Q1.
Examine the plate map (Fig. 2.3) with the earthquake and volcano map (Fig. 2.4). What correlation do you observe?
Answer
The correlation is almost exact: the dots and triangles in Fig. 2.4 trace the plate boundaries of Fig. 2.3. Earthquake origins (red dots) and active volcanoes (black triangles) are not scattered evenly over the Earth — they form narrow belts, and every belt sits on a line where two plates meet.
Match them belt by belt:
- Around the whole rim of the Pacific — from New Zealand up through Indonesia, the Philippines, Japan, Kamchatka, across the Aleutians, and down the west coast of North and South America — Fig. 2.4 shows the densest band of both dots and triangles anywhere on the map. In Fig. 2.3 this is the edge of the Pacific plate against the Eurasian, Indo-Australian, North American, South American and Antarctic plates. This belt is the Ring of Fire.
- A second belt runs west to east through the Mediterranean, Turkey, Iran, the Himalaya and on to Indonesia. In Fig. 2.3 this is the boundary where the African and Indo-Australian plates press north into the Eurasian plate.
- A thin line of red dots runs down the middle of the Atlantic, exactly where Fig. 2.3 shows the North and South American plates separating from the Eurasian and African plates. Notice this belt has many earthquakes but few marked volcanoes — it is the Mid-Atlantic Ridge, and its eruptions happen underwater.
- A line of volcanoes runs down East Africa, where the African plate is being pulled apart.
- The interiors of the plates are almost empty — central Siberia, the Sahara, the Australian interior, the Canadian shield, peninsular India, most of the deep ocean floor.
Why the correlation exists: an earthquake is stored strain released suddenly, and strain builds only where rock is being pushed, pulled or sheared — that is, at a boundary. A volcano needs a path for magma to reach the surface, and the cracked crust of a boundary provides exactly that: at a divergent boundary magma rises into the gap, and at a convergent boundary the sinking plate supplies molten material from below. In the middle of a plate the crust is neither strained nor cracked, so it is quiet.
Did you notice? The two maps are drawn on different centres — Fig. 2.3 is centred on the Atlantic, Fig. 2.4 on the Pacific. Find a common landmark (say Japan, or the tip of South America) on both before you start comparing.