Which one of the following sedimentary rocks has not been formed mechanically ?
- A.Sandstone
- B.Conglomerate
- C.Loess
- D.Geyserites
Correct answer
Explanation
The correct answer is D, Geyserites. Geyserite is a chemically formed sedimentary rock: it is silica that comes out of solution from hot spring and geyser water and is deposited around the vent.
Sedimentary rocks are grouped by how they form. Mechanically formed, or clastic, rocks are made of fragments of older rock broken by weathering, carried by water, wind or ice and then compacted and cemented. Organically formed rocks come from the remains of plants and animals, like coal and some limestones. Chemically formed rocks are deposited when dissolved minerals precipitate out of water, like rock salt, gypsum and geyserite. Geyserite builds up as white or grey crusts and mounds around geysers, such as those in Yellowstone in the United States and in Iceland.
Option A is wrong because sandstone is made of sand grains cemented together, a mechanical rock. Option B is wrong because conglomerate is made of rounded pebbles and gravel cemented together. Option C is wrong because loess is fine silt carried and laid down by wind, as on the Loess Plateau of China.
Exam tip: Mechanical: sandstone, conglomerate, loess, shale; organic: coal; chemical: rock salt, gypsum, geyserite.
Practice Questions
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- B.A manure that supplies nutrients to the soil
- C.A weedicide that kills the forest roots
- D.A means of levelling the slope
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- B.Dairy farming
- C.Shifting cultivation
- D.Commercial grain farming
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- B.Collective farming
- C.Plantation agriculture
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- B.The monsoon lands of Asia
- C.The steppes of Eurasia
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