What are Metamorphic Rocks – Foliated and Non-foliated
The common characteristic for metamorphic rocks is they are formed by existing igneous or sedimentary rock that has undergone a chemical or physical change. This change is due to great amounts of heat and pressure which make sedimentary and igneous rocks become a metamorphic rock. This heat and pressure changes the rock into a more denser form.
Metamorphic rocks are the most resistant to weathering because they are more dense, making them difficult to erode or break down.
Keep in mind metamorphic rocks are not igneous or sedimentary rocks that are entirely melted, this by definition would be an igneous rock. Though sometimes they may be partially melted, metamorphic rocks undergo a great amount of pressure and heat which changes its composition without breaking it down to magma.
Some changes that can occur during the creation of metamorphic rocks are the introduction of minerals or rearrangement of minerals during heat and pressurization. Additionally, chemical reactions from other minerals and fluids that enter into the rock can be a part of that change as well.
Fluids play a pretty big role in the creation of metamorphic rocks. Sedimentary rocks will hold the most fluids, but most rocks do hold some kind of water. This fluid can help with the mixing of minerals locally or even transferring minerals too far off places.
Identifying Metamorphic Rocks
Metamorphic rocks are identified by their denser, stronger form, which was caused by intense heat and pressure but not enough to melt the rock into magma. The minerals that are in the rock can show some very colorful variations, this at times can be coloring with banding or layering.
If you are out in nature and stumble upon some rocks, it can be a little difficult at times to determine if something is metamorphic, igneous, or sedimentary. For metamorphic rocks, look for that layering in the rock. Be observant, as sedimentary rocks can also have layering by nature.
Metamorphic rocks will be harder than sedimentary rocks which will help with identification. Additionally, note what environment you are in. If you are in mountainous regions, you will likely be finding more metamorphic rocks. If you are in desert regions, you might be finding more sedimentary rocks.
Types of Metamorphic Rocks
There are two types of metamorphic rocks. Foliated metamorphic rocks which have layers and banding, and Non-foliated metamorphic rocks which do not. Some common types of metamorphic rocks that can be found in these two categories are; amphibolite, argillite, cataclasite, eclogite, gneiss, greenstone, hornfels, marble, migmatite, mylonite, phyllite, and schist
Foliated Metamorphic Rocks
Foliated metamorphic rocks are metamorphic rocks that are banded or layered. Foliation occurs due to the force of heat and pressure given on a single plane of axis or in one direction. Re-crystallization of the minerals in the rock are elongated along that axis.
The foliated metamorphic rock will show the banding and layering of the different mineral colors that exists in the rock. Some foliated metamorphic rock types include schist, gneiss, slate, and phyllite.
Non-foliated Metamorphic Rocks
Non-foliated metamorphic rocks are metamorphic rocks that do not have any layering or banding. The heat and pressure was relatively uniform from all sides. Some non-foliated rocks will simply lack minerals that give it the ability to have any layering or banding.
Though banding or layering is not present, non-foliated metamorphic rocks have some very nice patterns formed from the minerals contained within them. Some non-foliated metamorphic rock types include the popular marble, hornfels, quartzite, and amphibolite.
