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Q81. The apparent dip of a plane is measured to be 45º towards NE. The true dip of the plane is
The correct answer is 48º towards ENE. The 'true dip' is the absolute steepest angle a rock layer tilts into the earth. If you look at the rock from a slightly angled perspective (apparent dip), it will always look less steep than it truly is. Since the apparent dip is 45º, the true dip MUST be a number greater than 45º. 40º is mathematically impossible. 55º SSW is in the completely opposite compass direction. 48º ENE is the only option that is steeper and facing the correct general direction.
Q82. The Triassic-Jurassic boundary lies within which one of the following stratigraphic units?
The correct answer is Dharmaram Formation. In Indian stratigraphy, the Dharmaram Formation contains a continuous sequence of rocks and fossils that transition perfectly from the Late Triassic period into the Early Jurassic period. The Panchet, Pachmarhi, and Denwa Formations were all laid down entirely within the Triassic period before this boundary happened, making them incorrect.
Q83. What is the minimum number of forms that an actual crystal must contain in Class 1 (Pedial) of the Triclinic System?
The correct answer is 4. The 'Pedial' class of crystals has absolutely zero symmetry—no mirror reflections and no repeating sides. Because of this, every single flat surface (face) on the crystal is totally unique and represents its own separate 'form'. To physically enclose a 3D object in space so it has an inside and outside, you need a minimum of 4 sides (like a pyramid with a triangle base). Since each side is a unique form, the crystal requires at least 4 forms. 1, 2, or 3 forms would just be open flat planes.
Q84. Which one is the correct order of stability of the minerals (arranged from the most stable to the least stable) during chemical weathering?
The correct answer is Quartz > Muscovite > Amphibole > Olivine. Minerals that form in the blistering heat of Earth's deep mantle (like Olivine) are the least stable in the cool, wet surface weather and rot away quickly. Minerals that form at cooler temperatures (like Quartz) are extremely tough and survive forever as beach sand. Following this rule (the Goldich Dissolution Series), the correct order from toughest to weakest is Quartz, then Muscovite, then Amphibole, and finally Olivine. Any other order violates this chemical law.
Q85. Which one of the following is a geochronologic unit?
The correct answer is Period. In geology, we separate physical rock layers from the actual 'time' during which they formed. A 'Period' (like the Jurassic Period) is a 'geochronologic' unit, meaning it represents a specific chunk of time. 'System', 'Member', and 'Formation' refer to the physical rock layers themselves, not the time concept, making them incorrect.
Q86. Which one of the following must have thickness less than 1 cm?
The correct answer is Lamina. When mud or sand settles into rock, it forms layers. In geological terms, a 'lamina' is defined specifically as the thinnest possible distinct layer of rock, measuring less than 1 centimeter thick. A 'Bed' is thicker than 1 cm. 'Stratum' and 'Layer' are general words for any sheet of rock regardless of its thickness, making them incorrect.
Q87. Which one of the following organisms became extinct during the Cretaceous-Tertiary mass extinction event?
The correct answer is Ammonite. Ammonites were shelled sea creatures that completely died out when the asteroid hit Earth at the end of the Cretaceous period (the same event that killed the dinosaurs). Trilobites went extinct much earlier, before dinosaurs even existed. Brachiopods and Echinoderms (like starfish) survived the extinction and still exist in oceans today, making them incorrect.
Q88. Which one of the following seismic waves involves compression and rarefaction (but not rotation) of the material that it passes through?
The correct answer is P-waves. When an earthquake happens, P-waves (Primary waves) travel by pushing and pulling the rock straight forward and backward, similar to a slinky toy being stretched and squeezed (compression and rarefaction). S-waves shear the rock side-to-side. Rayleigh and Love waves are surface waves that involve complex rolling and rotating motions. Therefore, only P-waves act purely through straight compression without rotation.
Q89. Which one of the given statements is correct?
The correct answer is Sulfide minerals form by covalent bonding between metal and sulfur. In minerals like pyrite (fool's gold), metal atoms deeply share their electrons with sulfur atoms to create a very strong connection known as a covalent bond. Statement A is wrong because weak van der Waals bonds do exist in silicates like soft clay and mica. Statement C is wrong because silicates don't use metallic bonds (which make things shiny and conductive like copper wire). Statement D is an incorrect chemistry assertion regarding atomic orbitals.
Q90. Which one of the given textural features results from exsolution?
The correct answer is Perthitic. 'Exsolution' is like a hot mixture of liquids separating as they cool down (like oil separating from vinegar). In rocks, when a hot mixed crystal cools, it separates into two distinct minerals. 'Perthitic' texture is the classic result of this, where tiny sodium-rich stripes separate out inside a potassium-rich feldspar crystal. Ophitic, Graphic, and Glomeroporphyritic textures happen during the original freezing of the magma, not from solid crystals unmixing, so they are incorrect.