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..
Hereof, 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.
Also Know, what does a rock with porphyritic texture look like? 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.
Similarly, it is asked, how does a fine grained porphyritic texture form?
Many rocks with an overall fine-grained texture display scattered minerals that are more than 1 mm across. 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 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.
Related Question Answers
What is the texture of gabbro?
Gabbro is mafic, intrusive, coarse-grained rock with allotriomorphic texture. Gabbros contain mainly ferromagnesian minerals and plagioclase, the amount of ferromagnesian minerals equaling or exceeding that of the plagioclase.What three factors affect the texture of igneous?
A: Water content, silicate content, and the most important part speed of cooling are the three factors that influence the texture of igneous rocks. 2. The three major compositional groups for igneous rocks are mafic, intermediate and felsic.What is Panidiomorphic texture?
Definition of hypidiomorphic texture. A texture of igneous rocks in which the greater proportion of the crystallized minerals have subhedral forms.What is the texture of basalt?
Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix.What is granular texture?
Definition of granular texture. A rock texture resulting from the aggregation of mineral grains of approx. used to describe an equigranular, holocrystalline igneous rock whose particles range in diameter from 0.05 to 10 mm.What is Ophitic texture?
Ophitic texture is the association of lath-shaped euhedral crystals of plagioclase, grouped radially or in an irregular mesh, with surrounding or interstitial large anhedral crystals of pyroxene; it is characteristic of the common rock type known as diabase.What is fine grained texture called?
Fine-Grained (Aphanitic) Textures. Fine-grained textures generally indicate magmas that rapidly cooled at or near the Earth's surface. Fast cooling prevents crystals from growing very large. The cutoff between fine- and coarse-grained textures is about 1 mm. All four photos below are of the mafic rock basalt.What is Intergrowth texture?
A graphic texture is an igneous rock texture in which an intergrowth of two minerals has the appearance of runic writing. Graphic textures are most commonly intergrowths of quartz and alkali feldspar in which the quartz appears as v-shaped inclusions enclosed by the feldspar.Is gabbro intrusive or extrusive?
Gabbro is a coarse-grained and usually dark-colored igneous rock. It is an intrusive rock. It means that it formed as magma cooled slowly in the crust. Igneous rocks with similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead).Is gabbro a porphyritic?
Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.What is the texture of pegmatite?
Pegmatitic Textures. A pegmatitic texture is one in which the mineral grains are exceptionally large. This texture is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly. Instead, the large crystals of a pegmatite formed in a magma that was extra rich in dissolved water.Is Rhyolite a porphyritic?
Rhyolite, extrusive igneous rock that is the volcanic equivalent of granite. Most rhyolites are porphyritic, indicating that crystallization began prior to extrusion. The glassy rhyolites include obsidian, pitchstone, perlite, and pumice.What does Equigranular mean?
: having or characterized by crystals of nearly the same size a rock of equigranular texture.Is pegmatite intrusive or extrusive?
Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals.What is the texture of hornblende?
Physical properties Hornblende has a hardness of 5–6, a specific gravity of 2.9–3.4 and is typically an opaque green, greenish-brown, brown or black color. Its cleavage angles are at 56 and 124 degrees.How is porphyritic rhyolite formed?
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 is rhyolite formed?
Rhyolite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magma, usually when it erupts onto the Earth's surface. When rhyolite erupts quietly it forms lava flows. If it erupts explosively it often forms pumice.What determines the texture of an igneous rock?
What is the main factor that determines the texture of an igneous rock? The cooling history is the main factor. A rock that cools slowly has time to build large crystals. A rock that cools quickly may form small crystals or even small glass.What is an Amygdaloidal rock?
noun Petrology. a volcanic rock in which rounded cavities formed by the expansion of gas or steam have later become filled with deposits of various minerals. 
