What Does it Mean to See Porphyry?

In teaching, one often learns where the great misunderstandings are. Geology has its own set of problems, most of which I face each Fall when I teach my introduction to the geological sciences course.

Sometimes it’s not apparent, even after teaching a course for years, that certain things are never explained properly.

It’s  been pointed out to me that there’s come confusion about the term ‘porphyry.’ First of all, this word seems to lack an appropriate number of vowels, but the y’s stand in for missing vowels.

It’s pronounced “POUR-furree” or “Pour-IF-urry,” depending on how a person might feel on a given day. Makes it sound a little cuddly, doesn’t it.

Porphyry is a texture in igneous rocks.

Texture refers to how individual mineral grains (or crystals) relate to each other in a rock.

Igneous rocks are those that form from the crystallization of minerals from cooling molten rock – much like how ice forms on a puddle as temperatures drop below freezing.

And that’s important. The whole puddle doesn’t freeze at once. All of the molten rock (called magma) doesn’t crystallize at once either. Parts of it remains liquid for a while.

When crystals grow from a cooling or freezing liquid, their size is dictated by how quickly the cooling happens. When the cooling is rapid, the crystals tend to be very small, because the atoms that make up the crystal don’t have much time to move themselves into proper places to make crystals larger. The crystals are usually so small that they are not easy to see without magnification.

Basalt is an igneous rock that cooled quickly. The individual crystals are not visible without a microscope. Credit: USGS

When cooling is slow, the crystals that form tend to be larger. They can be very large, like in pegmatites. But really, ‘large’ to a geologist usually only means that they are easy to see with the naked eye.

Gabbro is an igneous rock that cooled slowly enough that crystals are visible to the naked eye. Credit: Mark Wilson

Some igneous rocks are given a name with the word porphyry in it.

Porphyry (or porphyritic) is a texture that has both very small crystals that are invisible to the naked eye and very large crystals that are easy to see.

A basalt porphyry with big crystals of plagioclase. Credit: Lynn Fichter

When we see a porphyry or porphyritic rock, we know that there were two stages of cooling and crystal formation of the magma.

First, the cooling was slow and large crystals started to form. But there was still liquid around.

Then something happened to cause rapid cooling. With igneous rocks, this usually means a volcanic eruption. The remaining liquid crystallized quickly, resulting in tiny crystals that cannot be seen with the naked eye.

Thus, but just looking at a single rock, we can know quite a bit about what happened in an area. Pretty cool, eh?

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