Metamorphic Environments

Limestone is tricky when it changes. If it is completely pure and undergoes contact metamorphism, you get nice white marble. If there are impurities, other reactions will occur. Silica as a contaminant leads to wollastonite, which has calcium silicate in it. Aluminum as a contaminant will lead to anorthite in the rock, which is calcium aluminum silicate.

Finally, dolomite, which is a calcium magnesium carbonate, provides a metamorphic rock that has pure calcite or calcium carbonate in it but also has periclase, which is magnesium oxide. Mix this with silica and then you get calcite and magnesium silicate, which is called forsterite.

METAMORPHIC ENVIRONMENTS

You need to understand that the temperature and pressure will vary with location and circumstances for any rock undergoing a metamorphic change. There will be a magic combination of these that will create the different things you see. Geologists have created a pressure temperature grid that shows the relationship between different pressures and temperatures involved in changing rock. This is called the P-T diagram. It looks complicated but it provides a range of temperatures and pressures necessary to create the different rock types.

A geologist named George Barrow lived in the 1800s and mapped out regional metamorphic zones and designed the Barrovian sequence that named rocks in order of increasing metamorphism. It starts with the chemical chlorite seen in slates and phyllites. Then it goes to biotite, also seen in phyllites but also in schists. Next is garnet, seen only in schists. The ones with higher grades of metamorphism are staurolite and kyanite, seen in schists. The highest grade is called sillimanite, seen in schists and gneisses. This fits well with where he lived in Scotland.

Green coloration or greenschist rocks are seen in low pressure and temperature situations. Chlorite, epidote, and serpentine are all green so the schist will be green. These rocks are called schists because there is muscovite mica in them.

There are many environments that nuance the rock types coming out of metamorphism. We talked about some of these in the beginning but now that you know these rocks

better, you should be able to see what happens with greater clarity. These are a few examples of environments you will see having metamorphism of different types:

• Burial metamorphism – this is an environment where rocks are deep within the earth. Sedimentary rocks often get buried below other sediment so that clay becomes a mineral known as illite. Sandstone becomes quartzite under such high pressure situations.

• Contact metamorphism – you remember this as being more heat-related than pressure-related. You will get different things, depending on how much pressure is added. Hornfels comes with low pressure but if you add pressure, you can get amphibolite, greenschist, or granulite. The parent rock matters too. If you start with basaltic rock like shale, you'll get hornfels, but if you start with sandstone, you'll get quartzite. Limestone, as you know, becomes marble.

• Regional metamorphism – this you remember is when pressure and temperature both act over larger areas. Mountain ranges are good spots for this because continental crusts are converging. Obviously, this will vary over time and distance from the converging plates. Sediment becomes slate that becomes phyllite, schist, gneiss, migmatite, and then granite. This is the sequence to remember when it comes to regional pressure and temperature variations in continental plate convergent zones.

• Subduction zone metamorphism – this is also regional but occurs in subduction regions. The descending crust has an increasing temperature as it sinks, but the pressure factor is greater than the temperature factor. Blueschist is a common rock that comes out of this; it contains a blue mineral called glaucophane. Figure 46 is what this blueschist glaucophane looks like:

Figure 46.

These conditions also create epidote, almandine, chlorite, garnet, and jadeite. These are all metamorphic rocks occurring when pressure exceeds temperature in subduction areas. Very deep areas where pressure and temperature are the greatest lead to shearing of the minerals; this gives rise to things like tiger's eye.

• Shock metamorphism – these are rare circumstances you'll get when a meteorite hits earth. The pressure is very high and delivered quickly so you get interesting things like shatter cones and unusual things representing the shattering and deforming of crystals. Glassy material is a common complication of shock metamorphism. Tektites are interesting pieces of glass ejected after an impact.

They look like volcanic glass but are chemically different. They also are round and look like large pebbles. Rapid heating and cooling are the things necessary to make glassy objects in geology.