Metamorphic Rocks In Greek the word morphos means form or shape. The word metamorphism means the change of form. Within the Earth we find temperatures and pressures much high than we find on the surface. These increased temperatures and pressures and the percolating fluids that surround the rocks and pass through them can change the chemistry, mineralogy, and even the structure of the rocks without melting them. In other words they are Metamorphosed.

Agents of Change There are three agents responsible for altering the rocks. –1. temperature –2. pressure –3. fluids

Temperature High temperatures can alter the internal structure of a mineral thereby changing them into new minerals. There are two basic sources of high temperatures: –1. Geotherm- As you drill deeper into the Earth you increase the temperature at a rate of 20 to 60°C for every Km. This is called the geotherm.

Temperature 2. Intrusions—as magma come in contact with cooler rocks it increase the rocks temperature. Those rocks that are located at or very near an intrusion change their mineralogy. This kind of change is called contact metamorphism.

Contact Metamorphism

Pressure also known as Stress There are two main kinds of pressure (stress) that you should know; –1. Confining Pressure– Pressure due to the overlaying rock. This kind of pressure is roughly the same in all directions. Confining pressure is responsible for compacting rocks during diagenesis. Example olivine compacted to spinel, to perovskite. Each become a more tightly bonded crystal due to confining pressures.

Confining Pressure

–2. Directed Pressure: this is pressure which is stronger in a certain direction. Sort of like squeezing Play-Doh between two bricks. This kind of force is seen in tectonic forces. It changes rocks by changing the structure of minerals and changing the orientation of mineral grains (such as platy mineral – mica or clay).

Directed Pressure

Fluids The fluids that metamorphose rocks are not the pore fluids they were buried with but rather fluid which come form two other sources. –1. hydrothermal fluids – which are heated by magmatic intrusions. –2. dehydration of minerals (clays) which contain water in their chemical makeup (hydrous minerals).

Regardless of the source the fluids change the chemical makeup of the rocks they come in contact with. This kind of change is called Metasomatism.

Types of Rock Metamorphism There are 5 kinds of metamorphism you should know: –1. Burial—rocks buried deep enough will be exposed to increased heat which will cause changes. –2. Regional-This type of metamorphism is found in areas of mountain building and subduction zones volcanism. Regional metamorphism encompasses a widespread area under going moderate to high temperatures and pressures.

Types of Rock Metamorphism –3. Contact– This type of metamorphism is found near magmatic intrusions which have high temperatures. Typical area are volcanic regions (subduction zones, hot spots) and mountain building zones. –4. Cataclastic– Results in grinding along fault zones, mountain building zones, and deformation zones associated with subduction zones. –5. Hydrothermal– Found anywhere hot fluids can percolate through rocks. A good example would be the Mid-Atlantic ridge.

Metamorphic Rock Types There are three major metamorphic rock types: –1. Foliated Rocks—These are rocks that undergo directed pressure and have a preferred growth orientation of platy minerals. These rocks show parallel planes of mineral growth (Striped: schist and gneiss) –2. Non-foliated Rocks– these rocks do not have parallel planes of mineral growth but have cubic or spherical mineral grains. Example would be marble and amphibolites. –3. Deformational– caused by cataclastic metamorphism. Frequently gives the rock a broken appearance, and is associated with faults produced by crustal folding.

Grading Metamorphic Rocks Metamorphic rocks do not have sharp lines separating one type from another. Instead they grade from low grade (where the rock is hardly changed) due to low pressures and temperatures to high grade (rocks are highly altered) due to high pressures and/or temperatures. Geologist have determined in the lab certain minerals that form under particular conditions. These are called index minerals and can be used in the field.

Metamorphic Facies Two fundamental concepts: –1. If you start with two different types of rocks (basalt, and sandstone) the minerals will be different for the different rocks, even though they were under identical pressures and temperatures. –2. Starting with identical rocks which undergo different temperatures and pressures, the minerals will be different for each rock even though they were the identical in the beginging.

Summary You should know the following: 1. What is metamorphism? 2. What are metamorphic rocks? 3. The major kinds of metamorphism and what kinds of plate tectonic environments are they found. 4. What does Metamorphic grade mean? 5. what are index minerals? 6. What are the two major points abouy metamorphic facies?

Sites Used Thanks to Greg Anderson for use of lecture notes. Also: