Rocks so formed possess a course or phaneritic texture. Intrusive Igneous Rock. Since the rocks are poor conductors of heat, the heat of the magma cannot escape rapidly and the magma cools slowly. Intermediate lavas contain plagioclase feldspar and amphibole (sometimes called alkali feldspar) and quartz. Explosive volcanism creates highly distinctive features in igneous rocks. In this case, the rock formed consists of very large crystals without any matrix of smaller crystals around them. The silica content of the felsites is about the same as that of the granites and diorites. Practically all common igneous rocks contain at least a little feldspar. Where they are not resistant they get eroded forming long narrow trenches. Composite plutons are a special and common class of batholithic intrusive bodies representing multiple pulses of intrusion. The crystals can be seen with the aid of a magnifying glass or a microscope. They are intruded between sedimentary beds. Black dark grey and dark green rocks are dark coloured rocks. A phaneritic texture is characteristic of a(n) _____ igneous rock that cooled _____. If the fragments are large (exceeding 4 mm diameter) the rock formed is called breccia. In many cases the lopoliths are composed of well layered intrusions of mafic to ultramafic rock types. These rocks have resulted from the slow solidification of magma. The erupted lava cools and hardens on the surface as fine grained rock constituting extrusive rock, (volcanoes, volcanic products, volcanic features etc.) This rock is formed at great depths where the magma cools very slowly. Mafic igneous rocks have more than 50% dark minerals (primarily pyroxene) plus plagioclase feldspar. The mineral crystals do not have enough time to grow to large size. These lavas produce rocks like dacite, rhyolite, obsidian. The rock itself is termed a porphyry. Solidification of lava forms the extrusive or volcanic rock. Basalts were formed from magnesium-iron-rich lavas, which are very fluid were able to flow for long distance. Igneous rocks are extrusive and intrusive. Add to Library ; Share with Classes; … Due to slow cooling the crystals grow to large size and have approximately same size. When basalt magma cools rapidly on the earth’s surface, it is fine grained; when it cools at depth its crystals will be larger-this form is called dolerite (or diabase). Intrusive rocks are classified on the basis of depth of emplacement, the nature and geometry of contacts and the size of the body. There are six different igneous rock textures, which tell us where the rock formed and how quickly it cooled: glassy, aphanitic, pyroclastic, … Describes the features of intrusive and extrusive igneous rocks that cool slowly and rapidly, respectively. Texture of Igneous Rocks By “texture” is meant the general physical appearance of a rock, especially the size, shape, and arrangement of constituent mineral grains. The size and shape of the mineral grains or crystals and the pattern of their arrangement give a texture to the rock. Dikes and sills are small bodies compared to batholiths and they have much more surface for their volume. Composition 5. Typically they are emplaced into already existing fracture systems. It occurs in certain dark mafic rocks, notably peridotite. At greater depths the crust is not so brittle and will not crack. Basalts, which rarely have any quartz, contain feldspar, micas and hornblende and are darker still. The intrusion of sills appears to lift the overlying sediments causing considerable uplift at the ground surface. When magma cools within the Earth, the cooling proceeds slowly. Report a Violation, Science Fair Project on Rocks | Engineering Geology, 4 Main Divisions of Metamorphic Processes | Geology. This texture is created when an extrusive rock cools extremely fast from a lava flow. The upper surface of a batholith where it cooled in contact with overlying rocks is broadly dome shaped. In some rare cases vertical or outward dipping ring dikes or inward dipping cone sheets occur in oval or circular pattern. A lopolith consists of a large lenticular centrally sunken but generally concordant funnel shaped intrusive mass or basin. intrusive and extrusive depending on whether they were formed from magma or lava. I. Igneous Textures A. Consequently the rock shows some large crystals mixed with crystals of small size which cooled fast. These rocks are formed from the material ejected from the explosive type of volcanoes. During a volcanic eruption, along-with the lava, fragments of rocks from the walls of the volcano and ash are erupted. Grained rocks solidified under conditions that favoured the growth of large grains. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. Igneous rocks are identified by knowing two main features, texture and color. Granite pegmatite typically contains feldspar, mica, and quartz. The rocks formed from such erupted material are called pyroclastic rocks. Deeper in the crust, the rise of buoyant magma is resisted by the overlying crust, which acts like a cap. Examples include obsidian. Textures of Igneous Rocks 3. Intrusive rocks cool inside the Earth, which means they have time for crystals to grow big; Extrusive rocks cool on the exterior of the earth, meaning they have little time for crystal growth before the lava solidifies. As magma cools slowly the minerals have time to grow and form large crystals. 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. Micas are easily identified because they have shiny cleavage faces, split readily in one direction into extremely thin sheets and are soft. Content Filtrations 6. Feldspars are white, pink, red, grey and rarely dark grey or black. Hence these bodies cool much more rapidly and are fine grained or even glassy if cooled so rapidly that no crystallization occurs. Basalt wells up from oceanic spreading ridges and builds new ocean floor. A liquid lava sprayed out can take the form of glassy strands called pele’s hairs. There is no defined mineralogy for pegmatite; any plutonic rock may form pegmatite. There are other rocks which have resulted due to interaction of hot solutions and vapours with pre-existing rocks which are usually rich in silica. The dense rocks occur commonly in lava flows. In some cases gases dissolved in thick viscous lava form tiny bubbles. Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. Gradational contacts generally contain well developed foliations and lineation’s. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. In the first case it constitutes a fissure eruption issuing in quiet flows with little or no explosive activity and producing extensive lava fields or plateau basalts. Plus, they are, by definition, exposed to the elements of erosion immediately. Within the igneous rock family, there are two different types: intrusive rocks (also called plutonic) which solidify from magma below the surface of the Earth, and extrusive rocks (also called volcanic) which erupt or solidify from lava above the Earth’s surface. They are the dominant rocks in batholiths, laccoliths and large sills and dikes. The magma loses heat to the wall rock, which both rises the temperature of this surrounding rock and melts it contaminating the magma. Please note that you can expand this image to fill the screen by clicking on the blue arrows on … Rocks may be dark coloured or light coloured. This rock is called pumice. Laccoliths may occur singly or in clusters. Most igneous rocks have well developed crystal structures, although a microscope may be needed to see them. 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. The mineral composition and colour of rocks are related to their chemical composition. A batholith is a huge deep seated dome-shaped intrusion commonly composed of silica-rich igneous rocks (granites and similar rocks). Slow solidification creates large crystals and fast solidification creates small fine crystals. 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. Sills vary in thickness from a few centimetres to hundreds of thousands of kilometres. These are the kind of rocks which erupt onto the surface which results in small crystals as the cooling happens very rapidly. As magma cools slowly the minerals have time to grow and form large crystals. Igneous textures include the rock textures occurring in igneous rocks. In case the magma cools and solidifies while it is still trapped underground the rock formed is called an intrusive or plutonic rock. An igneous rock with over 66 per cent SiO2 it termed acidic, with 52 to 66 per cent it is intermediate, with 45 to 52 per cent it is termed as basic and with less than 45 per cent it is termed as ultrabasic. Minor plutons include dikes and sills. The four textural groups of rocks may be subdivided on the basis of colour. As a result, magma tends to rise. After this stage the magma rises and comes out of the surface as lava which undergoes fast solidification. Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. The diameter of lopolith may be tens to thousands of kilometres with thickness upto thousands of metre. If the fragments are small the rock is called tuff which is due to the consolidation of volcanic dust and ash. The key difference between intrusive and extrusive rocks is that the intrusive rocks are formed from magma whereas the extrusive rocks are formed from lava. It has a greasy lustre and is about as hard as feldspar. The individual grains are commonly less than 0.5 millimetre in diameter and cannot be distinguished with the naked eye. A high degree of fluidity is required to produce this sheet like form. Formation of Igneous Rocks: Igneous rocks are formed by the solidification of magma or lava. What three components make up most magmas? The heat is supplied by the intruding magma which consequently loses heat and solidifies. The two field occurrences, viz. Furthermore, if the magma should break through to the surface and cool under atmospheric conditions, it literally freezes so fast that various atoms cannot arrange themselves into the different structural arrangements of silicate minerals and hence there will be no formation of crystals and the rock is said to have a glassy texture. Intrusions that follow near horizontal cracks parallel to the layers of near-surface rocks, rather than cutting across them are called sills. Most of the earths surface is covered with rocks of various types. Ask Doubt. Aphanitic (a = not, phaner = visible) rocks, in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near Earth's surface. These formations are due to large difference in the melting points of the constituents. Hornblende crystals may be longer and more slender than those of pyroxene. Other factors involved are: … Textures 5. Answered by Social Science Expert | 16th Jul, 2019, 08:32: AM. Some igneous rocks show evidence of two stages of cooling. Igneous rocks can be easily identified with their texture, density, color, and mineral composition. Each mineral occurs in numerous varieties. When the chemical analysis of an acid rock like granite and of a basic rock like basalt are compared, important differences are seen such as, the greater proportion of silica and alkalies (Na2O and K2O) in the acid rock and the higher content of lime, magnesia and iron oxide in the basic rock. 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. There occur large crystals indicative of slow cooling embedded in a matrix of microscopic crystals indicative of fast cooling. Example: Obsidian is a volcanic glass. Privacy Policy 8. 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. https://en.wikipedia.org/w/index.php?title=Igneous_textures&oldid=952668499, Creative Commons Attribution-ShareAlike License, This page was last edited on 23 April 2020, at 13:12. Light grey, light green, white, red, pink, brown and yellow rocks are light coloured rocks. During this time various minerals start to crystallize and these crystals are the phenocrysts of the rock that is formed after further movement toward the surface takes place. Igneous rock is formed through the cooling and solidification of magma or lava. Intense heat that liquefies inside the Earth makes hot molten material called magma. Different ways in which magma can ascend through the crust and solidify to become an intrusive rock: The main driving force behind the movement of magma is buoyancy. Both minerals are black or dark green and have a hardness of 5 to 7. Glassy rocks are always formed at the earth’s surface where the lava cools very rapidly. The table below shows the averages of a large number of analyses. Crystal size primarily reflects the rate of cooling, but is also often strongly affected by rock composition (especially water or gas content). Intrusive igneous rocks are formed from slow cooling of magma. First the magma at depth begins to solidify slowly. Classification of Igneous Rocks 3. Extrusive Igneous Rock . Plutonic rock formations covering over 100 sq km area are called batholiths. Dikes are commonly vertical or steeply- inclined. Igneous Textures. It is narrow with relatively small thickness. The cooling rate of magma is highly important in terms of the physical appearance of the igneous rock formed. When extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The differences between these two are given in the table below. Pluton refers to deeper intrusive bodies while intrusion is a more generalized term that may be used for both shallow and deep bodies. If the lava is thin, the gas bubbles move out during the solidification of the rock creating pot marked surface with many small openings called vesicles. They are less viscous and they flow easily. Big lava globes thrown out of the volcano, solidify while they are hurling in the air are called volcanic bombs. The dense rocks are very commonly porphyritic because most magmas that finally reach the surface are halted for a time on their way up. These rocks have a fine grained or aphanite texture. Degree of Crystallinity Holocrystalline - composed wholly of crystals Hypocrystalline - contains both glass and … There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex. Magma is generated where the requisite pressure and temperature conditions for rock melting are reached. Their thickness may vary from a few centimetres to thousands of metres. Its hardness is 7. Thus, rapid cooling at the Earth’s surface of extrusive molten material, or lava, results in the growth of smaller crystals, or prevents crystal growth altogether. Thus, plutons include dikes, laccoliths, batholiths, sills, and other forms of intrusions. Two other properties useful in identifying rocks are texture and mineral composition. The best known product is basalt which accounts for over 90 per cent of all volcanic rocks. One has to make many observations in the field when he comes across an out crop of an igneous rock. This rock is created by slow cooling followed by fast cooling of magma. As the gas escapes later, the rock is full of holes or vesicles. The texture of these crystals is indicative of the manner a rock is formed. It moves inside the earth forcing its way into cracks and crevices. A rock of this texture may be extrusive or intrusive. Acidic lavas are rich in silica and are explosive and slow-flowing. Its texture depends on the shape, size, time period to cool down and solidify, and the arrangement of crystals in the rock. Such rocks are termed "intrusive" rocks. As a result a porphyritic texture is developed. 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. Igneous rocks are called intrusive when they cool and solidify beneath the surface. When magma and lava cool and harden, they create igneous rocks. Mineral composition refers to the various minerals present in the rock. They form from magma rich is silica. Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar, and mica minerals. This is typical of pegmatites. At depth in the crust, the upward movement of magma takes place by diapiric rise. There is usually an alternation of lava flow with explosions and periods of inactivity of greater or less duration intervene. Match the textural term to its correct definition: Intrusive or Extrusive? Image Guidelines 5. Porphyritic: Large … The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. This texture is seen in rocks formed from volcanic eruptions. Mode of Occurrence of the Different Kinds of Igneous Rocks: Mode of Occurrence of the Different Kinds of Igneous Rocks. Properties of Igneous Rock Textures What determines igneous rock texture? Thus batholiths are composed mainly of granitic rocks with crystals large enough to be easily seen. The specimen above is about two inches (five centimeters) across. As the name implies this texture is that of glass and slag which has amorphous structure without definite crystals. 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. It occurs in all colours, but transparent, white, pink, red, violet and green quartz are the most common varieties. If a sill passes from one horizontal level to another horizontal level, it is called a transgressive sill. The magma that has reached the earth’s surface through cracks and fissures is called lava. A body of intrusive igneous rock that crystallizes from cooling magmas beneath the Earth's surface is called a "pluton". The volume of most intrusions that solidify at moderate depths in the crust are generally small, so they cool rapidly. In such a case the cooling magma is blanketed by the surrounding rocks. It is possible that a single rock may be made up of several minerals differing in density, differing in colour and differing in hardness. This is a fine grained dark coloured rock containing the minerals plagioclase feldspar, pyroxene, olivine and magnetite. Copyright 10. On the other hand, lava issuing from a central vent builds up a volcanic cone and subsidiary cones. It is composed of silica (SiO2) and is the hardest of the common minerals found in rocks. TEXTURES OF THE IGNEOUS ROCKS* Structure - large-scale features recognizable in the field, such as banding, lineation, jointing, and vesicularity. In general igneous rocks formed underground have mineral of larger size than the igneous rocks formed above the ground. We know magma is molten rock inside the earth. The coarse fragments and lapilli which form the volcanic breccia settle near the volcano. In a dike swarm the various dikes may run parallel, radiating, intersecting and may also be branching. Sills are either single, multiple (more than one injection of magma) or differentiated. The sides of the batholiths slope away and make them bigger at greater depths. Mode of Occurrence. Sometimes the lava is forcibly extruded explosively in a volcano creating many rock material forms. IGNEOUS ROCK TEXTURES - PRINCIPLEThe fundamental principle behind igneous rock textures is that grain size is controlled by cooling rate. Batholiths are generally surrounded by metamorphic rocks. An exception is in the case of volcanic glasses. (Massive glass is called obsidian.). The size and shape of the mineral grains or crystals and the pattern of their arrangement give a texture to the rock. 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. The thicker sills are coarser than the thin sills. Thus, they result in large crystals as the cooling process happens very slowly. Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. This is a general term for injected intrusions having shapes so irregular that terms like dike, laccolith etc. For pyroxene the cleavage angles are 93° and 87°. A sill or sheet is an injected layer of igneous rock intruded between strata. Features may range in size from kilometer scale relationships on a geologic map, through metre scale characteristics such as layering, down to individual grains a millimetre or less across. The outer margins of these bodies in contact with the relatively cold, surrounding wall rock actually chill to a fine grained or glassy texture. Pumice has fine closely spaced pores. Rapid cooling on the other hand, results in microscopic crystals which can be seen only under a magnifying hand lens or a microscope. Intrusive and Extrusive Igneous Rocks. This is a coarse grained texture in which all the leading mineral constituents can be easily seen. All rocks can be placed in one of four textural groups as follows: Further subdivision of these groups will be necessary, as any rock of the first three groups may occur as an even grained rock or as a porphyry. A dike is such a formation that cuts across the layering of the rocks it intrudes. Preview; Assign Practice; Preview. Andesite containing feldspar, hornblende, quartz and mica is darker and forms from magma of moderate silica content. Required considerable factors for the description of texture 1. Textures of Igneous Rocks The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt) Other factors involved are: The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid. Intrusive rocks are igneous rocks which have forced their way into fissures or partings in older rocks or which have displaced or absorbed portion of them. This texture is most commonly seen in the solidification of lava having a high silica content. There are no huge holes for magma to fill. 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. They have two smooth cleavage surfaces at right angles to each other. The igneous rocks have tightly interlocking mineral crystals. The crust fractures allowing the magma to fill the cracks. They stem from partial melting of certain minerals in sub ducted oceanic crust. This rock is formed below the earth’s surface, but close to the earth’s surface under conditions of low temperature with large amount of water mixed with magma. This is magnesium-iron silicate (Mg Fe SiOn). In differentiated sill, the denser intrusion are close to the base. That is, it deals with size, shape, nature and arrangements of igneous rocks. Lacoliths are thicker bodies and they arch-up the overlying sediments. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Lava ascending to the surface may rise through many fissures in a certain area or through a central conduit and associated channels. These are silicates of potassium, sodium, calcium and aluminium. Engineering Uses. Dikes are tabular discordant intrusive bodies. The contact of an intrusive rock can be either concordant or discordant. The basic lavas are rich in metallic elements but relatively poor in silica. It is a complex high temperature solution which is liquid or molten rock present at considerable depth within the earth. If a penetrating intrusion cuts across the geological layers it is called a dike. Forms 4. Dikes vary in thickness from less than a metre to over 50 metres and may run for long distances of several kilometres. Quartz has no cleavage, but breaks usually with an uneven surface that may look like glass. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. This is a tabular body that has emplaced parallel to the bedding in the country rock. Texture of a rock is the appearance of the rock and how one feels touching it. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. These may be of the size of mountain ranges. Slow cooling allows the growth of megascopic crystals that is crystals large enough to be identified with the naked eye. Content Guidelines 2. This texture is created when the molten lava cools very fast. Absolute size of the crystal, the grain or granularity 3. But, the volcanic dust and pumice may be carried for long distances by wind. Example: Basalt. Examples of phaneritic igneous rocks are gabbro, diorite and granite. These rocks were mostly formed at considerable depth below the earth’s surface. The result is a natural amorphous glass with few or no crystals. Disclaimer 9. 7. These magmas have rich viscosity and have great resistance to the uniform lateral spread needed to form a sill. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. These rocks are called porphyry’s. Very large discordant bodies are called batholiths. In this case, the rock is full of holes presenting a spongy appearance, as the magma cools with gas bubbles trapped in it. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. 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. Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. They are discordant if the intrusive body cuts across the older rocks. The minerals that make up most of the igneous rocks are, quartz feldspar, biotite, amphibole, pyroxene and olivine. Sills are intruded parallel to the layering and tend to be horizontal. In most cases, this interpretation is based on careful observations of the grain size and other field characteristics of the rock. Classification by texture: Index Igneous rock concepts Reference Lutgens & Tarbuck Ch 3 . If the pluton is large, it may be called a batholith or a stock. The broad form in some cases is obscured by dikes presenting an irregular distribution of formations. Lava is mostly forced out or extruded in volcanoes or through large cracks present in the earth’s crust. Basaltic lavas rarely form glassy rocks because on account of their extreme fluidity crystals grow in them rapidly. Extrusive rocks, because of their small crystals and glass, are less durable. Most sand consists dominantly of Quartz grains. The cooling … Degree of Crystallization or Crystallinity 2. 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. Pegmatite is an extremely coarse-grained intrusive igneous rock composed of large interlocking crystals. This texture showing large sized crystals within a matrix of small crystals is the porphyritic texture. The large crystals, because of their prominence in the rock are called phenocrysts. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. The thickness of a lopolith is generally 1/10 to 1/20 of the width. Being mostly quartz and feldspar, the granites are light in colour. There are two common feldspars- orthoclase which contains potassium and plagioclase which contains sodium and calcium. 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. Examples include granite, gabbro, diorite and dunite. The igneous rock exhibits mineral crystals too small to see with the naked eye. Where the dikes are resistant to weathering and erosion, the dikes may stand out as narrow walls with steep or vertical sides. A pluton is an igneous intrusive rock body that has cooled in the crust. At times, magma rises beneath a volcano along a simple cylindrical channel and solidifies to form a volcanic neck. These two minerals are commonly called ferromagnesian or the newer term mafic (ma for magnesium, f for iron). Intrusive rocks form plutons and so are also called plutonic. Texture is best studied in thin section under microscope rather than in the outcrop. 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 … Dust from volcanoes may settle as tuff beds, thousand of metres thick. This is a mineral of somewhat rarer occurrence in igneous rocks. Rocks form when mineral compounds react with heat, water or pressure. Sills normally occur in relatively unfolded country rocks at shallow crustal levels. The term felsic (Fel for feldspar, for silica or quartz) is in common use for these minerals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Some granites and diorites are porphyritic however, especially those occurring in dikes and sills, but the magmas which gave rise to the mafic rocks were so fluid even at low temperatures that most of these rocks are wholly crystalline. Granites are very common in this category. Most of the lopoliths are found in underground or gently folded regions. Extrusive rocks are formed from magma above the surface, while intrusive igneous rocks form from magma beneath the surface. Since batholiths are large and also they probably were emplaced at least several thousand kilometres below the surface, they cooled very slowly. Below the zone in which they are found, is a zone rich in olivine (the peridotitic zone). Intrusive igneous rock has a coarse, visible crystalline texture. Felsite (composed of feldspar and quartz) generally has an aphanitic texture. 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. Thus granite does not posses a single characteristic colour or hardness. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.