Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
Topic 3 igneous rocks
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Topic 3 igneous rocks

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  • - Textbook Chapter 3 - Review Questions at the end of Chapter 3   - 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 shown as Overhead during Lecture #1
  • - 2 main types of igneous rocks: - volcanic (extrusive) - plutonic (intrusive)   (a) Volcanic - cool at the surface of the Earth - extrusive igneous rocks - common along continental margins of active tectonic plates   - “Pacific Ring of Fire” - very significant in the ocean - volcanic islands (Hawaii, Iceland, etc) and mid-oceanic ridges   - subject of Topic 4 (Chapter 4)     (b) Plutonic - cool below the surface of the Earth - intrusive igneous rocks - common in mountain chains and continental areas   - subject of second half of Topic 3 (Chapter 3)  
  • Characteristics of Igneous Rocks - 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   - 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   - texture is very important in igneous rocks - finely crystalline (volcanic) vs coarsely crystalline (plutonic)  
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • (2) 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: (A) Key Characteristics (B) Types of Magma (C) Gases Dissolved in Magma (D) Temperature of Magma (E) Viscosity of Magma
  • Figure 3.5 Both intrusive and extrusive igneous activity take place at divergent plate boundaries (spreading ridges) and where plates are subducted at convergent plate boundaries. Oceanic crust is composed largely of dark igneous rocks in plutons that cooled from submarine lava flows. Magma forms where an oceanic plate is subducted beneath another oceanic plate or beneath a continental plate as shown here. Much of the magma forms plutons, but some is erupted to form volcanoes (see Chapter 4).
  • (A) Key Characteristics - 3 key characteristics of magma: - range in composition - high temperatures - ability to flow   (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”      
  • (B) Frou Types of Magma (i) Ultramafic Magma - silica content = < 45 % - silica poor - 1300° to 1600° C   - ultramafic magma generates peridotite (igneous rock) - intrusive rock name    (ii) Mafic Magma - silica content = 45-52 % - silica poor - 1000° to 1300° C   - mafic magma generates basalt and gabbro (igneous rocks) - extrusive and intrusive rock names - more Ca, Fe and Mg    (iii) Intermediate Magma - silica content = 53-65 % - intermediate silica content - 700° to 1000° C   - intermediate magma generates andesite and diorite (igneous rocks) - extrusive and intrusive rock names    (iv) Felsic Magma - silica content = > 65 % - silica rich - 600° to 800° C - felsic magma generates rhyolite and granite (igneous rocks) - extrusive and intrusive rock names - considerable Na, K and Al - little Ca, Fe and Mg      - these four magmas are not formed in equal abundance - 80 % of all magmas erupted by volcanoes is basaltic ( e.g. Hawaiian volcanoes)   - 10 % are andesitic ( e.g. Mt. St. Helens is an andesitic volcano)   - e.g. Krakatau is an 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   - 10 % are rhyolitic - e.g. dormant Yellowstone National Park is rhyolitic   - minor concentration of ultramafic magma (< 1 %)  
  • (C) Gases Dissolved in Magma - magma consists of three different phases - solid: crystals (silicate minerals) - liquid: molten material (mainly silicate) - gas: volatiles   - small amounts of gas (0.2 to 3.0 % by weight) are dissolved in all magmas - these gases strongly influence the properties of magma   - gas consists of water vapour, carbon dioxide, nitrogen, chlorine, sulphur and argon - principal gas is water vapour - water vapour and CO 2 constitute 98 % of all gases emitted dissolved in magma   - effect of volatiles on the behaviour of magma and lava is important   - generally speaking gases: (a) increase the fluid nature of lava (b) lower the melting point of the lava (c) increase the likelihood of explosive eruption   - highly viscous (sticky) silica-rich lavas tend to trap gases - they are trapped at high pressures and temperatures beneath the Earth’s surface - when the gases reach near the surface they tend to expand explosively - therefore rhyolite lavas tend to erupt explosively - these explosions produce great clouds of ash and dust, rather than runny lava flows - e.g. Mt. St. Helens (1980)    
  • (D) Temperature of Magma - difficult to measure - no direct measurements have been taken beneath the surface - occasionally can be done during volcanic eruptions - only for mafic lavas because felsic lavas are too explosive    - use optical devices for measurement - optical pyrometer   - temperature average 1000° to 1200° C - some magmas reach temperatures of 1400° C - or up to 1600° C in certain cases   - temperature and composition are linked: 1300° to 1600° C Ultramafic 1000° to 1300° C Mafic 700° to 1000° C Intermediate 600° to 800° C Felsic   - ash flows from Mt. St. Helens were 300 to 420° C two weeks after the eruption!    
  • Viscosity of Magma viscosity - internal property of a substance that offers resistance to flow - water has a low viscosity - porridge or spaghetti sauce has a higher viscosity   - 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   - most magmas will move considerably slower - meters per hour or day  
  •   - 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   - 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)
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • Figure 3.8 The various textures of igneous rocks. Texture is one criterion used to classify igneous rocks. (a,b) Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture. (c,d) Slower cooling in plutons yields a phaneritic texture. (e,f) These porphyritic textures indicate a complex cooling history. (g) Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form. (h) Gases expand in lava and yield a vesicular texture. (i) Microscopic view of an igneous rock with a fragmental texture. The colorless, angular objects are pieces of volcanic glass measuring up to 2 mm. Texture of Igneous Rocks Consequence of rate of cooling Intrusive rocks… cool slowly at depth Extrusive rocks… cool quickly at the surface Most obvious feature of an igneous rock is the size of the mineral grains intrusive igneous rocks tend to be coarse grained extrusive igneous rocks tend to be fine-grained
  •   - at least 6 different textural terms: (A) Phaneritic (B) Aphanitic (C) Glassy (D) Porphyritic (E) Vesicular or Amygdaloidal (F) Pyroclastic or Fragmental
  • (A) Phaneritic Texture Rock in which the mineral grains are of such a size as to be visible with the unaided eye or the help of a hand lens is phaneritic. - magmas which cool slowly (plutonic) tend to form coarse grained rocks (phaneritic) - because the crystals have enough time to grow large   - intrusive igneous rocks - e.g. granite    
  • (B) Aphanatic Texture Rock in which the grain size is so small that the individual grains can be seen only with a microscope is aphanitic.   - magmas which cool fast (volcanic) tend to form fine grained rocks (aphanitic) - because the crystals do not have time to grow large   - extrusive igneous rocks - e.g. basalt  
  • The magma cooled so quickly, it did not have time to grow any crystals… it just quenched!… freezed!… glass!!!   - magmas which cool extremely fast (volcanic) tend to form glassy rocks - because the crystals do not have time to form at all   - extrusive igneous rocks that are largely or wholly glassy are called obsidian - such rocks display a distinctive fracture pattern on a broken surface - fracture pattern consists of a series of smooth curved surfaces  
  • - 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   - and the final product is said to be porphyritic - “larger crystals set in a finer grained groundmass”   - 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 tantalun (mineral name is tantalinite)   - e.g. Bernic Lake Pegmatite, Eastern Manitoba - Tanco Mine - cesium is mined for use in drilling mud - used for very deep drilling (> 20,000 to 30,000 feet deep)
  • (E) Vesicular or Amygdaloidal Texture - some lavas contain gas bubbles which are preserved when they cool - gas is mostly water vapour or CO 2   - these old gas bubbles are called vesicles - rocks that possess numerous vesicles are termed vesicular       - sometimes the vesicles have been filled in later by other minerals - produces an amygdaloidal texture - i.e. gas bubble vesicles or cavities are filled with a solid mineral  
  • (F) 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/solified upon descent    
  • Figure 3.15 Igneous rocks for which texture is the main consideration in classification. Composition is shown, but it is not essential for naming these rocks.
  • Composition: -second most important characteristic -first: texture - magma has many elements within it - as the magma cools, a number of different minerals can 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   - 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  
  • Composition: -second most important characteristic -first: texture - magma has many elements within it - as the magma cools, a number of different minerals can 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   - 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  
  • (A) 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   - 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
  • Figure 3.3 Bowen’s reaction series consists of a discontinuous branch along which a succession of ferromagnesian silicates crystallize as the magma’s temperature decreases, and a continuous branch along which plagioclase feldspars with increasing amounts of sodium crystallize. Notice also that the composition of the initial mafic magma changes as crystallization takes place along the two branches.
  • (i) Discontinuous - 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”   - 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    
  • (ii) Continuous - 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   - plagioclase grains in many igneous rocks have concentric zones of differing compositions - innermost layers are Ca-rich, outermost 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”       - 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  
  •     - 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  
  • (B) Change in Magma Composition - three mechanisms by which a magma can change composition: (i) mixing with other magmas (ii) crystal settling (iii) assimilation of surrounding rock material
  • (i) Magma Mixing - obvious - two different magmas are mixed - forms a third magma - different composition from the parent magma  
  • Figure 3.7 Magma mixing. Two magmas mix and produce magma with a composition different from either of the parent magmas. In this case, the resulting magma has an intermediate composition.
  • (ii) Crystal Settling - 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  
  • To view this animation, click “View” and then “Slide Show” on the top navigation bar.
  • assimilation - 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 intrusive body - blocks of country rocks can be observed at the margins of the instrusive body and these are termed inclusions - 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
  • Figure 3.6 (a) Early-formed ferromagnesian silicates are denser than the magma and settle and accumulate in the magma chamber. Fragments of rock dislodged by upward-moving magma may melt and be incorporated into the magma, or they may remain as inclusions. (b) Dark inclusions in granitic rock.
  • CLASSIFICATION - igneous rocks are formed from the cooling of molten matter - two main kinds: plutonic and volcanic   (a) Plutonic - liquid material (magma) both forms and cools within the Earth - intrusive igneous rocks   (b) Volcanic - liquid material (magma) forms within the Earth - but erupts (lava) and cools at the Earth’s surface - extrusive igneous rocks  
  • - most igneous rocks classified on the basis of texture and composition - usually 2 names given for one rock with the same composition: - extrusive name (basalt) - intrusive name (gabbro)   - textures are usually different between the two types: - extrusive (aphanitic) - intrusive (phaneritic)    - furthermore, igneous rocks are classified on silica content:   Low Silica = Ultramafic < 45 % Mafic 45 – 52   Intermediate Silica = Intermediate 53 – 65   High Silica = Felsic > 65    
  • Figure 3.9 Classification of igneous rocks. This diagram shows the relative proportions of the main minerals and the textures of common igneous rocks. For example, an aphanitic (fine-grained) rock of mostly calcium-rich plagioclase and pyroxene is basalt.
  • (A) Low Silica Igneous Rocks - ultramafic and mafic rocks   (i) Ultramafic - 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   - upper mantle origin - very rare at the surface - rare in rocks younger than 2.5 billion years   - required very hot temperatures for formation (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   - diamonds are contained in kimberlite pipes - originate 100 to 300 km below the surface, upper mantle - carrot-shaped pipe, no more than a few hundred meters in diameter - diamond mine in the NWT is mining a kimberlite pipe   - 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      
  • Figure 3.10 This specimen of the ultramafic rock peridotite is made up mostly of olivine. Notice in Figure 3.9 that peridotite is the only phaneritic rock that does not have an aphanetic counterpart. Peridotite is rare at Earth’s surface but is very likely the rock making up the mantle. Source: Sue Monroe
  • (A) Low Silica Igneous Rocks - ultramafic and mafic rocks   (i) Ultramafic - 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   - upper mantle origin - very rare at the surface - rare in rocks younger than 2.5 billion years   - required very hot temperatures for formation (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   - diamonds are contained in kimberlite pipes - originate 100 to 300 km below the surface, upper mantle - carrot-shaped pipe, no more than a few hundred meters in diameter - diamond mine in the NWT is mining a kimberlite pipe   - 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      
  • (ii) Mafic - temperatures 1000° to 1200° C - low viscosity (i.e. runny) - generally dark colour - relatively dense - basalt (extrusive) - gabbro (intrusive) - e.g. Hawaii       - 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)  
  • (ii) Mafic - temperatures 1000° to 1200° C - low viscosity (i.e. runny) - generally dark colour - relatively dense - basalt (extrusive) - gabbro (intrusive) - e.g. Hawaii       - 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)  
  • Figure 3.11 Mafic igneous rocks. (a) Basalt is aphanetic. (b) Gabbro is phaneritic. Notice the light reflected from crystal faces.
  • Figure 3.11 Mafic igneous rocks. (a) Basalt is aphanetic. (b) Gabbro is phaneritic. Notice the light reflected from crystal faces.
  • (B) Intermediate Silica Igneous Rocks - 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) - e.g. Mount St. Helens produced andesitic lava   - 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  
  • (B) Intermediate Silica Igneous Rocks - 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) - e.g. Mount St. Helens produced andesitic lava   - 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  
  • Figure 3.12 Intermediate igneous rocks. (a) Andesite. This specimen has hornblende phenocrysts and is thus andesite porphyry.
  • Figure 3.12 Intermediate igneous rocks. (b) Diorite has a salt-and-pepper appearance because it contains light-colored nonferromagnesian silicates and dark-colored ferromagnesian silicates.
  • (C) High Silica Igneous Rocks - silica rich - 600° to 800° C - high viscosity (i.e. not runny, sticky)   - light colour - relatively low density (light) - rhyolite (extrusive) - granite (intrusive) - e.g. Yellowstone National Park   - both granite and rhyolite are derived from felsic magmas - silica content of > 65 % - silica-rich 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 northeastern Manitoba - granites are the most common intrusive igneous rock      
  • (C) High Silica Igneous Rocks - silica rich - 600° to 800° C - high viscosity (i.e. not runny, sticky)   - light colour - relatively low density (light) - rhyolite (extrusive) - granite (intrusive) - e.g. Yellowstone National Park   - both granite and rhyolite are derived from felsic magmas - silica content of > 65 % - silica-rich 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 northeastern Manitoba - granites are the most common intrusive igneous rock      
  • (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, Eastern Manitoba  
  • Figure 3.14 (a) This pegmatite, the light-colored rock, is exposed in the Black Hills of South Dakota. (b) Close up view of a specimen from a pegmatite with minerals measuring 2 to 3 cm across. (c) Tourmaline from the Dunton Pegmatite in Maine.
  • (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 - a rock formed from fragments that are ejected during a volcanic eruption   tuff - 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 - 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    
  • (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
  • Figure 3.16 Examples of igneous rocks classified primarily by their texture. (a) Tuff is composed of pyroclastic materials such as those in Figure 3.8i. (b) The natural glass obsidian. (c) Pumice is glassy and extremely vesicular. (d) Scoria is also vesicular, but it is darker, heavier, and more crystalline than pumice.
  • (6) Intrusive Igneous Bodies: Plutons - all bodies of intrusive igneous rocks, regardless of shape and size are called plutons - after Pluto, the Greek god of the underworld pluton - defined as an intrusive igneous body     - 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 five main categories: (A) Dike (B) Sill (C) Batholith (D) Stock (E) Laccolith
  • (A) Dike - tabular, parallel-sided sheets of intrusive igneous rock - cut across the layering of the intruded rock - disconcordant: boundaries that cut across the 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”
  • This volcanic neck in Le Puy, France, rises 79 m above the surface of the town. Workers on the Chapel of Saint Michel d’Aiguilhe had to haul building materials and tools up in baskets.
  • (B) 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”) - sketch a diagram on the blackboard - does not push the crust upwards into a dome - if a dome develops, then the body is called a laccolith
  • (C) 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 diorite   - 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   - 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
  • Figure 3.18 Emplacement of a hypothetical batholith. As the magma rises, it shoulders aside and deforms the country rock.
  • Emplacement of a batholith by 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 (see Figure 3.6).
  • These granitic rocks in Yosemite National Park in California are part of the Sierra Nevada batholith that measures 640 km long and up to 110 km wide. This near-vertical cliff is El Capitan, meaning “The Chief.” It rises more than 900 m above the valley floor, making it the highest unbroken cliff in the world. Source: Courtesy of Richard L. Chambers.
  • (D) 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     (E) 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
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