Pegmatites,which often occur in dikes,are unusual among shallow intrusive rocks in that they ________.
A)possess porphyritic texture
B)possess exceptionally coarse grains
C)are mineralogically identical to the extrusive rock basalt
D)are glassy,cooling so rapidly that crystals do not have time to form

Question

Quizplus · Accepted Answer

Pegmatites are unusual among shallow intrusive rocks because they possess exceptionally coarse grains, often larger than 1 centimeter in diameter. This is due to the slow cooling and slow crystallization of the magma, which allows for the growth of large individual crystals. Porphyritic texture is a common characteristic of volcanic rocks, not shallow intrusive rocks like pegmatites. Pegmatites are mineralogically different from basalt, which is a common extrusive rock. Glassy texture indicates rapid cooling without crystal growth, which is not the case with pegmatites.

Pegmatites,which Often Occur in Dikes,are Unusual Among Shallow Intrusive Rocks