What do Phenocrysts represent in a porphyritic texture?

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.

What is phenocrysts of a porphyritic igneous rock?

A phenocryst is a large crystal that visually stands out compared to the rest of the minerals. Phenocrysts are found in porphyritic textures where large grains are visible in volcanic extrusive rocks. Imagine hiking in Colorado when you come upon a rock with crystals, phenocrysts to be exact, the size of your fist.

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.

What does the presence of phenocrysts in an igneous rock indicate?

In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix. These volcanic phenocrysts often show flow banding, a parallel arrangement of lath-shaped crystals. These characteristics provide clues to the rocks’ origins.

Do porphyritic rocks have phenocrysts?

Porphyritic – This texture describes a rock that has well-formed crystals visible to the naked eye, called phenocrysts, set in a very fine grained or glassy matrix, called the groundmass.

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 are phenocrysts made of?

A phenocryst is a conspicuous, large crystal embedded in a finer-grained matrix of smaller crystals in a porphyritic igneous rock. Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more).

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

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).

What is the cooling history of porphyritic texture quizlet?

Porphyritic texture indicates two-stage cooling: slow, then fast. Define glassy texture. Glassy texture is characteristic of extrusive rocks and forms by very rapid cooling (quenching) of the magma. There are no crystals because the atoms are “frozen” in a random pattern.

What does the porphyritic texture tell you about the cooling history of the magma?

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.

What is the cooling history of an igneous rock with a 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 kind of cooling history do porphyritic textures indicate?

multi-stage cooling

Porphyritic texture indicates the magma body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.

Why do porphyritic igneous rocks have the most complex cooling history?

Some igneous rocks undergo a complex cooling history as a result of the movement of magma to a different environment from which the initial cooling began resulting in large crystals (phenocrysts) embedded in a fine grained small crystal matrix (groundmass) resulting in a porphyritic texture.

What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?

What does the mixture of grain size in a porphyritic igneous rock indicate about its cooling history? This indicates that some cooled slowly at depth, so that large phenocrysts form, then the melt erupted and the remainder cools quickly.

How does the grain size of intrusive igneous rocks differ from that of extrusive igneous rocks Why?

Crystal Size and Texture

The most obvious difference between extrusive rocks and intrusive rocks is crystal size. Because extrusive rocks cool quickly, they only have time to form very small crystals such as basalt or none at all. On the other hand, intrusive rocks grow larger crystals because they take longer to cool.

What does the grain crystal size in igneous rock indicate?

The size of crystals in an igneous rock is an important indicator of the conditions where the rock formed. An igneous rock with large crystals probably indicates that the rock formed deep within the Earth, since it is typically warmer deep inside the Earth than near the surface.

What does a pegmatitic texture indicate about the rate of cooling?

Pegmatitic texture indicates very slow cooling in the presence of H2O.

What is the difference between porphyritic and pegmatitic textures?

If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals). If water was present during cooling, the texture may be pegmatitic (very large crystals). Magma intrudes into country rock by pushing it aside or melting through it.

What is pegmatitic texture?

A pegmatitic texture is one in which the mineral grains are exceptionally large. The largest ones are, by convention, more than about 3 cm long. This texture is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly.

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 extra large mineral grains are called phenocrysts.

How does a porphyritic texture form?

Porphyritic rocks are a product of igneous differentiation, and are generally 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.

What is the definition of porphyritic?

Definition of porphyritic

1 : of or relating to porphyry. 2 : having distinct crystals (as of feldspar) in a relatively fine-grained base.

What rock is porphyritic?

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.

What minerals are in porphyritic?

Porphyritic Granite

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.