All rocks on the planet originally derived from igneous rocks. As we know there are a few select silicate minerals that are common in igneous rocks. (quartz, feldspar, augite, hornblende, mica, olivine).
There are however a vast majority of uncommon silicate minerals (thousands of them). Where did they come from?
Metamorphism involves changing rocks in the SOLID state through extremely high temperatures and/or pressures.
The extremely high pressures literally break atomic bonds and force them into NEW, more stable minerals. The chemical structure of the parent mineral can be converted into a totally new minerals which are stable at the higher pressures.
Metamorphic rocks are also more DENSE than their parent rock. Pressure and temperature force air pockets and water OUT of the parent, increasing the density of the metamorphic rock.
Metamorphic rocks are thus derivatives of their parent (the rock before metamorphism). The metamorphic rock minerals are made from the starting materials present from the parent rock—new elements cannot be added to the metamorphic rock. They are simply made from the ingredients present in the parent.
Parent Rock: Mineralogy of the metamorphic rock is controlled by the mineralogy of the parent. Rarely new elements are introduced (just rearrange the atoms that are there currently, or squeeze ions out that happen to be dissolved in water).
Temperature: Recall that every mineral is stable at a certain temperature (Bowen’s reaction series as an example). Minerals unstable at high temperature will form new ones. (Clay is unstable at high temps, but micas are).
Pressure: Two types, confining (applied equally in all directions) and directed (pressure in a given direction forcing minerals to align perpendicular to the directed pressure). Directed pressure forms a foliated texture.
Confining pressure is pressure applied equally in all directions. Directed pressure is pressure applied in a preferential direction. Directed pressure involves minerals aligning PERPENDICULAR to the direction of the pressure.
High temperatures in ‘contact’ with surrounding rock tends to ‘bake’ the rock. Yields non-foliated textures (Appears as one mineral of equidimensional crystals). Zone of metamorphism is usually skinny and small forming a band around the intrusive igneous body.
Non-Foliated texture: form under higher temperature regimes (not so much ‘pressure’ related). Smaller crystals in the parent rock simply recrystallize into larger ones without being melted. They are more dense than the parent rock. This is an example of marble (parent is limestone with the dominant mineral calcite). Because calcite is the dominant mineral in these rocks, they will react to dilute hydrochloric acid.
Quartzite also comes in a variety of colors, depending on the parent rock (red sandstone, white sandstone etc.). Quartz is the dominant mineral in quartzite.
Remember, quartzite was originally a sandstone that was ‘baked by higher temperatures and has a non-foliated texture. Below check out the rocks in thin section (under the microscope). The left is a sandstone, note the dark spaces between the grains. The right is quartzite. Look at how closely packed the grains are. Removing the ‘spaces’ between the grains makes metamorphic rocks DENSER than their parents.
Metamorphism caused by high temperatures and pressures (directed and confining) over large areas by ‘mountain building’ (orogenic) processes. Squeezing rocks in a particular direction will result in the minerals aligning perpendicular to that force. Most metamorphic rocks (quantitatively) are produced by tectonism and are located in the earth’s major mountain belts.
Regional metamorphism produces foliated texture. The more heat and pressure, the more ‘foliated’ the rocks appear (easier it is to see the minerals aligned.
Foliated Rocks are rocks that were metamorphosed by regional processes—high heat and temperature. By looking at the degree of foliation, you can tell how much metamorphism took place.
This specimen of slate formed at lower temperatures and pressures. Minerals are just beginning to align perpendicular to the direction of the pressure. Slate can come in a variety of colors.
Slate (right): low grade regional metamorphism showing foliated texture with parent of shale (left).
Add some more pressure to slate (right) and end up with phyllite (left).
Increasing the temperatures and pressures would force the minerals to recrystallize and align perpendicular to the pressure. Mica crystals begin to form and they give PHYLLITE its sheen (can’t really see it here).
Increasing the pressures and temperatures even further causes the crystals to grow even larger and align even more. SCHIST
An extremely high grade metamorphic rock that has undergone very high temperatures and pressures but didn’t melt is a rock called GNEISS. If enough temperatures and pressures are applied the rocks behave plastically (recall the mantle) and can actually fold during mountain building events. Gneiss has large minerals that have segregated into layers (gives its characteristic zebra pattern).
Different foliated metamorphic rocks will form under different temperature and pressure regimes. Starting with shale (sedimentary rock) as the parent, we can crank up the temperatures and pressures to form slate, phyllite, schist then gneiss. To form gneiss, the rock must pass through the other stages first. Note that the more pressure and higher the temperature, the larger the crystal size and the more ‘foliated’ the texture.
Index Minerals: The presence of a given mineral can indicate the maximum T and P the rock experienced. We can find a specific mineral in a metamorphic rock—knowing properties about those minerals, we can calculate the maximum temperatures and pressures the rock was subjected to. For example, we will see minerals like garnet and biotite mica in schist. If we know the range of temperatures and pressures these minerals are STABLE at, the temperature and pressure the rock formed under is where their ranges overlap.
Garnet Schist: Foliated rocks form under high pressures (accompanied by high temperatures). At what temperature did this rock form?
Compressional Stress: Results in folding of layers of rocks indicating plastic behavior. Strain produces structures called anticlines and synclines. Anticlines look like an A in cross section and synclines have a U shape. Usually anticlines and synclines (folds) form together. Rocks AT THE EARTH’s surface are under such HIGH and CONSISTENT pressures that they can even behave PLASTICALLY and fold like taffy (note majority of crust rocks behave elastically).
The axis of the fold (either anticline or syncline) runs parallel with the fold. In anticlines the limbs of the fold dip down away from the fold axis. In synclines the limbs dip up away from the fold axis. Folding occurs over a long period of time with slow and consistent compressional stresses.
Strike and Dip: We can measure the degree of dipping beds from the horizontal 0 degrees (90 degree dip occurs when the rock layers are vertical). If you pour water down dipping beds that is the DIP measurement. Perpendicular to that is the STRIKE (compass direction). Both strike and dip tell us the ‘attitude’ of rock layers—(their orientation with respect to compass direction).
Strike and dip of anticlines and synclines. Note: erosion at the surface will expose different layers of sedimentary rocks. In an ANTICLINE, what is the relative ages of rocks along the axis? In a syncline?
Overturned folds may occur when the anticline is overturned.
Anticlines and synclines can PLUNGE into the earth resulting in unequal weathering of these rock layers at the surface. We can tell what rock layers are doing INSIDE the crust, based on these weathering patterns and the ‘attitude’ measurements.
Folding results when rocks behave plastically, but if near the surface, rocks will behave like elastic, they will FAIL under stress. Failure results in fracture (faults).