Interior of the Earth

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 3.

How do you know what you cannot see?

The earth's radius is about 6,378 km, yet the deepest hole ever drilled reaches only about 12 km, less than 0.2 per cent of the way to the centre. No one has seen the mantle or the core. Still, geography books tell you the core is liquid on the outside and solid on the inside, and that it is mostly nickel and iron. This chapter is about the detective work behind such statements, and about why it matters: the shape of the surface is largely a product of processes at work inside the earth, so you cannot understand why the earth shakes or how a tsunami starts without knowing the interior.

Where the information comes from

Most of what we know about the interior comes from estimates and inferences, though some comes from direct observation of materials.

Why gravity differs. Gravity is greater near the poles and less at the equator, because the equator is farther from the centre of the earth than the poles are. Uneven mass inside the earth also changes the reading. A gravity anomaly therefore reveals how the mass of the crust is distributed.

Beyond the textbook. For the exam: the Kola drill reached 12 km, and volcanoes are a direct source while seismic waves, gravity and magnetism are indirect sources. In reality: the Kola drill is on the Kola Peninsula in the Russian Arctic, on land, not in the ocean. Why it matters: the point to remember is how small 12 km is against 6,378 km, which is why indirect evidence does most of the work.

Earthquakes: a sudden slip along a fault

An earthquake is shaking of the earth, caused by the release of energy that sends waves out in all directions.

Why the earth shakes. The energy is released along a fault, a sharp break in the crustal rocks. Rocks on either side tend to move in opposite directions, but the weight of the strata above presses them and friction locks them together. Eventually the tendency to move apart beats the friction. The blocks deform and slide past each other abruptly, and energy is released.

All natural earthquakes occur in the lithosphere, which here means the portion up to 200 km from the surface. A seismograph records the arriving waves.

Earthquake waves: three kinds, three behaviours

Earthquake waves are of two types. Body waves begin at the focus and travel through the body of the earth. When they meet surface rocks they generate surface waves, which move along the surface. A wave's velocity changes with the density of the material, and the denser the material the faster the wave. Waves also reflect (rebound) and refract (change direction) when they meet materials of different densities.

Pause and predict: a seismograph records the first jolt, then a stronger jolt a little later, then the strongest and most damaging shaking. In what order did P, S and surface waves arrive?

Check your answer: P first (fastest), S next (with a time lag), surface waves last. The last ones are the most destructive.

The shadow zone: how a missing wave reveals the core

Waves from an earthquake are recorded at faraway stations, but in some areas certain waves are not recorded at all. This region is the shadow zone, and it is different for every earthquake.

Stations within 105° of the epicentre record both P and S waves. Stations beyond 145° record P waves but not S waves.

The detective story in three steps. First, S waves can only pass through solid material. Second, S waves are not recorded anywhere beyond 105°, so something in the middle of the earth stops them. Third, that something must be a liquid. This is how scientists inferred the liquid outer core. Changes in wave direction at deeper levels also show the core exists as a separate layer.

Pause and predict: if the whole earth were solid, how would the S wave shadow zone change?

Check your answer: it would not exist, because S waves can travel through solid material and would reach stations all around the globe. The shadow zone is the evidence for a liquid layer.

Types of earthquake and how they are measured

Earthquakes are scaled in two ways.

Beyond the textbook. For the exam: Richter measures magnitude on a 0 to 10 scale, and Mercalli measures intensity on a 1 to 12 scale. In reality: the Richter scale is logarithmic, so each whole number step means roughly ten times larger wave amplitude and about thirty times more energy. Modern seismologists mostly report a related measure called moment magnitude. Why it matters: magnitude is one number for the whole quake, while intensity differs from place to place depending on distance and local damage.

Effects of earthquakes. 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 affect landforms, while the others are of immediate concern for life and property. A tsunami occurs only if the epicentre lies under oceanic waters and the magnitude is high enough, and it is a wave generated by the tremor, not an earthquake in itself. The shaking lasts a few seconds, but a quake of magnitude more than 5 on the Richter scale has devastating effects.

Not every part of the globe has major shocks. Quakes of magnitude 8 and above are quite rare and occur about once in 1 to 2 years, while tiny ones occur almost every minute.

Layers of the earth

The earth's material is arranged in layers, with the density rising towards the centre.

The lithosphere is the crust plus the uppermost part of the mantle, and its thickness ranges from 10 to 200 km. The core mantle boundary lies at 2,900 km. The core is made up of very heavy material, mostly nickel and iron, which gives the nickname nife.

Magma and lava. The asthenosphere is the main source of the molten rock that reaches the surface during eruptions. Material in the upper mantle is magma. Once it moves towards the crust or reaches the surface it is called 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 small amounts of chlorine, hydrogen and argon.

Volcanoes: where the interior reaches the surface

A volcano is a place where gases, ashes or molten rock (lava) escape to the ground. It is active if these materials are being released or have been released in the recent past. Volcanoes are classified by the nature of the eruption and the form developed at the surface.

Two views. For the exam: the Deccan Traps formed from flood basalt eruptions, fed by dykes, and are the answer to "which type of volcanic eruption formed the Deccan Traps?" Another reading: many geologists link the Deccan eruptions to the period of the great extinction that ended the age of dinosaurs, and scientists still debate how large a role that volcanism played compared with an asteroid impact. The textbook deals with the eruption type, not that debate.

What happens to the lava that never erupts

Lava that cools on the surface forms volcanic rocks. Lava that cools within the crust forms plutonic rocks in intrusive forms.

Exam traps

60-second revision

Where this chapter connects

• Previous: The Origin and Evolution of the Earth explains how differentiation produced the crust, mantle and core.
• Next: Distribution of Oceans and Continents uses this layered interior to explain moving plates.
• Hazard link: Natural Hazards and Disasters applies earthquakes and tsunamis to India.
• Process link: Geomorphic Processes shows how the endogenic forces from inside the earth shape landforms.