Howdy and happy first day of Pisces, readers! It’s also the first day of reading break for my fellow UVic students and I — indeed, a much needed breath of fresh air — and to celebrate, I spent my day designing jewellery, walking around my very windy neighbourhood, and revisiting my geology textbooks to ID some of my favourite rocks from my ever-growing beach stone collection. Behold, a post attributed to my love of basalts, and the fun but challenging task of identifying the minerals present in their porphyritic inclusions.
Ah, the art of note taking. For my other fellow visual learners out there, I hope you’ll agree in me saying that any topic can be easily absorbed when drawn/written in a fun way. I certainly enjoyed doodling my fair share of basalts and their funky inclusions, hand-picked from the beaches near my home here on southern Vancouver Island (unceeded Esquimalt & Songhees territories, whose stewardship and relations to the land, I recognize, continue to this day).
The beaches that line the Strait of Juan de Fuca harbour such a wide formational variety of igneous, sedimentary and metamorphic rocks, it’s breathtaking. My feet melt into the cobble-sized stones that coat the shores of Dallas Road beach, home to the infamous “Dallasite”, a breccia-jasper made up of quartz, basalt, epidote, pumpellyite and, occasionally, moss agate. It’s thought to be associated with the formation of pillow basalts: extrusive flow along the sea floor, where lava cools rapidly before bursting open again and flowing like stuffing lumping from a burst pillow; or at least, that’s my mental image of it. Pillow basalts typically have a fine-grained core with a glassy crust and radial jointing, well represented by the breccia shards seen in Dallasite (BC Rock Hound, 2020). Other common igneous stones include a variety of granites, granodiorites, diorites, andesites, basalts and gabbros — occasionally with inclusions of olivine. This observation is based on my day-to-day explorations of the rocks that hug the Dallas Road coastline; I’m eager to identify each and every one.
Something that has troubled me? Basalt ‘spots’ (porphyrys) and what makes their matrix colour so variable. Introductory geology classes will teach you to identify mafic igneous rocks from their texture and composition – “texture” referring to the mineral properties, such as grain size and colour. The rule of thumb in ID-ing igneous rocks is that lighter coloured igneous rocks are felsic, meaning they have higher concentrations of silica and feldspars, whereas mafic rocks contain darker coloured minerals, the dark greys and browns attributable to the higher concentrations of pyroxenes (a chain silicate with high amounts of Ca, Na, Mg, Fe and Al). An easier way to think of it is ‘Ma-f-ic’, where the Ma = magnesium and the F = iron. Same goes for breaking down ‘Fel-sic’, where Fel = feldspar, and sic = silica. Anyway, what troubles me is the lighter coloured basalts, and their polka-dot looking inclusions. How can basalt, a mafic igneous rock, be the colour of rhyolite, a light coloured felsic stone similar to granite, or an iron-rich sandstone?
I’ve learnt that a class of lighter-coloured basalts are indeed referred to as the ‘Leucobasalts’, appearing lighter in colour from their high plagioclase (feldspar) content and lower pyroxene concentrations. Mystery solved.
The white bubble features? Amygdules. New word for me too. Amygdaloidal basalts occur when gas-filled cavities in cooling basalts are filled with minerals — typically feldspars in the amygdaloidal basalts we see here on Vancouver Island — where the largest amygdules are called phenocrysts. Yes, this is where the term porphyritic comes in: porphyritic basalts meaning those with crystal inclusions. They’re probably the coolest of the basalts (and arguably one of the most important types of igneous rock) because they can pick up unmelted fragments of the mantle, called xenoliths, which provide geologists the only direct observational method to study mantle composition (Haynes, 2017).
Finally, I had to connect the dots with the highly-regarded Vancouver Island Flowerstone. I’ve learnt through the Vancouver Island Rock-Hounding Facebook Group (a fantastic group of people that I highly recommend you become a part of if you’re interested in learning about rocks) that flowerstone is typically a basalt, its ‘flowers’ a porphyry of feldspar crystals. The flower shape is highly speculated, but thought to be associated with the electromagnetic attraction between feldspar crystals. I have a beautiful, bagel-sized porphyry basalt specimen (commonly referred to as a ‘Rice Stone’) from Looper Canyon, looking exactly as its name entails, with big (1-2cm) strands of feldspar porphyrys decorating the pyroxene-rich, dark-coloured matrix, though without any strands of “rice” coming together to take the shape of a flower. A theory as to what causes these flower shapes to arise? The density of the feldspar crystals. My ‘Rice Stone’ specimen, though highly weathered from the freshwater environment it was retrieved from, appears to have lighter/more faded feldspar concentrations in its porphyrys. Perhaps it’s when the feldspar inclusions reach a certain density that they start behaving in a flowerlike-fashion, and creating such a unique and unquestionably beautiful pattern. The more truly being the merrier.
This introductory overview on common beachstone basalts of southern Vancouver Island is brought to you in part by my burning interest in reading geology textbooks, my obsessive nature of absorbing every last detail of my EOS and GEOG courses, and pure internet scouring (mostly to fill in the missing details, like feldspar fills in the gas filled cavities of molten basalt).
Thanks for reading,