Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass.
It seems simple enough but unfortunately there are many different interpretations. Sometimes it is assumed that porphyry is granitic in composition 1 while other sources claim that composition plays no role whatsoever2. Some authors make a difference between porphyry and porphyritic rock. This is based on field relations. True porphyry according to this interpretation is an intrusive rock. Extrusive (lava) rock may have a porphyritic texture but it should be named porphyritic rock, not porphyry3.
Another system which I am familiar with because it seems to be widespread in continental Europe is to contrast between “porphyry” (feldspar phenocrysts are alkaline) and “porphyrite” (feldspar phenocrysts are plagioclase)4. So, according to this system, we can talk about “rhyolite porphyry” and “basalt porphyrite” for example but never “basalt porphyry” and “rhyolite porphyrite”. Nowadays it seems to be common to use appropriate rock type name in addition to “porphyry” (two examples in the previous sentence) but it used to be very common to use the name of the minerals that form the porphyritic texture. “Plagioclase porphyrite” and “quartz porphyry” have been used instead of “basalt porphyrite” and “rhyolite porphyry”.
Yet another source of confusion are the terms “porphyritic” and “porphyraceous”. Rocks are said to be porphyritic if their groundmass is fine-grained ar aphanitic and porphyraceous if their groundmass in visible to the naked eye. Hence, rhyolite and basalt as fine-grained volcanic rocks are porphyritic and granite, syenite, etc. as coarse-grained plutonic rocks are porphyraceous. Phenocrysts that make up porphyry should be felsic (quartz, feldspar).
Well, as you can see, mankind is very good when a need arises to make simple concepts difficult to understand. What is in my opinion important to undestand and what is agreed upon by all is that porphyritic rocks are always igneous rocks and they contain crystals that are noticeably larger than the crystals surrounding them.
How do porphyritic rocks form? This is in most cases fairly simple concept to grasp. Crystals need time to grow. In porphyritic rocks some large crystals had this time while others (groundmass) solidified quickly. Hence, porphyritic rocks started to solidify as normal intrusive rocks but something happened that resulted in quick loss of heat and rapid crystallization. This something may have been the emplacement of magma into narrow cracks near the surface or maybe volcanic eruption that brought magma to the surface.
This interpretation, however, is unable to give an adequate explanation to the question why are deep-seated plutonic rocks sometime porphyritic (or porphyraceous). They definitely cooled slowly and it is highly unlikely that there were noticeable changes in the cooling rate. These rock probably exhibit porphyritic texture because some crystals started to form before others and had therefore more time and room to grow. The role of volatile components in magma is probably important as well. Hence, we do have a reason to believe that there are several different mechanisms involved and in many cases it may be a complicated task to unravel the cooling history of a particular igneous rock.
Pictures of porphyry
Basalt porphyrite, plagioclase porphyrite or diabase? Probably all of them, it is mostly a matter of preference and depends on local traditions. White phenocrysts are plagioclase crystals. The Isle of Mull, Scotland. The rock is 8 cm in length.
Porphyritic basaltic rock from Tenerife. Phenocrysts are plagioclase (white) and pyroxene (black). Width of sample 14 cm.
This one from Oahu is clearly not the most classic version of porphyry because it is mafic, it is extrusive, and the phenocrysts are mafic. It is often assumed that phenocrysts that form the porphyritic texture must be felsic (feldspars (preferably alkali feldspars) or quartz). The green mineral here is olivine. But it does meet the most important and universal characteristics of porphyritic rocks — it is igneous and some minerals are clearly larger than the groundmass. Width of sample 6 cm.
Porphyry from Scotland with K-feldspar and quartz phenocrysts. Width of sample 8 cm.
Andesite porphyrite with plagioclase phenocrysts from Santorini. Width of sample 7 cm.
Porphyritic rhyolite from Estonia. It is locally known as quartz porphyry. Width of sample 9 cm.
1. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
2. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
3. Rose, W. I. (2007). Porphyry. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 14. 254-255.
4. Spry, A. (1990). Porphyry. In: The Encyclopedia of Igneous and Metamorphic Petrology (Ed. Bowes, D. R.). Springer. 479–480.