How do porphyritic rocks form?

Answer 1

I believe that porphyritic andesite is formed from volcanic ash and bits and pieces of minerals that eventually become ejected from the volcano. How the andesite becomes porphyritic is that it formed inside the volcano before being blown out of along with the magma and hot gases. You can find silver and gold specs as well as a few other metals encased by a smooth texture of solidifyed ash. Lee Durst (WVC 08)

Answer 2

A porophyritic rock is igneous by classification. Porphyritic rocks begin to form below the surface when minerals with a higher melting temperature crystallize and are carried to or near the surface where the surrounding minerals with lower melting temperatures crystallize, resulting in a porphyritic texture.

Answer 3

Porphyritic textures are formed when magma begins to cool while it is still underground. Minerals that crystallize at higher temps will start to separate out and form crystals. As the magma cools, these crystals get larger, some other minerals will crystalize and then the magma is erupted. Once the magma is erupted it will cool very quickly resulting in a finer grained matrix. When the magma is erupted, the water that was in the magma is suddenly ejected/expelled which also results in a halt of crystallization.

Answer 4

Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye