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Q71. A watershed has area 74 km2. Stream lengths: 1st order=19.5km, 2nd order=32km, 3rd order=30km. Drainage density is (km/km2):
Drainage Density = Total Stream Length / Basin Area = (19.5 + 32 + 30) / 74 = 81.5 / 74 = 1.10.
Q72. Index of Alteration (IA) = 100*[Al2O3]/([Al2O3]+[Na2O]+[K2O]). Correct statement:
During weathering, mobile cations (Na, K) are leached out, leaving immobile Al. This drives the ratio towards 100.
Q73. Which one of the following mineral assemblages is stable under eclogite facies conditions?
Eclogite facies is high-pressure/high-temperature and is defined by the absence of Plagioclase feldspar. The typical assemblage is Garnet + Omphacite (Cpx).
Q74. Which of the following physical properties of rocks has the widest range of variation?
Electrical resistivity in rocks can vary by many orders of magnitude (from 10^-8 to 10^14 Ohm-m) depending on porosity and fluid content.
Q75. In electromagnetic (EM) sounding, the depth of investigation [?] with increasing frequency.
EM depth of investigation (Skin Depth) is inversely proportional to the square root of frequency. Higher frequencies penetrate less.
Q76. Negative Eu anomalies in granite are explained by:
Europium is compatible in Plagioclase. If plagioclase is removed (fractionation) or left in the residue, the resulting melt (Granite) is depleted in Eu.
Q77. The most abundant mineral in the Earth's crust is:
Feldspars (including Plagioclase and K-Feldspar) are the most abundant mineral group in the Earth's crust, making up about 60% of it.
Q78. If the dissociation constant of pure natural water at 50°C is 10^-13.10, the pH of the water will be:
For neutral water, pH is half of pKw. At 50°C, pKw is 13.10, so the neutral pH is 13.10 / 2 = 6.55.
Q79. A Wenner array with 60 m spacing between current electrodes (total length L=3a) is placed over inhomogeneous ground. Measured V=10 mV, I=5 mA. Calculate apparent resistivity (ohm-m). (Note: Wenner spacing 'a' is L/3 = 20m).
For Wenner array, Rho = 2 * pi * a * (V/I). Spacing a = 60/3 = 20m. Rho = 2 * 3.14 * 20 * (10/5) = 125.6 * 2 = 251.2 ohm-m.
Q80. NOT true marine organism statements
Forams are unicellular and having higher benthic diversity