NCERT Solutions for Class 9th Social Science Chapter 2 The photograph in Fig. 2.6 — LET'S EXPLORE

A volcanic eruption — the caption of Fig. 2.6 confirms it: the photograph shows the deposition left by a volcanic eruption.

The evidence in the photograph itself:

• Everything — the ground, the roofs, the walls, the tree trunks, the hillside behind — is coated in the same uniform grey. Only something that fell out of the air could coat every surface equally like that.
• The houses are half-buried and several roofs have sagged or broken; the grey material is deep enough to bury the lower parts of the walls.
• The trees are stripped bare of leaves and still standing. A flood or a landslide would have knocked them over; a hot ash fall kills the leaves but leaves the trunk upright.
• The whole slope behind the village is smothered, and the sky is hazy — ash is still in the air.

So: a nearby volcano erupted, threw out ash and fine fragments, and the wind carried them over the settlement, where they settled as a thick grey blanket.

It is volcanic ash — the fine dust of the eruption.

When magma reaches the surface, the dissolved gases in it expand violently and shatter the molten rock into tiny fragments. Those fragments cool almost instantly in the air into fine, gritty particles of rock and glass, and that is what falls as ash. It is not soft like wood ash: it is sharp, abrasive and surprisingly heavy — which is why the roofs in the photograph have collapsed under it.

It tells us that the Earth is hot and active inside, and that its internal heat can reach the surface with enough force to remake a landscape in hours.

Four things this photograph proves:

1. The interior is hot enough to melt rock. Ash is shattered molten rock. Fig. 2.2 shows where that heat comes from and how it travels outward — by advection, convection and conduction — with the crust and mantle together supplying 78% of the total interior heat flow and the core the remaining 22%.
2. Internal forces are constructive. External agents like wind and rivers remove material; this eruption has added a whole new layer of it. Over time, repeated ash and lava falls build volcanic cones, plateaus and islands.
3. They act suddenly. Weathering and erosion take thousands of years to change a valley. An eruption buries a village in a single day. Both are shaping the Earth, but on completely different timescales.
4. They are tied to plate boundaries. A volcano needs a crack in the crust for magma to escape through, and page 15 tells us where such cracks are — where an oceanic plate sinks beneath a continental plate, and where plates pull apart. That is why Fig. 2.4 shows volcanoes lined up along boundaries and almost none in plate interiors.