Metamorphic Rocks Flashcards
How does metamorphic rocks compare to the other rocks?
Metamorphic rocks are the hardest rock to form and only form in some parts of the world.
They are often GEMSTONES (e.g garnet, sapphite, ruby, greenstone, kyanite and labrocute).
How are Metamorphic rocks formed?
They are often formed when either igneous or sedimentary rocks (or other metamorphic rocks) are altered by TEMPERATURE and PRESSURE.
Note that both temperature and pressure increase with depth.
What increases with depth?
Temperature and pressure.
Spaces between mineral grains close, The thickness of the overlying rock increases.
What is metamorphism?
Means to “change form.”
Metamorphism leads to changes in mineralogy, texture (e.g grain size), and sometimes the chemical composition of rocks.
The rock gradually changes until a state of equilibrium with the new environment is reached.
Metamorphism often occurs at elevated temperatures and pressures that exist a few km below Earth’s surface, extending to the upper mantle.
When does metamorphism stop or the rock stops changing form?
The rock gradually changes until a state of equilibrium with the new environment is reached.
Where does metamorphism often occur?
Metamorphism often occurs at elevated temperatures and pressures that exist a few km below Earth’s surface, extending to the upper mantle.
What are the three processes resulting in metamorphic rocks?
- NO pressure present and onLy heat (triggers chemical reactions resulting in RECRYSTALLISAION). The geothermal gradient is about 30oC/km (so, 20km=600oC).
- Pressure is present due to BURIAL and TECTONIC plates (movement).
Up to 1000’sca atmosphere, pressure gradient = 285bar/km - Need chemically active fluid (moving through the rocks).
What are the two types of metamorphic rocks?
Regional and contact.
Describe regional metamorphic rocks
Occur during MOUNTAIN BUILDING, great quantities of rock are subjected to directed high pressures + temperatures associated with large scale DEFROMATION.
Lots of sediments here get metamorphised.
Describe contact metamorphic rocks
These rocks had contact with a hot body of MAGMA (base of the magma chamber).
When the magma is pushed up through SILLS, the contact with EXISTING rocks change them. Heat alteration around INTURSITON.
E.g Antarctica light rock = old sandstone
E.g Antarctica dark rock = younger magma (baked the older sandstone).
What does low grade and high grade mean?
Hint: Includes example
The higher the grade the coarser the grains are in the rock.
E.g from low to high grade.
- Mudstone
- Shale (squashed)
- Shicst (new minerals grown and formed by regional)
- Gneiss (extreme pressure + temperatures)
During this process, it is getting squashed more and more resulting in an INCREASING GRAIN SIZE and FOLLIATION.
What does foliation mean?
Foliation is present in some igneous rocks and sedimentary rocks BUT is FUNDAMENTAL in metamorphic rocks (regional ones).
The rock units have been STRONGLY deformed mostly by folding.
Driven by compressional pressures /stresses that shortened rock units, resulting in mineral grains developing PARALLEL or nearly alignments.
What is fundamental in metamorphic rocks?
Foliation.
What does non-foliation mean?
Non-foliation also exists in some metamorphic rocks such as quartz or calcite.
They typically develop in environments where deformation is minimal and the parent rocks are composed of minerals that exhibit EQUIDIMENTAL CRYSTALS (randomly orientated).
What does low pressure and temperature (200oC) result in?
Hint: A sequence
Low pressure and temperature (200oC) = LOW lithifiaciton (more solid rock) = LOW grade = chemical changes in rock
