beneath earth's surface
Rhyolite Porphyry is an intrusive rock, formed from the slow cooling of magma underground. It is characterized by large crystals (phenocrysts) surrounded by a fine-grained matrix.
I don't know about the rest of the world, but in Sweden the main porphyry supplier is/was Älvdalen, Dalarna. I heard it's the place with the most porphyry in the world but I don't know if that's just the locals exaggerating.
Two types of igneous rock are intrusive (also called plutonic) and extrusive. There is also porphyry rock which is partly intrusive and partly extrusive. Porphyry rock has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground as does intrusive rock, and were carried in lava when it erupted. The mass of smaller crystals formed around the large crystals when the lava cooled quickly above ground, as does extrusive rock.
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Large crystals are diagnostic of an intrusive igneous rock, as large crystals form as the source magma cools slowly.
The rock is called porphyry, which forms when magma slowly cools beneath the Earth's surface. The large crystals are known as phenocrysts, and the smaller surrounding crystals are called groundmass or matrix.
A rock with large crystals scattered on a background of much smaller crystals is called a porphyry. This rock texture is the result of two stages of cooling, where large crystals formed during the slower cooling stage, followed by smaller crystals forming during rapid cooling.
a porphyritic rock. This texture indicates that the rock underwent two stages of cooling: slow cooling deep underground (resulting in the large crystals) followed by faster cooling near the surface (resulting in the small crystals).
Rhyolite Porphyry is an intrusive rock, formed from the slow cooling of magma underground. It is characterized by large crystals (phenocrysts) surrounded by a fine-grained matrix.
Two types of igneous rock are intrusive (also called plutonic) and extrusive. There is also porphyry rock which is partly intrusive and partly extrusive. Porphyry rock has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground as does intrusive rock, and were carried in lava when it erupted. The mass of smaller crystals formed around the large crystals when the lava cooled quickly above ground, as does extrusive rock.
I don't know about the rest of the world, but in Sweden the main porphyry supplier is/was Älvdalen, Dalarna. I heard it's the place with the most porphyry in the world but I don't know if that's just the locals exaggerating.
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The type of igneous rock that has large crystals surrounded by smaller crystals is not a rock but a porphyritic texture. This texture describes igneous rock that has porphyrites, or large crystals, with surrounding tiny particles, or groundmass.
Large crystals are diagnostic of an intrusive igneous rock, as large crystals form as the source magma cools slowly.
The variety of igneous rock that is called porphyry is usually formed when sections of magma rise up from the Earth's mantle and cool down. As the magma slowly cools down in the crust, large crystals are created (phenocrysts). Then the magma cools far more quickly as it is ejected through the crust, for example, in a volcanic eruption. This fast cooling freezes any new crystals into solid rock, which prevents them from becoming large. These large and small crystal grains are characteristic of porphyry rocks. The links below describe the origin of porphyry deposits in detail.
No, melted rock that cools quickly result in igneous rock with large minerals because large crystals take time to form and the rocks that usually have them are the ones that were formed inside the earth.
A hollow rock with crystals might be called a geode. A geode is a type of sedimentary rock that has crystals formed inside due to volcanic heating and cooling or by other chemicals seeping into the rock.