Module 8 - Metamorphic Processes and Products Flashcards
What does Isochemical mean?
It means that no elements are added or removed, with the exception of volatiles such as water and carbon dioxide
What is metamorphism?
The isochemical process by which rocks are changed by either heat or pressure, or both heat and pressure.
What is the same for metamorphic rocks?
The chemical composition of the parent rock is the same as the metamorphic rock produced
How does metamorphism work?
The rock undergoes the very slow process of solid state recrystallisation without melting. Different temperatures and pressures cause new minerals to grow in rocks that have the same composition
What is the lower limit for metamorphism?
Between 200 and 150 degrees C.
What happens if the temperature is below 150 degrees C?
The changes in the rock are now part of diagenesis.
What is the upper limit of temperature for metamorphism?
Around 800 degrees C where melting occurs
What can the processes of metamorphism result in?
- destruction of fossils, beds and sedimentary structures
- hardening of the rock
- change in colour
- alignment of minerals
- growth of new metamorphic minerals
How does temperature affect metamorphism?
- High temperatures occur near to igneous intrusions, where the magma heats the surrounding rocks
- Temperature also increases with depth due to the geothermal gradient
What happens as temperature increases?
The rate of metamorphic reactions also increases. This is because many of the chemical reactions require heat to take place. It increases the rate at which ions diffuse between minerals, though it is still a slow process because the ions have to move through solid rock during metamorphism.
What speeds up the rate of metamorphism?
Water because it allows the ions to diffuse more rapidly
What are the three ways that pressure is applied to rocks?
- Pore pressure - pressure is exerted by fluids between grains in a porous rock. Water speeds up the reaction by acting as a catalyst
- Load pressure - the weight of overlying rocks and the physical bringing of minerals into contact with each other very long periods of time
- Tectonic stress - caused as the rocks undergo folding or faulting and very high pressures are exerted. Usually over short periods of time
How long does it take for metamorphism to occur?
Usually takes millions of years to occur
What are the different types of metamorphism?
- Contact
- Burial
- Regional
What is country rock?
The rock into which an igneous rock has been intruded
What is contact metamorphism?
It occurs adjacent to igneous intrusions, which increase the temperature in the surrounding country rock. On a local scale it produces a metamorphic aureole. Temperatures are generally high but pressure is low. As pressure isn’t a significant factor, the minerals are not aligned in contact metamorphic rocks
What is Burial metamorphism?
It occurs in conditions of medium to high pressure and relatively low temperature. It affects rocks deeply buried by the weight of overlying sediments. It also occurs in subduction zones where sea floor sediments and basalts are buried. Almost always contain the blue mineral glacucophane. They are blueshists
What is regional metamorphism?
Affects larger areas than contact metamorphism, extending over hundreds or thousands of square kilometers. Caused by low to high temperature and low to high pressure at convergent plate margins. Results from either subduction or continental collision. Minerals have preferred alignment. They occur in the core of fold mountain belts where mountain ranges have been eroded
What is foliation?
It is the texture in metamorphic rocks, formed by the preferred alignment of flat, platy minerals
What is slaty cleavage?
It’s a texture bin fine grained rocks formed by low grade regional metamorphism. Platy minerals recrystallise perpendicular to the direction of stress applied during metamorphism so that the rock splits into thin sheets
How are all metamorphic rocks formed?
From parent rocks, the original rocks that existed prior to metamorphism. The composition of these rocks affect the mineralogy of the meatamorphic rocks so they have a likeness
Which type of metamorphism produces foliated rocks?
Regional metamorphism.
They have been affected by pressure, to some degree. Any Platy minerals they contain take on preferred alignment known as foliation
What is the most common Platy mineral and what does this mean most foliated rocks have as a parent rock?
Clay and so they have shale as a parent rock
What are the foliated rocks produced by regional metamorphism?
Slate, schist and gneiss