Porphyritic basalt or basalt porphyry describes basalt rocks with large crystals known as phenocrysts dispersed in a fine-grain matrix or groundmass. So, the word porphyry (pronounced as POR-fə-ree) is a textural term, and rocks with this texture are said to be porphyritic.
Regarding composition and mineralogy, porphyritic basalt rocks are like ordinary, fine-grained basalt counterparts. Therefore, the texture only indicates slow cooling deep within the Earth’s crust, forming larger crystals and fast cooling. Earth’s surface formed the embedding fine-grained matrix.
If you are hearing about basalt porphyry for the first time. You should know they occur in most of the igneous rock. Other common ones include granite, diorite, andesite, and rhyolite porphyry.
This article covers basalt porphyry rocks’ meaning, definition, formation, occurrence, and uses. But before that, let us start with a brief overview of basalt rocks.
What is basalt?
Basalt is a fine-grained (aphanitic) dark-colored, mafic extrusive, or volcanic igneous rock. However, it may have a porphyritic texture, and some, like pillow lava basalts, may have a glassy surface with a fine-grained core.
This hard (Mohs hardness of 6) dark gray to black rock is high in plagioclase feldspar and pyroxene (augite) and low in quartz (less than 20%). However, it may have other minerals like olivine, biotite, and hornblende.
Basalt forms from low-viscosity mafic (rich in iron and magnesium) lava flow. But can be from sills, dykes, or other igneous small intrusive bodies. The most common basalt rock-forming environments are oceanic hotspots and divergent oceanic boundaries or hotspots underneath continents and mantle plumes.
Lastly, basalt and its igneous rock equivalent, gabbro, form the most abundant rocks on the Earth’s surface.
What are porphyritic basalt rocks?
Porphyry is a term that describes igneous rock texture with consistently large crystals dispersed or embedded in a finer-grain or glassy matrix. It doesn’t reference composition (mineralogy), chemistry, characteristics, or even absolute sizes of the larger crystals or finer-grained matrix.
The large crystals are known as phenocrysts, and the finer-grained matrix groundmass. But sometimes, you may find terms like megaphenocrysts when very large (more than 5mm) or microphenocrysts (if 0.03 to 0.3mm).
Therefore, if a basalt rock shows porphyry texture, we call it basalt porphyry rock or, more accurately, porphyritic basalt rock since porphyritic is an adjective describing basalt.
In basalt porphyry, there are larger crystals or phenocrysts of olivine, augite (a form of pyroxene), plagioclase felspar (calcium-rich), or hornblende within a fine-grained groundmass. Usually, olivine and augite are the most common phenocrysts in porphyritic basalt. However, those of plagioclase feldspars phenocrysts do exist.
Identifying them shouldn’t be daunting as the phenocrysts are easily visible.
If you observe a basalt porphyry textured rock, plagioclase feldspar will appear as tubular light-grayish crystals. They will have a euhedral structure on a thin section characterized by sharp, easily recognized faces.
On the other hand, augite (pyroxene) will have smaller, black to very dark green euhedral crystals. What about olivine? It will form greenish, yellow-green, or brownish phenocrysts, while hornblende is brown to greenish in color. However, in rare cases, hornblende may appear yellowish or reddish-brown.
Lastly, in terms of composition, the groundmass will have plagioclase feldspar, olivine, pyroxene, and other rock-forming minerals and will be dark gray to black.
Naming these rocks
Besides the names porphyritic basalt or basalt porphyry, the naming may indicate abundant phenocrysts. For instance, a hornblende basalt porphyry or olivine basalt porphyry indicates that the specific basalt porphyry has hornblende and olivine phenocrysts as the most abundant or dominant.
How are porphyritic basalt rocks formed?
A basalt porphyry indicates that the larger crystals formed during a slow cooling phase before an eruption, while the groundmass formed during a faster cooling phase after a volcanic eruption. Therefore, porphyritic basalt forms from two-stage cooling stages.
During the slow cooling phase deep within the Earth’s crust, phenocrysts form via fractional crystallization from the basaltic lava. Usually, these crystals (plagioclase feldspar, augite, olivine, or hornblende) will start crystallizing out of the magma melt, starting with those with the highest melting temperatures.
Since the cooling rate is slow, there is time for these crystals to grow large. Also, they will be well-formed as the remnant magma melt will not limit their growth.
In case of a volcanic eruption, the remaining magma already formed will move to the Earth’s surface. This remaining magma will rapidly cool, forming a fine-grained groundmass with the phenocrysts.
The groundmass will have fine grains since the rapid cooling doesn’t give time for larger crystals to form.
If the basaltic magma has trapped gas, it will form a vesicular basalt porphyry. Such a rock will have vesicles (small holes) and large crystals in a fine-grain matrix.
Lastly, you may have heard about Flowerstone (Flower Feldspar). This is a basalt porphyry rock with flowery feldspar Phenocryst and a dark gray groundmass,
Occurrence
Like ordinary basalt rocks, basalt porphyry rocks occur on divergent oceanic boundaries, oceanic hotspots, or mantle plumes and hotspots below the continents.
In the US, porphyritic basalt occurs in various places, especially as lava flows. Some of these locations include:
- Isle Royale (Scoville Point, Tobin Harbor, Huginnin, Grace Island, and Hill Point)
- The Wanapum Basalt that covers Washington, Northern Nevada, and Western Idaho has mainly the plagioclase porphyritic basalt.
- Menominee Group; Blair Creek Formation (Michigan)
- North Shore Volcanic Group (Minnesota), Ossipee Mountain Complex of Kingsley (New Hampshire),
- Hook Mt. Basalt (New Jersey)
- Several locations in Oregon state
Uses of porphyritic basalt
The uses of porphyritic basalt rocks include making aggregate for road ballast, construction, or drain field filters. Also, they make good dimensional stones, and you can use them to make building veneer, floor tiles, monuments, landscaping, or paving.
References
- Department of Earth Sciences. (n.d.). L162 – Porphyritic basalt. University of Cambridge. Retrieved August 29, 2022, from https://wserv3.esc.cam.ac.uk/p1acollections/exhibits/show/igneous-rocks/l162
- Basalt. (2022, August 15). In Wikipedia https://en.wikipedia.org/w/index.php?title=Basalt&oldid=1104437424
- The United States Geological Survey. (n.d.). Geologic units containing Basalt. Retrieved August 29, 2022, from https://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=basalt
- basalt | Definition, Properties, & Facts. (1998, July 20). Encyclopedia Britannica. Retrieved August 29, 2022, from https://www.britannica.com/science/basalt