Metamorphic Rocks Flashcards
Most likely cause of metamorphism
An igneous intrusion
Metamorphic rocks
Subjected to temperature and pressure changes
contact metamorphism
“The simplest form of metamorphism results from heating when hot magma intrudes into colder rocks.”
geothermal gradient
“a measure of the rise in temperature with depth in the Earth”
regional metamorphism.
Metamorphism over a large area
country rocks
Surrounding rocks in metamorphism
metamorphic aureole
The zone of contact metamorphism “around an igneous intrusion”
hornfels
A fined grained splintery rock produced by intense metamorphic contact
foliation
The process of being split into thin sheets
lineation
“directional alignment of the long axes of prismatic minerals”
Metamorphic grade
high-grade metamorphism takes place at higher temperatures and pressures.
low-grade metamorphism occurs at lower temperatures and pressures
metamorphic banding
With an increase in grade of metamorphism felsic and Mafic minerals segregate into alternating bands
Summary
“Metamorphic rocks are formed from pre-existing rocks by the growth of new minerals in the solid state (without melting) as a result of increased pressure and/or temperature.
Metamorphism normally involves little or no change to the overall chemistry of a rock, apart from the expulsion of aqueous fluids and gases. However, except in the case of monomineralic rocks, the minerals of a metamorphic rock are not usually the same as those present in the original rock before metamorphism.
Contact metamorphism occurs when hot magma is intruded into cooler country rocks. The hard and splintery rock that forms (known as a hornfels) may be spotted (with diffuse patches of new minerals) and/or may contain porphyroblasts (larger crystals) that grew in random orientations.
Regional metamorphism occurs when rocks are buried and subjected to high pressures and temperatures during tectonic activity such as mountain building. New mineral growth responds to compression so that platy and elongate minerals line up more or less parallel to one another to produce a foliation or lineation.
In regionally metamorphosed rocks, the development of foliation and banding is used with grain size to describe the texture and hence to classify the rock. At high pressures and temperatures, segregation of felsic and mafic minerals may give the rock a banded appearance.
Under extreme conditions of regional metamorphism, a rock may begin to melt to form a migmatite.
The factors that control the minerals formed during metamorphism are the chemical composition of the original rock, and the temperature and pressure to which the rock was subjected and remained at long enough for equilibrium between minerals to be attained.
The minerals occurring in a metamorphic rock (the metamorphic assemblage) reflect the particular conditions at the peak of metamorphism. Most metamorphic minerals subsequently remain stable at lower temperatures and pressures unless affected by fluids during hydrothermal activity or weathering.
