Presentation on theme: "Objectives Compare and contrast intrusive and extrusive igneous rocks. What are igneous rocks? Describe the composition of magma. Discuss the factors."— Presentation transcript:
Objectives Compare and contrast intrusive and extrusive igneous rocks. What are igneous rocks? Describe the composition of magma. Discuss the factors that affect how rocks melt and crystallize. –igneous rock –lava –extrusive –intrusive Vocabulary –partial melting –fractional crystallization –Bowens reaction series
Lava is magma that flows out onto Earths surface. What are igneous rocks? Igneous rocks are rocks that are formed from the crystallization of magma. What are igneous rocks?
Types of Igneous Rocks Extrusive igneous rocks are fine-grained igneous rocks that cool quickly on Earths surface. What are igneous rocks? Intrusive igneous rocks are coarse-grained igneous rocks that cool slowly beneath Earths surface. Granite is the most common intrusive igneous rock.
Types of Igneous Rocks Careful study of granite rock formations revealed that they cut across other rock formations. What are igneous rocks? These cross-cutting relationships are evidence that the granite was intruded, or forced into, existing rocks. Magma cools slowly beneath Earths surface and forms course-grained igneous rocks such as granite.
Composition of Magma Magma is often a slushy mix of molten rock, gases, and mineral crystals. What are igneous rocks? The elements found in magma are the same major elements found in Earths crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).
Composition of Magma Magmas are classified as basaltic, andesitic, and rhyolitic, based on the amount of SiO 2 they contain. What are igneous rocks? Of all the compounds found in magma, silica (SiO 2 ) is the most abundant and has the greatest effect on magma characteristics. Silica content affects melting temperature and also impacts how quickly magma flows.
Origins of Magma In the laboratory, most rocks must be heated to temperatures of 800°C to 1200°C before they melt. What are igneous rocks? These temperatures are found in the upper mantle and lower crust. Scientists theorize that the remaining energy from Earths molten formation and the heat generated from the decay of radioactive elements are the sources of Earths thermal energy.
Origins of Magma Factors That Affect Magma Formation What are igneous rocks? –The main factors involved in the formation of magma are temperature, pressure, water content, and mineral composition. –Temperature generally increases with depth in Earths crust, a phenomenon known as the geothermal gradient.
Origins of Magma Factors That Affect Magma Formation What are igneous rocks? –Pressure increases with depth as a result of the weight of overlying rock. –As pressure on a rock increases, its melting point increases. –Rocks and minerals often contain small percentages of water. –As water content increases, the melting point decreases.
Origins of Magma Factors That Affect Magma Formation What are igneous rocks? –Mineral content also impacts how magma is formed as different minerals have different melting points. –In general, oceanic crust is rich in iron and magnesium and therefore melts at higher temperatures than continental crust, which contains higher levels of silicon and aluminum. –For rocks to melt, the right combination of temperature, pressure, and composition must be present.
Origins of Magma Factors That Affect Magma Formation What are igneous rocks? –Granites higher water content and mineral composition cause it to melt at a lower temperature than basalt.
How Rocks Melt Partial Melting What are igneous rocks? –Because different minerals have different melting points, not all parts of a rock melt at the same time. –Partial melting is the process whereby some minerals melt at low temperatures while other minerals remain solid.
How Rocks Melt Partial Melting What are igneous rocks? –If temperatures are not great enough to melt the entire rock, the resulting magma will have a different chemistry from that of the original rock. –This is one way in which different types of igneous rocks form.
How Rocks Melt Fractional Crystallization What are igneous rocks? –When magma cools, it crystallizes in the reverse order of partial meltingthe first minerals to crystallize from magma are the last minerals to melt during partial melting. –Fractional crystallization is the process wherein different minerals form at different temperatures.
Bowens Reaction Series Bowens reaction series illustrates the relationship between cooling magma and mineral formation. What are igneous rocks? Bowen discovered two main patterns, or branches, of crystallization. –The first pattern is characterized by a continuous, gradual change of mineral compositions in the feldspar group. –The second pattern is characterized by an abrupt change of mineral type in the iron- magnesium groups.
Bowens Reaction Series What are igneous rocks?
Bowens Reaction Series Feldspars What are igneous rocks? –In Bowens reaction series, the right branch represents the feldspar minerals, which undergo a continuous change of composition. –As magma cools, the first feldspars to form are rich in calcium. –As cooling continues, their calcium-rich compositions change to sodium-rich compositions.
Bowens Reaction Series Feldspars What are igneous rocks? –In some instances, as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma. –The result is a zoned crystal that has sodium-rich outer layers and calcium- rich cores.
Bowens Reaction Series Iron-Rich Minerals What are igneous rocks? –The left branch of Bowens reaction series represents the iron-rich minerals. –These minerals undergo abrupt changes during fractional crystallization.
Bowens Reaction Series Iron-Rich Minerals What are igneous rocks? –As minerals form in the order shown in Bowens reaction series, elements are removed from the magma. –Silica and oxygen are left over at the end of the reaction series. –When the remaining melt, enriched with silica and oxygen, finally crystallizes, quartz is formed.
–Geologists hypothesize that under certain conditions, newly formed crystals can be separated from magma. –This stops the chemical reactions between the magma and the minerals. –Crystal separation can occur when crystals settle to the bottom of the magma body, and when liquid magma is squeezed from the crystal mush to form two distinct igneous bodies with different compositions. Bowens Reaction Series Crystal Separation What are igneous rocks?
–In some magma bodies, the minerals form into distinct bands in the order shown in Bowens reaction series resulting in a layered intrusion. Bowens Reaction Series Layered Intrusions What are igneous rocks? –Geologists are uncertain how these layers form.
–Layered igneous intrusions can be valuable sources of rare metals. Bowens Reaction Series Layered Intrusions What are igneous rocks? –Some have very high concentrations of elements such as platinum, chromium, nickel, or gold.
Section Assessment 1.Match the following terms with their definitions. ___ extrusive ___ intrusive ___ lava ___ fractional crystallization What are igneous rocks? A.coarse-grained igneous rocks that cool slowly beneath Earths surface B.magma that flows out onto Earths surface C.fine-grained igneous rocks that cool quickly on Earths surface D.the process wherein different minerals form at different temperatures
Section Assessment 2.Describe partial melting. What are igneous rocks?
________ In Bowens reaction series feldspars undergo abrupt changes. ________ The melting point of basalt is higher than granite. ________Increased pressure decreases the melting point of rock. ________ Quartz is the last mineral to crystallize. Section Assessment 3.Identify whether the following statements are true or false. What are igneous rocks?
End of Section 1
Objectives Classify different types and textures of igneous rocks. Classifying Igneous Rocks Recognize the effects of cooling rates on the grain sizes of igneous rocks. Describe some uses of igneous rocks. –felsic –mafic –ultramafic –porphyritic Vocabulary –pegmatite –kimberlite
Classifying Igneous Rocks Igneous rocks are broadly classified as intrusive or extrusive. Classifying Igneous Rocks Igneous rocks are further classified by their mineral compositions. Physical properties such as grain size and texture serve as clues for the identification of various igneous rocks.
Mineral Composition The three main groups of igneous rocksfelsic, mafic, and intermediateare classified according to their mineral compositions. Classifying Igneous Rocks –Felsic rocks, such as granite, are light-colored and have high silica contents. –Mafic rocks, such as gabbro, are dark-colored, have lower silica contents, and are rich in iron and magnesium. –Intermediate rocks, such as diorite, have some characteristics of both felsic and mafic rocks.
Mineral Composition Classifying Igneous Rocks
Mineral Composition Ultramafic Rocks Classifying Igneous Rocks –Ultramafic rocks are unusual in that they have low silica contents and very high levels of iron and magnesium. –Some scientists theorize ultramafic rocks are formed by the fractional crystallization of olivine and pyroxene. –Another hypothesis is that ultramafic rocks represent pieces of the upper mantle that have been brought close to Earths surface.
Grain Size In addition to differences in their mineral compositions, igneous rocks differ in the sizes of their grains. Classifying Igneous Rocks Cooling Rates –When lava flows on Earths surface, it cools quickly and there is not enough time for large crystals to form. –Extrusive igneous rocks have no visible mineral grains. –When magma cools slowly beneath Earths surface, there is sufficient time for large crystals to form. –Intrusive igneous rocks may have crystals larger than 1 cm.
Texture Often, its easier to observe the sizes of mineral grains than it is to observe their shapes. Classifying Igneous Rocks Many mineral grains have interlocking edges. As the grains crystallize from magma, they grow together and form irregular edges. During fractional crystallization, the minerals that form early in the process float in a liquid and have space in which to grow distinct crystal shapes.
Texture Porphyritic Texture Classifying Igneous Rocks –A rock that has a porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals. –Porphyritic textures indicate a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.
Igneous Rocks as Resources Igneous rocks have several characteristics that make them especially useful as building materials. Classifying Igneous Rocks –The interlocking grain textures of igneous rocks help to give them strength. –Many of the minerals found in igneous rocks are resistant to weathering.
Ore Deposits Ores are minerals that contain a useful substance that can be mined at a profit. Classifying Igneous Rocks Valuable ore deposits are often associated with igneous intrusions. These deposits sometimes occur as veins.
Ore Deposits Veins Classifying Igneous Rocks –Some important metallic elements that are not included in common minerals are gold, silver, lead, and copper. –These elements, along with the dissolved silica, are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rock. –This fluid solidifies to form metal-rich quartz veins, such as the gold-bearing veins.
Ore Deposits Pegmatites Classifying Igneous Rocks –Vein deposits may contain other valuable resources in addition to metals. –Pegmatites are veins of extremely large-grained minerals. –Ores of rare elements, such as lithium and beryllium, are found in pegmatites. –Because these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes.
Ore Deposits Kimberlites Classifying Igneous Rocks –Diamond is a valuable mineral found in rare, ultramafic rocks known as kimberlites. –Kimberlites are a variety of peridotite and likely form deep in the crust at depths of 150 to 300 km or in the mantle. –Minerals found in kimberlites can form only under very high pressures. –Geologists hypothesize that kimberlite magma is intruded rapidly upwards towards Earths surface, where it forms long, narrow, pipelike structures.
Section Assessment 1.Match the following terms with their definitions. ___ felsic ___mafic ___ ultramafic ___ porphyritic Classifying Igneous Rocks A.igneous rocks that have low silica content and very high levels of iron and magnesium B.igneous rocks that are light-colored, have high silica contents, and contain quartz and feldspars C.texture characterized by well- formed crystals surrounded by finer-grained crystals D.igneous rocks that are dark- colored, have lower silica contents, and are rich in iron and magnesium
Section Assessment Classifying Igneous Rocks 2.What characteristic would indicate that kimberlites are formed deep within the crust or in the mantle?
Section Assessment 3.Why are some of the worlds most beautiful crystals found in pegmatites? Classifying Igneous Rocks
End of Section 2
Section 5.1 Main Ideas Igneous rocks are formed by the cooling and crystallization of magma. Intrusive rocks form inside Earths crust, and extrusive rocks form on Earths surface. Extrusive rocks, which cool more rapidly than intrusive rocks, are generally more fine grained. Magma is a slushy mix of molten rock, gases, and mineral crystals. The elements found in magma are the same major elements found in Earths crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na). Section 5.1 Study Guide
Section 5.1 Main Ideas Silica (SiO 2 ) is the most abundant compound in magma. Magmas are classified as basaltic, andesitic, and rhyolitic, based on the amount of SiO 2 they contain. Different minerals melt and crystallize at different temperatures in the processes of partial melting and fractional crystallization. Minerals crystallize from magma in a sequential pattern known as Bowens reaction series. Section 5.1 Study Guide
Section 5.2 Main Ideas Igneous rocks are classified as felsic, mafic, intermediate, and ultramafic, depending upon their mineral compositions. Felsic rocks such as granite are light- colored, have high silica contents, and contain quartz and feldspars. Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. Intermediate rocks have moderate silica levels. Ultramafic rocks have low silica contents and very high levels of iron and magnesium. Igneous groups can be further identified by crystal size and texture. Early forming minerals may have well-shaped crystals, while later-forming minerals have irregular shapes. Porphyritic textures contain both large and small crystals. Section 5.2 Study Guide
Section 5.2 Main Ideas Igneous rocks such as granite are often used as building materials because of their strength, durability, and beauty. Valuable ore deposits and gems are often associated with igneous intrusions. Ores of rare elements such as lithium and beryllium are found in veins of extremely large- grained minerals called pegmatites. Diamonds are found in rare types of igneous intrusions known as kimberlites. Section 5.2 Study Guide
1.The process wherein different minerals form at different temperatures is called ____. a.partial melting b.Bowens reaction series c.fractional crystallization d.crystal separation Multiple Choice Chapter Assessment Partial melting is when some minerals melt at low temperatures while others remain solid. Bowens reaction series describes in what order minerals form. Crystal separation is a hypothesis that answers questions about Bowens reaction series.
2.Andesitic magmas SiO 2 concentration is ____. a.30 percentc.60 percent b.45 percentd.70 percent Basaltic magma has an SiO 2 concentration of 50 percent. Rhyolitic magma has an SiO 2 concentration of 70 percent. Multiple Choice Chapter Assessment
Multiple Choice 3.What minerals are associated with the left branch of Bowens reaction series? a.iron-magnesiumc.intrusive b.feldsparsd.felsic Chapter Assessment Feldspars are represented on the right branch of Bowens reaction series. Intrusive describes igneous rocks that cool slowly beneath Earths surface. Felsic rocks are one classification of igneous rocks.
Multiple Choice 4.What is the texture of ultramafic rocks? a.glassyc.coarse-grained b.fine-grainedd.very coarse- grained Chapter Assessment Ultramafic rocks include peridotite and dunite which both have a coarse grain.
Multiple Choice 5.Extrusive igneous rocks generally have small mineral grains because ____. a.they cool slowly. b.the minerals in extrusive rocks do not form crystals under any condition. c.they cool quickly. d.there is too much water in the magma. Chapter Assessment By quickly cooling, the minerals in extrusive igneous rocks do not have enough time to form large crystals.
Short Answer 6.What characteristics make igneous rocks useful as building materials? Chapter Assessment
Short Answer 7.What causes a porphyritic texture to form in certain rocks? Chapter Assessment
True or False 8.Identify whether the following statements are true or false. ______ Diamonds are found in kimberlites. ______ Fe-Mg minerals undergo a continuous change according to Bowens reaction series. ______Silica is the most common compound in magma. ______ A rock that melts at 1000ºC on the surface of Earth may not melt until 1200ºC at a depth of 100 km. ______ Fractional crystallization occurs in the reverse order of partial melting. Chapter Assessment