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Chapter 6: Rocks

The sea has cut a wall of pale rock along the coast of Normandy, in northern France where the land meets the English Channel. From a distance it looks like the most permanent thing on that shore. Walk to its foot, though, and the beach is littered with fallen blocks of the same pale stone, broken off the cliff and waiting for the next tide to take them. Split one of those blocks open and you may find a shell inside it, whole and ribbed, set in the stone like a coin in a wall. That shell is the clue. The cliff is limestone, and limestone is built from the shells and skeletons of sea animals — corals, clams, oysters — that lived in shallow water, died, sank, and settled layer upon layer on the sea floor. The animals are long gone. The rock is what they left behind. So the cliff is not the beginning of the story. It is the middle of one. The material in it was loose sediment before it was rock, and living before it was sediment. The pieces breaking off it now will be buried and pressed into rock again somewhere else.

Diagram

Fig. 6.A1 Caption: The cliff is made of layers, and each layer was once loose material lying on a sea floor. Brief: Cross-section of a coastal cliff — horizontal bands of limestone with shell fragments drawn inside them, fallen blocks at the base, arrows pointing from cliff face down to the blocks and from the blocks out to sea.

NoteCuriosity Corner

Q1. Why are fossils found in sedimentary rocks and almost never in igneous or metamorphic ones? Q2. What decides whether cooling magma makes a rock with large crystals or one with tiny ones? Q3. The cliff is limestone. What would have to happen to it to turn it into marble? Q4. Rocks have been breaking down for 4.5 billion years. Why has the earth not run out of them? This chapter follows rock through every one of these changes, and by the end you will be able to answer all four.

6.1 Rocks And Minerals

A rock is an aggregate of minerals that forms more or less a definite unit of the earth’s crust. An aggregate is simply a mass made of many separate pieces held together. The word rock does not refer only to hard solid matter like granite. Soft and loose particles like sand, silt and clay derived from the earth are rock material too, even though you can pour them through your fingers.

The earth’s crust — the thin outer shell of solid material we live on, build on and dig into — is not made of one kind of rock. It is made of many types, and they differ from one another in ways that can be seen, felt and measured.

Rocks are told apart from one another by six properties. (a) Texture is the size and arrangement of the grains or crystals in the rock, which is why granite looks coarse and speckled while clay feels smooth between the fingers. (b) Structure is the way the rock is built in bulk — whether it lies in separate layers, or forms one solid mass with no layering at all. (c) Colour depends on which minerals went into the rock, so a rock rich in iron looks dark while one rich in quartz looks pale. (d) Permeability is the ability of a rock to let water pass through it, and a rock with connected pore spaces between its grains lets water through where a tightly crystallised one does not. (e) Mode of occurrence is where and in what form the rock is found — deep inside the crust, spread in sheets at the surface, or filling a crack in older rock. (f) Degree of resistance to denudation is how well a rock survives denudation, the wearing away and removal of rock from the earth’s surface, and it decides which rocks stand up as high ground and which are stripped down to lowland.

What A Mineral Is

A mineral is a naturally occurring inorganic solid with a definite chemical composition and a crystalline structure. Inorganic means it was not made by a living thing. Definite chemical composition means that the same mineral always contains the same elements in the same proportions, wherever in the world it is found. Crystalline structure means its atoms are arranged in a regular repeating pattern.

Rocks are aggregates of one or more minerals. The relationship runs in one direction only: rocks are composed of minerals, and not the other way round. Behind the minerals lie the elements. An element is a pure substance; elements are organised to form compounds, which are known as minerals; and minerals are organised to form rocks.

What The Earth Is Made Of

The most abundant mineral elements of the earth as a whole are iron, silicon, magnesium, nickel, sulphur and calcium. The most abundant mineral elements of the crust are silicon, aluminium, iron, magnesium, calcium, potassium and sodium. The two lists overlap, but they are not the same list, and the difference matters. The earth’s crust contains more of silicon and aluminium, whereas the earth as a whole contains more of iron, silica and magnesium.

What The Whole Earth Is Made Of Evidence: Every rock sample we hold comes from the crust, yet the earth’s average density is far higher than the density of crustal rock. Reasoning: Material much denser than crust must lie beneath it, and iron-rich material fits that density. Limit: Nobody has sampled the deep interior, so its composition is inferred, never weighed directly.

The difference between a rock and a mineral now comes down to one idea. A mineral is a single substance with a fixed recipe, so its chemical composition is definite; a rock is a mixture of such substances, so it has no definite chemical composition of its own. Both occur naturally in the crust, and both are solid, but they sit at different levels of organisation. There are three chief types of rocks — igneous, sedimentary and metamorphic — while the four chief mineral groups are silicates, carbonates, sulphides and metallic minerals. Table 6.1 sets the two side by side.

Table 6.1 Caption: Difference between rocks and minerals. | Rocks | Minerals | |—|—| | 1. Rocks are aggregates of minerals. | 1. Minerals are solid inorganic substances occurring naturally. | | 2. A rock has no definite chemical composition. | 2. Minerals have a definite chemical composition. | | 3. Minerals are organised to form rocks. | 3. Elements are organised to form compounds which are known as minerals. An element is a pure substance. | | 4. Three chief types of rocks are Igneous, Sedimentary and Metamorphic. | 4. Four chief mineral groups are silicates, carbonates, sulphides and metallic minerals. | | 5. Basalt, granite, sandstone, slate and quartz are some important types of rocks. | 5. Iron, silicon, magnesium, nickel, calcium, potassium, sodium are abundant minerals of the earth. |

Table 6.1 The rows are matched pairs, so read across, not down: row 2 sets ‘no definite chemical composition’ against ‘definite chemical composition’. Every pair traces back to one fact — a rock is a mixture, a mineral is not. What the table does not tell you is how any of these rocks or minerals came to be, which is the rest of this chapter.

6.2 Types Of Rocks

Rocks can be classified in several ways — by colour, by hardness, by the minerals in them. The important classification, and the one this chapter follows, is done on the basis of their origin. Origin means the way a rock came into being, and it is the most useful test because almost every other property of a rock follows from it.

Three main types of rocks are recognised on the basis of origin. (a) Igneous rocks are formed when hot molten material from inside the earth cools and hardens, either below the surface or on it. (b) Sedimentary rocks are formed when loose material worn away from older rocks settles in layers and is slowly pressed and cemented into solid stone. (c) Metamorphic rocks are formed when a rock that already exists is altered in form by heat, pressure or both, without ever becoming loose material.

Q. Why classify rocks by how they formed rather than by how they look?

A. Because origin explains the looks. Once you know how a rock formed, its texture, its layering and its hardness all follow from that.

The three photographs in Fig. 6.1 show one specimen from each of these groups. Look at the surfaces rather than the shapes: the layering, the grain and the banding are the clues that a trained eye reads.

Diagram

Fig. 6.1. (a) Caption: Igneous rocks. Brief: Photograph of an igneous rock specimen; label the visible crystal grain and keep the whole specimen in frame against a plain background.

Diagram

Fig. 6.1. (b) Caption: Sedimentary rocks. Brief: Photograph of a sedimentary rock specimen; the horizontal banding of separate layers must be clearly visible.

Diagram

Fig. 6.1. (c) Caption: Metamorphic rocks. Brief: Photograph of a metamorphic rock specimen; the wavy bands running through the rock must be legible at printed size.

The three groups do not stand as equals. Igneous rocks are called primary rocks, because they were the first to form, and both of the other groups are built from material that was igneous to begin with. Sedimentary and metamorphic rocks are therefore called secondary rocks. Each group also divides further, according to where or how the rock formed, and Fig. 6.A2 sets out the whole classification in one branching chart.

Flowchart

Fig. 6.A2 Caption: Rocks sorted by origin, with a common example named at the end of every branch. Nodes: classification of rocks → igneous rocks (primary rocks) → intrusive (granite) / extrusive (basalt); classification of rocks → sedimentary rocks (secondary rocks) → mechanically formed (sandstone) / organic (coal, limestone) / chemically formed (gypsum); classification of rocks → metamorphic rocks (secondary rocks), heat and pressure altered rocks (marble, slate)

Each of the three branches is taken in turn in the sections that follow, and the branch names shown here are the ones used throughout the rest of the chapter.

6.3 Igneous Rocks

The word igneous is derived from the Latin word ‘ignis’, meaning fire, and the name is a fair description of where these rocks come from. Igneous rocks were formed by the cooling, solidification and crystallisation of hot molten material of the earth, known as magma, which is found at great depths in the interior of the earth. Cooling is the loss of heat, solidification is the change from liquid to solid, and crystallisation is the growth of mineral crystals inside that solidifying mass. All three happen together as the melt loses its heat.

The chemical composition of igneous rocks varies. It runs from alumino-silicate minerals like quartz and feldspar to granite and rhyolite, which are collectively known as Sialic rocks. Since the igneous rocks were the first to be formed, they are called primary rocks, and they form the basis of formation of the other types of rocks. Every grain of sand on a beach and every slab of marble in a building can be traced back, through however many steps, to a rock that was once molten.

6.3.1 Characteristics Of Igneous Rocks

Igneous rocks share nine characteristics, and almost all of them follow from the single fact that the rock cooled from a melt. (a) They are hard and compact, because the mineral crystals grew tightly against one another as the melt solidified and left no loose space between the grains. (b) They are formed by the solidification of molten magma, so every igneous rock, without exception, began as a liquid. (c) They are granular and crystalline, since the minerals in them grew as crystals rather than being laid down as ready-made grains. (d) They are either fine grained, smooth and compact, or they may have large crystals with a coarse texture. (e) The size of their crystals depends upon the rate of cooling of the molten material, because crystals need time to grow and fast cooling does not give them that time. (f) These rocks are less affected by chemical weathering, the breaking down of rock by chemical change, because water does not percolate into them — there are no gaps for it to seep through. (g) Most of them consist of silicates, the group of minerals built around silicon and oxygen. (h) They do not have layers, and they are generally weathered by mechanical weathering, which breaks rock into smaller pieces without altering what it is made of. (i) Igneous rocks are associated with volcanic activity and are found mostly in volcanic regions, because that is where molten material reaches the surface or comes close to it.

6.3.2 Classification Based On Origin

Based on their origin, the igneous rocks are classified into two types — extrusive igneous rocks and intrusive igneous rocks. The dividing line is simply whether the magma cooled above the ground or below it, and everything else about the two groups follows from that one difference.

Extrusive Igneous Rocks

Extrusive igneous rocks are formed by the cooling of molten magma on the earth’s surface. The magma is brought to the surface through fissures, which are long cracks in the crust, or through volcanic eruptions, and it solidifies there to form rock. Because the open air and the surface are far cooler than the interior of the earth, this cooling is quick. Hence such rocks are smooth, crystalline and fine grained, and they are also called volcanic rocks.

Basalt is a common extrusive igneous rock. It forms when lava — the name given to magma once it has reached the surface — cools on the earth’s crust, whether it arrives as a lava flow, a lava sheet or a lava plateau. Some kinds of basalt solidify to form long polygonal columns, that is, columns with several straight sides. The Giant’s Causeway, on the north coast of Northern Ireland, is a well-known example, and Fig. 6.4 shows the columns packed side by side like stacked pillars.

Diagram

Fig. 6.4 Caption: Giant’s Causeway, Northern Ireland. Note the columns of basalt. Brief: Photograph of the basalt column pavement at the Giant’s Causeway; the many-sided tops of the columns and their tight packing must both be clearly visible.

Intrusive Igneous Rocks

Intrusive igneous rocks form when the magma solidifies within the earth’s crust instead of reaching the surface. Surrounded by warm rock on every side, the magma loses its heat very slowly, and the slow cooling gives the growing crystals a long time to enlarge. The result is coarse textured, hard rock with large crystals — granite, gabro and dolerite are examples. When magma solidifies below the earth’s surface it also builds different types of intrusive rock bodies, shown in Fig. 6.2.

Q2. What decides whether cooling magma makes a rock with large crystals or one with tiny ones?

A. The rate of cooling. Magma trapped in the crust cools slowly, so crystals have time to grow large and the rock is coarse. Magma poured out on the surface cools fast, so the crystals stay small and the rock is fine grained.

Five intrusive forms are named in this chapter, each defined by the shape the magma took as it hardened. (a) Batholiths are deep-seated intrusions of igneous rock, usually dome-shaped with no definite floor, and they form the cores of mountain ranges — the name comes from bathos, meaning depth. They are chiefly composed of granite, and continued erosion may in time strip away the rock above them and expose them at the earth’s surface. The Ranchi Batholiths, in Jharkhand in eastern India, are an important example. (b) Laccoliths are formed when the magma does not reach the crust of the earth and becomes solid just below it, and because different types of intrusions of magma form differently shaped landforms, the laccolith has a shape of its own — a dome-shaped upper surface with a flat bottom, from the Greek lakkos, meaning cistern. (c) Sills are formed when magma flows horizontally between existing layers of rock and hardens there, so a sill lies flat, following the layers it pushed between. (d) Dykes are formed when magma is forced upwards into vertical cracks or fissures in existing rocks and hardens there, so a dyke cuts straight across the layers instead of running along them. (e) Necks are formed when the passage of an extinct volcano is filled with magma that solidifies inside it, leaving a solid plug of rock in the pipe the lava once used, and this is known as a volcanic neck or plug.

Diagram

Fig. 6.2 Caption: Intrusive igneous rocks. Brief: Cross-section through the crust showing magma below, with batholith, laccolith, sill, dyke and volcanic neck labelled, a lava flow at the surface, and arrows showing magma rising upwards from the magma body into each form.

Fig. 6.2 Depth increases downwards, so the band at the top is the surface and the mass at the bottom is magma. Read each form by its shape and position — a sill lies flat between layers, a dyke cuts vertically across them, a batholith sits deepest. The drawing shows neither true scale nor time: these forms are not all the same size and did not form together.

Intrusive igneous rocks are also called plutonic rocks, because of the slow cooling of magma at great depth, which leads to the formation of crystals of large size. Not all intrusions are equally deep, though. Hypobyssal rocks are formed when magma cools at intermediate depth and forms minor rocks like sills and dykes — shallower than a batholith, deeper than a lava flow.

6.3.3 Classification On The Basis Of Chemical Composition

Igneous rocks are also classified on the basis of their chemical composition, into acid igneous rocks and basic igneous rocks. Here the test is not where the rock cooled but how much silica it contains.

Acid Igneous Rocks

Acid igneous rocks have a silica content between 65 to 85 per cent. They generally lack in iron and magnesium. Granite is an acid igneous rock. It is made up of large crystals of quartz, feldspar and mica, and it is commonly used as a building material.

Basic Igneous Rocks

Basic igneous rocks have a higher percentage of oxides of denser elements, and their silica content varies between 40 to 60 per cent. Such rock is heavy and dark in colour. Basalt, dolerite and gabro are typical examples of this group.

Q. Why does a piece of basalt feel heavier than a piece of granite of the same size?

A. Because basalt is a basic rock. It holds more oxides of dense elements and less silica, so the same volume weighs more.

Igneous rocks are usually compact and are of interest to builders and sculptors, who need stone that will take a clean cut and hold its shape. They may also be associated with ores of metals. The important igneous rocks are gabro, basalt, granite, dolerite, feldspar, pumice, obsidian, permadite and soria. Pumice, one of the rocks in that list, is shown in Fig. 6.3.

Diagram

Fig. 6.3 Caption: Pumice (an igneous rock). Brief: Photograph of a pumice specimen against a plain background; the pitted, hole-filled surface of the rock must be clearly legible.

6.4 Sedimentary Rocks

When igneous rocks are exposed on the surface of the earth, they are broken down by weathering, the breaking down of rock where it stands, and the loose material is then carried away by forces of gradation like rivers, glaciers and ocean waves. Gradation simply means the levelling work these agents do, wearing high ground down and filling low ground in. The material they carry is deposited as sediments.

Any exposed rock, worked on by the various agents of erosion, may weather and disintegrate in this way. The sediments are deposited over long periods in seas, lakes, streams and similar low-lying places. Subsequent layers are added on top, and due to their own weight from above the sediments get cemented together to form sedimentary rocks.

Sedimentary rocks are secondary rocks, because they are formed by the solidification of sediments derived from original igneous, sedimentary or metamorphic parent rocks. Nothing in a sedimentary rock is new material. It has all been rock before, somewhere else.

6.4.1 Characteristics Of Sedimentary Rocks

Sedimentary rocks share five characteristics. (a) They are formed from materials derived from other rocks, and they may also hold plant and animal remains, so these rocks may contain fossils. (b) They may be formed through different processes, and they are also classified on the basis of their formation; the process that turns loose sediments into rocks is called lithification. (c) They are the most widespread rocks on the surface of the earth and constitute about 75 per cent of the surface area of the globe. (d) They are generally not crystalline, and they are soft as compared to igneous rocks and layered, because they are formed by the deposition of sediments one load after another; the different types of sediments are consolidated and compacted by different types of cementing elements like calcium, silica and others. (e) They make extensive landforms, spreading widely but not standing deep, and they do not make massive rock forms like batholiths and laccoliths.

Q. Why is sandstone so much softer than granite?

A. Because its grains merely settled and were glued together afterwards, while granite’s crystals grew locked into one another as the melt cooled.

6.4.2 Processes Involved

Lithification means turning the loose sediments into hard rock, and the word carries its own explanation — ‘lith’ means stone. Transported sediments are deposited layer upon layer, and each new layer presses down on the ones beneath it. This builds up pressure and temperature at depth. Three processes are involved in turning the loose materials into hard rocks.

The three processes are evaporation, compaction and cementation. (a) Evaporation is the escape of water from the loose material, as in the case of rock-salt, whose accumulation mainly takes place through evaporation before the material is compacted and cemented. (b) Compaction is the squeezing of the sediments after they have piled up, and it is caused by the weight of the overlying layers pressing down and the hardness of the underlying layers resisting from below; sand is compacted in this way to sandstone. (c) Cementation is the bonding together of the compacted sediments by natural materials such as calcium compounds, silica and iron, all of which have the property of bonding loose materials into one mass.

Fig. 6.A3 sets out the whole journey, from a solid rock standing exposed to a new rock formed out of its fragments.

Flowchart

Fig. 6.A3 Caption: A sedimentary rock is an old rock taken apart, carried away and put back together somewhere else. Nodes: exposed igneous rock → weathering and disintegration → fragments carried away by rivers, glaciers and ocean waves → deposited as sediments in seas, lakes and streams → later layers added on top → weight of overlying layers raises pressure and temperature → lithification by evaporation, compaction and cementation → sedimentary rock

6.4.3 Classification On The Basis Of Formation

Sedimentary rocks are classified into three groups on the basis of how the sediment came to be lying there in the first place.

Mechanically Formed Rocks

Mechanically formed rocks contain fragments from the breaking up of other rocks, and these sediments are also called Clastic Sediments. The breaking up of older rocks takes place through denudation by agents like running water, moving glaciers and wind. Mechanically formed rocks include shale, sandstone, clay, conglomerates and loess.

In this process, the sediments are gradually squeezed by the weight of the overlying sediments, and the lower layers harden to form rocks. Since such rocks are formed in layers, they are known as Stratified Rocks. For example, sand becomes sandstone, which is shown in Fig. 6.5.

Diagram

Fig. 6.5 Caption: Sandstone. Brief: Photograph of a sandstone specimen against a plain background; the granular surface must be visible at printed size.

Chemically Formed Rocks

Chemically formed rocks are formed by the direct precipitation of mineral matter from solution — that is, the mineral is dissolved in water, and it comes out of the water as a solid and settles. The accumulation takes place in lakes and lagoons, and the deposits are compacted through evaporation. Gypsum, rock salt and potash are examples.

Gypsum, formed mainly by the evaporation of sea water, is found in sedimentary rocks all over the world. Rock salt is found in the Dead Sea and the Aral Sea, both in Asia, as well as in Sambhar Lake in Rajasthan. What these three have in common is that water enters them and leaves mainly by evaporating, so whatever was dissolved in it stays behind.

Organically Formed Rocks

Organically formed rocks contain the remains of dead plants and animals. Limestone, also called calcareous rock, is formed by skeletons, shells and animal remains, and it contains large proportions of lime. Fig. 6.6 and Fig. 6.7 show limestone in the landscape and in the hand.

Diagram

Fig. 6.6 Caption: Cliffs of Normandy. These are made of limestone. Brief: Photograph of the pale sea cliffs at Normandy; keep both the cliff face and the sea at its foot in frame.

Diagram

Fig. 6.7 Caption: Limestone. Brief: Photograph of a limestone surface showing the blocky, pale rock face at close range.

Rocks like peat, lignite, bituminous and anthracite are termed carbonaceous rocks, because carbon from once-living material is what they are largely made of. Other organically formed rocks are Siliceous Rocks, formed due to the dominance of silica contents. Chalk is a form of carbonate rock, and it is formed due to the precipitation of carbonate materials.

In the ancient past there were vast swamps of forest regions that got buried and underwent changes to yield fossil fuels. The vegetable matter undergoes changes after being submerged in underground water, and the formation into rocks proceeds in stages — peat, lignite and coal.

The products of the first phase are peat, which is used as fuel. Lignite is a more decomposed rock of organic matter, and it is also used as fuel. Several new products have been obtained from these rocks in recent years, including wax and resins. In coal, the percentage of carbon is very high, and two types of coal, bituminous and anthracite, are sources of fuel for power generation.

Flowchart

Fig. 6.A4 Caption: The same buried plant material at four stages of change, each richer in carbon than the one before. Nodes: swamp forest buried and submerged in underground water → vegetable matter decomposes under pressure → peat (used as fuel) → lignite (more decomposed, used as fuel) → coal (very high carbon) → bituminous and anthracite (fuel for power generation)

What Coal Was Before It Was Coal Evidence: Coal lies in layers inside sedimentary rock, and the same material can be traced back through lignite and peat, which are less decomposed. Reasoning: A series running from buried vegetation to peat to lignite to coal shows coal is the far end of one process. Limit: This tells us what coal was made from, not how long any one seam took to form.

The remains of animals and plants which have become hard and turned into sedimentary rocks or fuels, such as coal or petroleum, are known as fossil fuels. Every time a power station burns coal, it is releasing energy that a swamp forest captured from sunlight and locked away long before there were people to use it.

6.4.4 Classification On The Basis Of Agents Of Formation

Sedimentary rocks are also grouped by the agent that carried the sediment and dropped it, because each agent leaves its deposit in a place and a form of its own. (a) Riverine rocks are formed by the alluvial deposits brought by the flowing water of streams, alluvium being the fine material a river carries and lays down along its course. (b) Lacustrine rocks are found on the bed of a lake, laid down in bands that correspond to successive periods of deposition. (c) Glacial rocks are formed by glacial deposits in the form of debris or tills, till being the unsorted mixture of material a glacier carries; the glaciers erode the surface and the sides of a valley and transport the eroded material further, and when the glacier melts due to heat, the debris it brought is left behind in the form of moraines, which form glacial rocks that include boulders, gravels and sand. (d) Aeolian rocks are formed with sand particles brought by winds, and the deposition of sand particles one over the other makes them hard and forms sedimentary aeolian rocks; such rocks are found in deserts, and loess is an important aeolian deposit. (e) Marine rocks are formed at the ocean floor, and they are of two types. (i) Calcarious sedimentary marine rocks are formed by the deposition of shells and skeletons of sea organisms — corals, clams and oysters — which live on the ocean floor and extract calcium carbonate from ocean water, and chalk and limestone are examples of such rocks. (ii) Carbonaceous sedimentary rocks are formed by the sea plants which remain buried for a very long period, and these long preserved remains, known as fossils, are converted into the form of coal, lignite and peat due to the pressure of overlying rocks on them.

Fig. 6.8 shows a hollow worn into a sandstone wall, the kind of shape that forms where a layered rock is attacked unevenly.

Diagram

Fig. 6.8 Caption: Alcove in the Navajo Sandstone. Brief: Photograph of a large curved hollow cut into a sandstone rock face; the layering of the rock and the scale of the opening must both read clearly.

Classification of sedimentary rocks is also done in other ways, such as by transportation agents. Their basic characteristics, however, remain the same whichever scheme is used.

How A Fossil Is Made

Fossils are found only in sedimentary rocks, and the reason lies in how a fossil is made. Petrification is the process by which fossils are formed, and it involves six steps. (a) The animal dies. (b) Scavengers consume the body. (c) The soft parts — tissues, muscles, organs and the rest — start to decompose. (d) Mud, sand and silt cover what is left, and usually only bones and teeth remain by this stage. (e) More layers of sediment build up above, putting pressure on the layers below, and this eventually leads to the formation of sedimentary rock. (f) During this build-up, water seeps into the bones and teeth and leaves behind mineral crystals in them.

Fossilisation therefore requires two things together: the animal must be buried, and material must go on being deposited over the site. Land is mostly eroded rather than built up, so fossilisation there is rare; in fact deposition on land occurs only in river valleys. In the open sea the strong currents simply break and wear down animal remains, and in a water body decomposition is rapid. For animals without skeletons, fossilisation is rare because there are no bones to survive.

Q1. Why are fossils found in sedimentary rocks and almost never in igneous or metamorphic ones?

A. Only sedimentary rock forms by quiet burial, which covers remains and holds them in place. Igneous rock forms from molten material, and nothing survives that heat. In metamorphic rock, heat and pressure destroy whatever fossils the parent rock held.

6.5 Metamorphic Rocks

The word metamorphic is derived from ‘metamorphose’, which means change in form. Metamorphism refers to the alteration of the composition or structure of a rock by heat, pressure or other natural agency. Nothing is added from outside and nothing melts away; an existing rock is simply remade where it lies.

The rocks that undergo this change were once igneous or sedimentary, and they were altered through physical or chemical processes. The factors that take part in changing the form of rocks are temperature and pressure, or both together. Since rocks are composed of minerals, those minerals may change their composition and texture under great pressure and temperature, and when the minerals change, the rock changes with them. Fig. 6.10 shows the wavy bands this remaking often leaves behind.

Diagram

Fig. 6.10 Caption: Metamorphic rocks. Brief: Photograph of a metamorphic rock outcrop; the folded and banded appearance of the rock must be the dominant feature in frame.

6.5.1 Characteristics Of Metamorphic Rocks

Metamorphic rocks share five characteristics, and each one is a record of what heat and pressure did to the parent rock. (a) They are harder and more compact than their original form, because pressure squeezes out the spaces the parent rock had; marble made from limestone, for example, is harder than limestone. (b) Most of them are impermeable, so they do not allow water to percolate through them. (c) They do not have fossils, as any fossils present in the parent sedimentary rock are destroyed during the formation of metamorphic rocks. (d) They are formed due to a change in the texture and composition of the pre-existing rocks, not by the arrival of any new material. (e) New minerals are formed during the process of metamorphism, as the old minerals rearrange themselves under the new conditions.

Q3. The cliff is limestone. What would have to happen to it to turn it into marble?

A. It would have to be buried and put under great heat and pressure. Its minerals would recrystallise, and its texture and composition would change. The rock that results, marble, is harder and more compact than the limestone it came from.

6.5.2 Thermal Metamorphism

Thermal metamorphism occurs when the transformation of the original rock takes place due to the influence of high temperature. Here heat does the work almost alone. For example, slate is formed from clay, and graphite is formed from coal.

The heat may come from hot magma pushing into the surrounding rock, or from the friction of moving rock layers grinding against one another. Friction is the resistance felt when two surfaces rub, and it turns movement into heat — the same effect you feel when you rub your palms together quickly.

6.5.3 Dynamic Metamorphism

Dynamic metamorphism occurs when the transformation takes place mainly because of pressure at a great depth within the earth’s crust. Chemically active hot gases, passing through the rocks, change their chemical composition as they go. Under the conditions of this change the minerals get arranged in a series of bands, known as foliation — the striped appearance you can see in Fig. 6.11, where shale has become slate.

Diagram

Fig. 6.11 Caption: Slate is formed from shale. Brief: Close photograph of a slate surface; the fine parallel banding running across the rock must be clearly legible.

The important metamorphic rocks and the original rocks they came from are given in Table 6.2.

Table 6.2 Caption: Important metamorphic rocks and the original rocks they are formed from. | Original Rock | Metamorphic Rock | |—|—| | Limestone | Marble | | Sandstone | Quartzite | | Shale | Slate | | Coal | Graphite | | Basalt | Schist | | Granite | Gneiss | | Dolomite/Chalk | Marble |

Table 6.2 Read each row from left to right: the parent rock first, then what heat and pressure turn it into. Two different rows end at marble, so one metamorphic rock can come from more than one parent. What the table does not give is any condition — not how hot, not how deep, not how long any of these changes took.

WC 6.A1 — The Marble Of The Taj Mahal The situation: The rocks used in the Taj Mahal at Agra, in Uttar Pradesh, are marbles metamorphosed from dolomites, and the Dilwara Temple at Mount Abu, in the Aravalli range of Rajasthan, is built of similar marbles. The working: dolomite is laid down as a sedimentary carbonate rock → burial carries it deep into the crust → great heat and pressure act on it → its minerals recrystallise and its texture changes → the rock is now marble, harder and more compact than its parent, and impermeable.

The stone in these two buildings, then, is not the rock that was originally deposited. It is that rock after metamorphism, and the properties the builders were working with — hardness, compactness, a surface that rainwater cannot percolate into — are exactly the properties the process created. The parent dolomite would not have served nearly as well. This is the practical value of knowing how a rock formed: it tells you what the rock will do.

6.5.4 Regional Metamorphism

Regional metamorphism happens on the largest scale of the three. Due to the mountain building process, igneous and sedimentary rocks are buried deep inside the crust. There the pressure of the overlying rocks, together with the intense heat caused by large-scale earth movements, changes such rocks. Because whole belts of rock are buried at once, the change affects a wide region rather than a single spot.

When only a small area is affected by such a change, this is known as local or contact metamorphism. It happens when hot magma affects the cracks or the layers of rocks around it, and those areas alone are transformed into metamorphic rocks. The word contact says it plainly: the change reaches only as far as the heat does.

Flowchart

Fig. 6.A5 Caption: The three kinds of metamorphism, sorted by what does the changing and over how wide an area. Nodes: metamorphism of an igneous or sedimentary rock → thermal metamorphism (high temperature from magma or friction; clay → slate, coal → graphite) / dynamic metamorphism (pressure at great depth; hot gases alter composition; minerals arranged in bands, foliation) / regional metamorphism (burial by mountain building; heat and pressure over a wide belt) → regional metamorphism affecting only a small area → local or contact metamorphism

Metamorphic rocks matter for another reason as well. Many metallic minerals, such as gold and silver, are found in metamorphic rocks, so these rocks are hunted not only as building stone but as a source of metal.

6.6 Economic Significance Of Rocks

Rocks are not only objects of study. Almost every solid thing a town is built out of began as rock, and so did the soil the town’s food is grown in. Their value comes in two ways: some rocks are useful in themselves, and some are useful for what can be taken out of them.

Rocks are of economic significance in five ways. (a) Like minerals, rocks are of great resource value, some directly as the material itself and some as the constituents of minerals that are extracted from them. (b) Soils are derived from the weathering of rocks, so the loose fertile layer that farming depends on is simply older rock broken down where it stood. (c) Almost all types of building materials come from rocks, including the materials used for paving roads, laying floors, raising the walls of houses and constructing other structures such as bridges. (d) Rocks are a source of precious metals like gold, silver and platinum, which occur locked inside them and have to be separated out. (e) Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles, so the material left over after the useful part has been taken is no longer simply discarded.

Getting building stone out of the ground is done by quarrying, shown in Fig. 6.9, in which rock is cut and lifted from an open working rather than from a tunnel. The choice of rock for each job goes back to the properties studied earlier in this chapter — hardness, compactness and resistance to weathering decide what a stone is fit for.

Diagram

Fig. 6.9 Caption: Quarrying. Brief: Photograph of an open quarry with the cut rock faces and the excavating machinery both visible, and enough of the working shown to give a sense of its scale.

Q. How much of an ordinary classroom came out of rock?

A. The floor, the walls, the road outside — and, if the power station burns coal, the light overhead as well.

6.7 Rock Cycle

The earth is said to be 4.5 billion years old. Over so long a stretch of time, no rock stays as it is, and rocks obviously undergo cyclic transformation. This continuous process of transformation of old rocks into new ones is known as the rock cycle.

The cycle may pass through the formation of sedimentary, metamorphic and igneous rocks in turn, and it does not have to run in a fixed order. A rock may take a short path or a long one, but it never leaves the system.

The cycle can be followed in four stages. (a) Igneous rocks are the primary source of all other rocks, and they are formed on the outer surface of the earth due to volcanic activities; on the surface, rocks are disintegrated and eroded by climatic factors, and the disintegrated material is removed and transported to lower levels or to the sea by the forces of running water, winds, glaciers and ocean currents, until the sediments accumulate in basins, lakes or ocean beds. (b) The accumulations undergo lithification through the processes of evaporation, compaction and cementation, as discussed earlier in this chapter. (c) Once the loose sediment is transformed into sedimentary rock, it is carried into depressions or on the sea floor further down under the earth, where it may undergo disintegration and be again recrystallised into igneous rock and then transformed into metamorphic rock; in the course of metamorphism, remember, rock changes its form due to intense heat and pressure. (d) In case the rock melts, it may again result in the formation of igneous rock, and the disintegrated material may again form sedimentary rock, so the two halves of the cycle keep feeding one another.

In completing any of these changes, hundreds and thousands of years are involved. Nothing here happens at a speed a person could watch.

In the operation of the rock cycle the earth is kept young, and movements of adjustment — especially horizontal flow or convection currents, the slow circulation set up inside the earth by heat — maintain the balance between the various elements of the earth like oceans and continents. Fig. 6.12 draws the whole cycle in one picture, with the surface environment of low pressure and temperature at the top and the deep environment of high pressure and temperature at the bottom.

Diagram

Fig. 6.12 Caption: Rock cycle. Brief: Full-width cycle diagram with extrusive and intrusive igneous rocks, sedimentary rocks and metamorphic rocks as labelled boxes; solar energy entering at the top of the surface environment and radiogenic heat entering at the bottom of the deep environment; arrows running from igneous rocks through disintegration, decomposition and dissolution to sediment transportation and deposition, then down through burial and lithification to sedimentary rocks, across through metamorphism to metamorphic rocks, and back up through melting, rise of magma, intrusion and extrusion, with a return arrow from depth to the surface labelled uncovering by crustal uplift and denudation.

An Earth With No Internal Heat Remove: the heat inside the earth that melts rock and drives magma upwards. Then: nothing melts, so no new igneous rock forms; nothing buried is heated enough to be metamorphosed; weathering carries on until every rock has been reduced to sediment lying in the sea. Which tells us: internal heat is what bends the chain back on itself. Without it the rock cycle would be a one-way journey that ended in mud.

Q4. Rocks have been breaking down for 4.5 billion years. Why has the earth not run out of them?

A. Because breaking down is only half the story. Material stripped from one rock is deposited, buried, pressed, heated and sometimes melted, and it comes back as a new rock. Nothing leaves the system, so the same matter is used over and over.

Lithification — the process that turns loose sediments into rocks. Igneous rocks — rocks formed by cooling and solidification of magma on or below the surface of the earth. Sedimentary rocks — rocks formed in layers from sediments brought by rivers, glaciers and wind; they may also contain dead remains of plants and animals. Metamorphic rocks — rocks formed by the change in the form of igneous or sedimentary rocks under the earth’s crust by temperature and pressure.

6.8 Summary

  1. A rock is an aggregate of minerals that forms more or less a definite unit of the earth’s crust, and the term covers loose material like sand, silt and clay as well as hard stone like granite.
  2. Rocks are told apart from one another by texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation.
  3. Rocks and minerals are related but are not the same thing.
    1. A mineral is a naturally occurring inorganic solid with a definite chemical composition and a crystalline structure.
    2. A rock is an aggregate of one or more minerals, so it has no definite chemical composition of its own.
    3. Elements are organised to form minerals, and minerals are organised to form rocks; the order never runs the other way.
  4. The earth’s crust contains more silicon and aluminium, whereas the earth as a whole contains more iron, silica and magnesium.
  5. Rocks are classified on the basis of their origin into three main types — igneous, sedimentary and metamorphic. Igneous rocks are called primary rocks, and the other two are secondary rocks.
  6. Igneous rocks are formed by the cooling, solidification and crystallisation of magma. They are hard, compact, granular and crystalline, they carry no layers, and most of them consist of silicates.
  7. The size of the crystals in an igneous rock depends on the rate of cooling of the molten material.
    1. Extrusive igneous rocks cool quickly on the earth’s surface and are smooth and fine grained; basalt is the common example, and it can solidify into long polygonal columns.
    2. Intrusive igneous rocks cool slowly within the crust and are coarse textured with large crystals; granite, gabro and dolerite are examples.
  8. Magma that hardens below the surface builds batholiths, laccoliths, sills, dykes and necks. Intrusive rocks are also called plutonic rocks, while hypobyssal rocks form at intermediate depth.
  9. On the basis of chemical composition, igneous rocks are of two kinds.
    1. Acid igneous rocks have a silica content of 65 to 85 per cent and generally lack iron and magnesium; granite belongs here.
    2. Basic igneous rocks have a silica content of 40 to 60 per cent and more oxides of denser elements, so they are heavy and dark; basalt, dolerite and gabro belong here.
  10. Sedimentary rocks are formed when weathered material is carried away by rivers, glaciers and ocean waves, deposited in layers, and hardened under the weight of the layers above. They are secondary rocks and constitute about 75 per cent of the surface area of the globe.
  11. Lithification is the process that turns loose sediments into hard rock, and it works through three processes — evaporation, compaction and cementation.
  12. On the basis of formation, sedimentary rocks fall into three groups.
    1. Mechanically formed rocks are made of fragments of older rocks and are laid down in layers, so they are called stratified rocks; shale, sandstone, clay, conglomerates and loess belong here.
    2. Chemically formed rocks are made by the direct precipitation of mineral matter from solution; gypsum, rock salt and potash belong here.
    3. Organically formed rocks are made of the remains of dead plants and animals; limestone, chalk, peat, lignite and coal belong here.
  13. On the basis of the agent that deposited the sediment, sedimentary rocks are riverine, lacustrine, glacial, aeolian or marine.
  14. Fossils are found only in sedimentary rocks, because fossilisation requires the remains to be buried and material to go on being deposited over the site.
  15. Metamorphism is the alteration of the composition or structure of a rock by heat, pressure or other natural agency, and it acts on rocks that were once igneous or sedimentary.
    1. Thermal metamorphism is brought about mainly by high temperature; clay becomes slate and coal becomes graphite.
    2. Dynamic metamorphism is brought about mainly by pressure at great depth, and the minerals are arranged in bands known as foliation.
    3. Regional metamorphism follows the mountain building process and affects a wide belt, while a change confined to a small area is local or contact metamorphism.
  16. Metamorphic rocks are harder and more compact than their original form, mostly impermeable, and free of fossils. Limestone becomes marble, sandstone becomes quartzite, and granite becomes gneiss.
  17. Rocks are of great economic significance — as building material, as the parent material of soil, as a source of precious metals and fossil fuels, and even as rock waste used in manufacturing.
  18. The rock cycle is the continuous process of transformation of old rocks into new ones, and through its operation the earth is kept young.

Flowchart

Fig. 6.A6 Caption: The whole chapter in one chain — rock is never used up, only moved from one form into another. Nodes: magma → cooling, solidification and crystallisation → igneous rock → weathering and erosion by running water, wind, glaciers and ocean currents → sediment deposited in layers in seas, lakes and basins → lithification by evaporation, compaction and cementation → sedimentary rock → burial under heat and pressure → metamorphic rock → melting → back to magma; branch: igneous rock → heat and pressure → metamorphic rock; branch: sedimentary rock → weathering and erosion → back to sediment; branch: metamorphic rock → weathering and erosion → back to sediment

What This Chapter Cannot Tell You

The chapter names the changes but not their conditions. Table 6.2 tells you that limestone becomes marble, but not at what temperature, at what depth, or over how long, and the rock cycle diagram shows the paths available to a rock rather than the path any particular rock actually took.

Its figures are global ones. That sedimentary rocks cover about 75 per cent of the earth’s surface says nothing about which rock lies beneath any one district, and a silica range of 65 to 85 per cent describes a whole group, not the specimen in your hand.

Nor does the chapter reach the timing. It tells us that hundreds and thousands of years are involved in these changes, and that the earth is 4.5 billion years old, but it gives no method for dating a single rock.

6.9 Check Your Concepts

  1. Two igneous rock specimens are tested for silica. One returns a silica content of 70 per cent and the other 45 per cent. State which is an acid igneous rock and which is a basic igneous rock, and say what each result tells you about the colour and the weight of that rock.

  2. Look at Table 6.2.

    1. Name the two original rocks that both change into marble.
    2. State one thing this table does not tell you about the changes it lists.

  3. Loose sand lying on a river bed eventually becomes sandstone. Arrange the processes involved in the correct order, and explain what happens to the sand at each stage.

  4. A student finds a shell preserved inside a rock. Explain why the rock must be a sedimentary one, and why the same shell could not have survived inside an igneous or a metamorphic rock.

  5. Distinguish between plutonic and volcanic rocks.

  6. Give a reason for each of the following.

    1. Igneous rocks are also called the primary rocks.
    2. Igneous rocks are used by builders and sculptors.
    3. Sedimentary rocks are called secondary rocks.

  7. Explain the formation of sedimentary rocks on the basis of agents of formation.

  8. What is metamorphism? What are its causes? Give an example of rocks formed by heat and pressure.

  9. What is the rock cycle? How does it keep the earth young?

6.10 Case-Based Questions

Diagram

Fig. 6.A7 Caption: Study this section through the crust and answer the question that follows. Brief: Reproduce the intrusive igneous rocks cross-section exactly as printed in the book’s exercise — surface of the earth, lava flow, volcano, earth’s crust and magma below — with the letters A, B and C marked on the same three rock bodies the book marks, and no other labels.

  1. Study Fig. 6.A7. Identify the rocks formed in the case of A, B and C, and give reasons to support your answer.