CDS (II) 2020 General Knowledge · Question 84 of 120

Correct — A, Lonar Lake, Maharashtra. Lonar sits inside a meteorite impact crater blasted out of the Deccan basalt in Buldhana district, and the shock of that impact turned some of the rock's plagioclase feldspar into maskelynite, a natural glass that forms only under the enormous pressures of a hypervelocity impact. Maskelynite is found in Moon rocks and in meteorites, which is exactly why the lake is described as holding a substance found on the Moon; a 2019 study from IIT Bombay reported that the minerals in the lake soil closely resemble those in the samples the Apollo missions brought back. The crater has a mean diameter of about 1.2 km, the lake lies roughly 137 m below the rim, and the age is put at about 576,000 years.

• (b)Pangong Lake, Ladakh — Pangong Tso in Ladakh is a long, high-altitude brackish lake in a tectonic trough, not an impact crater, and its fame rests on its setting and its position astride the Line of Actual Control.
• (c)Chilika Lake, Odisha — Chilika in Odisha is Asia's largest brackish-water coastal lagoon, formed by a sandbar cutting off a bay, and is a Ramsar site known for its migratory birds and Irrawaddy dolphins.
• (d)Loktak Lake, Manipur — Loktak in Manipur is a freshwater lake famous for its phumdis, the floating mats of vegetation on which Keibul Lamjao, the only floating national park in the world, sits.

A hypervelocity impact drives a shock wave through the target rock that momentarily raises pressures far beyond anything geological processes produce. The rock does not melt in the ordinary way; instead crystalline minerals are converted to glass in the solid state. Maskelynite is plagioclase transformed like this, and its presence, along with planar deformation features in quartz and shatter cones, is the evidence geologists use to prove a crater is an impact structure rather than a volcanic one. Lonar was long thought volcanic, which is understandable since it sits in a lava plateau; the impact origin was established later.

The stem is deliberately roundabout — a substance found in the Moon rather than a mineral name — and the way in is to ask which of the four lakes has any extraterrestrial connection at all. Three of them are ordinary lakes of familiar kinds: a high-altitude tectonic lake, a coastal lagoon and a freshwater lake with floating islands. Only Lonar is a crater. A second Lonar fact worth keeping separate from this one is the colour change of June 2020, when the water turned from green to red within days; that was biological, caused by low water levels and high salinity encouraging Halobacterium and raising carotenoid levels, and it has nothing to do with the Moon.

• Lonar Lake lies in a meteorite impact crater in the Deccan basalt of Buldhana district, Maharashtra, about 576,000 years old.
• The impact converted plagioclase to maskelynite, a shock-formed glass also present in Moon rocks and meteorites.
• A 2019 IIT Bombay study found the lake's soil minerals very similar to those in Apollo lunar samples.
• The crater has a mean diameter of about 1.2 km, with the lake roughly 137 m below the rim.
• In June 2020 the water turned red within days because falling levels and high salinity favoured Halobacterium and raised carotenoid levels — a biological change, unconnected with the impact geology.

• Assuming a crater in a lava plateau must be volcanic. Lonar is an impact crater despite its basaltic setting.
• Attributing the 2020 red colour to the impact geology; it was caused by salt-loving microbes.
• Confusing Lonar with Sambhar or other salt lakes, which have no impact origin.

As an odd-one-out among Indian lakes, or as a lake-to-origin match.

• practice — not a real PYQ

  Lonar Lake, formed by a meteorite impact, is located in which State?

  • (a)Madhya Pradesh
  • (b)Maharashtra
  • (c)Rajasthan
  • (d)Gujarat

  Answer(b) Maharashtra — the crater lies in Buldhana district, blasted out of the basalt of the Deccan Traps.

• practice — not a real PYQ

  Maskelynite, found in the rocks around Lonar, is best described as

  • (a)a volcanic ash deposit
  • (b)a glass formed from plagioclase by shock pressure
  • (c)a variety of limestone
  • (d)an iron ore mineral

  Answer(b) a glass formed from plagioclase by shock pressure — its presence is one of the proofs that a crater is of impact origin.