Hey folks! We’re back with, well, it was supposed to be one post - but it turns out there’s a character limit on tumblr and apparently we over-exceeded it by a lot to talk about rocks, haha! So, this next series of posts will be going over the three main rock types and how we can identify them!
As some of you probably know, in geology, there are three main rock types. They are all formed by different processes, have different defining characteristics, and different ways of classifying them. Fortunately, they all have very particular “looks” to them, features that you can use to separate them from each other easily!
For our third and final post in this series, we will be examining . . .
Fig. 1: An image of the underside of a piece of Brittle Hollow's crust - note the distinct foliation in places, alongside evidence of deformation through faulting, features not present in the basaltic protolith on the surface.
Metamorphic rocks occur when pre-existing rocks undergo physical and chemical changes through a process known as, you guessed it, metamorphism! Metamorphism involves the original rock - known as a protolith - being subjected to increased temperatures and/or pressures, which cause these textural and chemical changes. Metamorphics are super interesting as they provide us with insight into some of the most spectacular geological processes that shaped our planet - from huge mountain building events, known as orogenies, to the ever changing map of our land masses and oceans through tectonic plate activity.
One of the most interesting aspects of metamorphic rocks is that they started life as other rocks that have then been deformed over time, and as such they display a surprising variety of colours, textures and chemical compositions! Gneisses, for example, are what we would describe as “high-grade” metamorphic rocks, because they were formed under high pressures and temperatures, and have changed significantly from their protolithic form!
Fig. 2 & 3: Brittle Hollow is primarily made up of columnar basalt, which forms these dark, blocky structures that fracture as the magma cools quickly (top). Note the deformation and distinct foliation present in Fig. 3 - evidence that these rocks have undergone some kind of metamorphism.
These high-grade conditions then cause more drastic changes in the protolith’s composition - forming new textures and appearances! Similarly, lower-grade metamorphics cause less dramatic changes to the protolith - slate for example, looks very similar to its protolith, shale. As such, metamorphic rocks are not as easily pinned down by the amateur geologist (this instead comes with experience and time in the field!) as their igneous or sedimentary protoliths - however, some tell-tale diagnostic features include:-
Mineral alignment - known as foliation
Evidence of folding or faulting
Fig. 4: Some of the Quantum Moon landscape shows evidence of extreme metamorphism in its outcrops - note the distinct banding and deformation present in this outcrop next to Solanum.
It is also important to note that the terminology used when describing metamorphic rocks are often wide and all encompassing. For example, a schist is not a specific singular rock, but a broad umbrella term for a variety of metamorphic rocks that display similar characteristics due to having formed under similar conditions.
The ever changing geological landscapes of the Outer Wilds system means that there are plenty of opportunities to see metamorphic features scattered among the planets.
Fig. 5: An image of some of the banded gneisses present on Ash Twin, note the shiny lustre that differentiates these from their twin's banded sandstone formations!
As you can see, the rocks of our own solar system and that of Outer Wilds come in a huge variety of shapes, sizes, textures and colours, which can make identifying them seem a little daunting at first; but hopefully this beginner’s guide to the main three rock types gives you the first stepping stones (haha) into your own geological journey!
As always, if you have any questions, feel free to reach out to us and we will do our best to answer!
We’ll catch you in the next loop!
[Igneous] [Sedimentary] [Metamorphic]
