Pressure and Chemical Changes in Metamorphic Rocks Help
There are three main factors that cause pressure increases and the formation of metamorphic rocks. These are:
- the huge weight of overlying sedimentary layers,
- stresses caused by plates clashing during mountain building, and
- stresses caused by plates sliding past each other, like the shearing forces along the San Andreas fault (western United States).
Pressure or stress from tectonic processes or the weight of overlying rock causes changes in mineral texture. The two types of pressure that are applied to existing rock are confining pressure and directed pressure .
Confining pressure is an all around pressure. Like atmospheric pressure at the surface of the Earth, confining pressure is present within the mantle’s depths. Extreme confining pressure changes a mineral’s structure by squeezing its atoms tighter and tighter until new minerals with denser crystalline structures are formed.
Directed pressure happens in a specific direction. When extreme squeezing pressure is applied in one direction, it’s like toothpaste in a tube, it is forced in one direction. When clashing plates are compressed, the force is applied in one direction. Since heat decreases a rock’s strength, when pressure is applied in one direction, a lot of folding and deforming goes on when temperatures are high.
Depending on the type of stress applied to a rock, the minerals in metamorphic rock are squeezed, stretched, and rotated to line up in a specific direction. This is how directed pressure affects the size and shape of metamorphic rock minerals undergoing change by heat and stress.
For example, during recrystallization of micas, crystals grow within the planes of their sheet-silicate structures and align perpendicular to the directed pressure. Geologists use this type of metamorphic mineral to figure out the pressures that specific samples have been exposed to during their history.
Characteristics that cause chemical changes in rocks also add to the formation of metamorphic rocks. Very hot liquids and gases can, under extreme pressures, fill the pores of existing rocks. These liquids and gases cause chemical reactions to occur, and over time, change the chemical composition of the existing rock. Metamorphism can take place instantly as in rock shearing at plate boundaries or can take millions of years as in the slow cooling of deeply buried magma.
It is important to remember that the changes that go on in metamorphism are mostly in rock texture. The chemical composition of metamorphic rock is altered very little. The basic changes that do occur include the addition or loss of water and carbon dioxide. The biggest changes of metamorphic transformation, then, have to do with the way minerals are rearranged.
A chemical shift in the composition of metamorphic rock can also be changed by the addition or removal of different elements. This can happen as a result of the intrusion of magma bringing new minerals into contact with existing rock. Sometimes this can be seen through color changes in minerals of the same basic chemical composition.
When hot, mineral-rich waters rise through magma, they carry a variety of elements. Some of these elements include sulfur, copper, sodium, potassium, silica, and zinc ions to name a few. These minerals come from magma and intruded rock, during the time that water is filtering upward through the crust. On this journey, they interact with other minerals and chemicals replacing some of their own minerals with others. This type of chemical interaction and substitution is called metasomatism . Metal deposits like copper and lead are formed in this way.
Practice problems of this concept can be found at: Metamorphic Rock Practice Test