We here at Backyard Geology have been enjoying the geology pun life, so we decided to keep that going with this week’s post about more fabulous metamorphic rocks! In last week’s post, Kristen discussed slate and phyllite; two types of metamorphic rocks that form at lower temperatures and pressures. This week I decided to tackle the two metamorphic rocks that form at higher pressures and temperatures, or the highest metamorphic “grades”; schist and gneiss.
What is Schist?
A schist is a specific type of metamorphic rock that shows distinct, visible (to the unaided eye) layering of platy minerals (e.g., micas) that are aligned parallel to one another1. We call this specific type of layering “foliation”. Outside of the alignment of platy minerals, a schist can have numerous minerals within it depending on the minerals present in the original rock (such as the sedimentary rock shale) before metamorphism.
Some of the minerals that can be found in a schist are:
- Quartz – very common mineral in schist, almost always exists.
Geologists will often name schists based on the minerals that are present within them, as each of these minerals provide important clues as to what the rock was before it was metamorphosed. For example, a schist with garnet, staurolite, and some combination of mica would be called a garnet – staurolite – mica schist. Based on the elements in these minerals, geologists would deduce this rock formed by metamorphosing a shale. Staurolite, garnet, and mica have a large amount of Aluminum in them and shales are composed of Aluminum-rich minerals (clays) whereas a rock such as a basalt lacks minerals very rich in Aluminum like clays.
What is Gneiss?
Gneiss is considered the highest grade of metamorphic rocks (things can get awesomely weird depending on how high the temperature gets before the rock eventually melts). It still has a foliation, but at this high grade not only is the grain size larger, but the rock has distinct banding of platy (micas) or elongated minerals (amphiboles or pyroxenes) and blocky minerals (feldspar and quartz) so the foliation is often referred to as compositional banding3. Some of the common minerals found in gneisses are:
- Amphibole – likely hornblende
- Micas – biotite predominately
We can also use the minerals present in our gneiss to determine what rock it was before metamorphism. Gneisses are often named based on what they formed from; a gneiss that forms from a basalt would become an amphibole-rich gneiss. Because geologists love different names, we would call this rock an amphibolite and that name would tell us the rock formed from a basalt rather than a shale. You will see the figure below in which our gneiss has been classified as a biotite gneiss and it formed from a shale originally.
Fun Fact: If the temperature gets hot enough, our gneiss will partially melt and become what is called a migmatite! These rocks can sometimes be really difficult to tell apart from gneisses in the field. The limits of metamorphism are actually still highly debated in the geology community.
Where on the Earth do Schists and Gneisses Form?
So we have talked about the fact that, as we increase temperature and pressure, we will create schists and gneiss, but how do we actually achieve this? How and where do schists and gneisses actually form? Metamorphism in itself is a very complex process and there are many facets of metamorphic processes that are still heavily debated within the geologic community today. There are several types of metamorphism and there are typically many components involved, some that are easy to tease out and others that are heavily debated; so I will keep this as simple as possible (because I could probably go on for many pages about metamorphism..teehee). For the purposes of this post we will focus on regional metamorphism and how it creates our gradient of foliated rocks.
Regional metamorphism occurs when two tectonic plates collide (convergent boundary) with one another where mountains (or even volcanoes!) are actively being formed. This type of metamorphism can take place over kilometers of areas and is responsible for creating our gradient of metamorphic rocks (slate to gneiss). The figure below shows how the collision process of two plates works.
In this regional metamorphic setting temperature, pressure, and fluids (water + ions/compounds that can be transported by water) play an important role working together to create our metamorphic rocks. There is a lot of pressure right where the two plates are colliding and it lessens away from that zone. The temperature along this zone is varying and thus we can end up with a special type of schist that forms under high pressure, but lower temperature. We call these blueschists – yes, they are actually the color blue and they do have a foliation. This schist is dominated by a blue amphibole called glaucophane. It is actually quite rare to see these on the surface of the Earth because of the tendency for them to be destroyed as the collision of the two plates continues. Within the overriding plate is where we typically see our gradient of rocks form as the pressure lessens a) as we move away from the collisional zone and/or b) as we move up within the crust. The temperature is also less as we move further toward the surface, so it is not all that surprising to see our lower grade metamorphic rocks closer to the surface. So it is within the intermediate depth of the crust in our overriding plate where we see our schists and gneisses forming from moderate amounts of heat and pressure.
We can get a variation in the minerals we see in our different foliated rocks not only because of the pressure and temperature, but due to the presence of fluids that migrate through fractures or are formed through the dehydration of hydrated minerals (minerals that already have water in them). These fluids paired with temperature create a perfect environment for new minerals to form in our rocks, leading to some very neat mineral treasures! By now you might be wondering how we ever see these rocks since they form below the surface (and that dear reader, is a totally valid question). If you got the chance to read my previous post about all things granite, you would recall we can see rocks formed below the surface in a few different ways: erosional processes removing overlying rocks, subsequent mountain forming processes that uplift regions exposing rocks, or a combination of the two (which is generally the case). Over time we get to see our beautiful foliated metamorphic rocks in all of their glory!
On a large scale we can get entire regions that are made up of these different foliated metamorphic rocks. If we want to take this a step further, regions can undergo multiple continental collisional events and metamorphose our pre-existing metamorphic rocks. We see this in the Appalachian Mountains in the Eastern U.S. which were formed through at least three different collision events – crazy awesome, I know!
1United States Geological Survey, Schist, 2018. https://www.usgs.gov/media/images/schist. Accessed 7/7/2021.
2Smith, G., and Pun, A., 2010, How Does the Earth Work; Pearson Education.
3United States Geological Survey, Gneiss, https://www.usgs.gov/media/images/schist. Accessed on 7/7/2021.
4Pachuck, K., 2018, Adapted from Steven Earle, Chapter 10. Metamorphism & Metamorphic Rocks. Physical Geology; First University of Saskatchewan Edition. https://openpress.usask.ca/app/uploads/sites/29/2017/07/Chapter-10_JA1019.pdf Accessed 7/8/2021.