When Landforms Turn Dangerous — Four Disasters to Know

A landslide is what happens when a slope loses its grip and a mass of rock, soil and debris suddenly slides downhill. A slope stays put only as long as friction can resist gravity — a landslide is the moment gravity finally wins. Heavy, continuous rain is the classic trigger: water soaking into the ground adds weight and acts like a lubricant, cutting the friction that holds everything together. Earthquakes can shake a slope loose in seconds. And people tip the balance too — clearing forests (whose roots bind the soil), mining, road-cutting and building on steep hillsides, and blocked drainage that traps water — all quietly weaken a slope until, one rainy day, it lets go.

An avalanche is a landslide made of snow — a vast mass of it breaking free and roaring down a steep mountainside. Snow piles up in layers over a winter, and the danger comes when those layers don't bond well to one another. A heavy fresh snowfall loads extra weight onto a weak buried layer; a sudden warm spell melts the snow just enough to slick the surfaces and cut friction; strong winds heap snow unevenly, over-loading one slope. After that, the smallest nudge — a skier, a loud crack, a falling cornice — can shatter the balance and send the whole slab thundering down.

A Glacial Lake Outburst Flood — a GLOF — is the disaster hiding behind those moraine dams you met earlier in this chapter. As a glacier melts, meltwater pools behind the ridge of debris (or ice) at its snout, forming a lake that grows bigger and heavier with every warm year. But that natural dam was never built to hold water — it's just piled-up rubble — so it's fragile. Add a burst of heavy rain, or a shock from an earthquake, avalanche or landslide crashing into the lake, and the dam can give way all at once, unleashing a wall of water and debris that tears downstream with terrifying speed. This is exactly the kind of glacier-linked flood that struck Chamoli.

A dust storm is wind erosion happening all at once and in your face: a strong wind sweeps up huge quantities of loose, dry soil and sand and drives them forward as a towering, choking wall of dust. The ingredients are simply dryness and bare ground. Long droughts and low rainfall parch the soil to powder; sparse plant cover — thinned by deforestation, overgrazing or careless farming — leaves nothing to hold that soil down, so the wind takes it. And a warming, more erratic climate is making these storms both more frequent and more fierce.