Can igneous rocks have a porphyritic texture

Crystals in aphanitic texture are not distinguishable by the naked eye, the crystals are microscopic in the left picture above those little points showing texture would only be visible under the microscope. Because these extrusive or volcanic rocks make contact with the ambient temperature they cool extremely quickly, so the minerals do not have time to form large crystals. Examples of aphanitic igneous rock include basalt, andesite and rhyolite.

Porphyritic - This type of texture develop when conditions during cooling of a magma change relatively quickly. Porphyritic basically means the mix of groundmass microscopic crystals with some larger crystals. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.

Figure 1. Different cooling rate and gas content resulted in these different textures. Let us start with textures associated with rocks formed by lava flows.

Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture. Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.

This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy.

Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice.

Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black.

Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture.

Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in. An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff.

A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture. However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye.

These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. 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.

Porphyry contains large crystals in the fine-grained matrix. Rhomb porphyry latite from Norway is a rock type associated with continental rifts. Intrusive equivalent of rhomb porphyry is larvikite monzonite. Width of sample 13 cm. Rhomb porphyry sample from the Oslo Rift. Another rhomb porphyry sample from the Oslo Rift. 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. Basaltic rock from Tenerife. Phenocrysts are plagioclase white and pyroxene black. Width of sample 14 cm. An introduction to the rock-forming minerals p. Igneous rocks. A classification and glossary of terms, 2.

Cambridge University Press.