Exposure to intense heat and pressure can result in the metamorphism of previously formed rocks. Learn about the factors that influence metamorphism and the metamorphic processes: recrystallization, neometamorphism and metasomatism in this lesson.
In this lesson, we are going to take a look at how metamorphic rocks form.
Metamorphic rocks are the unlucky rocks of the world because they got to their current state after being tortured by intense heat and pressure. In fact, if we look at the word metamorphism, we see that it comes from the word ‘morph,’ which means to be transformed or changed. So, metamorphic rocks are previously formed rocks that have been transformed by exposure to heat and/or pressure.
So, we already discussed two influencing factors that can transform pre-existing rocks into metamorphic rocks, which are heat and pressure. And, as you can image, it takes very intense heat and pressure to make rock change its form. In fact, the heat needed to make this change often comes from magma, which is the extremely hot fluid that forms below the surface of the Earth that you likely associate with volcanoes.
The pressure involved in metamorphism may come from the rock being deeply buried and this creates such intense pressure that crystals within the rock can grow together or collapse and rearrange into new mineral crystals. We also see that metamorphic rocks may be influenced by a third factor: the chemically active fluids that circulate around the newly forming rock and introduce different ions, which cause new and different mineral crystals to grow.
Metamorphic Processes: Recrystallization
There are different metamorphic processes that are responsible for the changes that we see in these rocks. Recrystallization is the most common and is defined as the metamorphic process by which crystals are packed together creating a new crystal structure.
Recrystallization requires a lot of heat and pressure, and the process changes the mineral’s size and shape, yet the basic composition remains unchanged.For example, limestone can go through the process of recrystallization to turn into marble. Limestone contains tiny calcite crystals that come from shells of marine creatures that were broken down and compacted into the sedimentary rock. When that limestone is buried and subjected to intense forces, the microscopic crystals pack together and slowly morph into the larger crystals of calcite that we find in marble. The same marble that you might use as a countertop in your kitchen.
So, we see with the recrystallization of limestone to marble that the size of the crystals change, but we still have the mineral calcite. However, with some processes, the minerals themselves change.
Neometamorphism is a metamorphic process that results in the formation of new minerals. It will help you to recall this term if you remember that the prefix ‘neo’ means ‘new.’ In fact, this process is sometimes called neocrystallization, or in other words ‘new crystals.
‘For example, shale is a sedimentary rock that contains clay minerals, such as quartz and feldspar. When shale undergoes metamorphism, the clay minerals can change to produce the new mineral garnet. This is kind of like cooking up a pan of lasagna in your oven.
You take separate ingredients, such as noodles, sauce and cheese, subject them to high temperatures and some pressure and what comes out of your oven 45 minutes later is a new substance, lasagna.
There is one last process for us to consider, and that is metasomatism. This is the metamorphic process by which the chemical composition of a rock is changed by fluids and heat. It may help you to recall this term if you remember that ‘soma’ can refer to a hallucinogenic or mind-altering beverage that was used as an offering to the Hindu gods. And, with metasomatism we have rock-altering fluid involved.
This process is seen in the creation of anthracite coal. The parent rock, bituminous coal, is formed from the compaction of dead plant matter, usually in a wet, swampy environment. Over time, the bituminous coal loses chemical substances and matures into anthracite coal, which is mostly just carbon.
- Metamorphic rocks are previously formed rocks that have been transformed by exposure to heat and/or pressure. Along with heat and pressure we see that chemically active fluids are also one of the influencing factors that can transform pre-existing rock into metamorphic rock.
- Recrystallization is the metamorphic process by which crystals are packed together creating a new crystal structure. With recrystallization the mineral’s size and shape may change, but the basic composition remains unchanged.
This is seen when limestone recrystallizes into marble.
- Neometamorphism, or neocrystallization, is a metamorphic process that results in the formation of new minerals. This is seen when shale that contains clay minerals change to produce the new mineral garnet.
- Metasomatism is the metamorphic process by which the chemical composition of a rock is changed by fluids and heat. This is seen in the creation of anthracite coal.
When this lesson is done, you should be able to:
- Understand metamorphism in rocks
- Identify recrystallization like the creation of marble
- Describe how neometamorphism works to create garnets
- Recognize how metasomatism results in anthracite coal