Earth's Interior, Earthquakes and Volcanoes: Structure, Seismic Waves, Nepal 2015 and Vulcanicity, ISC Class 11

What Earth's Interior, Earthquakes and Volcanoes is about

Weightage: CISCE gives no Class XI unit weightage for ISC Geography. Earth's Interior, Earthquakes and Volcanoes is topic 2 of the Class XI syllabus (Formation of the Earth) and can be asked anywhere in the 70-mark Paper I (Theory, 3 hours). Class XI is examined by the school, so your school sets the paper and decides how many marks each topic carries.Source: CISCE ISC Geography (853) syllabus, examination year 2028, Class XI

Key concepts

Structure of the earth's interior

The earth's mean radius is about 6,371 km. Its interior has three main layers that grow denser with depth.

1. Crust

• The thin, brittle outermost layer. Oceanic crust averages about 5 km thick and is made largely of basalt (sometimes called sima: silica and magnesium). Continental crust averages about 30-35 km and reaches about 70 km under the Himalayas; it is mainly granitic (sial: silica and aluminium).
• Its lower boundary is the Mohorovicic (Moho) discontinuity.

2. Mantle

• Extends from the Moho to about 2,900 km; it makes up most of the earth's volume.
• The upper part includes the asthenosphere (to about 400 km), a weak, partly molten zone from which magma rises. The crust and the rigid top of the mantle together form the lithosphere (about 10-200 km thick).
• The lower mantle is solid. The mantle-core boundary is the Gutenberg discontinuity.

3. Core

• Outer core (about 2,900-5,150 km): liquid, mainly iron and nickel ('nife'); its movement is thought to generate the earth's magnetic field.
• Inner core (about 5,150 km to the centre): solid because of enormous pressure, though very hot.

Temperature, pressure and density

• Temperature rises with depth, by roughly 25-30 degrees C per km in the upper crust (the geothermal gradient), and reaches several thousand degrees C in the core.
• Pressure rises with the weight of overlying rock and is millions of times atmospheric pressure at the centre.
• Density rises from about 2.7-3 g/cm3 in the crust to over 10 g/cm3 in the core; the earth's average density is about 5.5 g/cm3.

Worked example

[4] Describe the three main layers of the earth's interior with their depth and characteristics. (Recall)

Try it first, then show the solutionSolution

1. Crust: oceanic about 5 km (basaltic), continental about 30-35 km (granitic), to 70 km under Himalayas; Moho - 1 mark.
2. Mantle: to about 2,900 km; asthenosphere to about 400 km source of magma; lower mantle solid - 1 mark.
3. Core: outer liquid (2,900-5,150 km), inner solid; iron and nickel (nife) - 1 mark.
4. Temperature, pressure and density increase with depth - 1 mark.

Answer: The crust is the thin outer layer, about 5 km thick under the oceans (basaltic) and 30-35 km under the continents (granitic), reaching about 70 km beneath the Himalayas, and ends at the Moho. The mantle extends to about 2,900 km; its upper weak zone, the asthenosphere, supplies magma, while the lower mantle is solid. The core of iron and nickel has a liquid outer part and a solid inner part. Temperature, pressure and density all increase with depth.

Sources of information and seismic waves

Direct sources

• Mines and deep drilling: the deepest borehole (Kola, Russia) reached about 12 km, a tiny fraction of the radius.
• Volcanic eruptions bring up material from depth; deep-sea drilling projects sample the ocean floor.

Indirect sources

• Rising temperature and pressure with depth in mines and boreholes.
• Meteorites, believed to be made of material similar to the earth's interior.
• Gravity anomalies and the magnetic field.
• Above all, seismic waves.

Seismic waves
1. P-waves (primary): the fastest; they vibrate parallel to the direction of travel (compression and expansion); they pass through solids, liquids and gases.
2. S-waves (secondary): slower; they vibrate at right angles to the direction of travel; they pass only through solids.
3. Surface waves (L-waves): slowest; they travel along the surface and cause most damage.

Inferences

• Waves speed up or slow down and bend (refract) when they pass through materials of different density, showing that the earth is layered.
• Shadow zones: for any earthquake, stations within about 105 degrees of the epicentre receive both P and S waves; stations beyond about 145 degrees receive only P-waves; the band between 105 and 145 degrees receives no direct P or S waves. S-waves are absent everywhere beyond 105 degrees.
• Because S-waves cannot pass through the core, the outer core must be liquid; the bending of P-waves at the core boundary creates the P shadow zone.

Worked example

[4] Explain how the behaviour of seismic waves shows that the earth's outer core is liquid. (Analysis)

Try it first, then show the solutionSolution

1. P-waves travel through solids and liquids; S-waves only through solids - 1 mark.
2. S-waves are not received beyond about 105 degrees from the epicentre - 1 mark.
3. So the material they meet at about 2,900 km (outer core) must be liquid - 1 mark.
4. P-waves are refracted at the core boundary, creating a shadow zone between 105 and 145 degrees - 1 mark.

Answer: P-waves pass through solids and liquids, but S-waves pass only through solids. Seismographs more than about 105 degrees from an epicentre record no S-waves, which means the S-waves are stopped by a layer at about 2,900 km depth; that layer, the outer core, must therefore be liquid. P-waves are bent as they enter the core, leaving a shadow zone between about 105 and 145 degrees.

Earthquakes: causes, distribution, effects and measurement

Terms: the focus (hypocentre) is the point inside the earth where energy is released; the epicentre is the point on the surface directly above it.

Causes
1. Tectonic: movement of plates along faults; stress builds up until rocks break and rebound suddenly (most earthquakes).
2. Volcanic: movement of magma and explosive eruptions.
3. Collapse: roofs of mines or caverns collapsing.
4. Explosion: nuclear or chemical explosions.
5. Reservoir-induced: loading by large reservoirs (for example, Koyna, Maharashtra, 1967).

World distribution

• Circum-Pacific belt (the 'Ring of Fire'): most of the world's earthquakes.
• Alpine-Himalayan (Mid-continental) belt: from the Mediterranean through Iran to the Himalayas and Indonesia.
• Mid-ocean ridges such as the Mid-Atlantic Ridge.

Effects: ground shaking; landslides and avalanches; soil liquefaction; ground displacement and cracks; collapse of buildings; fires; floods from dam failures; tsunamis when the epicentre is under the sea (as in the Indian Ocean, 2004).

Isoseismal and homoseismal lines

• Isoseismal lines join places that experienced the same intensity of shaking; they form rough rings around the epicentre.
• Homoseismal (coseismal) lines join places where the shock waves arrived at the same time.

Measurement

• Seismographs record the waves.
• Magnitude (Richter scale, 1935): measures energy released, from instrument readings. It is logarithmic: each whole number up means ten times the wave amplitude and about 32 times the energy. Scientists today often use the moment magnitude scale, which works on the same idea.
• Intensity (Modified Mercalli scale, I to XII): measures the observed effects and damage at a place; it varies from place to place for the same earthquake.

Worked example

[4] Distinguish between the magnitude and the intensity of an earthquake, and between isoseismal and homoseismal lines. (Analysis)

Try it first, then show the solutionSolution

1. Magnitude: energy released, from instruments; Richter scale, logarithmic - 1 mark.
2. Intensity: observed effects at a place; Mercalli scale I-XII; varies with distance - 1 mark.
3. Isoseismal lines: join places of equal intensity - 1 mark.
4. Homoseismal lines: join places where shock waves arrived at the same time - 1 mark.

Answer: Magnitude measures the energy released at the source, read from instruments on the logarithmic Richter scale, and is the same for one earthquake everywhere; intensity measures the observed shaking and damage at a place on the Mercalli scale of I to XII and changes with distance and ground conditions. Isoseismal lines join places of equal intensity, while homoseismal lines join places where the shock waves arrived at the same time.

Case study: the Nepal earthquake of 2015

Event

• On 25 April 2015 an earthquake of magnitude 7.8 struck central Nepal; its epicentre was in Gorkha district, north-west of Kathmandu.
• A major aftershock of magnitude 7.3 followed on 12 May 2015, and hundreds of smaller aftershocks occurred.

Cause

• Nepal lies on the boundary where the Indian Plate pushes northward beneath the Eurasian Plate. Stress built up on the great thrust fault under the Himalayas and was released suddenly; the ground near Kathmandu rose by about a metre in places.

Effects

• Nearly 9,000 people were killed and many thousands injured; hundreds of thousands of houses were destroyed or damaged, especially poorly built brick and stone homes in villages.
• Historic monuments in the Kathmandu valley, including the Dharahara tower and temples in Durbar squares, collapsed or were damaged.
• Landslides blocked roads and rivers in the hills; an avalanche killed climbers at Mount Everest base camp.
• Shaking was felt in northern India, Bangladesh and Tibet.

Response and lessons

• International relief, including India's Operation Maitri, reached the country quickly.
• Lessons: enforce earthquake-resistant building codes, retrofit old structures, prepare communities and plan for landslides in mountain areas. The region had suffered earlier great earthquakes (such as 1934), showing that future shocks are certain.

Worked example

[4] Explain the cause and effects of the Nepal earthquake of 2015. (Understanding)

Try it first, then show the solutionSolution

1. 25 April 2015, magnitude 7.8, epicentre in Gorkha district - 1 mark.
2. Cause: Indian Plate pushing under the Eurasian Plate; release of stress on a Himalayan thrust fault - 1 mark.
3. Effects: nearly 9,000 deaths; houses and Kathmandu monuments destroyed - 1 mark.
4. Landslides, Everest avalanche; aftershock of 7.3 (12 May) - 1 mark.

Answer: On 25 April 2015 a magnitude 7.8 earthquake struck central Nepal, with its epicentre in Gorkha district. It was caused by the sudden release of stress on the Himalayan thrust fault where the Indian Plate pushes beneath the Eurasian Plate. Nearly 9,000 people died, hundreds of thousands of houses and historic monuments in the Kathmandu valley were destroyed, landslides blocked mountain roads, an avalanche struck Everest base camp, and a 7.3 aftershock followed on 12 May.

Vulcanicity: materials, processes, types and forms

Vulcanicity includes all the processes by which magma forms, rises and either erupts at the surface or cools within the crust.

Processes: magma forms in the asthenosphere or at subduction zones; being lighter than surrounding rock and full of gas, it rises through cracks into magma chambers and erupts through vents. Magma at the surface is called lava.

Materials

• Lava: molten rock.
• Pyroclasts: fragments thrown out: dust and ash, cinders (lapilli) and large volcanic bombs.
• Gases: mainly water vapour, with carbon dioxide, sulphur compounds, nitrogen compounds and others.

Properties of lava

• Basic (basaltic) lava: low in silica, very hot, fluid; flows long distances; gentle eruptions; forms broad shields and lava plateaus.
• Acidic (rich in silica) lava: cooler and viscous; traps gas; explosive eruptions; forms steep cones and domes.

Types of volcano

• By activity: active (erupting now or recently, e.g., Barren Island in the Andamans, Etna, Kilauea), dormant (not erupted for a long time but may erupt), extinct (not expected to erupt again).
• By form and eruption: shield volcanoes (fluid basalt, gentle slopes, e.g., Mauna Loa, Hawaii); composite or strato-volcanoes (layers of lava and ash, explosive, e.g., Fujiyama, Vesuvius); cinder cones; calderas (huge depressions after explosive collapse); flood basalt provinces (e.g., the Deccan Traps); mid-ocean ridge volcanoes (e.g., Iceland).

Volcanic forms

• Extrusive: cones, craters, calderas, lava plateaus and plains.
• Intrusive (cooled inside the crust): batholiths (huge granite masses), laccoliths (dome-shaped with a level base), lopoliths (saucer-shaped), phacoliths (wavy, in folds), sills (horizontal sheets) and dykes (vertical walls cutting across rock layers).

Worked example

[4] Distinguish between shield volcanoes and composite volcanoes. (Analysis)

Try it first, then show the solutionSolution

1. Shield: fluid basaltic lava, low silica; composite: viscous, silica-rich lava - 1 mark.
2. Shield: gentle eruptions; composite: explosive eruptions with ash - 1 mark.
3. Shield: broad, gently sloping; composite: steep cone of alternate lava and ash layers - 1 mark.
4. Examples: Mauna Loa (shield); Fujiyama, Vesuvius (composite) - 1 mark.

Answer: Shield volcanoes are built by very fluid, low-silica basaltic lava that erupts gently and spreads widely, forming broad, gently sloping mountains such as Mauna Loa in Hawaii. Composite volcanoes erupt cooler, viscous, silica-rich lava together with ash and other pyroclasts in explosive eruptions, building steep cones of alternate layers, as in Fujiyama and Vesuvius.

Why volcanoes are more common at converging plates

1. Subduction: where an oceanic plate meets another plate, the denser oceanic plate sinks (is subducted) into the mantle.
2. Melting: at depths of about 100 km the descending plate releases water, which lowers the melting point of the overlying mantle rock; heat and friction also contribute. Magma is produced.
3. Rise of magma: the magma is lighter than surrounding rock and rises through cracks to the surface.
4. Explosive volcanoes: this magma is often rich in silica and gas, producing viscous lava and explosive composite volcanoes.
5. Chains of volcanoes: a line of volcanoes forms parallel to the plate boundary, as volcanic island arcs (Japan, Indonesia, the Aleutians) or mountain chains (the Andes).
6. Distribution: the Pacific Ocean is ringed by converging boundaries, so most of the world's active volcanoes lie in the 'Ring of Fire'.

Other settings: volcanoes also form at divergent boundaries (mid-ocean ridges, Iceland) and at hot spots in the middle of plates (Hawaii), but the most numerous and dangerous active volcanoes are at converging boundaries.

Worked example

[4] Why are volcanoes more common in areas of converging plates? (Analysis)

Try it first, then show the solutionSolution

1. Oceanic plate subducted into the mantle at convergent boundaries - 1 mark.
2. Water released lowers melting point; magma forms at about 100 km - 1 mark.
3. Light magma rises through cracks to form volcanoes parallel to the boundary - 1 mark.
4. Hence the Ring of Fire around the Pacific (Andes, Japan, Indonesia) - 1 mark.

Answer: At converging boundaries the denser oceanic plate is subducted into the mantle; at about 100 km depth it releases water that lowers the melting point of the mantle rock above, producing magma. This light, gas-rich magma rises through cracks and erupts, building chains of volcanoes parallel to the boundary, such as the Andes, Japan and Indonesia, which is why the Pacific 'Ring of Fire' has most of the world's active volcanoes.

Earth's Interior, Earthquakes and Volcanoes in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Earthquakes: Causes, Distribution and Measurement
• Structure of the Earth's Interior and Seismic Waves
• Vulcanicity: Volcanoes and Their Landforms

Earth's Interior, Earthquakes and Volcanoes: key facts

Richter amplitude

$Amplituderatio=10^{(}M1-M2)$

Magnitude 7 vs 5: 100 times.

Richter energy

$Energyratio=10^{(}1.5(M1-M2)),about31.6perunit$

Magnitude 7 vs 5: about 1,000 times.

Distance from S-P lag

$d=lag/(1/vS-1/vP)$

Lag 10 s, vP 6, vS 3.5 km/s: 84 km.

Geothermal gradient

$T(depth)=T(surface)+gradientxdepth$

Roughly 25-30 degrees C per km in the upper crust.

Layer depths

Crust 5-70 km; mantle to 2,900 km; outer core to 5,150 km; inner core to 6,371 km

Moho, Gutenberg boundaries.

Shadow zones

$P:105-145degrees;S:beyond105degrees$

Outer core liquid.

How the school exam asks Earth's Interior, Earthquakes and Volcanoes

The question patterns to expect, and the method that works for each.

Tricks to remember Earth's Interior, Earthquakes and Volcanoes

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Layers: 'C-M-O-I' with 'Moho, Gutenberg, Lehmann'

  Crust - Moho - Mantle - Gutenberg (2,900 km) - Outer core - Lehmann (5,150 km) - Inner core.

  Use it when: Layers and boundaries in order.

• Waves: 'P = Push, S = Shake, L = Last'

  P pushes (longitudinal, fastest), S shakes sideways (solids only), L arrives last (surface).

  Use it when: Wave properties.

• Shadow numbers: '105 and 145'

  S stops at 105; P hides between 105 and 145.

  Use it when: Shadow zones.

Earth's Interior, Earthquakes and Volcanoes: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Earth's Interior, Earthquakes and Volcanoes

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying S-waves cannot pass the inner core only

  The liquid outer core stops them.

  ✓Do this: S-waves stop at the outer core (2,900 km), so none appear beyond about 105 degrees.

  Usually costs: 1 mark.

• ✗Mistake: Treating the Richter scale as linear

  Students think 7 is 'slightly more' than 6.

  Wrong working

  Magnitude 7 is 7/5 = 1.4 times stronger than magnitude 5.

  Right working

  Amplitude 10^2 = 100 times; energy about 1,000 times.

  ✓Do this: Use powers of 10 for amplitude and about 31.6 per step for energy.

  Usually costs: 2 marks.

• ✗Mistake: Confusing isoseismal and homoseismal lines

  Similar names.

  ✓Do this: Iso-SEISMAL = same intensity (damage); homo-seismal = same arrival time.

  Usually costs: 1 mark.

• ✗Mistake: Wrong layer depths

  Many numbers.

  ✓Do this: Moho at crust base; 2,900 km mantle-core; 5,150 km outer-inner core.

  Usually costs: 1 mark.

• ✗Mistake: Case study without date or magnitude

  A case study needs specific facts.

  ✓Do this: 25 April 2015, magnitude 7.8, Gorkha; nearly 9,000 deaths; 7.3 aftershock 12 May.

  Usually costs: 1-2 marks.

Practice questions on Earth's Interior, Earthquakes and Volcanoes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · Very short answerExam level

   What is the Mohorovicic discontinuity?

   Hint

   Below the crust.

   Answer

   The boundary between the crust and the mantle.

2. Q2 · 1 mark · Very short answerExam level

   What is the epicentre of an earthquake?

   Hint

   Above the focus.

   Answer

   The point on the earth's surface directly above the focus.

3. Q3 · 1 mark · MCQExam level

   The Deccan Traps are an example of:
   (a) A shield volcano
   (b) A flood basalt province
   (c) A caldera
   (d) A batholith

   Hint

   Thick basalt flows.

   Answer

   (b) A flood basalt province.

4. Q4 · 1 mark · Very short answerHarder exam level

   Which intrusive form is a vertical wall of igneous rock cutting across rock layers?

   Hint

   Not a sill.

   Answer

   A dyke.

5. Q5 · 4 marks · Short answerHarder exam level

   [4] Explain four effects of earthquakes.

   Hint

   Ground, slopes, water, structures.

   Answer

   (1) Ground shaking and cracking that topples buildings. (2) Landslides and avalanches in hilly areas, blocking roads and rivers. (3) Soil liquefaction, which makes buildings sink or tilt. (4) Tsunamis when the epicentre is under the sea, and fires or floods from damaged gas lines and dams.

6. Q6 · 4 marks · Short answerHarder exam level

   [4] At what temperature would rocks be at a depth of 4 km if the surface temperature is 20 degrees C and the geothermal gradient is 25 degrees C per km? What does this suggest about deep mines?

   Hint

   Add gradient x depth.

   Answer

   (1) Increase = 25 x 4 = 100 degrees C. (2) Temperature = 20 + 100 = 120 degrees C. (3) Deep mines become extremely hot. (4) They need strong cooling and ventilation, which limits how deep people can mine.

7. Q7 · 4 marks · Short answerHarder exam level

   [4] Distinguish between acidic and basic lava.

   Hint

   Silica, viscosity, eruption, landform.

   Answer

   (1) Acidic lava is rich in silica; basic lava is poor in silica. (2) Acidic lava is viscous and cooler; basic lava is fluid and hotter. (3) Acidic lava causes explosive eruptions; basic lava flows quietly over long distances. (4) Acidic lava builds steep cones and domes; basic lava builds shield volcanoes and lava plateaus such as the Deccan Traps.

8. Q8 · 4 marks · Short answerHOTS / competency

   [4] Why did the 2015 earthquake cause such heavy loss of life in Nepal?

   Hint

   Magnitude, buildings, terrain, time.

   Answer

   (1) Its high magnitude (7.8) and shallow source near populated areas. (2) Many rural houses were built of brick or stone without reinforcement. (3) Steep terrain caused landslides that buried villages and blocked rescue. (4) Weak enforcement of building codes and limited emergency resources; strong aftershocks added further damage.

9. Q9 · 4 marks · Short answerHOTS / competency

   [4] 'The earth's interior is known mostly by indirect evidence.' Justify.

   Hint

   12 km versus 6,371 km.

   Answer

   (1) The deepest borehole reached only about 12 km of a radius of about 6,371 km. (2) Volcanic material comes only from the upper mantle at most. (3) Seismic waves, gravity, magnetism and meteorites provide information about deeper layers. (4) The core's liquid outer part and solid inner part were inferred from wave behaviour, not observed.

10. Q10 · 7 marks · Long answerHarder exam level

    [7] (a) Explain the causes and world distribution of earthquakes. [4]
    (b) Explain why volcanoes are common at converging plate boundaries. [3]

    Hint

    (a) causes plus belts; (b) subduction chain.

    Answer

    (a) (1) Tectonic: sudden movement along faults at plate boundaries. (2) Volcanic, collapse, explosion and reservoir-induced earthquakes (Koyna, 1967). (3) Circum-Pacific belt has most earthquakes. (4) Alpine-Himalayan belt and mid-ocean ridges. (b) (1) Oceanic plate subducted; water lowers melting point; magma at about 100 km. (2) Light magma rises through cracks; silica-rich, explosive volcanoes. (3) Chains along the boundary: Ring of Fire (Andes, Japan, Indonesia).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is the thickness of the oceanic crust on average?Show answer

   About 5 km.

2. 2.Where is the Gutenberg discontinuity?Show answer

   At the mantle-core boundary, about 2,900 km deep.

3. 3.Which waves cause most damage at the surface?Show answer

   Surface (L) waves.

4. 4.What does the Mercalli scale measure?Show answer

   Intensity: observed effects and damage at a place (I to XII).

5. 5.What was the magnitude of the Nepal earthquake of 25 April 2015?Show answer

   7.8.

6. 6.What is a caldera?Show answer

   A large depression formed when a volcano collapses after a very explosive eruption.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Crust 5 km (oceanic) to 30-35 km (continental), 70 km Himalaya; mantle to 2,900 km; outer core liquid to 5,150 km; inner core solid.
• ☐Direct: mines, drilling (12 km), volcanoes; indirect: seismic waves, gravity, magnetism, meteorites.
• ☐P (push, all media), S (shake, solids only), L (surface, most damage); S shadow beyond 105; P shadow 105-145.
• ☐Causes: tectonic, volcanic, collapse, explosion, reservoir; belts: Circum-Pacific, Alpine-Himalayan, ridges.
• ☐Richter (energy, logarithmic: 10x amplitude, ~32x energy per unit); Mercalli (intensity I-XII); isoseismal vs homoseismal.
• ☐Nepal 25 April 2015: 7.8, Gorkha, nearly 9,000 dead; aftershock 7.3 on 12 May.
• ☐Volcanoes: shield, composite, caldera, flood basalt, ridge; acidic vs basic lava; intrusive forms; subduction explains Ring of Fire.

Earth's Interior, Earthquakes and Volcanoes: frequently asked questions