Fig. 1:- Gabbro and gabbro! Gabbro image from Geology is the Way.
Ahh, Gabbro, everyone’s favourite time buddy. Well known for their easy-going and nonchalant attitude towards, well, everything, it seems fitting that their namesake is that of an equally nonchalant igneous rock.
“How can a rock be nonchalant, OWGS Team?” we hear you ask. Well, let's try and explain that whilst also giving you an introduction to the geological version of gabbro!
First and foremost, gabbro is a slow-cooling, igneous rock with a phaneritic texture. Igneous rocks are formed when molten magma (or lava, if it has reached the Earth’s surface!) cools and solidifies. Remember how rocks are made up of individual, interlocking minerals? The size of the minerals that make up the rock depends on the speed at which our igneous rock cools, with fast-cooling producing fine-grained, or aphanitic, specimens such as basalt, dacite, and basanite; and slow cooling producing coarse-grained, or phaneritic, specimens such as granite, diorite, and - you guessed it, gabbro!
Fig. 2:- Hand sample of gabbro. Note the phaneritic texture made up of the large phenocrysts of pyroxene! Source.
Gabbro itself is an intrusive igneous rock, meaning it cooled beneath the surface of the Earth before being exposed via tectonic uplift or weathering/erosion; extrusive igneous rocks are instead formed specifically when magma erupts as lava via volcanic events before cooling - forming lava flows, pyroclastics or tuff. As the name suggests, intrusive rock intrudes into country rock (a fancy term for already in-situ rock), which acts as an excellent insulator, therefore allowing plenty of time for the intruding magma to, excuse the pun, chill out and solidify at its own pace. Which we think is pretty nonchalant. For a rock, anyway.
Gabbro can form from large magma intrusions associated with volcanism or continental rifts, but it's more commonly formed by the mid-ocean ridges that generate oceanic crust (which gabbro is a major component of!). While their island is a little less dense than oceanic crust, we still think it's pretty neat how Gabbro remains associated with the sea in-game!
Fig. 3: Diagram showing the origins of various intrusive and extrusive rocks. Source.
Okay, so what makes gabbro, gabbro? There are lots of intrusive igneous rocks out there! To be deemed worthy enough of the gabbro title, our rock must be made up predominantly of plagioclase feldspar. 90% predominantly. Which you can see on the infamous QAPF diagram here. Additionally, they are a mafic rock, meaning they are rich in magnesium and iron and formed from a magma with relatively low silica content.
Fig 4. The QAPF diagram, which geologists use to classify intrusive igneous rocks based on their relative concentrations of quartz, alkali (potassium) feldspar, plagioclase feldspar, and foidolite. You can spot several gabbros clustered around the rightmost point of the diagram! (by Lina Jakaitė-Darkšė)
Gabbro actually shares the same chemical composition and mineralogy as its aphanitic twin - basalt! They are both derived from the same mafic magma, and are made up of mainly pyroxene and plagioclase feldspar, alongside hornblende and olivine, albeit in smaller amounts - with the only difference being how fast they have cooled. You can actually see this under the microscope!
Fig. 5:- Thin section samples of gabbro (left) and basalt (right). Note that each sample contains the same minerals: plagioclase feldspar and pyroxene, but the size of the individual crystals differ between the samples. Source.
Gabbro is sometimes referred to as black granite, and that's because it shares the characteristic large, visible crystals that granites and other igneous intrusive rocks have, albeit with many more mafic minerals. Pyroxenes and hornblendes appear as dark green or black elongate crystals, and seeing lots of these in your sample - alongside a near complete absence of quartz and plenty of plagioclase feldspar - is what you can use to determine if your rock is indeed a gabbro!
Looking at our lovely QAPF diagram, you may be a little confused that gabbros appear represented by light-coloured rocks, when they typically contain so many dark minerals. When using a QAPF diagram, we ignore all minerals that aren't quartz, feldspar, or foidolite. Ignoring the mafic minerals in our gabbro leaves us with a very light-coloured rock!
Except, it's also important to note that plagioclase feldspars aren't always light! If you're interested in feldspar diversity, check out our Feldspar post, where we go into several different kinds of feldspar. The plagioclase feldspars found in gabbros can also be quite dark, so instead of looking for a striking black-and-white mosaic in your rock, you may need to keep an eye out for a rock that looks a little more...grey and plain.
Fig. 6:- Two gabbros with light (left) and dark (right) plagioclase feldspar makeups. Situations like these are why it's important to never rely on colour alone for mineral identification. (Psst. Click on the links to compare each sample under a microscope! You can see that despite their visual differences, they still both contain gabbro minerals!)
Instead of colour, we need to look at striations in the crystals to identify if they're plagioclase feldspars. Striations are small, thin, parallel grooves on the surface of a crystal, appearing almost like consistent scratch marks. Turning a crystal so that light reflects off of the surface can reveal these subtle marks. If you see them on a nice big blocky mineral with a porcelain-like lustre, you're probably looking at a plagioclase, regardless of whether it's got a light or dark colouration!
Fig. 7:- Striations on a crystal surface of hackmanite. Many minerals exhibit striations, such as plagioclase feldspar, pyrite, quartz, and sphalerite, which can help differentiate them from similar-looking minerals.
It may take learning a lot about other rocks and minerals to identify, but gabbro is a chilled-out rock that has a lot to say about the geologic history of the areas it's found in - and perhaps even a bit about philosophy and quantum physics, if we sit around long enough to listen!
