1. Topic 3 – Igneous Rocks

2. Topic 3 - Igneous Rocks Outline Introduction Magma and Lava Texture Composition Bowens Reaction Series Changes in Magma Composition Igneous Rock Classification Ultramafic Rocks Mafic Rocks Intermediate Rocks Felsic Rocks Other Igneous Rocks Plutons

3. Introduction Igneous rocks are formed by the crystallization of molten rock material called magma. Igneous rocks are one of the 3 main rock types. Sedimentary and metamorphic rocks are the other 2 types. Recall the rock cycle.

5. Characteristics of igneous rocks Rhyolite (extrusive, rapid cooling) vs. Granite (intrusive, slow cooling) Extrusive igneous rocks form by the solidification of lava at the Earth’s surface. Intrusive igneous rocks are formed when magma solidifies within the crust or mantle. Both types of igneous rocks are named and classified on the basis of rock texture and mineral assemblage . Igneous rocks form by cooling and solidification of magma. Crystallization of mineral grains. Larger mineral grains are formed during slow cooling. Smaller mineral grains grow during rapid cooling. Texture is very important in igneous rocks. Finely crystalline (volcanic) vs. coarsely crystalline (plutonic).

6. Igneous Features and Landforms

7. Magma and Lava magma Molten rock, together with any suspended mineral grains and dissolved gases, that forms when temperatures rise sufficiently high for melting to happen in the crust or mantle. volcano Vent from which magma, solid rock debris and gases are erupted onto the Earth’s surface and into the Earth’s atmosphere. Term volcano comes from the name of the Roman god of fire: Vulcan. lava Magma that reaches the Earth’s surface. Will cover the following subtopics : Key Characteristics of Magma Types of Magma Gases Dissolved in Magma Temperature of Magma Viscosity of Magma

8. Three key characteristics of magma: (i) Range in Composition (ii) High Temperatures (iii) Ability to Flow

9. (i) Range in Composition Silica (SiO 2 ) always occurs. But in varying proportions. Composition influences the viscosity or ability of the fluid to flow. (ii) High Temperatures Magma and lava can reach temperatures of 1400° C. Or up to 1600° C in extreme cases. (iii) Ability to Flow Magma has the properties of a liquid. Most magma is a mixture of crystals and liquid. Often referred to as a “melt”.

10. Parent magma plays a significant role in determining the mineral composition of an igneous rock. Four magma types : Ultramafic Mafic Intermediate Felsic

12. Krakatau is an example of andesitic volcano. Indonesia, August 27, 1883. Island disappeared. Sound of the volcanic eruption was heard 4600 km away. Giant tsunami or sea wave that was 40m high crashed into Java. 36,000 people were killed. 20 km 3 of volcanic debris was ejected into the atmosphere. Produced strangely coloured sunsets...green, blue, purple. Earth temperature dropped 0.5° C. Northern hemisphere had no summer for two years (snow in July). It took 5 years for all of the dust to settle back to the ground. Significant climate change.

16. Viscosity of Magma : Viscosity is an internal property of a substance that offers resistance to flow. Water has a low viscosity. Porridge or spaghetti sauce has a higher viscosity. Viscosity = 1/fluidity. Low viscosity = high fluidity. High viscosity = low fluidity. Some magmas are very fluid, others are not. Basaltic lava in Hawaii has been measured at 16 km/h. i.e. low viscosity. This is rare. Maximum speed 50-60 km/hr: confined flow, downhill. Most magmas will move considerably slower. Meters per hour or day.

17. Two factors effect the viscosity of magma: (a) temperature  temp  viscosity (b) silica content  silica  viscosity The higher the temperature, the lower the viscosity. Magma more readily flows at a higher temperature. As a lava cools from a volcano, the flow rate begins to slow down. It eventually slows to a complete halt. The lower the silica content, the more easily the magma will flow. Basalts flow more freely than rhyolites. Thinner lavas. Better flowing magmas/lavas allow gas to escape. Therefore not as explosive. e.g. mafic. If the magma has a high silica content, it is less runny (i.e. more viscous). Felsic lavas are thick and slow-moving. Therefore high silica = high viscosity (stickiness).

18. Magma Rising from the Mantle

24. Porphyritic Texture Some magmas are cool enough to have crystals floating around in them. If the whole lot is now cooled more rapidly, these early formed crystals will be preserved as phenocrysts . phenocryst Large mineral grains suspended in a finely crystalline groundmass. Final product is said to be porphyritic. Larger crystals set in a finer grained groundmass .

25. The name given to an intrusive igneous rock that contains unusually large mineral grains (average grain diameter > 2 cm) is a pegmatite . Pegmatites are found in eastern and northern Manitoba. Are mined for cesium and tantalum (mineral name is tantalinite). e.g. Bernic Lake Pegmatite, Eastern Manitoba. The Tanco Mine is a tantalum-cesium-lithium producer located east of Winnipeg. The pegmatite, which does not outcrop to surface, was originally discovered in the 1920s during a diamond drill program.

26. Tantalum is a very useful metal with unique properties. The major uses of tantalum are in the electronics industry and for cutting tools. High quality capacitors are the major single use for tantalum. Tantalum carbide is used in production of hard metal alloys for cutting tools. Other tantalum alloys are important constituents of aero engines as well as in acid resistant pipes for the chemical industry. Tantalum pins are used for medical purposes such as hip-joint replacement, since tantalum is the only metal that is not rejected by bodily fluids. Cesium formate is a water clear, water soluble fluid with a specific gravity of 2.3g/cc (two and one third times the density of water). It is used in the oil industry as a drilling fluid where the properties of low viscosity, high SG and complete solution have significant benefits over traditional bentonite/barite drill muds in deep wells (> 4,575m/15,000 feet!).

28. Pyroclastic or Fragmental Texture Characterizes igneous rocks that are formed by explosive volcanic activity. Made up of fragments from within the volcano or material that is ripped-up from the edges of the volcano. e.g. volcanic ash. e.g. volcanic bomb: “tear-drop” shaped, erupted as globs of lava and cooled/solidified upon descent.

30. Composition of Igneous Rocks Magma has many elements within it. As magma cools, a number of different minerals crystallize, all with different melting/freezing points. As magma cools, minerals with high melting/freezing points crystallize first. Followed by those with lower melting/freezing points. Parent magma plays a significant role in determining the mineral composition of an igneous rock. Recall the earlier definition of magma types: Ultramafic Mafic Intermediate Felsic

31. Magma types are defined based on silica content. Magmas have a wide range of compositions and are also exposed to a wide range of temperatures, pressures and cooling conditions. It is possible for the same magma to generate two completely different igneous rocks because its composition can change as a result of the sequence in which minerals crystallize, settle, assimilate and mix. Will examine two key topics related to how crystals form: (A) Bowen’s Reaction Series (B) Change in Magma Composition (i) Magma Mixing (ii) Crystal Settling (iii) Assimilation

32. Bowen’s Reaction Series Canadian born scientist N.L.Bowen. First recognized the importance of “ magmatic differentiation by fractional crystallization ” . Term used to describe the sinking of dense, early crystallized minerals to the bottom of a magma chamber thereby forming a solid mineral layer covered by melt. Minerals do not crystallize at the same time. Bowen suggested that a single magma could crystallize into both basalt (mafic rock) and rhyolite (felsic rock) because of fractional crystallization. Theoretically correct, but does not occur in significant volumes in nature. In nature, the crystallization of a basaltic magma occurs too quickly for the melt to become dominantly rhyolitic. Bowen proved that specific minerals crystallize from magma at different times under different temperature conditions. He studied this in the laboratory and also through observations in the field. He proposed a mechanism, which is now called the “ Bowen’s reaction series ” , to account for the derivation of intermediate and felsic rocks from a basaltic (mafic) magma.

33. Important part of his theory is that once a mineral is crystallized from a magma it can still chemically react with the liquid magma in order to form new minerals. Bowen identified 2 types of reactions: Discontinuous Continuous

34. Discontinuous Reaction Series Discontinuous reactions led to the formation of completely different minerals as the magma cooled and reacted with the crystallized minerals. Minerals associated with this process are olivine, pyroxene, amphibole and biotite mica (in order of decreasing temperature of formation). This branch of the Bowen’s reaction series is called the “ discontinuous branch ” .

35. For ferromagnesian minerals, olivine crystallizes first as the magma cools. Leaves a melt or magma enriched in SiO 2 because olivine has 40 % SiO 2 , while a typical basaltic magma has 50 % SiO 2 . As the temperature drops past a certain point, pyroxene will start to form. Solid olivine reacts with the melt to form a more silica-rich mineral, pyroxene. If the cooling is at a very slow rate, all of the olivine will react to form pyroxene. This is called a discontinuous reaction series . Early formed minerals form entirely new compounds through reaction with the remaining liquid. Reaction converts one mineral into another mineral. The reaction is not always complete. e.g. olivine may have a rim of pyroxene which would indicate an incomplete reaction.

36. Continuous Reaction Series In contrast, continuous reactions led to the gradual chemical change of a specific mineral group: the plagioclase feldspars. They continuously change from Ca-rich plagioclase to Na-rich plagioclase, as the temperature decreases. Ca Na Na Na Na Ca Ca

37. Plagioclase grains in many igneous rocks have concentric zones of differing compositions. Inner layers are Ca-rich, outer layers are Na-rich. Bowen pointed out the significance of these zoned crystals. Also, he observed that the main plagioclase associated with basalt is Ca-rich, while in contrast the main plagioclase associated with rhyolite is Na-rich. This branch of the Bowen’s reaction series is called the “ continuous branch ” or the “ continuous reaction series ” .

38. Both branches of the Bowen’s reaction series meet at a common mineral. Potassium feldspar or “K-spar”. Continue to form muscovite mica. Finally quartz. As the temperature of the magma cools even further. Then the crystallization of the magma is complete.

39. Change in Magma Composition Bowen showed that magmas change composition during their cooling history. Three mechanisms by which a magma can change composition: Mixing with other magmas. Crystal settling. Assimilation of surrounding rock material.

40. Magma Mixing Felsic magma Mafic magma Obvious. Two different magmas are mixed. Forms a third magma. Different composition from the parent magma. Intermediate magma

41. Mafic magma mixing with felsic magma. Produces an intermediate magma.

42. Crystal Settling crystal (e.g. olivine) magma chamber country rock One of the mechanisms by which a magma can change composition. Involves physical separation of minerals by crystallization and gravitational settling. Does occur in magmas. However, not as widespread as what Bowen first theorized. Involves physical separation of minerals by crystallization and gravitational settling. Occurs in large magma chambers, or large calm volumes of magma.

43. Fractional Crystallization

44. Assimilation magma chamber country rock inclusion One of the mechanisms by which a magma can change composition. Assimilation is a process whereby a magma reacts with pre-existing rock, called “country rock”, with which it comes in contact. Country rock is partially or completely melted by the instrusive body. Blocks of country rocks can be observed at the margins of the instrusive body and these are termed inclusions .

45. Fragments of rock dislodged by upward-moving magma could remain as inclusions .

46. Many inclusions are broken or wedged off the walls of the magma chamber and incorporated into the molten magma. Effect on the bulk composition of the magma is generally thought to be minimal. Only a limited amount of rock can be included. Inclusions tend to reduce the temperature of the magma and therefore speed up the process of crystallization. Relative age determination in the presence of included fragments . One of the fundamental principles of historical geology. “ Whenever two rock masses are in contact, the one containing pieces of the other will be the younger of the two”. Lyell 1830. Inclusions in an igneous rock represent older parent rock material, while the igneous rock that contains the inclusions is younger.

50. Ultramafic Rocks peridotite Composed largely of ferromagnesian silicate minerals. Peridotite is an ultramafic rock that contains mostly olivine, lesser amounts of pyroxene and minor amounts of plagioclase feldspar. Dark green or black in colour. Note: olivine is a green mineral. Upper mantle origin. Very rare at the surface. Rare in rocks younger than 2.5 billion years.

51. Peridotite. Mostly made up of olivine minerals (green).

52. Very hot temperatures associated with ultramafic magma (up to 1600° C). 1300° to 1600° C. These high temperatures are not common today. But were more typical of the Earth’s past. An example of an ultramafic rock is kimberlite . Host rock for diamonds. Named after Kimberly, South Africa. Kimberlite rock is generally unstable at the Earth’s surface and tends to weather very rapidly compared to the surrounding host or country rock. Many kimberlite pipes are therefore at the base of a lake or swamp in Canada. Very difficult to prospect for and find. Use geophysical (magnetic) surveys and geochemical trace element sampling to help pinpoint these valuable rocks. Latter involves sampling glacial overburden and identifying indicator mineral trains (pyrope garnet, chrome diopside, ilmenite). Diamond mines in the NWT are mining kimberlite pipes.

54. Mafic Rocks gabbro basalt Temperatures 1000° to 1200° C. Low viscosity (i.e. runny). Generally dark colour. Relatively dense. Basalt (extrusive). Gabbro (intrusive). e.g. Hawaii.

55. basalt Crystallize from mafic magmas. Silica content 45-52 %. Large proportion of ferromagnesian minerals. Basalt is the most common extrusive igneous rock. Basalt lava flows dominate the sea bed (ocean crust).

56. Basalt. Mafic extrusive rock. Aphanitic texture.

57. Gabbro. Mafic intrusive rock. Phaneritic texture.

58. Intermediate Rocks andesite diorite Intermediate silica content. 700° to 1000° C. Medium viscosity (i.e. a bit runny). Intermediate colour. Equal amounts of dark and light coloured minerals. Andesite (extrusive). Diorite(intrusive). Syenite and monzonite are similar to granite, but less quartz. e.g. Mount St. Helens produced andesitic lava. Close to felsic in composition: dacitic (62-63 % silica).

59. andesite diorite Intermediate in composition: 53 to 65 % silica. Mostly plagioclase feldspar (light minerals) with amphibole or biotite (dark minerals). “ Salt and pepper ” appearance. Andesite is common in the Cascade Range and Andes Mountains. Diorite is fairly common in the continental crust. Not as common as granite.

60. Andesite. Intermediate extrusive rock. Hornblende phenocrysts. Porphyritic texture.

61. Diorite. Intermediate intrusive rock. Salt and pepper appearance. Phaneritic texture.

62. Felsic Rocks granite rhyolite Silica-rich rocks. 600° to 800° C. High viscosity (i.e. not runny, sticky). Light colour. Relatively low density. Rhyolite (extrusive). Granite (intrusive). e.g. Yellowstone National Park. Both granite and rhyolite are derived from felsic magmas. Silica content of > 65 %.

63. rhyolite granite Felsic igneous rocks consist of the following minerals: - potassium feldspar (K-spar) - Na-rich plagioclase feldspar - quartz - some biotite - rare amphiboles Considerable Na, K and Al. Little Ca, Fe and Mg. Granites are common in the PC Shield areas of Canada. Many granitic rocks in northern and north-eastern Manitoba. Granites are the most common intrusive igneous rock.

64. Other Igneous Rocks (i) Pegmatites Very coarsely crystalline igneous rocks. Mentioned in the previous section. Minerals > 1 cm. Most pegmatites consist of the same minerals as granite. K-spar, plagioclase and quartz. Spatially associated with granite plutons. Thought to represent the minerals formed from the remaining fluid and vapour phases that existed after the granite had crystallized. Water-rich vapour phase contains rare elements, such as cesium and lithium. e.g. Bernic Lake Pegmatite, E. MB. Tanco mine. Will briefly discuss three other varieties of igneous rocks: (i) Pegmatites, (ii) Volcanic Tuff and Breccia, (iii) Obsidian and Pumice.

65. Pegmatite (light coloured rock) exposed in the Black Hills, South Dakota. Gem minerals, such as tourmaline, are observed in some pegmatites.

66. (ii) Volcanic Tuff and Breccia Fragmental material erupted from volcanoes which eventually turns into rock. Collective term used for these igneous rocks is “pyroclastic”. Pyroclastic rock is a rock formed from fragments that are ejected during a volcanic eruption. Tuff is used to describe volcanic ash-sized material that becomes a rock. < 2 mm in diameter (ash is < 2 mm in diameter). Use a prefix to describe the composition. e.g. rhyolitic tuff. Volcanic breccia is used to describe volcanic lapilli- sized (2-64 mm in diameter) and block- or bomb-sized (> 64 mm in diameter) material that becomes a rock. Larger sized material than tuff.

67. obsidian Scoria Pumice (iii) Obsidian and Pumice Varieties of volcanic glass. Obsidian was mentioned earlier. Black and conchoidal fracture pattern of glass. Pumice contains numerous vesicles or “bubbles”. Looks like an Aero chocolate bar. Porous. Gas escapes through lava. Forms a “ froth ” which solidifies into pumice. Pumice (felsic). Scoria (mafic).

69. Plutons Magma that forms a pluton did not originate where we now find the body. Originated much deeper into the crust and mantle. Was intruded upwards into the surrounding rock. Plutons are classified into categories: Dike Sill Batholith Stock Laccolith All bodies of intrusive igneous rocks, regardless of shape and size are called plutons. Named after Pluto, the Greek god of the underworld. Pluton is defined as an intrusive igneous body.

70. Dike Tabular, parallel-sided sheets of intrusive igneous rock. Cuts across layering of intruded rock. Discordant : boundaries that cut across layering of the country rock. Dike forms when magma squeezes into a fracture. Mostly small bodies (1 to 2 m across). Some greater than 100 m across. Later cools to fill the fissure. A dike can be a conduit for magma to travel to the surface and be erupted by a volcano as a lava flow. This is termed a “ volcanic pipe ”.

71. Examples of volcanic necks : Le Puy, France and Shiprock, New Mexico (with radiating dikes ).

72. Sill Tabular, parallel-sided sheets of intrusive igneous rock that are parallel to the layering of the intruded rock. Concordant : boundaries that are parallel to the layering of the country rock. Thicknesses are mostly a meter or less. Some are hundreds of meters thick. e.g. Palisades Sill, West Side of Hudson River, NY. Commonly occurs together with a series of dikes (dike “swarms”). Does not push the crust upwards into a dome. If a dome develops, then the body is called a laccolith.

73. Batholith A very large, igneous body of irregular shape that cuts across the layering of the rock it intrudes. Largest kind of pluton. Mostly granite. Some 1000 km long and 250 km wide. Most 20 to 30 km thick. Well known batholith exposed in Yosemite National Park (El Capitan), California. Cliff is 900 m high. Tallest unbroken cliff in the world. Most are composite masses. Comprise a number of separate intrusive bodies of slightly differing composition. Important mineral resources in batholiths include gold and copper deposits . Mineral rich solutions move through the cracks (fractures) in the granite and the ore minerals are precipitated into these pore spaces.

74. Emplacement of batholiths is somewhat analagous to salt dome or diapir emplacement. Less dense material rises upwards and laterally displaces the overlying country rock. In the case of a batholith, the magma could also melt or fragment the country rock, as well as laterally displacing the overlying rock.

75. xenolith Emplacement of a Batholith: Stoping : Magma is injected into fractures and planes between layers in the country rock. Blocks of country rock are detached and engulfed in the magma, thus making room for the magma to rise farther. Some of the engulfed blocks might be assimilated, and some might remain as inclusions: xenoliths .

76. Granitic rocks, Yosemite National Park, CA. Part of the Sierra Nevada batholith : 640 km long and up to 110 km wide. Near vertical cliff. El Capitan. Rises > 900 m above the valley floor. Highest unbroken cliff in the world.

77. Stock A small, irregular body of intrusive igneous rock, smaller than a batholith. Cuts across the layering of the intruded rock. No larger than 10 km in diameter. Could be a companion body to a batholith or even the top of an eroded batholith. Laccolith A lenticular pluton intruded parallel to the layering of the intruded rock, above which the layers of the invaded country rock have been bent upward to form a dome. Dome is recognized as an elevated area on the Earth’s surface.

78. Mantle Melting and Plate Tectonics