Physical Geography - Version 1 Unit 9 Igneous Rocks

is used under a license . UNIT IGNEOUS ROCKS Goals Objectives of this unit Describe the rock cycle and the types of processes that lead to the formation of igneous , sedimentary , and metamorphic rocks , and explain why there is an active rock cycle on Earth . Describe , in general terms , the range of chemical compositions of . Discuss the processes that take place during the cooling and crystallization of magma , and the typical order of crystallization according to the reaction series . Explain how magma composition can be changed by fractional crystallization and partial melting of the surrounding rocks . Apply the criteria for igneous rock classification based on mineral proportions . Describe the origins of , porphyritic , and pegmatitic textures . GEOGRAPHY

THE ROCK CYCLE The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle . The rock cycle is driven by two forces Earth internal heat engine , which moves material around in the core and the mantle and leads to slow but significant changes within the crust . The hydrological cycle , which is the movement of water , ice , and air at the surface , and is powered by the sun . The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving , our atmosphere is relatively thick , and we have liquid water . On some other planets or their satellites , such as the Moon , the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water . Weathering , erosion , The transportation , deposition Cycle ive ig Ea Surface Intrusive igneous rock Burial . compaction , cementation Cooling Sedimentary rock Deeper burial , heat and pressure Figure The Rock Cycle . Image is used under a Creative Commons Attribution International License . In describing the rock cycle , we can start anywhere we like , although it convenient to start with magma . Magma is a rock that is hot to the point of being entirely molten . This happens at GEOGRAPHY

between about and , depending on the composition and the pressure , onto the surface , and cool quickly ( within seconds to years ) forming extrusive igneous rock . Magma can either cool slowly within the crust ( over centuries to millions of years ) forming an intrusive igneous rock or can erupt onto the surface and cool quickly ( within seconds to years ) forming an extrusive igneous rock . An intrusive igneous rock typically crystallizes at depths of hundreds of meters to tens of kilometers below the surface . To change its position in the rock cycle , intrusive igneous rock has to be uplifted and exposed by the erosion of the overlying rocks . Through the various processes of mountain building , all types of rocks are uplifted and exposed at the surface . Once exposed , they are weathered , both physically ( by the mechanical breaking of the rock ) and chemically ( by weathering of the minerals ) and the weathering products , mostly small rock and mineral fragments are eroded , transported , and then deposited as sediments . Transportation and deposition occur through the action of glaciers , streams , waves , wind , and other agents , and sediments are deposited in rivers , lakes , deserts , and the ocean . Crystallization Igneous rocks form when molten material cools and hardens . They may form either below or above the Earth surface . They make up most ofthe rocks on Earth . Most igneous rock is buried below the surface and covered with sedimentary rock , and so we do not often see just how much igneous rock there is on Earth . In some places , however , large areas of igneous rocks can be seen at Earth surface . The first figure of this unit is California Sierra Nevada that consists entirely of granite , an igneous rock . Igneous rocks are called intrusive or plutonic when they cool and solidify beneath the surface . Because they form within the Earth , cooling occurs slowly . Such slow cooling allows time for large crystals to form , therefore , intrusive or plutonic igneous rocks have relatively large mineral crystals that are easy to see . Granite is the most common intrusive igneous rock . GEOGRAPHY

. Figure Collecting Diorite ( Granite ) Samples from a Fresh Rockslide near Lone Pine CA . image is used under a license . Granite is nearly always massive , hard , and tough . These properties have made granite a widespread construction stone throughout human history . The term granitic means like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin . In most cases , a body of hot magma is less dense than the rock surrounding it , so it move very slowly up toward the surface . It does so in a few different ways , including filling and widening existing cracks , melting the surrounding rock ( called country rock ) pushing the rock aside ( where it is somewhat plastic ) and breaking the rock . Where some of the country rock is broken off , it may fall into the magma , a process called stoping . The resulting fragments are known as ( Greek for strange rocks ) Figure Xenolith in Gran , ear Rock Creek ( Eastern , Ca ) image on Commons by , GEOGRAPHY

Figure Half Dome in California is an Example of Masses of Rocks , or , Creating a Batholith . Image by DAVID is used under a license . Some magma reaches the surface , resulting in volcanic eruptions , but most cools within the crust . The resulting body of rock is known as a . Large are called either stocks or . The distinction between the two is based on the area that is exposed at the surface if the body has an exposed surface area greater than 100 , then it a batholith smaller than 100 and it a stock . are typically formed only when a number of stocks coalesce beneath the surface to create one large body . One of the largest in the world is the Coast Range Plutonic Complex , which extends from the Vancouver region in Canada , to southeastern Alaska . More accurately , it many . Tabular ( are distinguished based on whether or not they are concordant with ( parallel to ) existing layering ( sedimentary bedding or metamorphic foliation ) in the . A sill is concordant with existing layering , and dyke is discordant . If the country rock has no bedding or foliation , then any tabular body within it is a dyke . Note that the designation is not determined simply by the orientation of the feature . A dyke can be horizontal , and a sill can be vertical ( if the bedding is vertical ) GEOGRAPHY

TEX Igneous rocks are also classified according to their textures . The textures of volcanic rocks will be discussed later , so here we only look at the different textures of intrusive igneous rocks . Almost all intrusive igneous rocks have crystals that are large enough to see with the naked eye , and we use the term ( from the Greek word meaning visible ) to describe that . Typically , that means they are larger than about , or the thickness of a strong line made with a ballpoint pen . If the crystals are too small to distinguish , which is typical of most volcanic rocks , we use the term aphanitic . In general , the size ofthe crystals is proportional to the rate of cooling . The longer it takes for a body of magma to cool , the larger the crystals will be . It is not uncommon to see an intrusive igneous rock with crystals up to a centimeter long . In some situations , especially toward the end of the cooling stage , the magma can become . The presence of liquid water ( still liquid at high temperatures because it is under pressure ) promotes the relatively easy movement of ions , and this allows crystals to grow large , sometimes to several centimeters . As already described , if an igneous rock goes through a cooling process , its texture will granite sample . be porphyritic , as seen below in the of a I . Figure ofa Granite Sump Specimen Cooled Slowly . Jeremy is used under a license . Igneous rocks are called extrusive or volcanic when they form above the surface . They solidify after molten material pours out onto the surface through an opening such as a volcano . Extrusive or volcanic igneous rocks cool much more rapidly and therefore have smaller crystals . Since the rapid cooling time does not allow time for large crystals to form , minerals are not easy GEOGRAPHY

to see within the rock . Some volcanic igneous rocks cool so rapidly that crystals do not develop at all . These form a glass , such as obsidian . Others , such as pumice , contain holes where gas bubbles were trapped when the material was still hot and molten . The holes make pumice so light that it floats in water . The most common extrusive igneous rock is basalt , a rock that is especially common below the oceans . Figure . of an Obsidian Sample . This Extrusive Rock Quickly from Lava . Jeremy is used under a license . MA Magma is a complex fluid substance . Temperatures of most are in the range of to . Magma can get forced into adjacent rocks ( intrusion or plutonic ) or forced out to the surface ( extrusion or volcanic ) as lava or blown out in explosions which include rock pieces ( Magma is made up of atoms and molecules of melted minerals . When magma cools the atoms and molecules rearrange to form mineral grains . Rock forms when mineral grains ( often crystals ) grow together . Granite , diorite , gabbro , and basalt are a few types of igneous rock . Quartz is one of the chief minerals produced by igneous action it is made of silica ( the most common molecule in igneous minerals . Some examples of igneous volcanic rock are pumice , obsidian ( volcanic glass ) and scoria , and much more . Once Magma reaches the surface , geologists identify it as lava . COMPOSITION Igneous rocks are classified according to how and where they formed , in other words , if plutonic or volcanic , and their mineral composition , describing the minerals they contain . The GEOGRAPHY

mineral compositions of igneous rocks are usually described as being , intermediate , mafic , or . As a mafic magma starts to cool , some of the silica combines with iron and magnesium to make olivine . As it cools further , much of the remaining silica goes into plagioclase , and any silica left may be used to convert some of the olivine to pyroxene . Soon after that , all of the magma is used up and no further changes take place . The minerals present will be olivine , pyroxene , and plagioclase . If the magma cools slowly underground , the product will be gabbro if it cools quickly at the surface , the product will be basalt . tend to be cooler than mafic when crystallization begins ( because they do have to be as hot to remain liquid ) and so they may start out pyroxene ( not olivine ) and plagioclase . As cooling continues , the various reactions on the discontinuous branch will proceed because silica is abundant , the plagioclase will become increasingly , and eventually , potassium feldspar and quartz will form . Commonly even very rocks will not have biotite or muscovite because they may not have enough aluminum or enough hydrogen to make the OH complexes that are necessary for mica minerals . Typical rocks are granite and rhyolite . The image below shows some common igneous rocks classified by mode of occurrence and mineral composition . Basalt Andesite Figure List of Common Igneous Rocks . Image is under a Creative Commons Attribution International License . GEOGRAPHY

The rocks listed in the table above are the most common igneous rocks , but there are more than 700 different types of igneous rocks . Granite is perhaps the most useful one for humans . We use granite in many building materials and art . As discussed in the introduction to this lesson , pumice is commonly used for . Peridotite is sometimes mined for peridot , a type of gemstone used in jewelry . Diorite is extremely hard and is commonly used for art , as it was used extensively by ancient civilizations for vases and other decorative artwork . SUMMARY Igneous rocks form either when they cool very slowly deep within the Earth or when magma cools rapidly at the Earth surface . The composition of the magma will determine the minerals that will crystallize forming different types of igneous rocks . Intrusive , or plutonic , igneous rock forms when magma is trapped deep inside the Earth . Great globs of molten rock rise toward the surface . Some of the magma may feed volcanoes on the Earth surface , but most remain trapped below , where it cools very slowly over many thousands or millions of years until it solidifies . Slow cooling means the individual mineral grains have a very long time to grow , so they grow to a relatively large size . Intrusive rocks have a texture . Extrusive , or volcanic , igneous rock is produced when magma exits and cools above ( or very near ) the Earth surface . These are the rocks that form at erupting volcanoes and oozing fissures . The magma , called lava when molten rock erupts on the surface , cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere . Quick cooling means that mineral crystals do have much time to grow , so these rocks have a very or even glassy texture . Hot gas bubbles are often trapped in the quenched lava , forming a bubbly , vesicular texture . GEOGRAPHY