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  • 1. Metamorphic RocksandCrustal Deformation

2. Metamorphism

  • 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?

3. 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 rocknew elements cannot be added to the metamorphic rock.They are simply made from the ingredients present in the parent.

4. Agents of Metamorphism

  • 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 (Bowens reaction series as an example).Minerals unstable at high temperature will form new ones.(Clay is unstable at high temps, but micas are).

5. Agents of Metamorphism

  • 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.

6. 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. 7. Agents of Metamorphism

  • 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.

8. Types of 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.

9. Contact Metamorphism 10. 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. 11. 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. 12. 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. 13. Types of 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 earths 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.

14. Foliated Rocks are rocks that were metamorphosed by regional processeshigh 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.

15. Slate (right): low grade regional metamorphism showing foliated texture with parent of shale (left). 16. Add some more pressure to slate (right) and end up with phyllite (left). 17. 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 (cant really see it here). 18. Increasing the pressures and temperatures even further causes the crystals to grow even larger and align even more.SCHIST 19. An extremely high grade metamorphic rock that has undergone very high temperatures and pressures but didnt 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). 20. 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. 21. 22. 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 rockknowing 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. 23. Garnet Schist:Foliated rocks form under high pressures (accompanied by high temperatures).At what temperature did this rock form? 24. Metamorphic Rocks Summary

  • Remember slate, phyllite, schist and gneiss all have foliated texture.The larger the minerals, the higher the temperatures and pressures (generally).
  • Foliated rocks can tell you the direction of pressure and the degree of metamorphism (or foliation) can tell you 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 quartz sandstone 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.

25. Crustal Deformation

  • Tectonics 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.

26. Stress and Strain

  • Deformation causes a change in volume or shape of a rock body.
  • STRESS is the amount offorceacting 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 andwill be have like ELASTIC causing faulting.

27. Stress

  • Confining Pressure
  • Uniform in all directions (i.e. rocks buried deep in the crust).
  • Increases with depth.

28. Stress

  • Stress Applied Unequally
  • Compressional stress
  • Tensional stress
  • Shear stress

29. Unequal Stress 30. 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 EARTHs 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). 31. 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