Metamorphism

• Types of Metamorphic Rocks: 2- Non-foliated


  no layers present

  -Hornfels: horneblende

  -Quartzite: quartz

  -Marble: calcite

  *Identify main mineral*Non-foliated = confining pressure
• Types of Metamorphic Rocks: 1- Foliated Metamorphic Rocks


  minerals are lined up in layers

  -Slate: low grade metamorphism, no crystals visible

  -Schist: intermediate grade; some crystals visible

  -Gneiss ("nice"): high grade, a lot of crystals visible, light and dark minerals alternating **Foliated = directed pressure
• Length of Metamorphism

  Changes w/in minerals can record P-T changes
• Length of Metamorphism

  -Prograde- all of the time when temp and/or pressure are increasing

  -Retrograde- all of the time when temp and pressure are decreasing
• Interpreting Facies

  Use facies info to reconstruct how metamorphism occurred
• 7 Major Metamorphic Facies

  -Zeolite

  -Blueschist

  -Hornfels

  -Amphibolite

  -Eclogite

  -Granulite
• metamorphic facies


  group of index minerals that all form at the same temperature and pressure

  -Ex- Blueschist facies
• high grade

  -Sillimanite
• intermediate grade

  -Garnet

  -Staurolite

  -Kyanite
• low grade

  -Chlorite

  -Biotite
• Index minerals


  one mineral that tells you the precise temperature and pressure
• Metamorphic Change

  Metamorphic Grade: the general increase in temperature and pressure

  -Low grade, intermediate grade, high grade

  -Never melt, always in solid state
• Met. Rks & Environments

  Q: What metamorphic rock forms?

  A: Parent rock's composition is key
• Types of Metamorphism: Seafloor metamorphism


  some temp increase but really deals with Metasomatism

  -Interacts with sea water, hot sea water circulating through rocks causing metasomatism
• Types of Metamorphism: Shock Metamorphism

  -pressure driven

  -occurs extremely fast, almost instant

  -occurs when a large meter hits the earth, immediately causes enough pressure for metamorphism to occur
• Types of Metamorphism: Fault Metamorphism

  breaks in the rocks when passing the fault line

  -pressure driven

  -small scale
• Types of Metamorphism: Regional metamorphism

  -pressure driven event

  -Very large scale

  -Happens at plate boundaries; collision zones
• Types of Metamorphism: Contact metamorphism


  occurs when the rock is in contact with magma

  -Very small amount of rock because rock is a good insulator of temperature
• Causes of Metamorphism: #3 Metasomatism

  the rock interacts with hot, high temperature ground water and breaks down old minerals and deposit new minerals in the rock

  -Ores: ore deposits; high concentration of something valuable
• That's deep! How do metamorphic rocks get back to the surface?

  Faults: cracks in rocks

  - where rocks come to the surface where they are weathered and eroded, so rocks lift up and this continues, eventually the metamorphic rocks can be exposed at the surface
• How Much Pressure is Needed?

  Most metamorphic rocks form at 10-30 km depth (mid-lower crust)

  -10 km = 6 miles

  -30 km = 18 miles
• Directed pressure (aka Differential)


  a lot of pressure from one primary direction

  -Most pressure is coming from one direction
• Confining pressure


  squeezing evenly from all directions

  -No matter which way you turn the pressure is the same
• Pressure gradient


  earth's pressure change over distance

  -average 300 bar / km depth
• Causes of Metamorphism: #2 Pressure


  Pascals and bars (units to measure pressure)

  -1 bar = atmospheric press @ surface
• Geothermal gradient


  earth's temperature change over distance (deeper and deeper into earth) Varys based on location

  -Avg: 30∞ C/km

  -Typical range: 20 - 60 C/km
• Causes of Metamorphism: #1 Temperature


  if you heat up the rock enough it can turn into a metamorphic rock; higher temperature
• Speed of metamorphism

  slow; multi million years

  -Form deeper, can show conditions deeper in planet
• Review- Things to Know

  -Temp & press gradients

  -Types of metamorphism

  -Grade vs facies

  -Foliated vs nonfoliated

  -Hornfels & blueschist facies examples