6.4 Metamorphic Grade

Metamorphic Rock Grade

Metamorphic grade describes the degree to which a rock has changed during metamorphism. The grade of a metamorphic rock reflects the intensity of metamorphism that is experienced. A low-grade rock has changed little from its parent rock, whereas a mid-grade rock retains only a few aspects of the parent rock. A high-grade metamorphic rock has changed so significantly that almost no aspects of the original parent rock remain. Higher grade rocks tend to be coarser-grained.

Take a close look at the videos of the rocks below. Within these videos, there is a parent sedimentary rock that all of these rocks originally began as.

In the exercise below, you will be asked to order the rocks from the videos above from parent rock to the rock with the highest grade metamorphism. Do not worry too much about rock color – focus instead on which rock demonstrates stronger or weaker foliation and shifts in mineralogy and how this is significantly different from what you have ID’d as the parent rock.

Parent Rock Identification

As you might have noticed in the last exercise, sometimes the parent rock can resemble the metamorphic rock. Sometimes it looks nothing like the parent rock! Based on what you discovered in the previous exercise, carefully look at the metamorphic rocks in the videos below, and then determine what their likely parent rock was in the exercise beneath it.

Possible Parent Rocks

Shale

Shale interactive model. Shale is gray and has some layering. It is a fine-grained rock
Figure 6.4.1 Shale. Click this image to go to an interactive model by Dr. Parvinder Sethi CC-BY.

Sandstone

Sandstone Interactive Model. Sandstone is light tan and has sand-sized grains
Figure 6.4.2 Sandstone. Click this image to go to an interactive model by Sara Carena CC-BY.

 

Limestone

Limestone outcrop interactive model. Limestone is gray and usually smooth but fizzes with acid
Figure 6.4.3 Outcrop of gray limestone that shows weathering. Click on this image to go to a 3D interactive model by Théobald GUFFON (CC BY-NC-SA)

BE SURE THAT YOU CLICK ON THESE MODELS AND INTERACT WITH THEM BEFORE YOU ATTEMPT THE PROBLEM!

Sample 1: Slate
Slate interactive model. Slate is often gray but can be many different colors, is smooth and finely foliated.
Figure 6.4.4 Slate. Click on this image to go to a 3D interactive model by rocksandminerals CC BY.

 

Sample 2: Quartzite
Quartzite interactive model. Quartzite is crystalline and non-foliated with large amounts of silica
Figure 6.4.5 Quartzite. Click on this image to go to a 3D interactive model by rocksandminerals CC BY.
Sample 3: Phyllite
Phyllite interactive model. Phyllite can be many colors. It is finely foliated and shimmers with mica minerals.
Figure 6.4.6 Phyllite. Click on this image to go to a 3D interactive model by Dr. Parvinder Sethi CC BY.
Sample 4: Marble
Marble Interactive Model. Marble is nonfoliated and crystalline with calcite, which often fizzes with acid.
Figure 6.4.7 Marble. Click on this image to go to a 3D interactive model by EDUROCK – EDUCATIONAL VIRTUAL ROCK COLLECTION CC BY.
Sample 5: Gneiss
Gneiss Interactive Model. Gneiss has lineation/banding of black and white minerals and is heavily foliated.
Figure 6.4.8 Gneiss. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

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