NAEP Science Grade 12

Category - Earth Science

In some desert regions, a thin layer of closely packed pebbles and cobbles covers the ground surface. This phenomenon, known as desert pavement, is primarily caused by the:
  1. Differential erosion of sediments by wind.
  2. Slow dispersal of material from the base of alluvial fans by gravity.
  3. Breakup of bedrock under hot and dry conditions.
  4. Deposition of coarse-grained sediments during flash flooding.
Explanation
Correct Response: A. Scientists believe desert pavement forms by different processes, however the most common process is believed to be differential erosion of the surface by wind; a process called deflation. Over thousands of years, smaller particles are removed from the surface by wind, leaving behind a flat surface primarily composed of pebbles and cobbles. Dispersal of alluvial fan deposits from the base of an alluvial fan as sediments move downward due to gravity is not thought to play a direct role in desert pavement formation (B) and although alternating hot and cold temperatures in the desert can support mechanical weathering (C), the process is not thought to play a role in desert pavement formation either. Although water carrying fine sediments in sheet flows may play a role in forming some types of desert pavement, flash floods carrying coarse sediments do not (D). 
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