Phaneritic Granite and Aphanitic Rhyolite Granite and rhyolite are both felsic in composition - both are lighter-toned. What does the term porphyritic mean? When extrusive rocks make contact with the atmosphere they cool quickly so the minerals do not have time to form large crystals. Granite is phaneritic and rhyolite is aphanitic. Rhyolite in Europe: Etsch The felsic rock in Figure 8.4C is called rhyolite. Rhyolite rocks have a hardness of 6 according to Mohs scale of hardness. Many rhyolites consist mainly of glass, and are termed obsidian, or are partially devitrified, and termed pitchstones. 1 : of or relating to porphyry. The viscosity can be so high that the gas can only escape by blasting the magma from the vent. The high gas content and high viscosity of these magmas are perfect for producing an explosive eruption. Also, note that rock with grain-sizes larger than 3cm is said to have pegmatitic texture and forms in the last stages of magma crystallization. Compare aphanitic. Alkali rhyolites are those in which >90% of feldspars are alkali feldspars. Quartz in rhyolite may be as low as 10% but is usually present in amounts of 25% to 30%. Explosive eruptions produce tuff or pumice. Aphanitic rocks are those that have very small crystals that are not visible to the naked eye. Granite is formed from cooled lava, while rhyolite is formed from cooled magma. It rarely is produced during oceanic eruptions. Glass rates a 5 to 5.5 on the Mohs scale, while rhyolite is harder, a 6-7. It is often vuggy or highly fractured. Read to classification of igneous rock page. near the town of Wenzhou, Zhejiang province, China. Aphanite ja phanerite ovat kaksi eri kivi. The Gondwana Rain forests of When extrusive rocks make contact with the atmosphere they cool quickly so the minerals do not have time to form large crystals. obsidian). Phaneritic vs. Aphanitic Rocks: What is the Difference? In contrast to this, the term phaneritic is normally used to refer to rocks that are also igneous but are not as finely grained as aphanites. The result is a natural amorphous glass with few or no crystals. The thick granitic lava that forms rhyolite often cools quickly while pockets of gas are still trapped inside of the lava. BASALT. It has been fashioned into scrapers, hoes, axe heads, spear points, and arrowheads. Rhyolite is the volcanic equivalent of granite. Chapter 8. Therefore, you can tell the cooling history by considering these two textures. These extremely fine-grained, crystalline fabrics are . document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); 2022 Science Drill. Basalt is an extrusive igneous rock formed by the rapid cooling of lava. How do Aphanitic rocks differ from phaneritic rocks? Rhyolite is a non-foliated rock because it does not have layers or bands. phaneritic How can a magma body change its composition? In contrast, sedimentary rocks are rocks that form from the deposition and consolidation of sediment. Generally Ferromagnesian, or dark, minerals are rare as phenocrysts, being mostly biotite when present. America. However they have been given separate names and fall under different categories for some reason. Rhyolite; Rhyolite is felsic igneous extrusive rock and it is a fine-grained and dominated by quartz (>20%) and alkali feldspar (>35%).Due to the high silica content, rhyolite lava is very viscous.It is often difficult to identify rhyolites without chemical analysis due to their glassy groundmasses. What is slaty? This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. The correct answer is Basalt. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. rhyolite Other specimens - Click the thumbnails to enlarge Group - volcanic . Rhyolite is also sometimes called rainbow obsidian because of its wide range of colors. LU Discontinuous Series Olivine \. Rhyolite is not a clastic rock because it is not made up of smaller pieces. The name rhyolite comes from the Greek word for flow, because it is often found in stream beds. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. View the full answer. Rhyolite is a volcanic rock because it forms from the cooling of magma at or near the Earths surface. This gives the magma a high viscosity and The minerals that are most common in this rock are sodium and calcium plagioclase (albite and anorthite), potassium feldspar (orthoclase), muscovite, biotite, and quartz. Rhyolite usually forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. How do Aphanitic rocks differ from phaneritic rocks? The magma cools down slowly deep under the ground in what is termed as plutonic environment. Aphanitic texture consists of small crystals that cannot be seen by the eye with or hand lens. Aphanitic ("AY-fa-NIT-ic") rocks have mineral grains that are mostly too small to be seen with the naked eye or a hand lens, like this rhyolite. Since 1900 only three rocks can be either foliated or non-foliated. It can also be used indoors as a decorative element in homes and offices. Plus, they are, by definition, exposed to the elements of erosion immediately. Rhyolite has composition similar to that of granite but with The typical examples of aphanitic rocks are basalt, rhyolite, and andesite, while those of phaneritic texture are granite, gabbro, and diorite. -granitic -andesitic -basaltic -ultramafic change slope as the volcano inflates and contracts. The aphanitic texture, when present, . Igneous rocks can be further classified as either extrusive or intrusive. It is the extrusive equivalent of granite, with a very similar mineralogical makeup. characterised the area. How does the rate of cooling affect rock formation? Indeed, they are so fine that their components (mineral crystals) cannot be detected by the human eye as long as some magnifying instrument has not been used. mountain. Rhyolite is extrusive equivalent of granite magma. This process of cooling and forming crystals is known as crystallization and it is what determines the end product. gray. central volcano. Rhyolite may be aphanitic or porphyritic. 37 Related Question Answers Found . coarse-grained 39-rhyolite (C)- granite 40-Gabbro (B)-basalt 41- and . Rhyolite can have a vitreous, pearly, or silky luster. Rhyolite. The texture is similar to metamorphic rocks. Each kind of texture has a variety of different characteristics that make them unique. Subvolcanic forms at depths less than 1.2 miles (2 km) from the Earths surface, especially in sills, dykes, or laccoliths. Aphanitic Texture Meaning, Formation and Rock Examples, Lava or Volcanic Bombs Meaning, Formation, and Types, Volcanic Glass Meaning, Formation, and Types, Vitreous or Glassy Rock Texture Formation and Examples, Examples of Pegmatites: Granite, Diorite, Gabbro and More, Coarse-grained or igneous rocks with large mineral crystals that are visible and distinguishable to a naked eye, Fine-grained or igneous rocks with fine mineral crystals, which you cannot distinguish or see with a naked eye, They form via a slow cooling rate that allows crystals to become larger. A rhyolite flow is a type of igneous rock that forms when magma flows over the surface of the Earth. 2014-10-07 23:47:36. As we have seen, the two main differences are in the size of the crystals and the quickness with which cooling takes place before they are formed. If you consider their chemical composition or mineralogy, basalt is the volcanic equivalent of gabbro, granite the equivalent of rhyolite, and andesite the equivalent of diorite. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. It is composed of light colour silicates. 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. It is often difficult to identify rhyolites without chemical analysis due to their glassy groundmasses. obsidian, or tuff. In contrast, aphanitic is mainly extrusive or volcanic igneous rock. many shapes, ranging from pumice to porphyritic. connect into complex molecules. Glassy textured r . To be considered a mineral, a material must have a consistent chemical composition. It was produced by Didier Descouens. feldspar. Rhyolite is an igneous, volcanic rock that forms when magma cools quickly at or near the Earths surface. rocks of the Rotliegendes. What type of rock has an aphanitic texture? Rhyolite, extrusive igneous rock that is the volcanic equivalent of granite. Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. A phaneritic texture is developed by the slow cooling and crystallisation of magma trapped within the Earth's crust and is characteristic of plutonic rocks. Feldspars often comprise 50% to 70% of rhyolite, with potassium feldspar present in at least twice the amount of plagioclase feldspar. These rocks have Top Educators. Phaneritic (Coarse-grained) Origin: Intrusive/Plutonic: Chemical Composition: Felsic: Color: Predominantly white: Mineral Composition: Potassium Feldspar, Quartz, Sodium Plagioclase, Biotite: Tectonic Environment: Convergent Boundary - Intruded into batholiths above Andean-type Subduction Zone: Most Slow rhyolitic lava piles up around a vent as it slowly exudes from a volcano, and as a result, produces a mound-shaped structure called a lava dome.. Others are granodiorite and dunite. When these magmas erupt, a rock with two grain sizes can form. Later, when the lava flow has cooled and hydrothermal gases or ground water move through, material can precipitate in the vugs. What is a rock made of unordered atoms called? (Think about that - several percent gas by weight is a LOT of gas!) APA 7 Rhyolite is a felsic rock, while granite is a mafic rock. The main difference between Aphanitic and Phaneritic is that the aphanitic rocks are very finely grained and their crystals cannot be seen with the naked eye. How do you tell if a rock is aphanitic or phaneritic? Porphyritic. Saxony, especially the north West. It has mainly volcanic or extrusive igneous rock with some subvolcanic. PHANERITIC TEXTURE - Igneous rocks with large visible crystals because the rock formed slowly in an underground magma chamber. a. phaneritic b. aphanitic c. porphyritic-aphanitic d. glassy. 2 : having distinct crystals (as of feldspar) in a relatively fine-grained base. Porphyritic textures are those that have large crystals (phenocrysts) embedded in a finer-grained matrix. Rhyolite rocks can be found in many countries including New Zealand, Germany, Iceland, India, and China, and the deposits can be found near active or extinct volcanoes. Wiki User. And metamorphic rocks are rocks that have been changed by heat, pressure, or chemically active fluids. Rhyolite is a rock that is rarely used in construction or manufacturing. Rhyolite is composed mostly of quartz and feldspar, with smaller amounts of mica, amphiboles, and other minerals. phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. pyroclastic flows and volcanic debris avalanches have been triggered by a lava The result is a natural amorphous glass with few or no crystals. Most rhyolites are porphyritic, indicating that crystallization began prior to extrusion. Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass). Black Forest e.g. St. Francois Mountains. As additional magma extrudes, CHOOSE ALL THAT APPLY. Discussion. The rate of cooling is proportional to the difference in temperature between the bodies in contact with heat flowing from the warmer body to the cooler body. Granitic magmas are rich in silica and often contain up to In contrast, mafic rocks are composed of mostly dark-colored minerals, such as olivine and pyroxene. As these magmas cool, the silica starts to This is how some of the world's best deposits of red beryl, topaz, agate, jasper, and opal are formed. He kuitenkin Effusive eruptions produce the rhyolite or the obsidian if the lava cools rapidly, but all the rocks can be found following a single eruption. 1382 to 1562 degrees Fahrenheit. Rhyolite Arrowheads: Rhyolite was often used to make stone tools and weapons when more suitable materials were not available. Diorite has a phaneritic, often speckled, texture of coarse grain size and is occasionally . Jemez Expert Answer. Rhyolite and granite are both igneous rocks, but they have different compositions. The Taupo Volcanic Zone in New Zealand has a large Granite is phaneritic and rhyolite is aphanitic. This sudden lowering of pressure can result in an explosion. Aphanitic (a = not phaner = visible) rocks in contrast to phaneritic rocks typically form from lava which crystallize rapidly on or near Earth's surface. O Aphanitic O Phaneritic O Vesicular O Glassy. This texture describes the presence of consistently larger crystals in a finer-grained matrix. Rhyolite is used in a variety of ways, including as a decorative stone, in jewelry, and as a construction material. The typical examples of aphanitic rocks are basalt, rhyolite, and andesite, while those of phaneritic texture are granite, gabbro, and diorite. These rocks are peralkaline and usually contain alkali amphiboles and/or pyroxenes. There is no need to resubmit your comment. In aphanitic texture, the cooling rate was fast, while phaneritic had a slow speed for phaneritic. Fire Opal is sometimes found filling cavities in rhyolite. Rhyolite rocks can be found in many countries including New Zealand, Germany, Iceland, India, and China, and the deposits can be found near active or extinct volcanoes. Both of these rocks show a difference in composition. the rock. These were at St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcano in Alaska, and Chaiten Volcano in Chile. dome collapse. Aphanitic/Phaneritic Rock Types with Images Igneous Rock Bodies Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. The specimen shown here is about two inches across. Phenocrysts of quartz, feldspar, hornblende Also, you can know the depths at which cooling occurred. Gem hunters have learned this and are always on the lookout for vuggy rhyolite. Effusive eruptions produce rhyolite or obsidian if the lava cools rapidly. aphanitic An igneous rock texture characterized by mineral grains which are too small to be identified without a petrological microscope. There could be differences between them at the atomic level (or the molecular level) which are not visible to the naked eye. conditions. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Many rhyolites form from granitic magma that has partially cooled in the subsurface. . rhyolite.) oforthoclase, biotite, amphibole ( augite), pyroxene ( hornblende), and glass; APHANITIC TEXTURE - Igneous rocks that form on the earth's surface have very fine-grained texture because the crystals are too small to see without magnification. While on the other hand, the Phaneritic rocks are large in size and can be easily seen from the naked eye. This activity can trigger a dome collapse. Papa Stour in Shetland. We have to be more precise if we have the two at hand and need to tell them apart. Cascade Range. Lava Dome: Photo of a lava dome in the caldera of Mount St. Helens. Is Rhyolite Aphanitic or Phaneritic? This answer is: Basalt is also darker in color than rhyolite and has a different chemical composition. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. But as we will now make clear, the two are indeed different from each other. or amygdales may be present. This texture is also known as coarse-grained and is common in plutonic or intrusive rocks like diorite, gabbro, or granite. Transcribed image text: Match the following rocks to their equivalent aphanitic or phaneritic igneous rocks. And fine-grained textures are those that have small crystals that are not visible to the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite. 2 : having distinct crystals (as of feldspar) in a relatively fine-grained base. No Related Courses. -crystal settling -magma mixing magnetic differentiation -assimilation Which of the choices below lists the four basic compositional groups of igneous rocks, ranging from the highest silica content to the lowest silica content? Difference Between Similar Terms and Objects. Rhyolite can be a range of colors, including white, gray, pink, and green. Rhyolite, with felsic minerals comprising >20% quartz and Rhyolite domes are often found in areas of volcanic activity. These occur for a logical reason. What is an aphanitic igneous rock? Phaneritic: coarse grain, individual crystals are visible - slow cooling Aphanatic: fine grained, individual crystals are not visible - rapid cooling Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass). rhyolites are uniform in texture, and their color ranges from gray to Sluggish rhyolitic lava can slowly exude from a volcano and pile up around the vent. Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. . (2014, November 6). GABBRO - phaneritic (large crystals) of mafic minerals: olivine, . Phaneritic. light-pink, depending on the striations made by the lava flow. Also, pumice and scoria may have this texture but with vesicles. Valley Vulcanite Group near Bolzano and the surrounding area. Rhyolite is a felsic rock because it is composed of mostly light-colored minerals, such as quartz and feldspar. .1 INTRODUCTION All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or metamorphic. Igneous rock composition chart: This chart shows that rhyolite is typically composed of orthoclase, quartz, plagioclase, micas, and amphiboles. The Yandang Shan mountain chain, The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Rhyolite is found in many different places around the world, including the United States, Chile, Iceland, and Australia. Rock Type: Igneous - A "mostly" extrusive igneous rock that is felsic in composition and porphyritic aphanitic in texture. On the other hand, phaneritic rocks have grains that are coarse and the grain size is usually between 1 millimetre and 10 millimetres. in a debris avalanche of material falling from the tall collapsing dome. Our eyes can see a grain of about 1/10mm. Odenwald. Examples of phaneritic igneous rocks are gabbro, diorite and granite. The large crystals are termed phenocrysts while the aphanitic rest of rock is called the groundmass. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. The phaneritic texture is an intrusive igneous rock texture since these rocks form deep inside the Earths crust. The phaneritic texture is an igneous rock texture characterized by mineral crystals you can see or distinguish with a naked eye. The specific gravity of rhyolite is 2.6-2.7. Phaneritic and aphanitic rocks are igneous rock textures, meaning they form from solidifying magma or lava (if extrusive). It only indicates two cooling stages, i.e., slow that forms the larger crystals and final faster cooling forms finer-grained matrix. Phaneritic: Granite: Diorite: Gabbro: Peridotite: Aphanitic: Rhyolite: Andesite: Basalt: Vesicular: Pumice: Scoria: Scoria: Glassy: Obsidian: PART I - Volcanic (Extrusive) Rocks. Cobalt, Ontario Canada. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. No, rhyolite is not basalt. They are customarily designated by the names at the left of this column. . Rhyolite is a rock because it is made up of multiple minerals. Eruptions of granitic magma can produce rhyolite, pumice, and updated on 2014, November 6, Difference Between Similar Terms and Objects, Difference between aphanitic and phaneritic, The difference between Royal icing and Buttercream icing, Difference between stuffed and deep dish pizza, Difference Between Igneous, Sedimentary and Metamorphic Rocks, Difference between Volcanic Rocks and Plutonic Rocks, Difference Between Badlands and Black Hills, Difference Between Dispersal and Vicariance, Difference Between Silicon Valley and Wall Street, The word, Aphanitic, that is used as an adjective to describe aphanites, is a term used to describe certain forms of igneous rocks that are very fine grained, so fine that their components (mineral crystals) cannot be detected by the human eye as long as some magnifying instrument has not been used; the term phaneritic is normally used to refer to rocks that are also igneous but are not as finely grained as aphanites, Aphanites have crystals that are very finely grained; phaneritic rocks, however, have crystals that are coarse grained, Rocks that are aphanitic have a grain size below 1 millimetre; phaneritic rocks have grains that are coarse and the grain size is usually between 1 millimetre and 10 millimetres, The texture of phaneritic rocks results from gradual cooling, the magma cools down slowly deep under the ground in what is termed as plutonic environment; the texture of aphanitic rocks results from rapid cooling, The grain size and cooling intervals are related; If the magma slows down in a longer period of time, the crystals that are formed are larger as they take time to form and can gain a considerable size (phanerites); when the magma cools rapidly, the crystals that are formed form very quickly and are hence very small textured (Aphanites). The grain size of phaneritic textured rock ranges from 1/16mm to 3cm, while that of aphanitic is below 1/16mm. You must be signed in to discuss. For example, the silica-rich extrusive rock, rhyolite , common in continental volcanic regions, is the fine-grained equivalent of intrusive granite . Rhyolites erupt from the Earths surface at temperatures of Effusive eruptions [5] The ground can also change slope as the volcano inflates and contracts. to being classified as felsic rocks, except that rhyolite has a fine-grained (Pumice is a highly vesicular variety of What is the meaning of Phaneritic? There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Previous question Next question. If the magma slows down in a longer period of time, the crystals that are formed are larger as they take time to form and can gain a considerable size. Ali, Allana. Rhyolite is an extrusive igneous rock with a very high silica content. May contain spherulites which are spherical bodies, often Categorized under Geography | Difference between aphanitic and phaneritic. These rocks are also said to be fine-grained, and to see the mineral crystals, you need to prepare a thin section and use an electron and a petrological polarized microscope or electron. Both these two textures are holocrystalline, i.e., crystalline or have mineral crystals as contrasted to a non-crystalline like glassy or vitreous texture. phenocrysts of plagioclase and quartz, often with amphibole and / or biotite, The Malani Igneous Suite, Rajasthan, India. Mountains. PHANERITIC TEXTURE - Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber. 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. Trapped gases often produce vugs in the rock. It is commonly erupted as passive lava flows due to its low viscosity (resulting from high magmatic temperatures . The large crystals that formed beneath the surface are called phenocrysts, and the small crystals formed at the surface are called groundmass. Examples include obsidian. These rocks have similar compositions but different cooling of the lava as well as the cooling period when it reaches the surface. Rhyolite in Germany: Pyroxene \ Amphibole \ _ Biotite _ cr o LU Q 1 LIS . Hence, rhyolite and basalt as fine-grained volcanic rocks are porphyritic and . The thick granitic lava that forms rhyolite cools quickly, and pockets of gas remain trapped inside of the lava, eventually forming the vugs, where the materials precipitate as ground water or hydrothermal gases move through. Granite and rhyolite are light-colored, felsic rocks or silica-rich with main quartz and felspar and smaller amounts of mica, hornblende, etc. For instance, a rock with 90% of the mineral crystals below 1/16mm is aphanitic, even if there are a few larger crystals. Eruptions of granitic magma are rare. It may have any texture from glassy, aphanitic, porphyritic, and by the orientation of small crystals reflecting the lava flow. We are obsessed with the outdoors, and work constantly to bring you the information you need to enjoy the rocks and minerals you've added to your collection. hundred meters. This dome is composed of dacite, a rock that is intermediate in composition between rhyolite and andesite. The result is a natural amorphous glass with few or no crystals. 02 of 09 Equigranular Texture James St. John/Flickr What is the difference between aphanitic and phaneritic, textures? Cite Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. As additional magma extrudes, the brittle dome can become highly fractured and unstable. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. This means that the size of the matrix grains in these rocks are quite large and unlike aphanites, they can be noted and distinguished by the naked eye. Some examples of the uses of rhyolite include: Decorative stone: Rhyolite is a popular choice for use in gardens and landscaping. The ground can also extinct central volcanoes, e.g. Moreover, as opposed to aphanitic rocks, the texture of phaneritic rocks does not result from rapid cooling but from gradual cooling. magma, and the crystallization of glass. There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex. Rockhounding Around The WorldScience QuestionsLearn About Different Kinds of Rocks, About UsPrivacy PolicyTerms Of UseArchives, Pumice: Description, Characteristics, and Other FAQs, Schist: Description, Characteristics, and Other FAQs, Rockhounding Louisiana: Places To Hunt Rocks, Crystals, and Fossils, Rockhounding Kansas: 5 Places To Hunt Rocks, Crystals, and Fossils, Rockhounding Rhode Island: Places To Hunt Rocks, Crystals, and Fossils, Rockhounding Mississippi: 11 Places To Hunt Rocks, Crystals, and Fossils, Rockhounding North Dakota: 10 Places To Hunt Rocks, Crystals, and Fossils. "Difference between aphanitic and phaneritic." the Knigstuhl (Pfalz) on the Donnersberg This gives the magma a high viscosity and causes it to move very sluggishly. Which rock is the fine-grained equivalent of granite (same composition but aphanitic)? This activity can trigger a false. What are the common examples of aphanitic and phaneritic rocks? This sudden lowering of pressure can result in an explosion. pile up around the vent. Spherulites are generally less than 0,5 cm in diameter, but they may Silica (SiO 2) content 69%-77%. Great Dividing Range. Phaneritic rocks have larger crystals that are visible to the naked eye. Aphanitic Aphanitic Texture Phaneritic Porphyritic Phaneritic Phaneritic Porphyritic Aphanitic Porphyritic Aphanitic Aphanitic Porphyritic Rock Name Granite Porphyritic Granite Granite Porphyry Rhyolite Porphyritic Rhyolite Rhyolite Porphyry. A dome collapse can lower the pressure on the extruding magma. the brittle dome can become highly fractured and unstable. [ fn-rtk ] Of or relating to an igneous rock in which the crystals are so coarse that individual minerals can be distinguished with the naked eye. As the lava quickly cools, the trapped gas is unable to escape and forms cavities known as "vugs."
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