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What is a porphyritic rock and how does it form?
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.
How does porphyritic andesite form?
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.
What is porphyritic structure?
Porphyry. — This name refers to the structure, which is formed of larger crystals set in a fine-grained or glassy ground-mass. Porphyritic structure may appear in any igneous rock, but the true porphyries are rocks in which there is a marked contrast between the large crystals and the ground mass.
How is porphyritic rhyolite formed?
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 causes porphyritic texture?
Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix.
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 is a porphyritic igneous rock?
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. An aphanitic texture is developed when magma is erupted at the Earth’s surface and cools too quickly for large crystals to grow.
What does a porphyritic texture mean?
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. Porphyritic textures are found in rocks with both coarse- and fine-grained groundmasses.
How are porphyritic rocks formed in the crust?
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.
What kind of texture does a porphyry rock have?
Porphyry. 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.
What is the meaning of the word porphyritic?
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group.
How are phenocrysts embedded in porphyry igneous rocks?
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. Width of view 7 cm.
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