Most igneous rocks have well developed crystal structures, although a microscope may be needed to see them. These different angles of cleavage are useful means of distinguishing them. I. Igneous Textures A. It is narrow with relatively small thickness. The igneous rocks have tightly interlocking mineral crystals. Pegmatite is an extremely coarse-grained intrusive igneous rock composed of large interlocking crystals. Copyright 10. Extrusive Igneous Rock . Describes the features of intrusive and extrusive igneous rocks that cool slowly and rapidly, respectively. This indicates how strong in your memory this concept is. Gabbro is a basic igneous rock, the intrusive equivalent of basalt. The best known product is basalt which accounts for over 90 per cent of all volcanic rocks. Generally they are much longer than their width and many have been traced to kilometres lengths. This is a general term for injected intrusions having shapes so irregular that terms like dike, laccolith etc. Mafic igneous rocks have more than 50% dark minerals (primarily pyroxene) plus plagioclase feldspar. Sills are particularly abundant in basins of thick unfolded sediments where conditions are ideal for widespread lateral intrusion. Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. Plagiarism Prevention 4. These rocks were mostly formed at considerable depth below the earth’s surface. If a penetrating intrusion cuts across the geological layers it is called a dike. Of the hundreds of named igneous rocks consider the three rocks, granite, andesite and basalt. The sunken feature of the lopolith may be due to the sagging of the enclosing rocks creating a structural basin. Example: Obsidian is a volcanic glass. In some cases gases dissolved in thick viscous lava form tiny bubbles. There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex. the sill and the extrusive lava flow may be confused one for the other. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Intrusive igneous rocks are formed when the magma cools and solidify below the surface of the Earth. This rock is an intrusive rock. Intermediate lavas contain plagioclase feldspar and amphibole (sometimes called alkali feldspar) and quartz. Rocks may be dark coloured or light coloured. A liquid lava sprayed out can take the form of glassy strands called pele’s hairs. During a volcanic eruption, along-with the lava, fragments of rocks from the walls of the volcano and ash are erupted. Conversely, slow cooling within the Earth’s crust of intrusive molten material, called … Texture Name: Crystal Sizes: Type of Magma Image: Pegmatic >1cm: very slow cooling, thick, crystals larger than the thickness of pencil lead: Phaneritic: 1-10mm: slow cooling, thick crystals the size of a sharpened pencil lead. Deeper in the crust, the rise of buoyant magma is resisted by the overlying crust, which acts like a cap. Basalt is the most common extrusive rock while granite is a very common intrusive rock. This is a coarse grained texture in which all the leading mineral constituents can be easily seen. The intrusion of sills appears to lift the overlying sediments causing considerable uplift at the ground surface. Igneous rock is formed through the cooling and solidification of magma or lava. This is a fine grained dark coloured rock containing the minerals plagioclase feldspar, pyroxene, olivine and magnetite. Textures 5. In these plutons intruding rock types grade from diorite to granite. In case the magma cools and solidifies while it is still trapped underground the rock formed is called an intrusive or plutonic rock. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. When magma cools within the Earth, the cooling proceeds slowly. Sills normally occur in relatively unfolded country rocks at shallow crustal levels. Dikes vary in thickness from less than a metre to over 50 metres and may run for long distances of several kilometres. Content Guidelines 2. Igneous textures include the rock textures occurring in igneous rocks. It is generally defined as the geometrical relationship among the component minerals of a rock and any amorphous materials (glass or gas in cavities) that may be present. They are the dominant rocks in batholiths, laccoliths and large sills and dikes. Since these rocks solidify at a faster pace, they are smooth, crystalline and fine grained. These rocks have resulted from the slow solidification of magma. Such a formation suggests that the magma was injected into a cooler environment of the first crystals formed. Basalts, which rarely have any quartz, contain feldspar, micas and hornblende and are darker still. They form from magma rich is silica. Slow solidification creates large crystals and fast solidification creates small fine crystals. Magma is generated where the requisite pressure and temperature conditions for rock melting are reached. … Classification of Igneous Rocks 3. Answered by Social Science Expert | 16th Jul, 2019, 08:32: AM. Mode of Occurrence. There are six different igneous rock textures, which tell us where the rock formed and how quickly it cooled: glassy, aphanitic, pyroclastic, … The two field occurrences, viz. Create Assignment. Texture is best studied in thin section under microscope rather than in the outcrop. Textures of Igneous Rocks: Texture of a rock is the appearance of the rock and how one feels touching it. Sheet like intrusions that cross cut pre-existing rocks are called dikes. For hornblende the cleavage angles are 124° and 56°. Volcanic dust becomes somewhat stratified as dust particles of the same size settle together to the earth. First the magma at depth begins to solidify slowly. Dikes are commonly vertical or steeply- inclined. The rock so formed is called volcanic glass. Prohibited Content 3. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Examples include obsidian. MEMORY METER. As this felsile lava was usually viscous it could not flow far from the opening, but solidified rapidly; hence, felsites are common in volcanic lava flows. As magma cools slowly the minerals have time to grow and form large crystals. Moreover, chilling at the leading thin edges increases the viscosity of the magma and encourages thickening or swelling and doming near the initial vertical magma conduit. These rocks have a fine grained or aphanite texture. In this case, the rock is full of holes presenting a spongy appearance, as the magma cools with gas bubbles trapped in it. The grain size of any igneous rock is increased by slow cooling and low viscosity, which allow elements to migrate through a melt and reach sites where crystals are growing. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Since batholiths are large and also they probably were emplaced at least several thousand kilometres below the surface, they cooled very slowly. The rock is crystalline, but so fine grained that it appears homogeneous. Lacoliths are thicker bodies and they arch-up the overlying sediments. As a result, magma tends to rise. This texture is most commonly seen in the solidification of lava having a high silica content. Rocks with visible crystals of roughly the same size are said to have a phaneritic texture. The heat from the crystallizing magma is enough to cause this metamorphism. Micas are easily identified because they have shiny cleavage faces, split readily in one direction into extremely thin sheets and are soft. The following terms are commonly used to describe the texture of igneous rocks: This is the texture of an intrusive rock whose crystals are large and can be seen with the naked eye. Magma is made up largely of silicates together with some oxides and sulphides along with considerable quantities of water and other gases in solution under great pressure. This rock is formed at great depths where the magma cools very slowly. After this stage the magma rises and comes out of the surface as lava which undergoes fast solidification. Examples of phaneritic igneous rocks are gabbro, diorite and granite. What three components make up most magmas? Due to enormous temperature and pressure, this molten material always has a tendency to penetrate into the cracks and lines of weakness of the thin … The crystals can be seen with the aid of a magnifying glass or a microscope. The first thing to know about an igneous rock is whether it is intrusive or extrusive, that is whether it formed below or on the surface of the earth. Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. Naming 6. So, there is no quartz, and about half the rock is made of minerals (pyroxene and olivine)… Feldspar (in anorthosite) Some varieties of gabbro have few dark minerals, and are made almost entirely of plagioclase feldspar. Therefore, most landforms and rock … The large crystals are called phenocrysts and the crystalline aggregate in which they are embedded is called the ground mass. Consequently the rock shows some large crystals mixed with crystals of small size which cooled fast. Intrusive Igneous Rock. The gabbroid rocks are fairly wide spread at the surface but become increasingly abundant downward. In igneous rocks the mineral crystal are scattered randomly, but they are tightly interlocked. Dolerite (or, in the USA especially, diabase) is the name … Disclaimer 9. Sometimes the lava is forcibly extruded explosively in a volcano creating many rock material forms. As the name implies this texture is that of glass and slag which has amorphous structure without definite crystals. Being mostly quartz and feldspar, the granites are light in colour. This rock is called pumice. These gradations may be expressed in terms of the size of grains as follows: Another important textural factor is the presence of certain substances in solution, notably water, boron, fluorine, chlorine, sulphur and carbon dioxide, all of which are termed mineralizers. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. Laccoliths are concordant, mushroom shaped intrusions ranging from 1 to 8 km in diameter with a maximum thickness of 1000 m. They occur in relatively unreformed sedimentary rocks at shallow depths. We use the term hypabyssal to describe very shallow intrusive bodies. Before publishing your articles on this site, please read the following pages: 1. This is typical of pegmatites. They are discordant if the intrusive body cuts across the older rocks. Both minerals are black or dark green and have a hardness of 5 to 7. Intrusive igneous rock has a coarse, visible crystalline texture. Felsic igneous rocks have less than 20% dark minerals (ferromagnesian silicates including amphibole and/or biotite) with varying amounts of quartz, both potassium and plagioclase feldspars, and sometimes muscovite. Igneous rocks are classified based on their mineral composition and texture. These formations are due to large difference in the melting points of the constituents. Dust from volcanoes may settle as tuff beds, thousand of metres thick. Science, Types, Geology, Rocks, Igneous Rocks. The basic lavas are rich in metallic elements but relatively poor in silica. On the other hand, lava issuing from a central vent builds up a volcanic cone and subsidiary cones. Sometimes the rock shows an unusual texture with coarse mineral grains embedded in a matrix of fine mineral grains. It occurs in all colours, but transparent, white, pink, red, violet and green quartz are the most common varieties. Both intrusive and extrusive rock textures are represented. Both biotite and muscovite are fairly common. Plutonic rock formations covering over 100 sq km area are called batholiths. The rock itself is termed a porphyry. The igneous rock exhibits mineral crystals too small to see with the naked eye. Igneous rocks are called intrusive when they cool and solidify beneath the surface. Magma that has reached the earth’s surface through cracks and fissures is called lava. Intrusive Igneous rock are a coarser grained rock and make up a majority of igneous rocks we would see today. In most cases, this interpretation is based on careful observations of the grain size and other field characteristics of the rock. feldspar, quartz, hornblende, pyroxene and olivine. Composite plutons are a special and common class of batholithic intrusive bodies representing multiple pulses of intrusion. intrusive; below the surface. Some magmas have formed in the earth’s mantle, other magmas have formed when the rocks of the lower part of the crust melted and other magmas have apparently consisted of mixtures from the mantle and the crust. Engineering Uses. Basalts were formed from magnesium-iron-rich lavas, which are very fluid were able to flow for long distance. Add to Library ; Share with Classes; … The table below gives an estimate of the relative abundance of these minerals. Acidic lavas are rich in silica and are explosive and slow-flowing. During the process of forcing its way into the surrounding and overlying hard rock, a process called intrusion, the magma cools.
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