1. Metamorphic rocks
Geology 101

2. Metamorphic rocks
Unlike what you may have heard, it’s not just “heat and pressure” applied to existing rocks
Also, not due to partial melting of rocks
What it is: “the solid-state reaction of minerals within the rock to produce new minerals and thus new rocks”

3. Metamorphism
Metamorphism is a series of chemical reactions that occur to stabilize minerals in relatively high temperature and/or pressure conditions
Notice this is not freezing minerals, like in igneous rocks

4. Conditions for metamorphism
Besides heat and pressure, time is needed to complete the chemical reactions and fluids (either water, or more rarely, carbon dioxide) are needed to transport ions

5. Recognizing metamorphic rx
Metamorphic rocks subjected to directed pressure typically result in foliated rocks
The rock fabric gives a sense of the pressure direction

6. Recognizing metamorphic rx
Metamorphic rocks subjected to confining (or lithostatic) pressure do not show foliation
However, because of metamorphic reactions, the rock tends to be the same hardness all the way through; cracks in a rock go through grains, rather than around them – intragrain fracture

7. Three types of metamorphism

8. Contact metamorphism
Contact metamorphism occurs when a hot body (pluton or lava flow) cools in contact with pre-existing cold “country rock”.
Relatively low pressures (high T, low P)

9. Dynamothermal metamorphism
Also known as “dynamic” or “subduction zone” metamorphism
Relatively low temperatures (high P, low T)

10. Regional metamorphism
“Standard” metamorphic conditions - both temperature and pressure rise due to increasing depth of burial

11. Regional metamorphism
Characteristic facies – an index mineral or combinations of minerals - are found that indicate a more precise maximum pressure and temperature
For instance, garnets are geobarometers

12. PT diagrams

13. Metamorphic grade diagram

14. Facies diagrams Units are °C and kb (kilobars) where 1 bar is
roughly 1
atmospheric
pressure

15. So what rocks do you find?

16. Sed/met rx boundary
If there is very hot (>200°C) water flowing through rocks, minerals may be hydrothermally altered, or, in some cases, deposited
Metasomatism creates ore deposits