Earth Science Extras
by Russ Colson
Kyanite crystals in a metamorphic rock tell a significant story about the temperature-pressure conditions of metamorphism.
This exercise is intended to provide you with practice and insight into
The emphasis is not on detailed and complicated interpretations of each rock, something that often requires more information from regional context, microscopic study, chemical analysis, mineralogical analysis, stratigraphic relationships, and so on than you can get from a hand sample (or the pictures in this lesson), but rather on identifying the basic stories told by the textures and minerals of the rock. For metamorphic rocks in this lesson, this mostly relates to
The story of temperature and pressure is told primarily by those characteristics of the rock that change (metamorphose) during exposure to high temperature and pressure events. Those two are 1) changes in rock textures and 2) changes in rock mineralogy.
Texture: In general, crystal size increases during metamorphism, going from smaller crystals, often too small to see, to larger crystals. Under regional metamorphism where temperature and pressure are both elevated in roughly equal measure, and with differential stress, rocks tend to develop foliation--that is, an orientiation of crystals with long axes perpendicular to the principle stress direction. In extreme cases, foliation develops into banding in which dark and light minerals segregate into distinct bands. Foliation won't develop in rocks not subjected to differential stress or in rocks lacking mica that contain only minerals that lack an assymetrical shape such as quartz or calcite.
Mineralogy: Reactions under metamorphic conditions tend to produce new crystalline phases that are more stable under the conditions of metamorphism (and are often slow to 'back react' to their former state when metamorphic conditions end). Higher temperature conditions favor minerals with high entropy and higher pressure conditions favor minerals with higher density.
Chemical composition: The detailed chemical composition of minerals is also a function of temperature and pressure. One can use chemistry of mineral pairs as "geothermometers' and "geobarometers' of the metamorphic conditions. In this lesson, looking only at hand samples without chemical analysis of mineral composition, we won't be able to apply this tool to our story-telling.
In these exercises, you can often figure out the 'answer' through context and simple clues, but the hope is that you pay attention to all of the observations and interpretations that are listed in the answers (and feedback) so as to hone your ablity to notice those features and to practice your ability to make interpretations of those features. There are quite a few "options" in the multiple choice answers, which hopefully encourages you to slow down and think about each rock in particular, what its features are and what stories those features tell. This lesson is not intended to be zipped through in an hour.
This lesson does not provide an introduction to metamorphic rocks. It is expected that you already know the basics of rock types and how to recognize textures, minerals, and tell stories from them.
The pictures provided are intended to approximate what you can see in an actual hand sample, where sometimes key features like cleavage, crystal form, etc are not quite visible. Of course, you can rotate a real sample in the light and get more information based on how what you see changes as you turn it, seeing glints off cleavage planes and getting a better sense of its three-dimensional character. However, knowing what to look for can often help you spot important features that are present in a hand sample, and this lesson provides some prompts to help with noticing some of the important features that might help you know what to look for when looking at a real hand sample.
Although naming the rocks is almost an incidental part of this lesson (given less importance than noticing and interpreting features), here are some classification charts to help you get oriented in naming the rocks.
The chart below summarizes some of the rock names we will use in this lesson, a long with relative metamorphic grade of different rock types.
Normal Geothermal Gradient, like in the roots of mountain ranges, both temperature and pressure increase roughly in line with the average geothermal gradient, sometimes called regional metamorphism
High Geothermal Gradient, like rock at a shallow depth in contact with a magma body, temperature increases more than pressure, sometimes called contact metamorphism
Low Geothermal Gradient: like in a slab of subducting crust, pressure increases more than temperature, may lead to blueschist facies metamorphism
Read each option carefully (yes, this will take time, that's the point). Some answers are very similar with only slight differences. Read and think about the feedback to the rock-story interpretations.
last updated 11/14/2022. Text and pictures are the property of Russ Colson.