Metamorphic Rocks and Crustal DeformationMetamorphismAll 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?Summary of Metamorphism• 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.Agents of MetamorphismParent 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.Fluids: If water is present in between the pore spaces of minerals in the parent rock, it may be squeezed out, carrying with it dissolved ions that can be used to make another mineral elsewhere.Types of MetamorphismCONTACT METAMORPHISM • 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. o 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. o 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. Removing the ‘spaces’ between the grains makes metamorphic rocks DENSER than their parents.REGIONAL METAMORPHISM • 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. • 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. o Slate is a low grade regional metamorphism showing foliated texture with parent of shale. o Add some more pressure to slate and end up with phyllite. 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. o Increasing the pressures and temperatures even further causes the crystals to grow even larger and align even more. SCHIST. o 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). • 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.Metamorphic Rocks Summary•Remember slate, phyllite, schist and gneiss all have foliated texture. The larger the minerals, the higherthe temperatures and pressures (generally).•Foliated rocks can tell you the direction of pressure and the degree of metamorphism (or foliation) can tellyou about the temperatures and pressures.
•Regional metamorphism is large scale and results from tectonism.•The parent rock for slate, phyllite, schist and gneiss is shale (sedimentary rock), quartzite is quartzsandstone and marble is limestone (calcite).•Non-foliated rocks form by ‘baking’ (primarily temperatures) from igneous intrusions in country rock.•All metamorphic rocks are denser than their parents; the spaces between grains and water are removed.Crustal DeformationTectonics produces mountains and metamorphic rocks but also deforms the crust forming geologic structures.We will now look at how the crust becomes deformed as the result of tectonic activity.Stress and StrainDeformation causes a change in volume or shape of a rock body.STRESS is the amount of force acting on a rock to change its shape or volume and the STRAIN is the actual change in shape or volume caused by the force (stress).The more confining pressure (with increase in depth) the more PLASTIC the rock behaves.Near the surface rocks behave elastically and will be have like ELASTIC causing faulting.StressConfining Pressure o Uniform in all directions (i.e. rocks buried deep in the crust). o Increases with depth.StressStress Applied Unequally o Compressional stress o Tensional stress o Shear stressCompressional StresAnticlines and Synclines:o 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).o 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.o 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).o 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?o Overturned folds may occur when the anticline is overturned.o 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.Faults:Folding results when rocks behave plastically, but if near the surface, rocks will behave like elastic, they willFAIL under stress. Failure results in fracture (faults).Types of Faults•Faults can be divided into several different types depending on the direction of relative displacement.There are 2 main categories. –Dip-slip faults-where the displacement is vertical –Strike-slip faults-where the displacement is horizontal.•Relative displacement is largely a function of the type of tectonic stress the rock is under.Normal Faults o Occurs when the headwall drops relative to the footwall. Result of TENSION. o Blocks that are uplifted are called horsts and those thrown down form a valley like structure called a graben. Fault Block Mountains.Reverse Faults o Reverse Fault occurs when the headwall moves up relative to the footwall. Result of COMPRESSION. o Commonly large scale overthrust faults move layers of rock almost horizontally over one another (dip of the fault is a small angle from the horizontal) near collision boundaries.Transform FaultsIf the fault is vertical and movement of rocks relative to one another remains in the horizontal, this is atransform fault. Caused by shear stress.