CDS (II) 2021 General Knowledge · Question 96 of 120

Correct — B, Terrestrial. An avalanche is a rapid flow of snow down a slope, which as it grows can pick up and carry ice, rock, soil and trees with it. That makes it a mass movement of material along the land surface under gravity — the same family as a landslide, a rockfall or a mudflow — and disaster classifications that sort events by the sphere they occur in put all of these together as terrestrial. The sorting rule is where the event happens, not what set it off. Atmospheric disasters are events of the air itself: cyclones, hailstorms, thunderstorms, frost, drought. Aquatic ones happen in water bodies: floods, tsunamis, storm surges. Biological ones involve living agents: epidemics and pest attacks. Snow sliding off a mountainside sits in none of those three; it is material moving on land, so it is terrestrial.

• (a)Atmospheric — The trap, and a reasonable one, because the weather is what loads and destabilises a snowpack — heavy snowfall, wind loading, a sudden thaw. But the trigger is not the classification; the event being classified is the movement of snow down a slope, which happens on the ground.
• (c)Aquatic — Aquatic disasters are events of water bodies — floods, tsunamis, storm surges. Snow is frozen water, but an avalanche is not an event of a river, lake or ocean.
• (d)Biological — Biological disasters involve living agents — epidemics, pandemics, locust and pest attacks. Nothing living is the agent in an avalanche.

Snow builds up on a slope in layers, and each layer bonds to the one beneath with a strength that depends on temperature, wind and the shape of the crystals. When a weak layer gives way, the slab above it releases and accelerates downhill; loose-snow avalanches, by contrast, start from a point and fan out. Triggers range from fresh snowfall and rapid warming to the weight of an animal or a skier, and earthquakes can set them off as well.

The four options are the standard sphere-based sorting of natural disasters, so the exam is checking whether you place an event by where it occurs. The commonest error is to reason from the cause — snow falls out of the sky, therefore atmospheric — but by that logic a flood would be atmospheric too, since rain causes it. Apply the rule consistently and the family becomes obvious: avalanches sit beside landslides, earthquakes and volcanic eruptions. In India this is not an abstract category. Avalanches are a live hazard along the high Himalaya in Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand and Sikkim, where they close roads and passes and threaten both settlements and troops posted at altitude.

• An avalanche is a rapid downslope flow of snow that may entrain ice, rock and trees as it grows.
• The two basic forms are slab avalanches, where a cohesive block releases over a weak layer, and loose-snow avalanches, which start from a point.
• Triggers include fresh snowfall, wind loading, rapid warming, earthquakes, and the weight of people or animals on the slope.
• Sphere-based classification places cyclones, hailstorms, frost and drought as atmospheric; earthquakes, volcanic eruptions, landslides and avalanches as terrestrial; floods, tsunamis and storm surges as aquatic; and epidemics and pest attacks as biological.
• In India, avalanches are a recurring hazard in the high Himalayan states and in Ladakh.

• Classifying by cause rather than by where the event occurs — this is what makes 'atmospheric' tempting.
• Treating snow as water and so calling the event aquatic.
• Assuming avalanches happen only in winter; a rapid spring thaw is one of the commonest triggers.

As a one-line classification item, or in reverse as 'which one is not an atmospheric disaster'.

• practice — not a real PYQ

  Which one of the following is classified as a terrestrial natural disaster?

  • (a)Hailstorm
  • (b)Storm surge
  • (c)Landslide
  • (d)Locust attack

  Answer(c) Landslide — a mass movement on land, in the same family as avalanches, earthquakes and volcanic eruptions; the other three are atmospheric, aquatic and biological respectively.

• practice — not a real PYQ

  A slab avalanche is best described as the release of

  • (a)loose surface snow spreading out from a single point
  • (b)a cohesive block of snow over a weaker layer beneath it
  • (c)meltwater trapped inside a glacier
  • (d)rock debris from a cliff face without any snow

  Answer(b) a cohesive block of snow over a weaker layer beneath it — the failure of that buried weak layer is what releases the slab, while a loose-snow avalanche starts at a point and fans outward.