when the organisms die. The most abundant minerals of chemical and biological sediments are carbonates such as calcite, the main constituent of limestone.

Figure 5 (a) Clastic sedimentary rock, coarse and medium grained sandstone, (b) Non clastic sedimentary rock, limestone

Different kinds of sedimentary rocks are identified on the basis of their mineral composition. According to some estimates, 70% to 85% of all sedimentary rocks on Earth are clastic, whereas 15%-25% are carbonate biochemical or chemical rocks.

Geologists can work backward using evidences provided by a sedimentary rock’s mineral content, texture, and physical structure to infer the sources of the sediments from which these rocks were formed and environment of their deposition.

3.5 Metamorphic Rocks

Metamorphic rocks take their name from the Greek words meta meaning ‘change’ (meta) and morphe, meaning ‘form’. A metamorphic rock is one that (a) forms when a pre-existing rocks or protolith; (b) undergoes a solid-state change in response to the modification of its environment. This process of change is called metamorphism. The rocks undergo metamorphism when they are subjected to high temperature and pressures deep within Earth. This results in changes in the mineralogy, texture or chemical composition of any kind of pre-existing rock-igneous, sedimentary or other metamorphic rock-while maintaining its solid form.

The temperatures of metamorphism are below the melting point of the rocks (about 700oC) but high enough (above 250oC) for the rocks to be changed by recrystallisation and chemical reactions. Metamorphism can produce a group of minerals which together make up a metamorphic minerals assemblage. Their texture is defined by the new or re-arrangement of mineral grains. Commonly, the texture results in metamorphic foliation defined by the parallel alignment of platy minerals (such as mica) and/or the presence of alternating light coloured and dark coloured bands. Metamorphic rocks can be grouped into two types (Figure 6)-foliated, e.g. phyllite, schist and nonfoliated, e.g. quartzite, marble. For example the metamorphism of granite, a rock with randomly oriented crystals can produce a metamorphosed rock like schist showing parallel alignment of platy minerals (such as mica) or gneiss with alternating light coloured and dark coloured bands.

The formation of metamorphic minerals and textures takes place slowly-it may take – millions of years. The most common processes are:

Recrystallisation, which changes the shape and size of grains without changing the identity of the mineral making up the grains.

Phase change, which transforms one mineral into another mineral with the same composition but with a different crystal structure.

Metamorphic reaction or neocrystallisation (from the Greek neos, for new) which results in the growth of new mineral crystals that differ from those of the protolith.

Pressure solution, which happens when a wet rock is squeezed more strongly in one direction than in others, producing ions that migrate through the water to precipitate elsewhere.

Plastic deformation, which happens when a rock is squeezed or sheared at elevated temperatures and pressures. Under such conditions minerals behave like soft plastic and change shape without breaking.

Figure 6. Metamorphic rocks (a) foliated-schist, (b) nonfoliated-marble

Common minerals of metamorphic rocks are silicate minerals like quartz, feldspar, micas, pyroxenes and amphiboles. Several other silicate minerals like kyanite, andalusite and some varieties of garnet, are good indicators of metamorphism. Calcite is the mineral of marble which is metamorphosed limestone. Similarly quartz is the mineral of quartzite which is metamorphosed sandstone.

3.6 Rock Cycle

Figure 6. Metamorphic rocks (a) foliated-schist, (b) nonfoliated-marble

Common minerals of metamorphic rocks are silicate minerals like quartz, feldspar, micas, pyroxenes and amphiboles. Several other silicate minerals like kyanite, andalusite and some varieties of garnet, are good indicators of metamorphism. Calcite is the mineral of marble which is metamorphosed limestone. Similarly quartz is the mineral of quartzite which is metamorphosed sandstone.

3.6 Rock Cycle

Earth has witnessed the transformation of the rocks innumerable times in the time span of 4.57 Ga in its history. Earth formed from a ball of melted material during the birth of the solar system. Thereafter the Earth cooled, and solidified at the surface forming a shell of solid igneous rock. While the Earth’s crust was forming it witnessed extensive volcanic activity and thus resulted in the formation of igneous rocks. The spewed gases and steam created the first atmosphere and oceans and therefore the first weather. The weather conditions generated on the surface of the planet wore down the igneous rock into sediments. The sediments collected into the low areas or depressions to accumulate as the mountains wore down. In some places the piles of sediment stacked up for hundreds or thousands of feet. The intense weight of the sediments began to coat the sediment grains as mineral cement. The combination of compaction and cementation caused the sediments to transform into solid sedimentary rock in a process called lithification. The sedimentary and igneous rock formed at the bottom was driven deeper into the Earth under increasing pressure and temperature. These rocks underwent recrystallisation and rearrangement thus forming new minerals giving rise to metamorphic rock. Even though a new rock has formed, the process of heating and pressurisation does not stop. Eventually, the minerals reached their melting points and the rocks turned into liquid magma. The return of the melted rock completes the cycle and the rocks go on to become igneous then sedimentary and metamorphic rocks again. We had discussed the basic concepts of rock cycle.

Now let us look at Figure 7 and discuss rock cycle.

Magma occurs as molten material inside the Earth and is the source of all the igneous rocks. Since Earth was largely molten state during its origin, magma may be considered the beginning of the rock cycle. The relationship between igneous, metamorphic and sedimentary rock constitute a “rock cycle” which is a continuous process. Rock cycle is considered to be operating through ages; it is intimately involved with other cyclic Earth processes. This is one of the basic concepts of geology emerging out of the principle of uniformitarianism principle (the present is the key to the past) was given by Hutton in 1785. Rock cycle is particularly closely related to the plate tectonic processes. It starts with cooling and consolidation of mantle derived magmas at the divergent or convergent boundaries or within intraplate tectonic setting. The erosion of lavas and exposed deep seated rocks produced clastic materials which are transported to low lying depositional basins. The deeply buried sediments in due course are deformed and metamorphosed. The tectonic cycle leads to deformation, reconstitution, uplift and accompanying erosion of fresh rocks so that the cycle continues. The movements of tectonic plates are the main forces driving the rock cycle.

The energy changes or redistribution of energy within the Earth systems is manifested by the operation of rock cycle. The rock cycle illustrates the role of various geologic processes operating and transforming from one type into another. The rock cycle helps us to visualise interrelationships among different components of the Earth systems. The rock cycle explains how geological processes can change a rock from one type to another through geological time.

Figure 7. Rock cycle- rocks are constantly forming, changing and reforming. The rock cycle helps us to understand the origin of three rock groups

3.7 Significance of Study of Rocks

A geologist’s primary aim is to understand the properties of rocks and to deduce their geological origin from those properties. Such deductions enhance understanding of many important aspects of our planet like its origin, information of economically important resources.

Understanding how rocks form also guides us in solving environmental problems. For example, the underground storage of radioactive and other wastes depends on analysis of the rock to be used as a repository. Modern society depends on mineral resources. They are essential for the construction of our cities and for power generation, transportation and communications. They enrich us in art, and we wear them as personal adornments. Rocks are naturally occurring chemical compounds that have been formed by geological processes. They provide chemicals that are essential for life on Earth.

The identity of a rock is determined partly depends on its mineralogy and texture. Largely the physical appearance of a rock depends on colour, grain size and also on the kind of minerals that compose them. Minerals are the building blocks from which rocks are made. Petrologist studies rocks and makes observations.

Why do they petrologists study rocks?

rocks provides clues about the formation of Earth and it’s past because different rocks are formed under specific conditions;

the events that shaped and continue to shape the Earth could be understood by studying rocks, and their constituent minerals;

rocks host valuable metals, mineral deposits, coal, oil, natural gas, ground water and building stones.

The study of the rocks begins with careful recording of observations of the outcrop in the field. Break off a hand specimen of a fist sized piece, so as to examine it more closely with hand lens. This is followed by preparation of thin section and examination under the petrological or polarising microscope. This enables us to study rock composition and texture. The branch of geology dealing with the description and systematic classification of rocks, especially by microscopic examination of thin sections is known as Petrography. Let us evaluate the importance of petrography in geological studies.

It helps to understand the environmental conditions of their formation be it igneous, sedimentary and metamorphic

It throws light on the mineralogical composition and interrelationship between mineral grains

It also enables to discern the kinetics (energy of a body derived from its movement) of their formation.

The petrography provides the basic knowledge of rocks. Beginning in the 1950s, sophisticated high-tech electronic equipments became available that enabled the petrologists to examine rocks on even finer scale. Modern research laboratories have instruments such as electron microprobes (which focus a beam of electrons to a small grain), X-ray diffractometers and mass spectrometers.

4. Summary

In this lecture we learnt about:

Definition of rock

Classification of rocks into three groups- igneous, sedimentary and metamorphic Characteristics and classification of igneous rocks

Process of formation of sedimentary rocks

Processes involved during the formation of metamorphic rocks Concept of rock cycle and its relation to plate tectonics

Significance of studying rocks