~A Very Simple Metamorphic Rock Classification~
Metamorphism is the alteration of a preexisting rock (the parent rock) due to heat and pressure caused by burial in the earth. The parent rock must adapt to the new conditions and it does so by changing mineral composition and texture. These rocks with new texture and composition are metamorphic rocks.
Metamorphic Textures and Compositions
Metamorphic Rock Texture
Metamorphic textures are of two types, 1) granular (also non-foliated) where the grains are equidimensional, and 2) foliated where the minerals are layered.
- Granular rocks are usually uniform in composition; they are all one mineral (e.g.Quartz = Quartzite, Calcite = Marble; exception = Hornfels) so the minerals do not segregate into layers (right).
- Foliated textures result when the new metamorphic minerals (many of which are platy micas such as Biotite and Muscovite) line up producing a distinct layering in the rock. The layering produces three distinctly different looking rocks, those with slaty cleavage (e.g. Slate), schistosity (e.g. Schist), and mineral banding (Gneiss Texture). Metamorphic rocks are identified on these textures (click image for large version, or go to Metamorphic Texture for more details).
The table below shows the relationships between texture names and rock names.
Metamorphic Rock Composition
Metamorphism is the alteration of a (parent) rock into a metamorphic rock. The simplest way to examine compositional changes during metamorphism is to start with the three end products of the Simple Model For Sedimentary Rocks, shown in the illustration below (click picture for full model). These weathering products are also the most common sedimentary rocks and form a very large percentage of the metamorphic rocks we see.
These three end products (Attractors) in the sedimentary model (Quartz Sandstone, Shale, and Limestone) are the three parent rocks of metamorphism. Compositionally simple sedimentary rocks, such as Quartz Sandstone and Limestone (Calcite) do not change composition with metamorphism - their chemistry is too simple. Quartz remains Quartz, and Calcite remains Calcite. The rocks recrystallize, however, forming granular rocks with fused grains; the rocks Quartzite and marble. It does not matter by what process you metamorphose Quartz Sandstone and Limestone, their composition does not change, and they produce the same kind of metamorphic rock.
Rocks with a more complex chemical composition, such as Shale (the mineral Clay), however, undergo many compositional, and textural changes. Shale produces the greatest diversity of metamorphic rocks. We can see this in the chart below where sedimentary Shale turns into Slate, then Phyllite, then Schist, then Gneiss.
Rocks are identified by making a series of decisions about their properties, such as texture, composition, hardness, etc. The Key To Common Metamorphic Rocks allows identification of a rock based on its physical properties. We are able to do this because the properties do not overlap completely.
1. Barrovian Metamorphism
Barrovian metamorphism takes place during mountain building Events when metamorphism produces both textural changes and compositional changes in the rock. The compositional changes take place because sedimentary minerals are stable only at the earth's surface. As they become buried the sedimentary minerals transform into new metamorphic minerals. At the upper end of metamorphism the rock melts and becomes igneous. Sedimentary rocks are buried, squeezed, and heated. Sometimes the metamorphism occurs due to depth, but often it is also associated with major igneous intrusions that supply most of the heat. For example, the small sketch to the right shows a collision between two continents.The texture of the rock also changes during metamorphism and the rocks become foliated.
Thus, metamorphic rocks not only tell us the kind of metamorphism, they are also a measure of the intensity of metamorphism. The closer we get to the source of heat and pressure the more altered the rock becomes.
2) Contact Metamorphism
(click image for enlargement)
Contact metamorphism occurs in the "country rock" (the rock intruded by and surrounding an igneous intrusion). Rocks are " baked" from heat escaping from intrusives, often enhanced by hot fluids. Unlike Barrovian metamorphism, pressure is not a significant factor in the contact process. Without directed pressure foliation does not develop. So, the Clay (Shale), which in Barrovian metamorphism develops a strong foliation (slate, Phyllite, schist, Gneiss), in contact metamorphism develops a granular texture (the rock Hornfels).
Contact metamorphism can occur next to any igneous intrusion, although it is most easily seen next to smaller intrusions. The intensity of metamorphism decreases with distance from the intrusion, until at some distance away the rock is unaltered country rock. However, large intrusions such as batholiths usually alter the country rock so much that other styles of metamorphism override the contact metamorphism.
The Relationships Between Rock and Process
There is no simple and conistent relationship between the processes of metamorphism and the rocks produced. Some rocks are confined to one kind of metamorphic process, but other rocks result from several types of metamorphism. These relationships summarized in the table to the right.
Observe that Hornfels, Quartzite, and marble are contact metamorphic rocks, but that Hornfels is the only strictly contact metamorphic rock. Hornfels, slate, Phyllite, schist, and Gneiss can indicate the type of metamorphism that created them. Quartzite and marble can be formed by both contact and Barrovian metamorphism, therefore neither of these can indicate the type of metamorphism that formed them.
Contributed by Lynn Fichter