Welcome back to Fossils on Friday, where we continue with our fossil-free theme of metamorphic rocks! Last week we learnt about what metamorphic rocks are – igneous or sedimentary rocks that underwent structural changes to create a new rock.
There are actually several different grades of metamorphic rock, from low-grade to high-grade. This reflects the amount of metamorphism that the rock has gone through. It can also reflect the depth, pressure amount, or heat level that the rock has gone through during the metamorphic process. The higher the grade of metamorphization, the more crystallised and unrecognisable the rock becomes.
A good rock to visualise this with is shale. Shale is a sedimentary mudstone, which gets deposited in many thin layers, many of them containing fossils. The layers are very fine and fragile, often easily broken up with your hands.
If shale undergoes low-grade metamorphism, it will transform into slate. This is a harder rock, and the mudstone has begun to crystallize, but it is still in many layers. These layers can be split up and used for many things, including roof tiles!
If shale or slate undergoes higher-grade metamorphism, it will form phyllite. This is a shiny and flaky rock. Here the slate has begun to crystallise in-between the layers. This forms the shiny layers, and leaves the phyllite with a very flaky texture.
The next level of shale metamorphization is schist. Schist is still shiny, although not as much as phyllite. It is also harder, as the crystals have begun to spread throughout the whole rock, and the layers have become less defined. All fossils have been destroyed by this point, very few would have made it past slate.
The final level of shale metamorphism is Gneiss (pronounced ‘nice’). Gneiss is a much harder rock, being completely crystallised. It is shinier than shale and slate, but not as much as phyllite or schist. Most layers have gone, although they may have created bands in the new rock.