Plate boundaries, earthquakes and volcanoes

The edge where two plates meet is called a plate boundary. Plates can meet in only three ways: they can push together, pull apart, or slide past each other. Use your two hands to act each one out as you read.

1. Convergent boundary: plates move towards each other. Push your hands together, and your fingers bend upwards.

• When two continental plates collide, the land crumples into fold mountains, such as the Himalaya.
• When an oceanic plate meets a continental plate, the heavier ocean plate sinks beneath the other. This brings volcanoes and earthquakes.

2. Divergent boundary: plates move apart. Pull your hands apart. In the gap, magma (melted rock) rises from below and cools into new crust. This builds mid-ocean ridges, such as the Mid-Atlantic Ridge.

3. Transform boundary: plates slide past each other. Rub your palms sideways. No crust is made and none is destroyed, but the rubbing and sticking causes earthquakes, as along the San Andreas Fault in the United States.

"Where do most earthquakes (bhukamp) and volcanoes (jwalamukhi) happen?" Compare the plate map (Fig. 2.3) with the earthquake and volcano map (Fig. 2.4). They match! Most happen along plate boundaries, especially all round the Pacific Ocean. That ring is called the Ring of Fire.

Watch out: a common wrong answer is that earthquakes happen mostly in the middle of plates. It is the edges, where plates push, pull and rub.

"Is India at risk?" Yes. India has had major earthquakes that killed thousands of people. In a crowded country, one big earthquake can do terrible damage. Fig. 2.5 shows the destruction of the Gujarat earthquake of 2001. India's own plate is pressing into the Eurasian plate, which is why the Himalayan region shakes so often.

People in India studied earthquakes long ago. They called them bhūkampa, "the shaking of the Earth". In the Bṛihatsaṁhitā, Varāhamihira wrote a section on earthquakes. He noted signs in wind, rain, clouds, animal behaviour and the planets, and linked earthquakes to four forces: Vāyu (wind), Agni (fire), Indra (heaven, thunder) and Varuṇa (water). It was an early attempt to join observation with reasoning.

A volcano can bury the land around it in grey ash (Fig. 2.6). And India has a rare kind: the mud volcano at Baratang Island in the Andaman and Nicobar Islands, India's only one. Instead of fire, mud bubbles out, pushed up by underground gases and pressure.