Question

The surface of the earth is still irregular inspite of millions of years of gradation, because ___________.

• A. Gradation is a slow process
• B. Gradation does not reduce irregularities
• C. Deposition of material re-elevates the land
• D. Parts of the crust have been uplifted

Answer 1

Answer: (D)

Parts of the crust have been uplifted

• The surface of the earth is still irregular in spite of millions of years of gradation, because— material re-elevates the land. Parts of the crust have been uplifted.
  
• Delamination of the lithosphere has two major geologic effects. First, because a large portion of dense material is removed, the remaining portion of the crust and lithosphere undergo rapid uplift to form mountain ranges.
  
• Second, the flow of hot mantle material encounters the base of the thin lithosphere and often results in melting and a new phase of volcanism.
  
• Delamination may thus account for some volcanic regions that have been attributed to mantle plumes in the past.