Structure of the Earth and Rocks: Layers, Waves and Types
Interior of the Earth and rocks for exams: crust, mantle and core, discontinuities, P and S waves, shadow zones, and igneous, sedimentary and metamorphic rocks.
By GK24 Editorial Team· Published · 4 min read
The interior of the Earth and the rocks that make its crust are the two most dependable scoring areas of physical geography, because almost everything in them is fixed: a boundary has one name, a rock has one parent, a wave has one property. No one has ever seen the inside of the planet, so the whole picture is built from evidence, and examiners like to ask what that evidence is as much as what it shows.
How we know what lies inside
The direct sources are shallow. Mines and deep boreholes reach only a few kilometres, and the deepest of them, the Kola borehole in Russia, stopped around twelve kilometres, a scratch on a planet whose radius is about 6,371 kilometres. Volcanic lava brings up material from the upper mantle, and meteorites, formed from the same cloud of matter as the Earth, suggest what the deeper layers are made of. The indirect sources do the real work: earthquake waves, the planet average density of about 5.5 grams per cubic centimetre against a crustal density of under 3, the pressure and temperature that rise with depth, and the magnetic field that points to a metallic, fluid core.
The layers and their boundaries
| Layer | Extent | Marks to remember |
|---|---|---|
| Crust | About 5 km under the oceans, 30 to 70 km under the continents | Thinnest and lightest layer; continental crust is called Sial, rich in silica and aluminium, and oceanic crust Sima, rich in silica and magnesium |
| Mantle | From the base of the crust to about 2,900 km | The largest layer by volume; its upper part, the asthenosphere, is the source of magma and the layer on which plates move |
| Outer core | About 2,900 km to 5,150 km | In a liquid state, which is why S waves stop here; its churning generates the magnetic field |
| Inner core | About 5,150 km to the centre | Solid despite the heat, because the pressure is enormous; core material is called Nife, nickel and iron |
The boundaries between layers are called discontinuities and each carries a name. The Conrad discontinuity separates the upper and lower crust; the Mohorovicic discontinuity, the Moho, separates the crust from the mantle; the Repetti discontinuity lies between the upper and lower mantle; the Gutenberg discontinuity separates the mantle from the core at about 2,900 kilometres; and the Lehmann discontinuity separates the liquid outer core from the solid inner core. The crust and the rigid top of the mantle together form the lithosphere, which is broken into the plates of plate tectonics.
Earthquake waves and the shadow zones
- Primary or P waves are longitudinal, the fastest, and the first to be recorded; they pass through solids, liquids and gases.
- Secondary or S waves are transverse and pass only through solids, which is the single strongest proof that the outer core is liquid.
- Surface or L waves travel along the surface, arrive last, and cause the greatest destruction to buildings.
Because the core bends and blocks these waves, a seismograph placed between about 105 and 145 degrees from an epicentre records neither P nor S waves, and beyond 105 degrees no S waves are received at all. This is called the shadow zone, and it is asked almost every year.
What the crust is made of
By weight the crust is nearly half oxygen, at about 46.6 per cent, followed by silicon at about 27.7 per cent, aluminium at about 8.1 per cent and iron at about 5 per cent, so silicate minerals dominate. Feldspar is the most abundant mineral group in the crust and quartz, made of silica, is the mineral that survives weathering best and therefore fills sands and sandstones.
The three families of rocks
| Family | How it forms | Examples |
|---|---|---|
| Igneous | From cooled magma or lava; called the primary or parent rock | Granite, gabbro and dolerite cool inside the crust; basalt and pumice cool on the surface, and basalt built the Deccan Trap |
| Sedimentary | From sediments compacted in layers; the only rocks that contain fossils | Sandstone and conglomerate are mechanically formed, limestone and coal organically, rock salt and gypsum chemically |
| Metamorphic | From existing rocks changed by heat and pressure without melting | Marble, slate, quartzite, gneiss, schist and graphite |
Metamorphism and the rock cycle
Metamorphism is of two kinds: contact or thermal metamorphism, where hot magma touches a rock, and regional or dynamic metamorphism, where the pressure of mountain building acts over a wide area. The pairs to memorise are limestone to marble, sandstone to quartzite, shale or clay to slate, slate to phyllite and then schist, granite to gneiss, and coal to graphite. The rock cycle joins the three families: igneous rock weathers into sediments that harden into sedimentary rock, heat and pressure turn either into metamorphic rock, and any of them can melt back into magma to begin again. Igneous and metamorphic rocks are hard, crystalline and poor in fossils; sedimentary rocks are layered, softer and hold the coal, petroleum and fossils that make them economically the most useful group.
Exam Point of View
The questions are short and factual. Expect a discontinuity to be matched with the layers it separates, a wave to be matched with its property, and a metamorphic rock to be matched with its parent. The regular traps are Gutenberg against Mohorovicic, P waves against S waves, and marble against quartzite. A second group asks which layer is liquid, which rock contains fossils, and which rock type is called the parent rock. Numbers worth carrying are 2,900 kilometres for the mantle and core boundary, 105 to 145 degrees for the shadow zone, and the order oxygen, silicon, aluminium and iron for the crust. Statement-based papers often test that basalt is extrusive while granite is intrusive.
Important Facts
| Radius of the Earth | About 6,371 km; the deepest borehole reaches only about 12 km |
|---|---|
| Mohorovicic discontinuity | Boundary between the crust and the mantle |
| Gutenberg discontinuity | Boundary between the mantle and the core, at about 2,900 km depth |
| Lehmann discontinuity | Boundary between the liquid outer core and the solid inner core |
| P waves | Longitudinal, fastest, travel through solids, liquids and gases |
| S waves | Transverse, travel only through solids, absent beyond 105 degrees |
| Shadow zone | About 105 to 145 degrees from the epicentre for both P and S waves |
| Crust composition | Oxygen about 46.6 per cent, silicon 27.7, aluminium 8.1 and iron 5 per cent by weight |
| Sial, Sima and Nife | Continental crust, oceanic crust and core respectively |
| Parent rock | Igneous rock, formed by the cooling of magma or lava |
| Fossil-bearing rocks | Sedimentary rocks alone; they are formed in layers or strata |
| Common metamorphic pairs | Limestone to marble, sandstone to quartzite, shale to slate, granite to gneiss, coal to graphite |
Practice MCQs on this topic
The boundary that separates the crust from the mantle is known as the
- A.Conrad discontinuity
- B.Mohorovicic discontinuity
- C.Gutenberg discontinuity
- D.Lehmann discontinuity
Show answer
Named after the Croatian seismologist who noticed that earthquake waves suddenly speed up at a certain depth, it marks the base of the crust and the top of the mantle, and is commonly shortened to the Moho.
Read the full explanation →Which of the following seismic waves cannot travel through liquids?
- A.Primary or P waves
- B.Secondary or S waves
- C.Surface or L waves
- D.All three types
Show answer
They are transverse waves, in which particles vibrate at right angles to the direction of travel, and such motion cannot be sustained in a fluid, so S waves die out at the outer core.
Read the full explanation →Marble is a metamorphic rock formed from which of the following parent rocks?
- A.Sandstone
- B.Shale
- C.Limestone
- D.Granite
Show answer
Limestone is a sedimentary rock made largely of calcium carbonate, and when heat and pressure recrystallise it without melting it, the result is marble, the stone used in the Taj Mahal and in the Dilwara temples.
Read the full explanation →The Deccan Trap of peninsular India is composed mainly of which rock?
- A.Granite
- B.Basalt
- C.Limestone
- D.Gneiss
Show answer
The Deccan Trap was formed by vast fissure eruptions in which lava spread over the surface and cooled quickly, giving a fine-grained extrusive igneous rock, basalt.
Read the full explanation →Fossils of ancient plants and animals are found mainly in which type of rock?
- A.Igneous rocks
- B.Sedimentary rocks
- C.Metamorphic rocks
- D.Plutonic rocks
Show answer
They are built up layer by layer as sediment settles over the remains of living things and hardens gently, so shells, bones and plant matter survive as fossils.
Read the full explanation →Which element is the most abundant in the Earth crust by weight?
- A.Silicon
- B.Aluminium
- C.Oxygen
- D.Iron
Show answer
Almost half the weight of the crust, about 46.6 per cent, is oxygen, because it is locked into the silicate minerals of which nearly all crustal rocks are made rather than existing as free gas.
Read the full explanation →The discontinuity that separates the outer core from the inner core is called the
- A.Repetti discontinuity
- B.Lehmann discontinuity
- C.Moho discontinuity
- D.Conrad discontinuity
Show answer
It marks the change from the molten outer core to the solid inner core, and is named after Inge Lehmann, who deduced from the behaviour of earthquake waves that the innermost part of the planet is solid.
Read the full explanation →Granite is an example of which type of rock?
- A.Intrusive igneous rock
- B.Extrusive igneous rock
- C.Chemically formed sedimentary rock
- D.Foliated metamorphic rock
Show answer
Granite forms when magma cools slowly deep below the surface, which gives the crystals time to grow, so the rock is coarse and speckled.
Read the full explanation →The zone between about 105 and 145 degrees from the epicentre of an earthquake, where seismic waves are not recorded, is called the
- A.Focus
- B.Shadow zone
- C.Benioff zone
- D.Fault zone
Show answer
The core bends P waves sharply and stops S waves altogether, so a belt of the Earth surface receives neither type of wave from a given earthquake.
Read the full explanation →The layer of the upper mantle over which the tectonic plates move is known as the
- A.Lithosphere
- B.Asthenosphere
- C.Barysphere
- D.Biosphere
Show answer
It lies in the upper mantle just below the rigid outer shell, is weak and partly molten, and is the main source of the magma that reaches the surface through volcanoes; the plates slide over it.
Read the full explanation →Under great heat and pressure, coal is changed into which of the following?
- A.Graphite
- B.Slate
- C.Quartzite
- D.Gneiss
Show answer
Coal is a sedimentary rock formed from buried plant matter, and prolonged heat and pressure drive off its volatile content and rearrange the carbon, first into harder anthracite and finally into graphite, a metamorphic rock of nearly pure carbon.
Read the full explanation →The term Nife is used for which part of the Earth?
- A.The continental crust
- B.The oceanic crust
- C.The mantle
- D.The core
Show answer
Nife is formed from the chemical symbols of nickel and iron, the two metals believed to make up the central part of the planet, and the flow of that metal in the liquid outer core is what generates the Earth magnetic field.
Read the full explanation →Frequently Asked Questions
Why is the outer core considered to be liquid?
Secondary or S waves are transverse and cannot travel through a liquid. Seismographs placed beyond about 105 degrees from an epicentre receive no S waves at all, which shows that the waves are stopped somewhere below the mantle. That barrier is the outer core, so it must be in a molten state, while the inner core stays solid because of the extreme pressure.
What is the difference between the lithosphere and the asthenosphere?
The lithosphere is the rigid outer shell of the Earth, made of the crust together with the uppermost solid part of the mantle, and it is broken into the tectonic plates. The asthenosphere lies just below it in the upper mantle, is weak and partly molten, and is the source of magma; the plates move because they ride on it.
Why are fossils found only in sedimentary rocks?
Sedimentary rocks form when sediment settles in layers over the remains of plants and animals and hardens gently, so the remains are preserved as fossils. Igneous rocks form from molten material and metamorphic rocks are reworked by heat and pressure, and both processes destroy any organic remains. This is why coal and petroleum too are found in sedimentary basins.
What is the rock cycle?
It is the continuous conversion of one rock family into another. Magma cools into igneous rock, weathering breaks it into sediments that compact into sedimentary rock, heat and pressure change either type into metamorphic rock, and deep melting turns any of them back into magma. No rock is permanent, which is why igneous rock is called the parent of the other two.
Which rock is the Deccan Trap made of?
Basalt, an extrusive igneous rock formed when lava spread over the surface and cooled quickly, so its crystals are fine. The weathering of this basalt produced the black regur or cotton soil of Maharashtra and adjoining states, which holds moisture well and suits cotton, and that link between rock and soil is itself a frequent question.
Sources
- Fundamentals of Physical Geography (Class XI), chapters on the interior of the Earth and on minerals and rocks — NCERT
- India: Physical Environment (Class XI), chapter on physiography — NCERT
