What is porphyritic texture

A porphyritic texture displays minerals in two distinct size populations: one or more minerals are consistently larger than the rest of the minerals in a rock. … The rest of the smaller mineral grains are called the groundmass

What is porphyritic texture and how does it form?

A porphyritic texture is developed when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly. This texture is characteristic of most volcanic rocks.

What is an example of porphyritic texture igneous rock?

Explanation: A porphyritic texture is one that is characterised by large crystals (phenocrysts) in a finer groundmass (aphanitic part). So, many rocks are porphyritic: andesite, granite and even some basalts.

What is an example of porphyritic texture?

Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice.

Which rock shows porphyritic texture?

Porphyritic texture in a granite. This is an intrusive porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada, Rock Creek Canyon, California.

Is porphyritic texture intrusive or extrusive?

Porphyritic texture — andesite: This is an extrusive igneous rock. The magma from which it formed cooled slowly for a while deep below the surface (forming the large crystals), then finished cooling very quickly when it was ejected at the surface, forming the fine-grained groundmass.

How do you identify porphyritic rocks?

Rocks are said to be porphyritic if their groundmass is fine-grained ar aphanitic and porphyraceous if their groundmass in visible to the naked eye.

What does porphyritic texture indicate about the history of an igneous rock?

This porphyritic texture indicates that the magma sat and cooled a bit below the Earth’s surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly. The large crystals are termed phenocrysts while the aphanitic rest of rock is called the groundmass.

Where are porphyritic rocks found?

Porphyritic textures are most common in volcanic rocks where one of the minerals will begin to crystallize prior to eruption (therefore producing larger grains), with the rest cooling more rapidly after eruption.

Is porphyritic basalt mafic or felsic?

TypeIgneous RockTexturePorphyriticOriginExtrusive/VolcanicChemical CompositionMaficColorDark Gray to Black

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Is pumice porphyritic?

This texture is called porphyritic. … Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals (glass by definition is not a mineral, because it does not have a crystal lattice).

What is a porphyritic granite?

Granite (porphyritic) In terms of its mineral content this is a typical granite, consisting of pink potassium feldspar, cream sodium feldspar (plagioclase), grey quartz and black biotite mica. … This texture, of large well-formed crystals in a finer matrix, is called porphyritic.

Is porphyritic rhyolite intrusive or extrusive?

TypeIgneous RockTexturePorphyriticOriginExtrusive/VolcanicChemical CompositionFelsicColorLight Green

What can porphyritic texture tell you about the history of an igneous rock and the magma that it came from?

What does a porphyritic texture indicate about the cooling history of an igneous rock? It indicates that crystals were formed at depth (slow cooling) and then the magma moved to a shallow depth or erupted (fast cooling).

Is porphyritic rhyolite mafic or felsic?

Rock textureName of felsic rockCoarse-grained (phaneritic)GraniteCoarse-grained and porphyriticPorphyritic graniteFine-grained (aphanitic)RhyoliteFine-grained and porphyriticPorphyritic rhyolite

Is porphyritic andesite intrusive?

Andesite most commonly denotes fine-grained, usually porphyritic rocks; in composition these correspond roughly to the intrusive igneous rock diorite and consist essentially of andesine (a plagioclase feldspar) and one or more ferromagnesian minerals, such as pyroxene or biotite.

What is the cooling history of porphyritic texture?

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 porphyritic rock 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.

What minerals are in porphyritic rocks?

Quartz-porphyry. Quartz-porphyry has phenocrysts of quartz and usually feldspar, in a fine-grained ground mass. …
Feldspar-porphyry. Feldspar-porphyry is characterized by crystals of feldspar in a fine-grained or glassy ground mass; it has the composition of syenite. …
Lamprophyre.

What distinguishes a porphyritic texture in igneous rocks?

The larger crystals will flow out with the lava. The lava will then cool rapidly, and the larger crystals will be surrounded by much smaller ones. An igneous rock with crystals of distinctly different size (Figure 7.14) is said to have a porphyritic texture, or might be referred to as a porphyry.

What is a Phenocrysts in geology?

« Back to Glossary Index. A large crystal within an igneous rock. These can be seen within phaneritic and porphyritic rocks.

Is porphyritic basalt volcanic or plutonic?

Atlas of Igneous RocksRock Name / ImageTextureOccuranceBasalt: Olivine Basalt PorphyryPorphyriticVolcanicBostonite (Trachyte Porphyry)PorphyriticVolcanicCarbonatite: Siderite CarbonatitePhaneriticPlutonic

What minerals are in porphyritic basalt?

Basalt is mainly made of two minerals: Plagioclase feldspar and pyroxene.

What is porphyritic rhyolite?

Most rhyolites are porphyritic, indicating that crystallization began prior to extrusion. … Crystallization may sometimes have begun while the magma was deeply buried; in such cases, the rock may consist principally of well-developed, large, single crystals (phenocrysts) at the time of extrusion.

Is Obsidian a Phaneritic?

The result is a natural amorphous glass with few or no crystals. Examples include obsidian. … The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite.

How is glassy texture different from vesicular texture?

Glassy texture refers to a quick/rapid cooling lava that does not have an underlying organized mineral structure visible under a conventional polarized light microscope. Vesicular texture occurs when gases are trapped within a lava flow and minerals crystallize around the gas pocket.

Is Obsidian porphyritic?

OBSIDIAN, a glassy volcanic rock of acid composition. … Few obsidians are entirely vitreous; usually they have small crystals of felspar, quartz, biotite or iron oxides, and when these are numerous the rock is called a porphyritic obsidian (or hyalo-liparite).

What is the texture of porphyritic granite?

TypeIgneous RockTexturePorphyriticOriginIntrusive/PlutonicChemical CompositionFelsicColorPredominately white

Is porphyritic rhyolite felsic?

Most rhyolites are porphyritic and are identified by the quartz phenocrysts and white to salmon K-feldspar phenocrysts. … The groundmass is aphanitic or even glassy.

What minerals are in porphyritic rhyolite?

It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent to granite.