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Q191. In metabasic rocks, plagioclase is not stable in
The correct answer is eclogite facies. When basalt rock gets buried incredibly deep under mountains, the crushing pressure becomes so extreme that the light, airy mineral 'plagioclase' can no longer survive. It completely breaks down and its chemical parts get absorbed into dense garnet and omphacite. This extreme pressure zone is the 'eclogite facies'. Plagioclase happily survives in the lower-pressure granulite and amphibolite facies, making them wrong.
Q192. Pressure (1GPa = 10 kbar) and temperature at the centre of the Earth are estimated to be
The correct answer is 360 GPa, 6000 K. Deep inside the Earth's solid iron inner core, thousands of miles down, geophysicists calculate that the crushing weight of the entire planet above creates a pressure of about 360 Gigapascals (GPa). Additionally, the heat trapped there from the Earth's formation is incredibly intense, estimated to be roughly 6000 Kelvin (about as hot as the surface of the Sun). The other numbers are either significantly too high or too cold.
Q193. The age of the Patcham Formation is
The correct answer is Jurassic. In Indian stratigraphy, the Patcham Formation is the oldest rock layer found in the Kutch basin. It is famous among geologists for being deposited during the Middle Jurassic period, rich with marine fossils. The Permian, Triassic, and Cretaceous are all incorrect geological time periods for this specific rock formation.
Q194. The facial suture of trilobites running through the genal angle is known as
The correct answer is gonatoparian. Trilobites were ancient sea bugs with shells that cracked along specific lines ('sutures') when they shed their skin to grow. If this crack runs perfectly through the sharp back corner of their head (the genal angle), it is strictly called a 'gonatoparian' suture. If it cuts the side it's proparian, and if it cuts the back edge it's opisthoparian, making the other options wrong.
Q195. The progressive metamorphic isograd sequence that explains Barrovian metamorphism in pelite is
The correct answer is chlorite -> biotite -> garnet -> staurolite -> kyanite -> sillimanite. When a clay-rich rock (pelite) gets slowly buried and heated, it changes minerals in a very specific, famous sequence discovered by George Barrow. It starts with low-heat Chlorite, gets hotter to form Biotite, then Garnet, then Staurolite, then Kyanite, and finally blistering-hot Sillimanite. The other options mix up this heat order or include low-pressure minerals like andalusite.
Q196. The tubefeet in echinoids emerge through
The correct answer is ambulacral plates. Echinoids, like sea urchins, have a hard shell made of plates. They move and breathe using tiny, fleshy tentacles called 'tubefeet'. The specific plates on their shell that have tiny holes for these tubefeet to poke out of are called 'ambulacral plates'. The 'interambulacral plates' are solid and do not have these holes, and the bourrelets and plastron are other specific structural parts of the shell.
Q197. The volcanic equivalent of nepheline syenite is
The correct answer is phonolite. Magma creates different rocks depending on where it cools. Nepheline syenite is a rock that cooled slowly deep underground (plutonic). If that exact same chemical mixture of magma erupts and cools quickly on the surface (volcanic), it forms a rock called phonolite. Rhyolite matches granite, andesite matches diorite, and basanite is related to basalt, making them incorrect.
Q198. Which one of the following lithostratigraphic units is of Phanerozoic Eon?
The correct answer is Uttatur Group. The Phanerozoic Eon is the most recent chunk of Earth's history, starting about 540 million years ago when complex life exploded. The Uttatur Group in South India contains dinosaur-era fossils, so it belongs to this eon. The Sargur, Semri, and Papaghni Groups are all extremely ancient (Precambrian) rocks formed long before the Phanerozoic even started.
Q199. Which one of the following primary sedimentary structures is NOT used for palaeocurrent analysis?
The correct answer is Symmetrical wave ripples. 'Palaeocurrent analysis' tries to figure out which single direction an ancient river or current was flowing. Current crescents, flute marks, and stacked (imbricated) pebbles all point like arrows downstream. Symmetrical ripples, however, are formed by waves gently rocking back and forth equally, so they don't point in one specific direction, making them useless for finding a one-way current.
Q200. Which one of the following sequences of silicate structures indicates an increasing degree of sharing of corners of (SiO4)4- tetrahedra?
The correct answer is Nesosilicate -> Single-chain inosilicate -> Phyllosilicate -> Tectosilicate. Silicate minerals are built from tiny silicon-oxygen pyramids. 'Nesosilicates' don't share any corners. 'Inosilicates' share 2 corners to form chains. 'Phyllosilicates' share 3 corners to form flat sheets. Finally, 'Tectosilicates' share all 4 corners to form a solid 3D grid. Option A is the only one that perfectly lists this progression from least shared (0) to most shared (4).