Earth's Interior, Plates, Earthquakes and Volcanoes

The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached only about 12 km, while the centre of Earth lies about 6,371 km below us. So nobody has seen Earth's interior. What we know comes from detective work: clues from lava, meteorites, gravity, magnetism and, above all, earthquake waves. The structure those clues reveal is simple: a thin crust, a thick mantle and a metal core.

Crust, mantle and core are layers by composition. Geologists also divide the top of Earth by strength: the rigid outer shell is the lithosphere (crust plus the uppermost solid mantle, about 100 km thick), and below it the soft, slow-flowing asthenosphere (roughly 100 to 400 km). The lithosphere is broken into plates and these plates float and slide on the asthenosphere. That is the key to the whole next topic.

Isostasy is the balance between the crust and the mantle. Lighter blocks of crust float on the denser mantle like icebergs on water: a high mountain has a deep light 'root' below it, and a low plain has a shallow one. When a mountain is worn down by erosion, its root rises; when ice melts off a land, the land slowly rises too. The idea was developed by Airy (roots) and Pratt (density differences) in the 1850s.

An earthquake is a sudden shaking of the ground. Rocks in the crust are slowly squeezed, pulled or twisted by the moving plates. Stress builds up for years, like bending a stick, until the rock suddenly breaks and snaps back along a fault. The stored energy is released as seismic waves that spread in all directions. Most earthquakes (the tectonic ones) are caused this way. Others are caused by volcanic eruptions, collapse of caves or mines, explosions, and even by filling a very large reservoir behind a dam.

Seismographs far from the epicentre show shadow zones, where some waves never arrive. S-waves are not recorded anywhere beyond 105 degrees from the epicentre, because the liquid outer core blocks them. P-waves are bent (refracted) by the core and leave a ring-shaped shadow zone between about 105 and 140 degrees. This pattern is how scientists proved that Earth has a liquid outer core.

India's old map had five zones; Zone I was merged with Zone II in 2002, leaving four zones, II to V, with V the most dangerous. The Himalayan belt is dangerous because the Indian plate is pushing into the Eurasian plate, a convergent boundary, so stress keeps building. The 2025 revision of the BIS code is reported to add a sixth, highest zone (Zone VI) for the Himalayan arc; older books and many exam papers still speak of Zones II to V. India's earthquakes in the stable peninsula (Koyna 1967, Latur 1993) are far fewer but can still be deadly.

A tsunami (Japanese for 'harbour wave') is a series of huge ocean waves, set off when a large volume of water is suddenly pushed up or down. The commonest cause is an undersea earthquake at a subduction zone, where the seabed jumps up or down. Volcanic eruptions, undersea landslides and meteorite impacts can also do it. Tsunamis are not tidal waves; they have nothing to do with tides.

Look at a world map: the bulge of Brazil seems to fit into the notch of West Africa, like two pieces of a torn page. In 1912 the German scientist Alfred Wegener asked a bold question: were the continents once joined, and have they drifted apart? That question, answered over the next sixty years, grew into plate tectonics, the single theory that explains mountains, earthquakes, volcanoes and ocean trenches together.

Continental Drift (Wegener): about 250-200 million years ago all the land formed one super-continent, Pangaea, surrounded by one ocean, Panthalassa. Pangaea split into Laurasia in the north (North America, Europe, Asia without India) and Gondwanaland in the south (South America, Africa, India, Australia, Antarctica), separated by the Tethys Sea. These pieces then drifted to where they are today.

Sea-floor spreading supplied the missing mechanism. Along the mid-ocean ridges hot rock rises from the mantle, cools and forms new ocean floor, which pushes the older floor outwards on both sides. So the youngest ocean rocks are at the ridge and they get older with distance; the oldest ocean floor is only about 200 million years old, while continents carry rocks four billion years old. Iron minerals in the new lava lock in Earth's magnetic direction at the time, so the ocean floor shows magnetic stripes that mirror each other on both sides of the ridge. Ocean floor that is made at ridges is destroyed at deep trenches, where it sinks back into the mantle.

Plate Tectonics: Earth's rigid lithosphere is broken into large and small pieces called plates. They ride on the soft asthenosphere and move a few centimetres a year, driven by mantle convection, slab pull (a heavy plate edge sinking at a trench pulls the rest after it) and ridge push. Almost all earthquakes, volcanoes and mountain-building happen at the edges of plates, because that is where plates interact.

India's own story is the best example. The Indian plate broke off from Gondwanaland, drifted northwards across the Tethys Sea, and began colliding with the Eurasian plate about 50 million years ago. The sediments of the Tethys were squeezed and lifted into the Himalaya, which are still rising, and that is why the Himalayan region is still shaken by earthquakes. Away from plate edges, hotspots (such as Hawaii) lie over fixed plumes of hot mantle; as a plate slides over them it leaves a chain of volcanoes. The Deccan Traps are linked to the Réunion hotspot.

A volcano is an opening in the crust through which hot molten rock from below, along with gas, ash and rock fragments, reaches the surface. Below the ground the molten rock is called magma; once it pours out it is called lava. Think of a shaken soda bottle: when the pressure of dissolved gas gets too high, the cap blows and the contents rush out. A volcano is the same idea on a planetary scale, and the 'cap' is usually a weak spot at a plate boundary.

Why one volcano is gentle and another explosive comes down to the lava. Basic (basaltic) lava has less silica; it is hot and runny, gas escapes easily, so it flows quietly (shield volcanoes, Hawaii). Acidic (rhyolitic, andesitic) lava has more silica; it is thick and sticky, traps gas, and the pressure builds until it blows violently (composite volcanoes, Fuji, Vesuvius). A fissure eruption, where lava pours out of long cracks over a huge area, builds lava plateaus like the Deccan Traps.

Which volcanoes does India have? Very few. Barren Island in the Andaman Sea, about 135 km north-east of Port Blair (now Sri Vijaya Puram), is India's only confirmed active volcano; it has erupted several times since 1787, including in 1991, 2005 and 2017. Narcondam Island, also in the Andaman Sea, is a dormant volcano. The Baratang island in the Andamans has mud volcanoes, which are not true volcanoes. On the mainland, the great lava flows of the Deccan Traps (about 66 million years old, fissure eruptions of basalt) cover Maharashtra and parts of Gujarat, Madhya Pradesh, Karnataka and Telangana, and weathering of this basalt gave the black regur soil. Remember that the Himalayas, though young, have no active volcanoes, because they formed by continent-continent collision.