Pariksha SaathiLesson 3 of 10 · Physical and World Geography
The Earth's Interior, Rocks, Continental Drift and Plate Tectonics
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How we know the interior
The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached only about 12 km of the Earth's radius of about 6,371 km. So the interior is studied indirectly, above all through earthquake waves, whose speed and path change with the material they cross. The Earth's average density, about 5.5 grams per cubic centimetre, is about twice that of crustal rocks (about 2.7), so the interior must be much heavier.
The crust
The crust is the thin, rigid outer layer and forms less than 1 per cent of the Earth's volume. Oceanic crust is about 5 km thick on average, while continental crust averages about 30 km and reaches about 70 km under the Himalayas. The upper continental part, rich in silica and aluminium, is called SIAL and is granitic; the lower part, which also forms the ocean floor, is rich in silica and magnesium, is called SIMA and is basaltic. The Conrad discontinuity separates the two. By weight the crust is about 46.6 per cent oxygen, 27.7 per cent silicon and 8.1 per cent aluminium, followed by iron, so aluminium is the most abundant metal in the crust.
The mantle
The mantle extends from the base of the crust to about 2,900 km and holds roughly 84 per cent of the Earth's volume. Its upper boundary is the Moho discontinuity, identified by the Croatian seismologist Andrija Mohorovicic in 1909. The crust and the uppermost solid mantle together form the lithosphere, 10 to 200 km thick. Below it lies the asthenosphere, a soft, partly molten zone of the upper mantle extending to about 400 km (the NCERT figure); it is the main source of magma and the layer over which the plates move.
The core
The core begins at the Gutenberg discontinuity, about 2,900 km deep, and continues to the centre at about 6,371 km. It is made mainly of nickel and iron (NIFE). The outer core is liquid, which is why S waves cannot pass through it; the inner core, whose existence the Danish seismologist Inge Lehmann deduced in 1936, is solid because of enormous pressure. Exam books call the boundary between them, at about 5,150 km, the Lehmann discontinuity; seismologists call it the inner-core boundary. Movement of molten iron in the outer core generates the Earth's magnetic field. Earthquake waves reveal this structure (NCERT figures): within 105 degrees of the epicentre both P and S waves arrive, beyond 145 degrees only P waves arrive, and the belt between 105 and 145 degrees is a shadow zone for both.
Three families of rocks
Igneous rocks form from cooling magma and are called primary rocks because all others come from them. Intrusive or plutonic rocks such as granite cool slowly underground and have large crystals; extrusive or volcanic rocks such as basalt cool quickly at the surface and are fine-grained. The Deccan Traps are basalt, which weathers into black cotton soil (regur). Sedimentary rocks such as sandstone, shale, limestone and coal form in layers (strata) from deposited particles and preserve fossils; they cover about 75 per cent of the land surface but make up only about 5 per cent of the crust's volume. Metamorphic rocks form when heat and pressure change existing rocks: limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss.
Continental drift
In 1912 the German meteorologist Alfred Wegener proposed that all continents once formed one supercontinent, Pangaea, surrounded by one ocean, Panthalassa. Pangaea split into Laurasia in the north and Gondwanaland in the south, separated by the Tethys Sea, from whose sediments the Himalayas later rose. His evidence was the jigsaw fit of South America and Africa, matching rock belts on both sides of the Atlantic, glacial deposits (tillite) in India and other southern continents, gold deposits in Ghana whose source rocks lie in Brazil, and fossils of the small reptile Mesosaurus found in southern Africa and in Brazil, now an ocean apart. Wegener could not convincingly explain the force behind the drift, which Arthur Holmes in the 1930s traced to convection currents in the mantle.
Sea-floor spreading and plate tectonics
Ocean-floor mapping after the Second World War found volcanic mid-ocean ridges, ocean rocks that grow older away from the ridges and are almost nowhere older than about 200 million years, and matching magnetic stripes on both sides. In the early 1960s Harry Hess explained this as sea-floor spreading: magma rising at the ridges forms new crust and pushes the old floor apart. In 1967 these ideas were combined into plate tectonics. The lithosphere is broken into seven major plates (Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic) and minor ones such as the Nazca and Arabian plates. At divergent boundaries plates move apart and new crust forms, as on the Mid-Atlantic Ridge and in the East African Rift. At convergent boundaries one plate sinks (subducts) under another, forming trenches, volcanoes and fold mountains such as the Andes; where two continents collide, as when the Indian plate met the Eurasian plate about 40 to 50 million years ago, ranges like the Himalayas rise. At transform boundaries plates slide past each other, as along the San Andreas Fault.
Key facts
- Layers and depths: crust (oceanic about 5 km, continental about 30 km, up to about 70 km under the Himalayas), mantle to about 2,900 km, core to the centre at about 6,371 km.
- SIAL: silica and aluminium, continental, granitic. SIMA: silica and magnesium, ocean floor, basaltic. NIFE: nickel and iron, the core.
- Crust by weight: oxygen 46.6%, silicon 27.7%, aluminium 8.1%, then iron; aluminium is the most abundant metal in the crust.
- Discontinuities: Conrad (SIAL-SIMA), Moho, 1909 (crust-mantle), Gutenberg (mantle-core, about 2,900 km), Lehmann (outer core-inner core, about 5,150 km).
- Shadow zone (NCERT figures): between 105 and 145 degrees from the epicentre; S waves are absent everywhere beyond 105 degrees because the outer core is liquid.
- Asthenosphere: soft, partly molten upper mantle up to about 400 km (NCERT figure), the main source of magma; the lithospheric plates (10-200 km thick) move over it.
- Granite is intrusive igneous, basalt extrusive igneous (Deccan Traps, black soil); sedimentary rocks hold fossils and cover about 75% of the land but only about 5% of the crust's volume.
- Wegener (1912): Pangaea and Panthalassa; Laurasia (north) and Gondwanaland (south) separated by the Tethys; evidence: jigsaw fit, tillite, Mesosaurus fossils, Ghana gold.
- Plate boundaries: divergent (Mid-Atlantic Ridge, East African Rift), convergent (Andes, Mariana Trench, Himalayas), transform (San Andreas Fault).
Quick revision
Earth's radius about 6,371 km; crust under 1% of volume, oceanic about 5 km, continental about 30 km; SIAL over SIMA with Conrad between; Moho (1909) crust-mantle; mantle to 2,900 km, about 84% of volume; asthenosphere up to about 400 km, source of magma; Gutenberg at 2,900 km; outer core liquid, inner core solid (found by Lehmann, 1936); NIFE core; shadow zone 105-145 degrees; granite intrusive, basalt extrusive; Wegener 1912 Pangaea; Hess sea-floor spreading; plate tectonics 1967; seven major plates; San Andreas transform fault.