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Q101. Siliceous (opaline) rather than calcareous sediments are found on a major portion of the ocean floor because:
Below the Carbonate Compensation Depth (CCD), cold temperatures and high pressure cause Calcium Carbonate to dissolve completely. Siliceous shells (like diatoms) are not affected by the CCD and settle to the deep ocean floor.
Q102. Which of the following river patterns is more common in the Precambrian successions?
Before the evolution of land plants with complex root systems in the Phanerozoic, there was no vegetation to stabilize river banks. This lack of cohesion caused rivers to easily wash out, leading to overwhelmingly braided channel systems.
Q103. Two cogenetic igneous rocks crystallized 1 billion years ago, yielded different present day 87Sr/86Sr ratios, this could be because these rocks:
Because they are cogenetic, their initial 87Sr/86Sr ratio was identical. The difference today is due to the radioactive decay of 87Rb into 87Sr; thus, differing initial Rb/Sr ratios cause different current isotopic signatures.
Q104. What is the value of the Earth's magnetic field (in Oe) at a place where its vertical and horizontal components are equal if the magnetic field at the equator is 0.30 Oe?
At equator, H = 0.30. At any latitude, H = 0.30*cos(L) and Z = 0.60*sin(L). Since H = Z, 0.30*cos(L) = 0.60*sin(L), so tan(L) = 0.5. From this, cos(L) = 2/sqrt(5). H = 0.60/sqrt(5). Total field F = sqrt(H^2+Z^2) = sqrt(2)*H = sqrt(2)*0.60/sqrt(5) = 0.379 Oe.
Q105. The X, Y and Z components of the remanent magnetism of a sample collected from a 120 my old rock unit at a place of magnetic latitude 45N are in the ratio 1:0:2. Consider these statements: (I) Inclination of remanent magnetism is along present Earth field. (II) Remanent magnetism is dominantly due to viscous remanent magnetization.
At 45N, the expected present-day inclination tan(I) = 2*tan(45) = 2. The rock's inclination is Z/X = 2/1 = 2. It perfectly matches the modern field, strongly implying that Viscous Remanent Magnetization (VRM) has overprinted the ancient signal.
Q106. One of the important consequences of the northward movement of Australia in the Cenozoic is:
As Australia moved north and collided with the Indonesian archipelago, it severely restricted the deep flow of warm equatorial water from the Pacific to the Indian Ocean. This caused warm water to pile up in the western Pacific, forming the Warm Pool and driving the Walker circulation.
Q107. Second boiling, which is important for certain types of ore deposits, refers to:
As anhydrous minerals crystallize from a magma, the remaining melt becomes supersaturated with volatiles. When vapor pressure exceeds confining pressure, the fluid exsolves aggressively, known as 'second' or 'retrograde' boiling.
Q108. A change in climate over a drainage basin from humid to arid conditions will lead to one of the following:
An arid climate leads to loss of vegetation. Without vegetation to intercept and slow down rainfall, water runs off rapidly over the baked soil. This dramatically shortens the lag time and causes a sharp, highly elevated flood peak (flash floods).
Q109. The buoyancy frequency is a measure of:
Also known as the Brunt-Väisälä frequency, it measures the vertical stability of a fluid (atmosphere or ocean). A higher frequency means a more statically stable environment.
Q110. What is the value of a + b, if the following set of equations are indeterminate? 3x+y+2z=4 ; 2x-y+z=a ; 5x+by+3z=6
Adding equations 1 and 2 gives 5x + 0y + 3z = 4+a. For the system to be indeterminate, this must perfectly match equation 3 (5x + by + 3z = 6). Therefore, b = 0 and 4+a = 6 (so a = 2). Thus, a + b = 2.