- 1 What are porphyritic rocks used for?
- 2 What is the cooling history of a porphyritic rock?
- 3 What igneous rocks are porphyritic?
What are porphyritic rocks used for?
Thus, “imperial”-grade porphyry was prized for monuments and building projects in Imperial Rome and thereafter. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition.
Why are porphyritic rocks called porphyritic?
Porphyry. — This name refers to the structure, which is formed of larger crystals set in a fine-grained or glassy ground-mass. The porphyritic structure may be plainly visible, the larger crystals being easily seen (phenocrysts), or it may require the microscope to bring it out.
What does a porphyritic rock contain?
Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyry is an igneous rock that contains larger crystals (phenocrysts) in a fine-grained groundmass. K-feldspar phenocrysts in this sample.
How do you identify porphyritic rocks?
Porphyritic igneous rocks have coarse crystals in a fine background. Crystals are two to three times size of the matrix, and less than 10% of rock is crystals.
What kind of rock is porphyritic?
Porphyritic texture is an igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma.
What minerals are in porphyritic rocks?
Pitchstone porphyry (vitrophyre) consists of a glassy base (groundmass) enclosing abundant large crystals (phenocrysts) of such minerals as quartz, alkali feldspar, and plagioclase, as well as fewer crystals of pyroxene or hornblende.
What is the cooling history of a porphyritic rock?
Porphyritic:a mixture of large and small crystals – perhaps a two-stage cooling history with the large crystals (the phenocrysts) cooling slowly and the small crystals (the groundmass) cooling more quickly. The rock to the right has a phaneritic texture; individual grains can be seen.
What is the differences between phenocrysts and porphyritic?
Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. granite, gabbro). Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass).
What is a porphyritic granite?
Granites are coarsely crystalline igneous rocks, having a phaneritic texture. Porphyritic granites have a mix of large & small crystals, but all crystals are still fairly decent-sized. Large crystals in a porphyritic rock are called phenocrysts. Porphyritic granites typically have K-feldspar phenocrysts (pinkish).
What is a phenocrysts in geology?
A relatively large crystal embedded in a finer-grained or glassy igneous rock. The presence of phenocrysts gives the rock a porphyritic texture (see illustration). Phenocrysts are represented most commonly by feldspar, quartz, biotite, hornblende, pyroxene and olivine.
Are porphyritic rocks intermediate?
Cooling of the magma can occur deep beneath the surface (plutonic), beneath the surface but at relatively shallow depths (hypabyssal) or on the surface (volcanic)….Igneous Rocks.
|Atlas of Igneous Rocks
|Rock Name / Image
|Andesite: Hornblende Andesite
What does the term porphyritic mean?
1 : of or relating to porphyry. 2 : having distinct crystals (as of feldspar) in a relatively fine-grained base.
What igneous rocks are porphyritic?
Andesite is an extrusive igneous rock that has a porphyritic texture. This rock is formed from molten magma, which has medium viscosity.
What is an example of porphyritic igneous rock?
Porphyritic is used as an adjective to modify the name of any fine-grained igneous rock that has less than 50% phenocrysts in it. Example: Porphyritic basalt, porphyritic andesite. Porphyry is used as a noun after the rock name if more than 50% of the rock is made of phenocrysts.
What does a porphyritic texture indicate about an igneous rock?
Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyry is an igneous rock that contains larger crystals (phenocrysts) in a fine-grained groundmass.
How are porphyritic igneous rocks formed?
Formation. Porphyritic rocks are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano,…