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Inside the Earth: layers, seismic evidence and rocks

मराठीत स्पष्टीकरण

The Earth: its place, motions and interior · Part 3 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

How the interior is known from direct and indirect sources, P, S and surface waves, the shadow zone, the crust, mantle and core with the Moho, Gutenberg and Lehmann boundaries, the elements and minerals of the crust, igneous, sedimentary and metamorphic rocks, and the rock cycle.

Each idea is explained first in Marathi (मराठी), ending with the English and Hindi exam terms, followed by the same point in exam English. UPSC Prelims is in English or Hindi, so understand it in Marathi and revise it in English.

भाग 1 आणि 2 मध्ये पृथ्वी बाहेरून पाहिली. आता आत जाऊ. पृथ्वीची सरासरी त्रिज्या सुमारे 6,371 किमी, पण माणसाने खोदलेले सर्वात खोल छिद्र 12 किमीच्या पुढे गेलेले नाही — म्हणजे कांद्याची वरची साल फक्त खरवडल्यासारखे. म्हणून आतली माहिती मुख्यतः भूकंप लहरी कशा प्रवास करतात यावरून काढली आहे. पृथ्वी थरांनी बनली आहे: बाहेर पातळ भूकवच, त्याखाली जाड खडकाळ प्रावरण, आणि मध्यभागी जड धातूचा गाभा. पुढे भूकवच कशापासून बनले आहे आणि तीन प्रकारचे खडक एकमेकांत कसे बदलतात ते पाहू. (→ English: crust, mantle, core · हिन्दी: भूपर्पटी, प्रावार, क्रोड)

In exam EnglishParts 1 and 2 looked at the Earth from outside; this part goes underneath. The Earth's mean radius is about 6,371 km, yet no hole drilled by people has gone much deeper than 12 km, so nearly all knowledge of the interior is reasoned out from indirect clues, above all the way earthquake waves travel. Those clues show a planet built in shells: a thin crust outside, a thick rocky mantle below it and a dense metallic core at the centre. The second half of the part turns to the crust itself: its elements and minerals, and the three families of rock that keep turning into one another.

आतल्या माहितीचे पुरावे दोन प्रकारचे. प्रत्यक्ष पुरावा म्हणजे हातात घेऊन तपासता येईल असा खरा पदार्थ: दक्षिण आफ्रिकेतील सोन्याच्या खाणी (सुमारे 4 किमी), रशियातील कोला खोल विंधनविहीर (1989 मध्ये सुमारे 12.2 किमी), ज्वालामुखी बाहेर आणतो तो शिलारस. अप्रत्यक्ष पुरावा म्हणजे वरून मोजून अंदाज बांधणे — डॉक्टर स्टेथोस्कोपने छातीच्या आत ऐकतात तसे. खोल जावे तसे तापमान, दाब आणि घनता वाढते; पृथ्वीची सरासरी घनता वरच्या खडकांच्या जवळजवळ दुप्पट, म्हणून आत जड पदार्थ असलाच पाहिजे. उल्कापिंड, गुरुत्व व चुंबकीय सर्वेक्षणे, आणि सर्वात महत्त्वाचे म्हणजे भूकंप लहरी. (→ English: magma · हिन्दी: मैग्मा)

In exam EnglishEvidence about the interior comes from direct and indirect sources. Direct sources are real material from below: deep mines, such as South Africa's gold mines at roughly 4 km; drilling, such as the Kola Superdeep Borehole in north-western Russia, which reached about 12.2 km in 1989; and volcanic eruptions, since magma rises from depths no drill can reach. Indirect sources are inferences from measurements at the surface. Temperature, pressure and density all increase with depth; the Earth's average density, about 5.5 g/cm³, is roughly twice that of surface rock, so heavier material must lie inside. Meteorites are thought to resemble the deep material, and gravity and magnetic surveys show how mass and magnetic minerals are spread. The richest indirect source is the record of earthquake waves.

ताणाखालील खडक तुटून घसरतात तेव्हा अचानक ऊर्जा मुक्त होणे म्हणजे भूकंप. आत जिथे ऊर्जा मुक्त होते ते केंद्र (फोकस); त्याच्या थेट वर जमिनीवरचे ठिकाण म्हणजे अपिकेंद्र. ऊर्जा भूकंप लहरी म्हणून पसरते. P लहरी सर्वात वेगवान — रेल्वेचे डबे एकमेकांना पुढे ढकलतात तशा; घन, द्रव, वायू तिन्हींमधून जातात. S लहरी नंतर येतात — दोरीचे एक टोक धरून झटका दिला की वर-खाली लाट जाते तशा; फक्त घन पदार्थातून जातात. पृष्ठीय लहरी हळू आणि सर्वात शेवटी येतात, पण घरांचे सर्वाधिक नुकसान त्याच करतात. (→ English: seismic waves, P waves, S waves, surface waves · हिन्दी: भूकंपीय तरंगें, P तरंगें, S तरंगें, धरातलीय तरंगें)

In exam EnglishAn earthquake is a sudden release of energy when strained rocks break and slip along a fault. The point underground where the energy is released is the focus, or hypocentre; the point on the surface directly above it is the epicentre. The energy spreads as seismic waves, recorded by a seismograph. P waves, or primary waves, are the fastest and arrive first; they push and pull material along their line of travel, as sound does, and cross solids, liquids and gases. S waves, or secondary waves, come next; they shake material at right angles to their path, like a rope flicked at one end, and move only through solids. Surface waves run along the Earth's outer skin; they are the slowest, arrive last and do the most damage to buildings.

The three kinds of seismic waves
Wave	Speed and arrival	Passes through	How it moves the ground
P (primary), body wave	Fastest; arrives first	Solids, liquids and gases	Push and pull along the direction of travel
S (secondary), body wave	Slower; arrives second	Solids only	Shakes at right angles to the direction of travel
Surface waves	Slowest; arrive last	Along the outer layer of the Earth	Rolling and sideways shaking; most destructive

जगभरातील भूकंपलेखांनी एक विचित्र नमुना दाखवला. अपिकेंद्रापासून सुमारे 105° पर्यंत P आणि S दोन्ही थेट पोहोचतात. 105° च्या पलीकडे S लहरी येतच नाहीत. 105° ते 145° दरम्यान P लहरीही थेट येत नाहीत — हाच छाया प्रदेश. दिव्यासमोर चेंडू ठेवला की मागे सावली पडते, तसा मधला गाभा लहरींना आड येतो. S लहरी द्रवातून जात नाहीत, म्हणून बाहेरचा गाभा द्रव आहे हे कळले. P लहरी गाभ्यात वाकतात, म्हणून तो पट्टा रिकामा राहतो. प्रत्येक भूकंपाचा स्वतःचा छाया प्रदेश असतो. (→ English: shadow zone · हिन्दी: छाया क्षेत्र)

In exam EnglishSeismograph records from around the world reveal a telling pattern. Measuring distance from the epicentre as an angle at the Earth's centre, both P and S waves arrive directly up to about 105°. Beyond about 105°, direct S waves never arrive. Between about 105° and 145°, direct P waves are missing too; this belt is the shadow zone. Beyond 145°, P waves appear again. Two conclusions follow. S waves cannot cross liquid, so their absence shows that the outer core is liquid. P waves bend sharply as they enter and leave the core, which steers them away from the 105°–145° belt. Because S waves are missing everywhere beyond 105°, their shadow is much larger than that of P waves. Every earthquake has its own shadow zone, measured from its own epicentre.

भूकवच म्हणजे पृथ्वीचे वरचे कवच — अंड्याच्या कवचासारखे अगदी पातळ, पृथ्वीच्या आकारमानाच्या एक टक्काही नाही. हे दोन प्रकारचे. खंडीय भूकवच जाड (सरासरी 30–40 किमी, हिमालयाखाली सुमारे 70 किमी), हलके, ग्रॅनाइटसारखा खडक — सिलिका व अॅल्युमिनियम जास्त, म्हणून SIAL. सागरी भूकवच पातळ (5–10 किमी), जड बेसाल्ट — सिलिका व मॅग्नेशियम, म्हणून SIMA. हलके वर, जड खाली — म्हणूनच खंड उंच आणि महासागर खोल. भूकवचाच्या तळाची सीमा मोहो; 1909 मध्ये मोहोरोविचिच यांनी ओळखली. (→ English: crust, Moho discontinuity · हिन्दी: भूपर्पटी, मोहो असंबद्धता)

In exam EnglishPicture the crust as the skin on a giant fruit. No layer lies above it, none below it is thinner, and all of it together fills under one per cent of the Earth's volume. Beneath the continents the crust averages roughly 30 to 40 km in depth and thickens to around 70 km below lofty mountain belts like the Himalaya; it is old, relatively light and made mostly of granite-like rock rich in silica and aluminium, hence the old name SIAL. Oceanic crust, under the deep sea floor, is only about 5–10 km thick, younger and heavier, made mostly of basalt rich in silica and magnesium, hence SIMA. Being lighter and thicker, continental crust stands high, while oceanic crust lies low and forms the ocean basins. The crust ends below at the Mohorovičić discontinuity, the Moho for short, a boundary at which earthquake waves abruptly travel faster; the Croatian seismologist Andrija Mohorovičić identified it in 1909 while studying an earthquake near Zagreb.

मोहोच्या खाली प्रावरण — सुमारे 2,900 किमी खोलीपर्यंत, पृथ्वीच्या आकारमानाच्या चार-पंचमांशपेक्षा जास्त. लहरींसाठी ते घनच वाटते, पण खूप गरम असल्याने लाखो वर्षांत घट्ट गुळाच्या पाकासारखे हळूहळू सरकते. वरच्या प्रावरणात दुर्बलावरण नावाचा मऊ थर आहे (सुमारे 100 किमीपासून काहीशे किमीपर्यंत) — तिथे खडक अंशतः वितळलेला असतो, ज्वालामुखीचा शिलारस बहुतेक तिथेच तयार होतो. त्याच्या वर भूकवच आणि प्रावरणाचा कडक वरचा भाग मिळून शिलावरण. तळ्यात तरंगणाऱ्या पानांसारख्या शिलावरणाच्या प्लेटा मऊ दुर्बलावरणावर सरकतात. (→ English: mantle, asthenosphere, magma, lithosphere · हिन्दी: प्रावार, दुर्बलतामंडल, मैग्मा, स्थलमंडल)

In exam EnglishBelow the Moho lies the mantle, reaching down to about 2,900 km and making up more than four-fifths of the Earth's volume. It is dense rock rich in silicates of iron and magnesium. S waves pass through it, so it behaves as a solid, yet it is hot enough to creep very slowly over millions of years, like an extremely stiff toffee. In the upper mantle lies the asthenosphere from roughly 100 km to a few hundred kilometres down; the rock there is near its melting point and partly molten in places, and much of the magma that feeds volcanoes forms there. Above it, the crust and the rigid uppermost mantle together form the lithosphere, commonly put at about 100 km thick. The lithosphere is broken into plates that move over the soft asthenosphere, the basis of plate tectonics in the next chapter.

सुमारे 2,900 किमी खोलीवर गुटेनबर्ग सीमेपाशी गाभा सुरू होतो; तिथे S लहरी थांबतात. बाह्य गाभा (2,900–5,150 किमी) द्रव धातू — त्यातील प्रवाह सायकलच्या डायनॅमोसारखे काम करून पृथ्वीचे चुंबकीय क्षेत्र तयार करतात; होकायंत्र उत्तर दाखवते ते यामुळेच. आंतरिक गाभा (5,150 किमीपासून केंद्रापर्यंत) सूर्याच्या पृष्ठभागाइतका गरम असूनही घन आहे — वरून दाबणाऱ्या प्रचंड दाबामुळे तो वितळत नाही. या दोघांमधील सीमा लेहमन यांच्या नावाने (1936). लोखंड आणि निकेल — म्हणून NIFE. (→ English: core, Gutenberg discontinuity, Lehmann discontinuity · हिन्दी: क्रोड, गुटेनबर्ग असंबद्धता, लेहमन असंबद्धता)

In exam EnglishThe core begins at the Gutenberg discontinuity, about 2,900 km down, where P waves slow sharply and S waves stop. The outer core, down to about 5,150 km, is liquid metal; slow currents in it act like a dynamo and generate the Earth's magnetic field, which shields the planet from charged particles from the Sun and makes a compass needle point north. The inner core, from about 5,150 km to the centre at about 6,371 km, is solid although its temperature is estimated at 5,000–6,000 °C, comparable to the surface of the Sun, because enormous pressure keeps it from melting. The boundary between the outer and inner core is commonly called the Lehmann discontinuity, after the Danish seismologist Inge Lehmann, who proposed a solid inner core in 1936. The core is mainly iron with some nickel, hence the old name NIFE, and it is the densest layer.

The layers of the Earth
Layer	Depth (approx.)	State	Main material	Boundary at its base
Crust	Surface to about 5–10 km (oceans) or 30–70 km (continents)	Solid	Granite-like rock (continents), basalt (oceans)	Moho (1909)
Mantle	Moho to about 2,900 km	Solid but able to creep; asthenosphere weak and partly molten	Rock rich in iron and magnesium silicates	Gutenberg discontinuity
Outer core	About 2,900–5,150 km	Liquid	Iron and nickel	Lehmann discontinuity (1936)
Inner core	About 5,150 km to the centre (about 6,371 km)	Solid	Iron and nickel	None: the centre

भूकवचात मूलद्रव्यांपासून खनिजे, खनिजांपासून खडक — विटांपासून भिंत, भिंतींपासून घर बांधतो तसे. आठ मूलद्रव्येच भूकवचाच्या वजनाचा सुमारे 98%: ऑक्सिजन (सुमारे 46%), सिलिकॉन (28%), अॅल्युमिनियम (8%), लोह (5%), मग कॅल्शियम, सोडियम, पोटॅशियम, मॅग्नेशियम. पण संपूर्ण पृथ्वी घेतली तर लोहच सर्वाधिक — कारण गाभा धातूचा आहे. खनिजाला ठरावीक रासायनिक रचना आणि अणूंची नियमित मांडणी असते. सर्वाधिक आढळणारी खनिजे फेल्डस्पार आणि क्वार्ट्झ. ग्रॅनाइट म्हणजे फेल्डस्पार + क्वार्ट्झ + थोडा अभ्रक. (→ English: elements, minerals, feldspar, quartz · हिन्दी: तत्व, खनिज, फ़ेल्डस्पार, क्वार्ट्ज़)

In exam EnglishThe crust is built from elements combined into minerals, and minerals combined into rocks. Count up the crust's weight and just eight elements together account for close to 98 per cent of the total. Top of that list by weight is oxygen, near 46 percent; silicon follows at close to 28, aluminium near 8 and iron near 5, and the remaining four, calcium, sodium, potassium and magnesium, supply somewhere between 2 and 4 percent apiece. For the planet as a whole the order changes: because the core is mostly metal, iron is the most abundant element of the whole Earth by mass. Each mineral has a fixed chemistry and an orderly, repeating arrangement of atoms. With oxygen and silicon dominant, most minerals are silicates; feldspars are the commonest group, and quartz is another very common mineral. A rock is a natural mixture of minerals: granite, for instance, is mainly feldspar and quartz with a little mica.

वितळलेला खडक थंड होऊन घट्ट झाला की अग्निजन्य खडक. बाकी सगळे खडक यांच्यापासूनच आले म्हणून प्राथमिक खडक. कुठे थंड झाला यावर फरक: जमिनीखाली शिलारस हळूहळू थंड झाला तर स्फटिक मोठे होतात — ग्रॅनाइट (अंतर्वेशी खडक). वर आलेला लाव्हा पटकन थंड झाला तर स्फटिक लहान — बेसाल्ट (बहिर्वेशी खडक). गुळाचा पाक हळू निवला तर रवाळ, थंड पाण्यात ओतला तर गुळगुळीत होतो तसेच. यांच्यात थर किंवा जीवाश्म नसतात. उदाहरण: महाराष्ट्राचा दख्खन ट्रॅप बेसाल्ट (सुमारे 6.6 कोटी वर्षांपूर्वी); त्यातूनच काळी कापसाची मृदा. (→ English: igneous rocks, magma, granite, basalt · हिन्दी: आग्नेय शैल, मैग्मा, ग्रेनाइट, बेसाल्ट)

In exam Englishigneous rocks are born when molten rock cools and sets solid; their name borrows the Latin 'ignis', fire. Geologists label them primary because sedimentary and metamorphic rocks are ultimately made, directly or indirectly, from material that first solidified this way. Where the cooling happens sets their texture. magma cooling slowly deep underground grows large crystals, giving coarse-grained intrusive, or plutonic, rocks such as granite. Lava cooling quickly at the surface keeps its crystals tiny, giving fine-grained extrusive, or volcanic, rocks such as basalt. Igneous rocks are generally hard, are not layered and contain no fossils. India's best-known example is the Deccan Traps, a vast pile of basalt lava flows covering much of Maharashtra and parts of neighbouring states, laid down about 66 million years ago, around the end of the age of dinosaurs. Weathering of that basalt across ages produced the dark Deccan soil, and a large part of the plateau's cotton crop now grows in it.

जुन्या खडकांचे तुकडे आणि सजीवांचे अवशेष साचून घट्ट झाले की गाळाचे खडक. नद्या, वारा, हिमनद्या, लाटा माती-वाळू वाहून नेऊन बहुधा पाण्याखाली थरावर थर टाकतात — पुस्तकांची चळत रचावी तसे. वरच्या थरांच्या वजनाने खालचे थर दबतात आणि नैसर्गिक सिमेंटने जोडले जाऊन खडक होतात. म्हणूनच यांच्यात थर आणि जीवाश्म दिसतात. यांत्रिक (वालुकाश्म, शेल), सेंद्रिय (कोळसा, चुनखडी) आणि रासायनिक (खडकमीठ, जिप्सम) पद्धतीने बनतात. कोळसा आणि पेट्रोलियम यांच्यातच सापडतात; दिल्लीच्या लाल किल्ल्याचा लाल वालुकाश्म हे एक उदाहरण. (→ English: sedimentary rocks, lithification, fossils · हिन्दी: अवसादी शैल, शिलीभवन, जीवाश्म)

In exam Englishsedimentary rocks are made from fragments of older rocks and the remains of living things. Rivers, wind, glaciers and waves carry weathered particles and drop them as sediment, mostly under water. Layer settles on layer; the weight above compacts the layers below and natural cements bind the grains, a hardening called lithification. So these rocks are usually stratified, and they are the ones that hold fossils. By origin they are mechanically formed from rock fragments (sandstone, conglomerate, shale), organically formed from plant and animal remains (coal, and limestone or chalk from shells and coral), or chemically formed when minerals come out of solution (rock salt, gypsum, some limestones). Coal, petroleum and natural gas occur in them, and the red sandstone of Delhi's Red Fort is a familiar example.

आधीचा खडक न वितळता प्रचंड उष्णता आणि दाबाने बदलला की तो रूपांतरित खडक — मातीचे मडके भट्टीत भाजले की कडक होते तसे, पण इथे लाखो वर्षे लागतात. खनिजे पुन्हा स्फटिक होऊन बहुधा पट्ट्यांत रचली जातात. शिलारसाच्या जवळच्या उष्णतेने बदलले तर संपर्क रूपांतरण; पर्वत तयार होताना मोठ्या प्रदेशात बदलले तर प्रादेशिक रूपांतरण. लक्षात ठेवण्याच्या जोड्या: चुनखडी → संगमरवर, वालुकाश्म → क्वार्ट्झाइट, शेल → स्लेट, ग्रॅनाइट → नाइस. ताजमहालचा संगमरवर राजस्थानातील मकराना येथून आला. (→ English: metamorphic rocks, marble, quartzite, slate, gneiss · हिन्दी: कायांतरित शैल, संगमरमर, क्वार्ट्ज़ाइट, स्लेट, नीस)

In exam Englishmetamorphic rocks are existing rocks, whether igneous, sedimentary or already metamorphic, changed by great heat, great pressure or both, without melting. Their minerals recrystallise and often line up in bands or sheets, a texture called foliation. Heat from nearby magma causes contact, or thermal, metamorphism; the squeezing and heating of wide areas during mountain building causes regional metamorphism. Four pairs are asked again and again: limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss. The white marble of the Taj Mahal, quarried at Makrana in Rajasthan, began as limestone laid down under water. Metamorphic rocks are usually harder and more compact than their parent rocks.

The three families of rock
Family	How it forms	Key features	Examples
Igneous	Molten rock cools and hardens	Crystalline, no layers, no fossils; primary rocks	Granite (intrusive), basalt (extrusive)
Sedimentary	Sediment is deposited in layers and hardened	Layered; contain fossils; host coal and oil	Sandstone, shale, limestone, coal, rock salt
Metamorphic	Existing rock changed by heat and pressure without melting	Harder, often banded (foliated)	Marble, quartzite, slate, gneiss

तिन्ही प्रकारचे खडक कायमचे नाहीत — कोणताही खडक दुसऱ्यात बदलू शकतो; या न संपणाऱ्या चक्रालाच खडक चक्र म्हणतात. जुना सोन्याचा दागिना वितळवून नवा घडवतात, तसा निसर्ग खडक पुन्हा पुन्हा वापरतो. अग्निजन्य खडक झिजून गाळाचा खडक; खोलवर उष्णता आणि दाबाने रूपांतरित खडक; आणखी खोल जाऊन वितळला की शिलारस, थंड झाला की पुन्हा अग्निजन्य. वर सूर्याची ऊर्जा (पाऊस, नदी, वारा), आत पृथ्वीची उष्णता हे चक्र फिरवतात. भूकंप लहरींवरून कळलेले आतले थर उष्णता आणि शिलारस देतात; वरच्या भूकवचात खडक तयार होत आणि बदलत राहतात. (→ English: rock cycle, magma, seismic waves · हिन्दी: शैल चक्र, मैग्मा, भूकंपीय तरंगें)

In exam EnglishThe three families are not fixed: over geological time any rock can become any other, a recycling called the rock cycle. Igneous rock exposed at the surface weathers into sediment and then sedimentary rock. Buried, heated or squeezed, igneous or sedimentary rock becomes metamorphic. Carried deeper still, any rock can melt into magma and cool into new igneous rock, and uplift brings rocks back to the surface to be worn down again. The Sun's energy, working through rain, rivers, wind and ice, drives weathering and erosion at the surface; the Earth's internal heat drives melting, metamorphism and the movement of plates. So the two halves of this part connect: the layered interior known from seismic waves supplies the heat and the magma, the crust is where rocks are made and remade, and the same forces shape the landforms, earthquakes and volcanoes of the next chapter.

Exam traps

Solids, liquids and gases all carry P waves, but S waves stop the moment they meet anything that is not solid. The absence of S waves beyond about 105° from the epicentre is the evidence for a liquid outer core.
Direct P waves are missing between about 105° and 145° from the epicentre, but S waves are missing everywhere beyond 105°, so the area left without S waves covers far more of the globe than the band without direct P waves.
Energy is released at the focus (hypocentre), below the ground; the epicentre is the spot on the ground lying straight above it.
Boundaries in order: Moho (crust and mantle), Gutenberg (mantle and core, about 2,900 km), Lehmann (outer and inner core, about 5,150 km). Questions shuffle these.
Oxygen is the most abundant element in the crust, followed by silicon; for the Earth as a whole, iron is the most abundant by mass.
Do not equate the lithosphere with the crust alone, since the rigid top layer of the mantle belongs to it too; beneath it sits the asthenosphere, a weak zone of the upper mantle from which much magma rises.
The inner core is solid although it is the hottest part, because of enormous pressure; it is the outer core that is liquid.
Fossils are found in sedimentary rocks. Granite is intrusive igneous and basalt extrusive igneous; marble comes from limestone, quartzite from sandstone, slate from shale and gneiss from granite.

Recap

The interior is known mostly indirectly. Direct sources: mines, drilling (Kola, about 12 km), volcanic material. Indirect: temperature, pressure and density, meteorites, gravity, magnetism and, above all, seismic waves.
Focus underground, epicentre on the surface above it; P waves fastest and pass through all states; S waves only through solids; surface waves slowest and most destructive.
Shadow zone: no direct P waves between about 105° and 145°; no S waves beyond 105°; hence a liquid outer core.
Crust: continental (thick, granitic, light, SIAL) and oceanic (thin, basaltic, dense, SIMA); the Moho (1909) at its base.
Mantle to about 2,900 km; asthenosphere weak, partly molten, a source of magma; lithosphere = crust + rigid upper mantle, broken into plates.
Core: liquid outer core (source of the magnetic field) and solid inner core; Gutenberg at about 2,900 km, Lehmann at about 5,150 km; iron and nickel (NIFE).
Crust by weight: O > Si > Al > Fe > Ca > Na > K > Mg; whole Earth: iron first. Feldspar and quartz are common minerals; rocks are mixtures of minerals.
Igneous (granite, basalt, Deccan Traps), sedimentary (layers, fossils, coal, Red Fort sandstone), metamorphic (marble, quartzite, slate, gneiss), all linked by the rock cycle.

Practice questions

Q1. Which of the following types of seismic waves cannot travel through liquids?

P waves
S waves
Both P and S waves
Neither P nor S waves

Show answer

Answer: B. S waves

S waves move material at right angles to their direction of travel, which needs a rigid medium, so they pass only through solids. P waves are compressional and travel through solids, liquids and gases, so (a) and (c) are wrong; (d) is wrong because S waves cannot cross liquids.

Q2. Consider the following statements:
1. The epicentre is the point inside the Earth where the energy of an earthquake is released.
2. Of all seismic waves, surface waves travel slowest and do the greatest damage to buildings.
Which of the statements given above is/are correct?

1 only
2 only
Both 1 and 2
Neither 1 nor 2

Show answer

Answer: B. 2 only

Statement 1 describes the focus (hypocentre), not the epicentre; the epicentre is the point on the surface directly above the focus. Statement 2 is correct. So (a) and (c) are wrong because they include statement 1, and (d) is wrong because statement 2 is correct.

Q3. Consider the following pairs of discontinuities and the layers they separate:
1. Mohorovičić discontinuity : crust and mantle
2. Gutenberg discontinuity : upper mantle and lower mantle
3. Lehmann discontinuity : outer core and inner core
How many of the pairs given above are correctly matched?

Only one
Only two
All three
None

Show answer

Answer: B. Only two

Pairs 1 and 3 are correct. The Gutenberg discontinuity, at about 2,900 km, separates the mantle from the core, not the upper from the lower mantle, so pair 2 is wrong. Hence 'only two'; 'only one' undercounts, and 'all three' and 'none' are wrong.

Q4. Which element is the most abundant in the Earth's crust by weight?

Silicon
Oxygen
Iron
Aluminium

Show answer

Answer: B. Oxygen

Oxygen makes up about 46 percent of the crust by weight. Silicon is second (about 28 percent) and aluminium third (about 8 percent). Iron is fourth in the crust, though it is the most abundant element of the Earth as a whole, which is the usual trap.

Q5. Consider the following statements:
1. The shadow zone of S waves is larger than the shadow zone of P waves.
2. The liquid state of the outer core was inferred from the absence of S waves beyond about 105° from the epicentre.
Which of the statements given above is/are correct?

1 only
2 only
Both 1 and 2
Neither 1 nor 2

Show answer

Answer: C. Both 1 and 2

Both are correct. Direct P waves are missing only in the belt between about 105° and 145°, while S waves are missing over the whole area beyond 105°, so the S-wave shadow is larger. Since S waves cannot cross liquid, their absence showed that the outer core is liquid. Options (a), (b) and (d) each reject a correct statement.

Q6. Match List I (original rock) with List II (metamorphic rock formed from it):
List I: A. Limestone B. Sandstone C. Shale D. Granite
List II: 1. Slate 2. Gneiss 3. Marble 4. Quartzite
Select the correct answer using the code given below.

A-3, B-4, C-1, D-2
A-4, B-3, C-1, D-2
A-3, B-4, C-2, D-1
A-1, B-4, C-3, D-2

Show answer

Answer: A. A-3, B-4, C-1, D-2

Limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss. Option (b) swaps marble and quartzite; (c) swaps slate and gneiss; (d) wrongly gives slate for limestone and marble for shale.

Q7. Consider the following statements:
1. The lithosphere consists of the crust together with the rigid uppermost part of the mantle.
2. The asthenosphere is a weak zone located in the outer core.
Which of the statements given above is/are correct?

1 only
2 only
Both 1 and 2
Neither 1 nor 2

Show answer

Answer: A. 1 only

Statement 1 is correct. Statement 2 is wrong: the asthenosphere lies in the upper mantle, beginning at roughly 100 km, far above the core. So (b) and (c) are wrong because they accept statement 2, and (d) is wrong because statement 1 is correct.

Q8. Which one of the following is an extrusive igneous rock?

Granite
Basalt
Marble
Sandstone

Show answer

Answer: B. Basalt

Basalt forms when lava cools quickly at the surface, so it is extrusive igneous. Granite is igneous but intrusive, formed by slow cooling underground; marble is metamorphic; sandstone is sedimentary.

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Sources

NASA Science: Earth facts and the Earth's interior and magnetic field explainers (National Aeronautics and Space Administration, US government) — https://science.nasa.gov/earth/facts/
United States Geological Survey: 'This Dynamic Earth' and educational material on the Earth's interior, minerals and the rock cycle (US government publication) — https://pubs.usgs.gov/publication/7000097
USGS Earthquake Hazards Program: seismic waves, focus and epicentre, seismographs, and the structure of the Earth from seismology — https://www.usgs.gov/programs/earthquake-hazards
Geological Survey of India, Ministry of Mines: geology of India, the Deccan Traps and rock types — https://www.gsi.gov.in