Answer :
Final answer:
A porphyritic texture in igneous rocks indicates two stages of cooling, characterized by large crystals set in a fine-grained matrix, revealing a complex cooling history first slower, then more rapidly (option d).
Explanation:
The two stages of cooling in igneous rocks are best represented by a porphyritic texture. This texture indicates that the rock has undergone two distinct cooling processes: a slower rate of cooling followed by a more rapid cooling phase. This is evident by the presence of larger crystals, known as phenocrysts, embedded within a finer-grained or aphanitic groundmass.
During the slower cooling phase that occurs deep within the Earth's crust, the magma allows for the formation of these larger crystals. As the magma then rises closer to or reaches the Earth's surface, the remaining liquid part of the magma cools much more rapidly, leading to the formation of the fine-grained matrix. Thus, a porphyritic texture indicates a complex cooling history and is characterized by two distinct sizes of crystals reflecting two stages of cooling.
