July 14, 2011

Geology Word of the Week: G is for Glomeroporphyritic

Posted by Evelyn Mervine

Glomeroporphyritic basalt from Ontario, Canada. Image courtesy of Ron Schott.

def. Glomeroporphyritic:
A textural term used to describe igneous rocks that contain clusters of phenocrysts, which are large crystals in a finer-grained matrix or groundmass.

Glomeroporphyritic is one of my favorite geology words. I love this word because it’s such a funny, complicated-looking word, and it’s so much fun to say. Sometimes, I manage to say “glomeroporphyritic” perfectly. Other times, I stumble over the tongue-twister of the word, saying something like “glomeroporphaglomeroporphyryyitic” before taking a breath and slowly, deliberately saying, “glomeroporphyritic.”

The word “glomeroporphyritic” is a modification of the word “porphyritic,” which is a textural term used to describe igneous rocks that have large crystals in a finer-grained groundmass.  How do porphyritic igneous rocks form?

Well, to quote an earlier blog post of mine,

How to porphyritic igneous rocks form? They generally form when magma that has been slowly cooling for a long time, possibly in a magma chamber, is suddenly erupted to Earth’s surface. Deeper in the Earth where magmas can cool more slowly, large crystals have time to form and grow.  Magma takes awhile to crystallize completely, so sometimes partially-crystallized magmas are brought to Earth’s surface and erupted as lavas. When these partially-crystallized lavas are erupted, the rest of the molten rock cools quickly, and there is no time for large crystals to form.

Here are a couple of pictures of regular porphyritic rocks:

Porphyritic granite. Photo courtesy of Ian Stimpson.

A rhomb porphyry (porphyritic trachyte) from the 'Sande cauldron', in the Oslo Graben, Norway. Image courtesy of Ian Stimpson.

Unfortunately, “glomeroporphyritic” is not in the Oxford English Dictionary, so I’m not sure when the word was first adopted into the English language (anyone know?). The word “porphyritic” was first used in English in the late 1700s and comes from the Latin word “porphyrites,” which comes from the Greek word for purple and was used by ancient Romans to describe a purple-colored (and apparently porphyritic!) stone found in Egypt and used as a building stone.

The prefix “glomero-” is also of Latin origin, coming from the Latin “glomerat-“,  which is the participial stem of the verb “glomerare,” which means “to form into a ball or mass” or “to collect.” So, a glomeroporphyritic rock is a rock which contains balls or collections of phenocrysts. As an aside, I have just discovered that “glomerate” is a real word in English, and I think it’s my new favorite verb. I’m actually headed off to a meeting tomorrow, and I think I’m going to say to people, “Hey, shall we glomerate over there?” just for fun. It’s a nerdy meeting, so I think people will like the word “glomerate.”

There are two other geology words which are related to the word “glomeroporphyritic.” The first word is “glomeroporphyroclast,” which describes a cluster or collection of phenocrysts. Although the word contains the singular “clast,” the term really describes a collection of  crystals. Actually, the “-clast” suffix is a misnomer. Technically,  a “clast” is a rock fragment found in a sedimentary rocks. Glomeroporphyroclasts don’t really contain clasts at all– they contain phenocrysts. So, I think the term should really be “glomeroporphyrocryst,” but as far as I can tell no one uses that.  The second word is “glomeroporphyroblast,” which describes a collection of “blasts,” which are large crystals found in a finer-grained matrix in a metamorphic rock. For more on crysts, clasts, and blasts, see this “About Geology” article.

You might be wondering– do geologists actually use these fun but complicated words? Absolutely. A quick search of the GeoRef database returns 52 papers when I search with the term “glomeroporphyritic.” If you google the term “glomeroporphyritic,” you will find numerous scientific papers and reports which include the word as a textural description for rocks. I suppose geologists could just say something like “clumps of crystals” or “globs of phenocrysts,” but “glomeroporphyritic” is such a fun word! I’ve even used the word in my own research to describe globs of plagioclase phenocrysts in basalts.

For example, below are some pictures of a geologic thin section which I described as having glomeroporphyroclasts of plagioclase phenocrysts:

Plagioclase glomeroporphyroclast (-cryst) in basalt. Thin section viewed through a microscope under plane light. Sorry there's no scalebar!

Plagioclase glomeroporphyroclast (-cryst) in basalt. Thin section viewed through a microscope under cross-polarized light. Sorry there's no scalebar!

The thin section pictured above came from a porphyritic basalt collected along the Ninetyeast Ridge in the Indian Ocean. In hand sample, the basalt looked something like this (not the same rock, but similar):

Porphyritic basalt with phenocrysts of plagioclase. Collected along the Ninetyeast Ridge, Indian Ocean in 2007.

Finally, back in May I identified (with the help of some fellow geologists!) a “Mystery Rock” with gorgeous plagioclase phenocrysts. This mystery rock has slight glomeroporphyritic texture– some of the phenocrysts look as if they are clumping together. The owner of this mystery rock actually decided to name her rock “Glomer.” You can see some more photographs of Glomer the rock here.

Plagioclase phenocrysts in the "Mystery Rock."


***Note: Thanks to Ron Schott and Ian Stimpson for pictures for this week’s geology word of the week.***

“glomerate, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.

“porphyritic, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.

“porphyrite, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.