Formation of the Earth | ISC Class 11 Geography Notes

This note covers the formation of the earth: the Big Bang theory, the nebular and planetesimal hypotheses, how radioactivity measures the age of the earth, the crust, mantle and core, direct and indirect sources of information, seismic waves and shadow zones, earthquakes and their measurement, the Nepal earthquake case study, vulcanicity and volcanic forms, the mineral groups of rocks, the three rock types and their distribution in India, and the rock cycle.

What are the main theories of the formation of the earth?

Several explanations have been offered for how the earth and the other planets came into being.

The older ones are grouped as the nebular hypothesis and the planetesimal hypothesis; the modern one is the Big Bang theory, which explains the origin of the whole universe rather than of the earth alone.

Definition: A hypothesis is a proposed explanation for a set of observations, put forward so that it can be tested against further evidence. A theory is a hypothesis that has survived such testing and is widely accepted.

The nebular hypothesis

The German philosopher Immanuel Kant put forward one of the earlier and popular arguments. The mathematician Laplace revised it in 1796, and the revised form is known as the nebular hypothesis.

It considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

In 1950, Otto Schmidt in Russia and Carl Weizsäcker in Germany somewhat revised the nebular hypothesis, though differing in details.

They considered that the sun was surrounded by a solar nebula containing mostly hydrogen and helium along with what may be termed as dust.

The friction and collision of particles led to the formation of a disk-shaped cloud, and the planets were formed through the process of accretion.

Definition: Accretion is the gradual building up of a larger body as smaller particles and lumps of matter collide and stick together under gravitational attraction.

The planetesimal hypothesis

The planetesimal hypothesis explains the growth of planets from small solid bodies. The stages are set out below, and they are the same stages that the modern account of planet formation uses.

1. The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud, and a huge rotating disc of gas and dust develops around the gas core.
2. The gas cloud starts getting condensed, and the matter around the core develops into small rounded objects.
3. These small rounded objects, by the process of cohesion, develop into what are called planetesimals.
4. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
5. In the final stage, these large numbers of small planetesimals accrete to form a fewer large bodies in the form of planets.

Definition: Planetesimals are a large number of small, rounded bodies of gas and dust that form around a young star and later collide and accrete to build planets.

What does the Big Bang theory say about the origin of the universe?

The most popular argument regarding the origin of the universe is the Big Bang theory. It is also called the expanding universe hypothesis. Edwin Hubble, in 1929, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.

The expansion of the universe means an increase in the space between the galaxies. An alternative to this was Hoyle's concept of steady state, which considered the universe to be roughly the same at any point of time.

However, with greater evidence becoming available about the expanding universe, the scientific community at present favours the argument of the expanding universe.

The stages of the Big Bang

1. In the beginning, all matter forming the universe existed in one place in the form of a tiny ball (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
2. At the Big Bang the tiny ball exploded violently. This led to a huge expansion. It is now generally accepted that the event of the Big Bang took place about 13.7 billion years before the present; the latest measurements put it at about 13.8 billion years. The expansion continues even to the present day.
3. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within the first three minutes from the Big Bang event, the first atom began to form.
4. Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent.

Draw a nested triangle, narrow at the bottom and widening upwards. Mark the bottom point Singularity.

Draw successive wider layers above it, each holding small spiral or cloud shapes standing for galaxies, and the widest layer at the top holding four larger spiral galaxies. Label the whole diagram "The Big Bang".

Note: The balloon experiment is only partly correct. If you mark points on a balloon to represent galaxies and inflate it, the points move away from each other, as the galaxies do. But the points themselves also expand, and observations do not support the expansion of galaxies.

How are stars and planets formed?

The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces, and this caused the matter to get drawn together.

These formed the bases for the development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years.

The diameters of individual galaxies range from 80,000 to 150,000 light years.

A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called a nebula. Eventually, the growing nebula develops localised clumps of gas.

These clumps continue to grow into even denser gaseous bodies, giving rise to the formation of stars. The formation of stars is believed to have taken place some 5 to 6 billion years ago.

Definition: A light year is a measure of distance and not of time. Light travels at a speed of 300,000 km per second, and the distance light travels in one year is taken to be one light year. This equals 9.461 × 10¹² km.

The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311 minutes.

How is the age of the earth measured by radioactivity?

Rocks carry their own clocks. Certain elements in a rock decay into other elements at a steady rate, and by comparing the amounts of the parent and the daughter element a scientist can work out how long the decay has been going on. This is the principle of radiometric dating.

Definition: Radioactivity is the spontaneous breakdown of an unstable atomic nucleus into a different element, giving out particles or energy. The original element is the parent and the element it becomes is the daughter.

Definition: The half-life of a radioactive element is the time taken for half the atoms in a sample to decay. After one half-life half the parent remains, after two half-lives one quarter, and so on.

Each radioactive pair has its own half-life, so each suits a different span of time. The table below gives the pairs commonly used in dating earth materials.

The oldest rocks so far found on the earth are dated at about 4,000 million years, and the earth as a whole is taken to be about 4,600 million years old.

The earth's oceans were formed within 500 million years from the formation of the earth, which tells us that the oceans are as old as 4,000 million years.

Worked example 1. A mineral from a volcanic rock contains potassium-40 and argon-40. Argon-40 makes up three quarters of the total of the two, and potassium-40 makes up one quarter. The half-life of potassium-40 is 1.25 billion years. Treat every potassium-40 atom that decays as becoming argon-40. How old is the rock?

Answer: One quarter of the parent remaining means two half-lives have passed, because 1 → ½ → ¼. Age = 2 × 1.25 billion = 2.5 billion years.

What are the sources of information about the earth's interior?

The earth's radius is about 6,378 km. No one can reach the centre of the earth and make observations or collect samples of the material.

Most of our knowledge about the interior is therefore largely based on estimates and inferences, yet a part of the information is obtained through direct observations and analysis of materials.

Direct sources

The most easily available solid earth material is surface rock or the rocks we get from mining areas. Gold mines in South Africa are as deep as 3 to 4 km.

Going beyond this depth is not possible as it is very hot at this depth. Besides mining, scientists have taken up a number of projects to penetrate deeper depths to explore the conditions in the crustal portions.

Scientists the world over are working on two major projects, the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project. The deepest drill, the Kola borehole on the Kola Peninsula in Arctic Russia,, has so far reached a depth of 12 km.

Volcanic eruption forms another source of direct information: as and when the molten material (magma) is thrown onto the surface during a volcanic eruption, it becomes available for laboratory analysis, though it is difficult to ascertain the depth of the source of such magma.

Indirect sources

Analysis of the properties of matter indirectly provides information about the interior. Mining activity shows that temperature and pressure increase with increasing distance from the surface towards the interior, and the density of the material also increases with depth.

Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and density of materials at different depths.

Another source of information is meteors that at times reach the earth. The material and the structure observed in meteors are similar to that of the earth; they are solid bodies developed out of materials same as, or similar to, our planet. The other indirect sources include gravitation, magnetic field and seismic activity.

The gravitational force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator, because the distance from the centre at the equator is greater than that at the poles.

Gravity values also differ according to the mass of material, and the uneven distribution of mass within the earth influences this value.

Readings that differ from the expected values are called a gravity anomaly, and gravity anomalies give us information about the distribution of mass of the material in the crust.

Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion.

What is the layered structure of the earth's interior?

The earth has a layered structure. From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform. The atmospheric matter has the least density.

From the surface to deeper depths, the earth's interior has different zones, and each of these contains materials with different characteristics.

Draw a cutaway wedge of the earth showing concentric layers from the surface to the centre. Label, from outside inwards: Lithosphere (crust and uppermost solid mantle); Crust 0-100 km thick; Asthenosphere; Mantle;

2,900 km; Liquid outer core; Core; 5,100 km; Solid inner core; 6,378 km. Show mountains and the lithosphere at the top surface.

The crust

The crust is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust.

The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km.

The continental crust is thicker in the areas of major mountain systems; it is as much as 70 km thick in the Himalayan region.

The mantle

The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho's discontinuity to a depth of 2,900 km. The upper portion of the mantle is called the asthenosphere. The word astheno means weak.

It is considered to be extending up to 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions.

The crust and the uppermost part of the mantle are called the lithosphere, and its thickness ranges from 10 to 200 km. The lower mantle extends beyond the asthenosphere and is in solid state.

The core

The earthquake wave velocities helped in understanding the existence of the core of the earth. The core-mantle boundary is located at the depth of 2,900 km.

The outer core is in liquid state while the inner core is in solid state. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.

Temperature and pressure both rise from the crust to the core. The crust is cool enough for mines 3 to 4 km deep to be worked, but beyond that depth it is very hot.

Pressure rises with the weight of overlying rock, and density increases from the crust to the core.

How do seismic waves reveal the earth's interior?

The study of seismic waves provides a complete picture of the layered interior. An earthquake in simple words is shaking of the earth. It is a natural event, caused due to release of energy, which generates waves that travel in all directions.

Why does the earth shake?

The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together.

However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually they slide past one another abruptly.

This causes a release of energy, and the energy waves travel in all directions.

The point where the energy is released is called the focus of an earthquake, alternatively called the hypocentre. The point on the surface nearest to the focus is called the epicentre. It is the first one to experience the waves, and it is a point directly above the focus.

Types of earthquake waves

Most earthquakes take place in the lithosphere, the portion of depth up to about 200 km from the surface of the earth, though some deep-focus earthquakes in sinking plates occur as deep as about 700 km. An instrument called a seismograph records the waves reaching the surface.

Earthquake waves are basically of two types, body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.

The body waves interact with the surface rocks and generate a new set of waves called surface waves, which move along the surface.

The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.

Reflection causes waves to rebound whereas refraction makes waves move in different directions.

Draw a boxed seismograph trace with three successive wave sections of increasing amplitude, labelled from left to right P-waves, S-waves and Surface waves, with arrows marking Arrival times and Amplitude.

The shadow zone

Earthquake waves get recorded in seismographs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the shadow zone. The study of different events reveals that for each earthquake there exists an altogether different shadow zone.

Seismographs located at any distance within 105° from the epicentre recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from the epicentre record the arrival of P-waves, but not that of S-waves.

Thus a zone between 105° and 145° from the epicentre was identified as the shadow zone for both types of waves. The entire zone beyond 105° does not receive S-waves.

The shadow zone of the S-wave is much larger than that of the P-waves, and it is a little over 40 per cent of the earth's surface.

Draw two circular cross-sections of the earth. In the upper one, titled P-wave shadow zone, show ray paths radiating from a quake point at the top through a core, with shaded shadow regions marked at 105° and 145° on both sides, labelled No direct P-waves and P-waves received here.

In the lower one, titled S-wave shadow zone, show a quake point at the top, a central Core circle, S-wave ray paths and a much larger shaded region below 105° on both sides, labelled No direct S-waves received here.

The behaviour of the two body waves gives two firm inferences. Because S-waves cannot pass through liquids, their absence beyond 105° shows that the outer core is liquid.

Because P-waves are refracted and delayed as they cross the core, their shadow band between 105° and 145° shows that the core has a different density and composition from the mantle.

What are the types, effects and measurement of earthquakes?

Types of earthquakes

1. The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
2. A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. However, these are confined to areas of active volcanoes.
3. In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
4. Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
5. The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.

Effects of earthquakes

An earthquake is a natural hazard. The immediate hazardous effects are ground shaking, differential ground settlement, land and mud slides, soil liquefaction, ground lurching, avalanches, ground displacement, floods from dam and levee failures, fires, structural collapse, falling objects and tsunami.

The first six listed have some bearing upon landforms, while the others may be considered the effects causing immediate concern to the life and properties of people in the region.

The effect of tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by the tremors and not an earthquake in itself.

Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.

Measuring earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale.

The magnitude relates to the energy released during the quake, and is expressed in numbers from 0 to 10. The intensity scale is named after Mercalli, an Italian seismologist.

The intensity scale takes into account the visible damage caused by the event, and its range is from 1 to 12.

Definition: Isoseismal lines are lines drawn on a map joining places that experienced the same intensity of shaking from one earthquake. Homoseismal lines join places where the earthquake was felt at the same time.

Quakes of high magnitude, that is 8 and above, are quite rare; they occur once in 1 to 2 years, whereas those of tiny types occur almost every minute.

Locating an earthquake

The speed difference between P-waves and S-waves lets a seismograph station work out how far away the focus is, because the gap between their arrival times grows with distance. One station knows the distance but not the direction. Drawing a circle of that radius around each of three stations gives one point where all three circles meet, and that point is the epicentre. The same quick location is the first step in issuing a tsunami warning.

Distribution of earthquakes

Earthquakes are not spread evenly. Most occur along plate boundaries, in three belts. The Circum-Pacific belt, the "Ring of Fire" around the Pacific Ocean, has the largest share, including the great earthquakes of Japan, Chile and Alaska. The Mid-Continental or Alpine-Himalayan belt runs from the Mediterranean through Iran and the Himalayas to Myanmar and Indonesia. The mid-oceanic ridges have frequent but mostly moderate, shallow earthquakes.

In India the Himalayan belt and the North-East are the most earthquake-prone regions. The peninsula is more stable, yet it has had damaging earthquakes such as Latur in Maharashtra in 1993 and Bhuj in Gujarat in 2001.

Draw a world outline. Shade the Circum-Pacific belt around the rim of the Pacific Ocean, the Alpine-Himalayan belt from the Mediterranean to Indonesia, and the mid-oceanic ridges, including the Mid-Atlantic Ridge. Mark Nepal, Sumatra, Latur and Bhuj, and use dots for the main volcanic chains.

Case study: the Nepal earthquake of 2015

Nepal lies along the collision zone where the Indian plate pushes northwards into the Eurasian plate, so the Himalayan belt is one of the most earthquake-prone regions of the world.

On 25 April 2015 a tectonic earthquake of magnitude 7.8 on the Richter scale struck with its epicentre in the Gorkha district, about 77 km north-west of Kathmandu, at a shallow depth, about 8 to 15 km by different estimates. A major aftershock of magnitude 7.3 followed on 12 May 2015.

The effects were severe. About 9,000 people died and more than 20,000 were injured. Entire villages in the hill districts were flattened, and heritage structures in Kathmandu, including the Dharahara tower, collapsed.

Landslides and avalanches buried settlements and climbers on Everest, and the shaking triggered an avalanche on the mountain that killed people at base camp.

The earthquake also caused ground displacement and damaged roads and bridges, cutting off remote valleys for days.

The Nepal case shows the main ideas of this unit at work: a shallow focus, a densely settled mountain region and weak buildings together turn a magnitude 7.8 event into a disaster.

It also shows why the study of seismic waves matters, because the same waves that record the event are the only means of knowing the structure of the interior through which they passed.

Case study: the Indian Ocean tsunami of 26 December 2004

The tsunami of 26 December 2004 followed a great earthquake with its epicentre near Sumatra, Indonesia. The earthquake was below oceanic waters and its magnitude was sufficiently high, so a tsunami was generated.

The earthquake, of magnitude about 9.1, struck off the west coast of northern Sumatra on the morning of 26 December 2004. The waves crossed the Bay of Bengal and the Indian Ocean within hours and struck the coasts of Indonesia, Sri Lanka, India, Thailand, the Maldives and even East Africa.

Early reports on the day spoke of about 10,000 deaths; the true scale became clear only over the following weeks. In India the worst damage was on the Tamil Nadu coast, including Nagapattinam and Chennai's Marina Beach, and in the Andaman and Nicobar Islands. No warning system existed in the Indian Ocean at the time, so in 2007 India set up the Indian Tsunami Early Warning Centre at INCOIS, Hyderabad.

The case makes clear that a tsunami is a wave generated by a tremor and not an earthquake in itself, and that its reach depends on where the epicentre lies.

What is vulcanicity and how are volcanoes formed?

A volcano is a place where gases, ashes and/or molten rock material, lava, escape to the ground. A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past.

The layer below the solid crust is the mantle, which has higher density than the crust.

The mantle contains a weaker zone called the asthenosphere, and it is from this that the molten rock materials find their way to the surface.

Definition: Magma is the molten rock material in the upper mantle portion. Once it starts moving towards the crust or reaches the surface, it is referred to as lava.

The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust, and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.

How a volcano is formed

1. Heat and pressure in the mantle melt rock in the asthenosphere, the weak zone up to about 400 km deep.
2. The molten material, being less dense than the surrounding rock, rises through cracks and fissures in the crust.
3. It collects in a magma chamber within the crust and builds up pressure.
4. The pressure forces the magma up a pipe-like conduit, called the vent, to the surface.
5. At the surface the magma becomes lava and escapes, together with ash, bombs and gases, and cools to build a cone or a lava flow around the vent.

Why volcanoes are more common near converging plates

Where two plates converge, one plate is forced down beneath the other in a process called subduction. The descending plate carries water and wet ocean-floor rock down with it.

The water lowers the melting point of the overlying mantle rock, so magma is generated above the sinking plate. This magma rises and feeds a chain of volcanoes parallel to the plate boundary.

That is why the great volcanic belts of the world, such as the rim of the Pacific Ocean, lie along converging plate margins.

What are the major volcanic forms?

Volcanoes are classified on the basis of the nature of eruption and the form developed at the surface.

Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.

They are mostly made up of basalt, a type of lava that is very fluid when erupted, and for this reason these volcanoes are not steep.

They become explosive if somehow water gets into the vent; otherwise they are characterised by low explosivity.

The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent, developing into a cinder cone.

Composite volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. They often result in explosive eruptions, and along with lava, large quantities of pyroclastic material and ashes find their way to the ground.

This material accumulates in the vicinity of the vent openings, leading to the formation of layers, and this makes the mounts appear as composite volcanoes.

Calderas are the most explosive of the earth's volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.

Flood basalt provinces outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows.

There can be a series of flows with some flows attaining thickness of more than 50 m, and individual flows may extend for hundreds of km.

The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province; it is believed that initially the trap formations covered a much larger area than the present.

Mid-ocean ridge volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins, and the central portion of this ridge experiences frequent eruptions.

Intrusive forms

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion.

Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).

The lava that cools within the crustal portions assumes different forms, called intrusive forms.

Draw a labelled block cross-section showing a volcano and the surrounding terrain, with surface and subsurface forms. Label: Lava Plateau; Volcanic neck with radiating dykes; Caldera with cinder cone on floor; Lava Mesa;

Ash cone with volcanic dome; Composite Volcano; Cinder Cone; Ash flows; Lava flow; Volcanic conduit; Country rock; Eroded laccolith; Sills; Laccolith; Dyke; Stock; Batholith.

• Batholiths are a large body of magmatic material that cools in the deeper depth of the crust and develops in the form of large domes. They appear on the surface only after denudational processes remove the overlying materials. They cover large areas and at times assume depth that may be several km. These are granitic bodies, and batholiths are the cooled portion of magma chambers.
• Laccoliths are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. They resemble the surface volcanic domes of composite volcanoes, only these are located at deeper depths. The Karnataka plateau is spotted with domal hills of granite rocks; most of these, now exfoliated, are examples of laccoliths or batholiths.
• A lopolith is a saucer-shaped body, concave to the sky.
• Phacoliths are a wavy mass of intrusive rocks, at times found at the base of synclines or at the top of anticlines in folded igneous country, with a definite conduit to source beneath in the form of magma chambers.
• Sills and sheets are the near-horizontal bodies of intrusive igneous rocks. The thinner ones are called sheets while the thick horizontal deposits are called sills.
• Dykes form when lava makes its way through cracks and fissures in the land and solidifies almost perpendicular to the ground, developing a wall-like structure. These are the most commonly found intrusive forms in the western Maharashtra area, and are considered the feeders for the eruptions that led to the development of the Deccan Traps.

Which mineral groups make up the rocks of the earth?

Rocks are made of minerals, and minerals are grouped by their chemical composition. Four groups account for most of the rocks formed on the earth.

Silicates are by far the most abundant group, and they make up the bulk of the crust and mantle. Carbonates form the sedimentary rocks of limestone and chalk and, when heated and pressed, marble.

Sulphides and native metals are far less common but are economically important because they carry the ores of lead, zinc, copper, gold and iron.

How are rocks classified by origin?

Rocks are classified by origin into igneous, sedimentary and metamorphic rocks.

Igneous rocks

Igneous rocks form from the cooling and solidification of magma or lava. Those that cool at the surface are volcanic and fine-grained; those that cool within the crust are plutonic and coarse-grained. Granite and basalt are the common examples.

Igneous rocks are the original rocks from which the other two types are derived.

Sedimentary rocks

Sedimentary rocks form from the deposition, compaction and cementation of sediments carried by water, wind and ice. They are usually layered and may contain fossils. Limestone, sandstone and shale are the common examples. They cover much of the plains and coastal lowlands.

Metamorphic rocks

Metamorphic rocks form when existing rocks are changed by great heat and pressure without melting. Limestone becomes marble, sandstone becomes quartzite and shale becomes slate. They are common in the old fold mountains and in the shield areas.

Distribution in India

• Igneous rocks: the Deccan Traps of Maharashtra, the granite domes of the Karnataka plateau, and the batholiths and dykes of western Maharashtra.
• Sedimentary rocks: the Himalayan belt, the Ganga plain and the coastal plains, with limestone, sandstone and shale.
• Metamorphic rocks: the old crystalline rocks of the peninsular plateau, the Aravalli and the Himalayan core, with marble, slate, quartzite and gneiss.

Draw an outline of India. Shade the Deccan Traps (igneous basalt) on the Maharashtra plateau, the old crystalline igneous and metamorphic rocks of the rest of the peninsular plateau and the Aravalli, the sedimentary rocks of the Indo-Gangetic plain and the coastal plains, and the folded sedimentary and metamorphic rocks of the Himalaya. Add a key.

Economic importance

Igneous rocks supply building stone and the ores of metals. Sedimentary rocks supply coal, petroleum, limestone for cement and sandstone for building. Metamorphic rocks supply marble, slate and graphite. The Deccan Traps also give the black cotton soil of the Maharashtra plateau, which is important for cotton.

What is the rock cycle?

The rock cycle is the continuous series of changes by which one rock type is transformed into another. It has no beginning and no end.

1. Magma rises from the mantle and cools, either at the surface or within the crust, to form igneous rock.
2. Weathering and erosion break the igneous rock into sediments, which are carried by water, wind and ice and deposited in low places.
3. Compaction and cementation of the sediments form sedimentary rock.
4. Heat and pressure deep in the crust change the sedimentary or igneous rock into metamorphic rock.
5. Further heat may melt the metamorphic rock back into magma, and the cycle begins again.

Worked example 2. A granite hill is weathered, its grains are washed into a river and deposited on the sea floor, where they are compacted into sandstone. The sandstone is later buried, heated and pressed, and becomes quartzite.

Trace the rock cycle stages and name each rock type at each stage.

Answer: Granite is igneous (cooled magma). Weathering, erosion, deposition, compaction and cementation make sandstone, a sedimentary rock. Heat and pressure make quartzite, a metamorphic rock. If the quartzite is melted, it returns to magma and the cycle restarts.

Glossary

• Nebula: a very large cloud of hydrogen gas and dust in space, from which stars and planets form.
• Accretion: the building up of a larger body as smaller particles collide and stick together under gravity.
• Planetesimal: a small rounded body of gas and dust that later collides to form a planet.
• Light year: the distance light travels in one year, equal to 9.461 × 10¹² km.
• Half-life: the time taken for half the atoms of a radioactive element in a sample to decay.
• Gravity anomaly: a reading of gravity that differs from the expected value, showing uneven distribution of mass.
• Focus: the point inside the earth where earthquake energy is released, also called the hypocentre.
• Epicentre: the point on the surface directly above the focus, first to feel the waves.
• Shadow zone: a zone from which earthquake waves are not reported, between 105° and 145° from the epicentre.
• Isoseismal line: a line on a map joining places that felt the same intensity of shaking.
• Asthenosphere: the weak upper part of the mantle, up to about 400 km deep, the main source of magma.
• Lithosphere: the crust and the uppermost solid part of the mantle, 10 to 200 km thick.
• Magma: molten rock material in the upper mantle, which becomes lava on reaching the surface.
• Batholith: a large dome of granitic magma that cooled deep in the crust and appears only after erosion.
• Dyke: a wall-like intrusive body formed when lava solidifies in a crack almost perpendicular to the ground.

Common errors and misconceptions

• Misconception: A light year measures time. Correct: It measures distance, the distance light travels in one year.
• Misconception: The Big Bang happened at a point in space. Correct: All matter existed in one tiny ball, and space itself has been expanding since.
• Misconception: S-waves travel through liquids. Correct: S-waves travel only through solids, which is why they are absent beyond 105°.
• Misconception: The Richter scale measures damage. Correct: Richter measures magnitude, the energy released; the Mercalli scale measures visible damage.
• Misconception: A tsunami is an earthquake. Correct: A tsunami is a wave generated by a tremor below oceanic waters.
• Misconception: The outer core is solid. Correct: The outer core is liquid and the inner core is solid.
• Misconception: Volcanoes are equally common everywhere. Correct: They are more common near converging plates, where subduction generates magma.

Exam-style questions with model answers

Q1. State the age of the earth and the age of the universe as given by the Big Bang theory. [2 marks]

1. The earth is about 4,600 million years old.
2. The Big Bang took place 13.7 billion years before the present.

Q2. Distinguish between the focus and the epicentre of an earthquake. [3 marks]

1. The focus is the point inside the earth where energy is released; it is also called the hypocentre.
2. The epicentre is the point on the surface directly above the focus.
3. The epicentre is the first to experience the waves, and the effects are strongest there.

Q3. Explain how the behaviour of S-waves proves that the outer core is liquid. [4 marks]

1. S-waves can travel only through solid materials.
2. Seismographs within 105° of the epicentre record both P and S-waves.
3. Beyond 105° no S-waves are recorded, although P-waves are recorded beyond 145°.
4. This absence shows that the S-waves meet a liquid layer, the outer core, which stops them.

Q4. Explain why volcanoes are more common in the areas of converging plates. [4 marks]

1. At a converging boundary one plate is forced down beneath the other in subduction.
2. The descending plate carries water and wet ocean-floor rock downwards.
3. The water lowers the melting point of the mantle rock above, generating magma.
4. The magma rises and feeds a chain of volcanoes parallel to the boundary.

Q5. Describe the structure of the earth's interior, giving the thickness and state of each layer. [5 marks]

1. The crust is the outermost solid, brittle layer; mean thickness 5 km under the oceans and about 30 km under the continents, up to 70 km under the Himalaya.
2. The mantle extends from Moho's discontinuity to 2,900 km; its upper part, the asthenosphere, is weak and extends up to 400 km and is the main source of magma.
3. The lithosphere is the crust and uppermost solid mantle, 10 to 200 km thick.
4. The outer core, from 2,900 km to 5,100 km, is liquid.
5. The inner core, from 5,100 km to 6,378 km, is solid; the core is made mostly of nickel and iron and is called the nife layer.

Q6. Evaluate the effects of the Nepal earthquake of 2015 in the light of the causes of earthquakes. [6 marks]

1. Nepal lies on the collision zone of the Indian and Eurasian plates, so the region is earthquake-prone.
2. The earthquake of 25 April 2015 was a tectonic earthquake of magnitude 7.8, with its epicentre in Gorkha district about 77 km north-west of Kathmandu and a shallow focus, about 8 to 15 km deep.
3. A shallow focus means the waves reach the surface with little loss of energy, so shaking is severe.
4. About 9,000 people died and more than 20,000 were injured; villages in the hill districts were flattened and heritage structures in Kathmandu collapsed.
5. Landslides and avalanches buried settlements and climbers, and damaged roads cut off remote valleys.
6. The disaster shows that magnitude alone does not decide the effect; settlement density, building quality and mountain terrain matter as much.

Q7. A mineral contains uranium-238 and lead-206. Lead-206 and uranium-238 are present in equal amounts. The half-life of uranium-238 is 4.5 billion years. Calculate the age of the mineral and compare it with the age of the earth. [3 marks]

1. Equal amounts of parent and daughter mean that half the parent remains, so one half-life has passed.
2. Age = 1 × 4.5 billion years = 4.5 billion years.
3. This is close to the age of the earth, about 4,600 million years, so the mineral is among the oldest earth materials.

Q8. Compare shield volcanoes and composite volcanoes. [4 marks]

1. Shield volcanoes are made of very fluid basalt lava and are broad and not steep; the Hawaiian volcanoes are the most famous examples.
2. Composite volcanoes erupt cooler, more viscous lava and are often explosive.
3. Composite volcanoes build layers of lava, pyroclastic material and ash around the vent, giving a layered cone.
4. Shield volcanoes have low explosivity unless water enters the vent, while composite volcanoes are frequently explosive.

Q9. Explain how the rock cycle links the three rock types, using granite, sandstone and quartzite as examples. [5 marks]

1. Granite is an igneous rock, formed when magma cools and solidifies, either at the surface or within the crust.
2. Weathering and erosion break the granite into grains, which are carried by water, wind and ice and deposited in low places.
3. Compaction and cementation of the deposited grains form sandstone, a sedimentary rock.
4. Heat and pressure deep in the crust change the sandstone into quartzite, a metamorphic rock.
5. Further heat may melt the quartzite back into magma, and the cycle begins again, so the rock cycle has no beginning and no end.

Q10. A rock sample contains potassium-40 and argon-40. Argon-40 makes up seven eighths of the total of the two and potassium-40 one eighth. The half-life of potassium-40 is 1.25 billion years. Calculate the age of the rock and state which dating method this illustrates. [3 marks]

1. One eighth of the parent remaining means three half-lives have passed, since 1 → ½ → ¼ → ⅛.
2. Age = 3 × 1.25 billion years = 3.75 billion years.
3. This illustrates radiometric dating, in which the amounts of parent and daughter elements are compared to find the age of a rock.

Key takeaways

• The Big Bang theory holds that all matter began in a tiny ball and exploded 13.7 billion years before the present.
• The nebular hypothesis of Kant and Laplace, revised by Schmidt and Weizsäcker, explains planets forming from a rotating solar nebula.
• Radioactivity gives the earth's age as about 4,600 million years, using the half-lives of elements such as uranium and potassium.
• The earth has a crust, mantle and core; the outer core is liquid and the inner core solid.
• Seismic waves are the chief indirect source of information about the interior.
• S-waves cannot pass through liquids, which proves the outer core is liquid.
• Earthquakes are measured by the Richter scale for magnitude and the Mercalli scale for intensity.
• Volcanoes are more common near converging plates because subduction generates magma.
• Rocks are grouped by origin into igneous, sedimentary and metamorphic, and the rock cycle links them.

Test yourself

What is a light year?

A light year is a measure of distance, not time. It is the distance light travels in one year at 300,000 km per second, equal to 9.461 × 10¹² km.

Name two direct sources of information about the earth's interior.

Mining and deep drilling, such as the Kola drill which reached 12 km, and volcanic eruptions that bring magma to the surface for analysis.

What is the shadow zone of an earthquake?

It is the zone between 105° and 145° from the epicentre from which earthquake waves are not reported. The S-wave shadow zone is larger and covers a little over 40 per cent of the earth's surface.

Which scale measures the visible damage of an earthquake?

The Mercalli intensity scale, named after an Italian seismologist, which ranges from 1 to 12 and takes into account the visible damage caused by the event.

What is the Deccan Traps?

A large flood basalt province in India, presently covering most of the Maharashtra plateau, formed by highly fluid lava that flowed for long distances.

What is the difference between magma and lava?

Magma is the molten rock material in the upper mantle portion. Once it starts moving towards the crust or reaches the surface, it is referred to as lava.

Name the three rock types by origin.

Igneous rocks, formed by cooling magma or lava; sedimentary rocks, formed from compacted sediments; and metamorphic rocks, formed by heat and pressure on existing rocks.

What is a gravity anomaly?

A gravity anomaly is a reading of gravity at a place that differs from the expected value. Such differences give information about the distribution of mass of material in the crust.