Earthquakes and Volcanoes: Waves, Belts and Types

Earthquakes and volcanoes are the two quick ways in which the earth's interior reaches its surface. Both are products of plate movement, both are concentrated along the same narrow belts, and both are asked in every general studies paper as short factual questions: which wave does not travel through a liquid, which scale measures magnitude, where is India's only active volcano. This note keeps the physics short and the examinable facts in front.

How an earthquake happens

Rocks under strain along a fault store elastic energy. When the strain crosses the strength of the rock, the rock snaps, the two blocks slide past each other and the stored energy leaves as waves. The point inside the earth where the rupture begins is the focus, also called the hypocentre. The point on the surface vertically above it is the epicentre, and it is where the shock is felt first and usually strongest. Most earthquakes are tectonic; others are volcanic, or caused by the collapse of mine roofs, by explosions, or by the weight of water in a large new reservoir.

Seismic waves and what they prove

Because S waves are transverse they cannot pass through a liquid, and they are not recorded anywhere beyond about 103 degrees from the epicentre. That single observation is the proof that the outer core is liquid. P waves are bent sharply as they enter the core, so the belt between about 103 and 142 degrees receives neither direct P nor S waves and is called the shadow zone. Surface waves arrive last but cause most of the destruction, since buildings resonate with their long, rolling motion.

Measuring earthquakes

A seismograph records the waves and the record itself is a seismogram. Two different things are measured. Magnitude is the energy released at the source and is given by the Richter scale, devised by Charles F. Richter in 1935, and now more often by the moment magnitude scale for large shocks. The Richter scale is logarithmic and open-ended: each whole number is a tenfold rise in wave amplitude and about thirty-two times more energy. Intensity is the damage felt at a place and is given by the modified Mercalli scale, based on the work of Giuseppe Mercalli, which runs in twelve degrees written in Roman numerals from I, not felt, to XII, total destruction. Magnitude has one value for one earthquake; intensity falls as you move away from the epicentre.

Where earthquakes and volcanoes occur

Three belts hold nearly all of them. The circum-Pacific belt, the Ring of Fire, rings the Pacific Ocean from the Andes through the Aleutians to Japan, Philippines, Indonesia and New Zealand, and has the largest share of both earthquakes and active volcanoes. The Mid-Atlantic belt follows the mid-ocean ridge, where plates move apart, and gives frequent but shallow shocks and quiet basaltic eruptions, as in Iceland. The Alpine-Himalayan or Mediterranean belt runs from the Atlas and the Alps through Turkey and Iran into the Himalayas and Myanmar, and it is where the Indian plate pushing north makes the Himalayan earthquakes. In India the Bureau of Indian Standards maps the country in four seismic zones numbered II to V, the risk rising with the number. Zone V, the highest, covers the north-eastern states, the Kutch region of Gujarat, parts of Himachal Pradesh and Uttarakhand, north Bihar and the Andaman and Nicobar Islands. A shallow undersea earthquake can displace the water column and raise a tsunami, as happened in the Indian Ocean on 26 December 2004.

Volcanoes: parts and types

Molten rock below the surface is magma; once it pours out it is lava. It rises through a pipe called the vent and leaves by a mouth called the crater; when the top of a cone collapses into the emptied magma chamber the wide hollow left behind is a caldera. By activity a volcano is active if it erupts or has erupted in recorded history, dormant if quiet for long but capable of erupting, and extinct if it has no record of eruption. Barren Island in the Andaman Sea is the only active volcano in India, and neighbouring Narcondam is treated as dormant.

Form depends on the lava. Basic lava is thin and runs far, building a broad shield volcano with gentle slopes, as Mauna Loa in Hawaii. Acidic lava is thick and traps gas, so it erupts violently and builds a steep composite or strato volcano of alternating ash and lava, as Fujiyama in Japan and Vesuvius in Italy. Loose fragments alone build a cinder cone. Fissure eruptions spread sheets of basalt to make lava plateaus, and the Deccan Trap of peninsular India, whose weathering gives the black regur soil, is the classic Indian example. Magma that cools inside the crust makes intrusive forms: the huge batholith, the dome-shaped laccolith, the saucer-shaped lopolith, the sheet-like sill that lies along the bedding and the dyke that cuts across it. Hot springs and geysers such as Old Faithful in Yellowstone, and Manikaran and Tattapani in India, mark the dying stage of volcanic activity.