Earth's Interior, Continental Drift and Plate Tectonics

Why the inside of the Earth matters

Mountains, earthquakes, volcanoes, ocean basins and even the location of minerals are all explained by what happens deep inside the Earth. Nobody has drilled beyond about 12 kilometres (the Kola Superdeep Borehole in Russia), yet the Earth's radius is about 6,371 kilometres. Our knowledge comes from indirect sources – mainly seismic waves from earthquakes, along with meteorites, gravity and magnetic surveys, and volcanic material. UPSC regularly asks about the layers of the Earth, seismic waves and shadow zones, and the theories of continental drift, sea-floor spreading and plate tectonics.

Sources of information

• Direct: mining, deep drilling and volcanic eruptions, which bring up material from the mantle.
• Indirect: temperature and pressure increase with depth (roughly 1°C for every 32 metres near the surface); meteorites, which are made of the same material as the early solar system; gravity anomalies; magnetic records in rocks; and above all seismic waves.

Seismic waves and shadow zones

Earthquakes release energy as waves.

• P-waves (primary) – the fastest; longitudinal (compressional) waves, like sound; travel through solids, liquids and gases.
• S-waves (secondary) – slower; transverse waves that move material at right angles; travel only through solids.
• Surface waves (Love and Rayleigh) – slowest, but cause most of the destruction.

Waves change speed and bend (refract) at boundaries between layers of different density. Because S-waves do not pass through the outer core, scientists concluded it is liquid. There is an S-wave shadow zone beyond about 105° from the epicentre, and a P-wave shadow zone between about 105° and 145° where P-waves are refracted away.

Layers of the Earth

Discontinuities: the Mohorovičić (Moho) between crust and mantle; the Gutenberg between mantle and core; the Lehmann between outer and inner core; and the Conrad within the crust. The lithosphere is the crust plus the rigid upper mantle (up to about 100 km); it "floats" on the asthenosphere.

Continental drift (Alfred Wegener, 1912)

The German meteorologist Alfred Wegener proposed that all continents were once joined in a supercontinent, Pangaea, surrounded by a single ocean, Panthalassa. About 200 million years ago Pangaea split into Laurasia in the north and Gondwanaland in the south (including India, Africa, South America, Australia and Antarctica), which later broke into today's continents.

Evidence:

• Jigsaw fit of the coastlines of South America and Africa (best matched at the continental shelf edge, as shown by Bullard in 1964).
• Matching rocks and mountain belts across oceans – for example, 2-billion-year-old rocks in Brazil and West Africa.
• Fossils of the same land animals and plants – *Mesosaurus*, a freshwater reptile, in South Africa and Brazil; the *Glossopteris* fern across Gondwana lands including India.
• Tillite (glacial deposits) of the same age in India, Africa, Australia and South America, showing past glaciation in now-tropical lands.
• Placer gold deposits in Ghana's coast with the source rock in Brazil.

Weakness: Wegener could not explain the force that moved continents. He suggested polar-fleeing force and tidal force, which were far too weak, so his theory was rejected for decades.

Sea-floor spreading (Harry Hess, 1960s)

Post-war ocean mapping revealed mid-oceanic ridges, deep trenches and young ocean floors. Harry Hess proposed that convection currents in the mantle (an idea earlier suggested by Arthur Holmes in the 1930s) bring magma up at the ridges, forming new crust that spreads outward and sinks back into the mantle at trenches.

Evidence:

• Rocks near ridges are youngest; age increases away from them; the oldest ocean floor is only about 200 million years old, much younger than continental rocks (about 4 billion).
• Palaeomagnetic stripes – the Earth's magnetic field has reversed many times; rocks on both sides of a ridge show symmetrical bands of normal and reversed magnetism (Vine and Matthews, 1963).
• Thin sediment near ridges, thicker away from them.
• High heat flow and volcanic activity along ridges, as in Iceland.

Plate tectonics (late 1960s)

The theory of plate tectonics, developed by McKenzie, Parker and Morgan (1967) among others, combined these ideas. The lithosphere is broken into seven major plates – Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic – and many minor plates (Nazca, Cocos, Philippine, Arabian, Caribbean, Juan de Fuca, Scotia). Plates move a few centimetres a year, driven by mantle convection, ridge push and slab pull.

Types of plate boundaries:

1. Divergent (constructive) – plates move apart; new crust forms. Examples: Mid-Atlantic Ridge, the East African Rift Valley.
2. Convergent (destructive) – plates collide.

• Ocean–continent: the denser oceanic plate subducts, forming trenches and fold mountains with volcanoes – the Andes (Nazca under South America).
• Ocean–ocean: subduction forms island arcs and deep trenches – Japan, the Mariana Trench (deepest point, Challenger Deep, about 11 km).
• Continent–continent: neither sinks easily; crust crumples into high fold mountains – the Himalayas (Indian plate meeting Eurasian plate).

1. Transform (conservative) – plates slide past each other; crust is neither created nor destroyed; earthquakes are common – the San Andreas Fault in California.

The journey of the Indian plate

About 140 million years ago India broke away from Gondwana and moved north at an unusually high speed of up to 15–20 cm per year. About 65 million years ago, as it passed over the Réunion hotspot, huge outpourings of lava formed the Deccan Traps. The Tethys Sea between India and Asia closed, and India collided with Asia around 50–40 million years ago, pushing up the Himalayas and the Tibetan Plateau. India is still moving north-north-east at about 5 cm per year, which is why the Himalayas are still rising and the region is earthquake-prone.

Hotspots and the rock cycle

Not all volcanic activity occurs at plate edges. Hotspots are plumes of hot mantle material that stay fixed while plates move over them, leaving a chain of volcanoes that become older away from the active spot – the Hawaiian–Emperor seamount chain is the classic example, and the Réunion hotspot created both the Deccan Traps and the island chain of the Lakshadweep, Maldives and Chagos ridge. Plate movements also drive the rock cycle: igneous rocks form from cooling magma, are weathered into sediments that become sedimentary rocks, and are changed by heat and pressure into metamorphic rocks, which may melt again in subduction zones.

Mains practice question

How did the theory of plate tectonics improve upon Wegener's continental drift theory? Explain with reference to the formation of the Himalayas. (250 words)

Answer framework

• Intro: Wegener's Pangaea and evidence.
• Gaps: no driving force; continents ploughing through ocean floor.
• Plate tectonics: lithospheric plates, mantle convection, sea-floor spreading evidence (palaeomagnetism, age of ocean floor).
• Himalayas: Indian plate drift, Tethys closure, continent–continent collision.
• Conclusion: unified explanation of earthquakes, volcanoes and mountains.