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Q141. A continuous 10 m thick sequence of shale was deposited in 10,000 years at uniform rate of sedimentation. The number of samples that must be collected at equal stratigraphic intervals to sample the succession every 500 years is ______.
The correct answer is 20. The total time for the mud sequence to be deposited was 10,000 years. If the geologist wants to take one sample representing every 500 years of that history, they simply divide the total time by the sampling interval. 10,000 divided by 500 equals exactly 20. The geologist must collect 20 samples spaced equally apart in the rock.
Q142. The dip slip on a fault 000º, 30ºE is 10 m. Assuming slip equals separation here, the throw on the fault is _____ m (answer in one decimal place).
The correct answer is 5.0. When a fault breaks, one side slides down the slanted crack (dip slip). The crack is slanted at a 30-degree angle, and the rock slid exactly 10 meters diagonally. The 'throw' is just the vertical, straight-down drop of that slide. Using simple trigonometry, we multiply the diagonal slide (10) by the sine of the angle (30 degrees). Since the sine of 30 is exactly 0.5, the vertical drop is 10 * 0.5, which equals exactly 5.0 meters.
Q143. The number of alpha (α) particles emitted to produce a daughter isotope of 206Pb from a parent isotope of 238U by radioactive decay is _____.
The correct answer is 8. Heavy, unstable elements like Uranium slowly break down into lighter, stable elements like Lead (Pb) by shooting out tiny pieces of themselves. An 'alpha particle' is a heavy chunk of atomic material that weighs exactly 4 atomic mass units. The Uranium atom starts with a mass of 238, and the final Lead atom has a mass of 206. The total mass lost is 32. Since each alpha particle carries away 4 units of mass, dividing 32 by 4 means exactly 8 alpha particles were fired out.
Q144. The absolute difference in the Moh’s hardness values of the two silicates among the minerals listed below is _____. Apatite, Corundum, Gypsum, Talc, Topaz
The correct answer is 7. First, identify the two 'silicate' minerals from the list. Talc and Topaz are both silicates. (Apatite is a phosphate, Corundum is an oxide, and Gypsum is a sulfate). On the Mohs hardness scale, Talc is the softest mineral with a hardness of 1. Topaz is a very hard mineral with a hardness of 8. The absolute difference between 8 and 1 is exactly 7.
Q145. A crustal rock is at a lithostatic pressure of 3 kbar and a temperature of 275ºC. If the lithostatic pressure increases at a uniform rate of 0.3 kbar/km, and the surface temperature is 25ºC, the geothermal gradient (in ºC/km) is _____ (answer in one decimal place).
The correct answer is 25.0. First, we find the depth of the rock. The pressure increases by 0.3 kbar per kilometer, and the rock is at 3 kbar. Dividing 3 by 0.3 means the rock is exactly 10 kilometers deep. Next, we find the temperature change. The surface is 25ºC and the rock is 275ºC, a difference of 250ºC. The 'geothermal gradient' is the heat increase per kilometer. Dividing the 250ºC difference by the 10 km depth gives exactly 25.0 ºC/km.
Q146. The valency of iron in hematite is _____.
The correct answer is 3. Hematite is an iron oxide mineral with the chemical formula Fe2O3. In chemistry, oxygen almost always has a valency (charge) of -2. Since there are 3 oxygen atoms, the total negative charge is -6. To balance this perfectly to zero, the 2 iron atoms must provide a total positive charge of +6. Therefore, each iron atom must have a valency of +3.
Q147. When plotted on a map of 1:50000 scale, a 2 km long dyke exposed on a horizontal surface has a length of _____cm (answer in one decimal place).
The correct answer is 4.0. The map scale '1:50000' means that 1 centimeter on the map equals 50,000 centimeters (which is 0.5 kilometers) in the real world. If the real dyke is 2 kilometers long, we simply divide 2 km by 0.5 km per centimeter. This mathematically gives exactly 4.0 centimeters on the map.