1 Rocks: Materials of the Solid Earth Presentation modified from: Instructor Resource Center on CD-ROM, Foundations of Earth Science,, 4 th Edition, Lutgens/Tarbuck, Rock Cycle Igneous Rocks Today 2 Rock Cycle Shows the interrelationships among the three rock types Earth as a system: The rock cycle Magma Magma Crystallization Igneous rock Weathering, transportation, and deposition 3 1
4 Rock Cycle Earth as a system: The rock cycle Sediment Sediment Lithification Sedimentary rock Sedimentary rock Metamorphism Metamorphic rock Melting Magma 5 Rock Cycle Earth as a system: The rock cycle Full cycle does not always take place due to "shortcuts" or interruptions e.g., sedimentary rock melts e.g., igneous rock is metamorphosed e.g., sedimentary rock is weathered e.g., metamorphic rock weathers The Rock Cycle 6 Figure 2.2 2
Rock Cycle Questions 7 What do the colored arrows in Figure 2.2 represent? Where does the rock cycle start and end? What is the source of energy for each arrow in Figure 2.2? Choices are Earth s s Internal Heat Gravity Sun s s Heat Figure 2.2 Igneous Rocks Crystallize from Magma (molten Earth material) Basic Rock Types 8 Sedimentary Rocks From pieces of other rocks, or Chemical break-down of other rocks or organisms Metamorphic Rocks Alternation of other rocks by high temperature and pressure Igneous Rocks Form as magma cools and crystallizes Rocks formed inside Earth are called plutonic or intrusive rocks Rocks formed on the surface Formed from lava (a material similar to magma, but without gas) Called volcanic or extrusive rocks 9 3
Igneous Rocks Form from magma Molten earth material Mineral crystals form as magma cools Like ice from water Magma on Earth s surface is Lava Cools rapidly Extruded Extrusive igneous rocks Magma below the Earth s s surface is magma Cools slowly Intruded into the crust Intrusive igneous rocks Photograph by C.Heliker on October 3, 2003 http://hvo.wr.usgs.gov/archive/spotlight_images/20031003-ch-0050.html 10 Igneous Rocks 11 Crystallization of magma Ions are arranged into orderly patterns Crystal size is determined by the rate of cooling Slow rate forms large crystals Fast rate forms microscopic crystals Very fast rate forms glass Igneous Rocks 12 Classification is based on the rock's texture and mineral constituents Texture Size and arrangement of crystals Types Fine-grained grained fast rate of cooling Coarse-grained grained slow rate of cooling Porphyritic (two crystal sizes) two rates of cooling Glassy very fast rate of cooling 4
Lava & Volcanic Rocks 13 Volcanic Rocks Devil s s Tower 14 Igneous Rocks - Granite 15 5
Fine-Grained Igneous Texture 16 Fig 2.4 A Coarse-Grained Igneous Texture 17 Fig 2.4 B Porphyritic Igneous Texture 18 Fig 2.6 6
Obsidian Exhibits a Glassy Texture 19 Fig 2.7 A 20 Igneous Compositions Composed mainly of silicate minerals Two major groups Dark silicates = rich in iron and/or magnesium Light silicates = greater amounts of potassium, sodium, and calcium Igneous Compositions 21 Granitic rocks Composed almost entirely of light-colored silicates quartz and feldspar Also referred to as felsic: feldspar and silica (quartz) High silica content (about 70 percent) Common rock is granite 7
Igneous Compositions 22 Basaltic rocks Contain substantial dark silicate minerals and calcium- rich plagioclase feldspar Also referred to as mafic: magnesium and ferrum (iron) Common rock is basalt Basalt 23 Igneous Compositions 24 Other compositional groups Andesitic (or intermediate) Common volcanic rock is andesite Ultramafic Ultramafic Peridotite 8
Classification of Igneous Rocks 25 Figure 2.8 How Different Igneous Rocks Form 26 Bowen s s reaction series Magma crystallizes over a temperature range of several hundred degrees Therefore, minerals crystallize in a predictable order Last minerals to crystallize are very different in composition from the earlier formed minerals Bowen s Reaction Series 27 Figure 2.9 9
How Different Igneous Rocks Form 28 Magmatic differentiation Differentiation refers to the formation of one or more secondary magmas from a single parent magma One example of this is crystal settling Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the magma chamber These 2 rocks have the same chemical and mineralogical composition 29 They are classified as different rock types How do they differ in appearance? Figure 2.4 These two rocks have the same texture 30 They are classified as different rocks How do they differ in appearance? Figure 2.4a 10
What criteria are used to classify igneous rocks? Texture Coarse grained or fine grained Mineral composition Light colored (lots of quartz and feldspar) Felsic Dark colored (lots of Magnesium and Iron minerals) Mafic 31 A Demonstration 32 I need a volunteer Texture: Coarse-grained 33 Individual mineral crystals are visible Cooled slowly (underground) Intrusive (or plutonic after Pluto Greek god of the underworld) Figure 2.4B 11
Texture: Fine-grained Cannot see individual crystals with naked eye Cooled rapidly Extrusive (or volcanic) 34 Figure 2.4A Texture: Glassy Did this rock cool slowly or rapidly? In what environment might this rock have formed? 35 Figure 2.7A Gas-escape bubbles (vesicles) Did this rock cool rapidly or slowly? Is this an intrusive (plutonic) or extrusive (volcanic) rock? Texture: Vesicular 36 Figure 2.5 12
Tell me about the origin of this rock 37 It is glassy It contains vesicles It is called pumice Figure 2.7B Tell me about the origin of this rock 38 It has some well- developed crystals, but It is mostly fine grained (It is called porphyry) Figure 2.6 Instant Recall Challenge 39 Close your notes and books. What are some igneous textures are what do they indicate about the origin of the igneous rock? 13
What criteria are used to classify igneous rocks? Appearance or Texture Coarse grained Fine grained Glassy Vesicular Porphyritic Mineral composition Light colored (lots of quartz and feldspar) Felsic Dark colored (lots of Magnesium and Iron minerals) Mafic 40 Two Compositions to worry about 41 Rhyolite (or Granite) Composition Basalt (or Gabbro) ) Composition Hand Samples What are these rocks? 42 Gabbro Basalt Granite Rhyolite Figure 2.4 14
Rocks to be able to identify these igneous rocks 43 Gabbro Coarse grained, Dark Colored Granite Coarse Grained Light Colored Basalt Fine grained Dark Colored Rhyolite Fine Grained Light Colored How and where did these rocks form? 44 Oceanic Crust Continental Crust Oceanic Versus Continental Igneous Rocks 45 15
Recall from plate tectonics 46 At what type of tectonic boundaries is new crust formed? Divergent Boundaries Is new crust usually (A) Oceanic (B) Continental (C) Either? A good question: 47 If new crust is formed at divergent boundaries, and If new crust has the composition of ocean crust Basalt Where does granite come from? (Or, how come continental crust is made of granite?) Can I get granite (or rhyolite) ) by re-crystallizing basalt? 48 I have a piece of basalt I melt it to make magma (basaltic magma) Now I let it cool (rapidly) What kind of rock will I end up with? Think about this individually, then compare your answer with your neighbor s. 16
To make a rock with a granite composition (granite or rhyolite) ) from a basaltic magma, I need to change the chemistry of the magma! 49 Granite (rhyolite( rhyolite) ) has more silica than basalt (and gabbro) Basalt (and gabbro) ) has more iron, calcium, and magnesium than granite (and rhyolite) Iron and Magnesium Gabbro & Basalt Granite & Rhyolite Silica A thought experiment I have a jar of jellybeans. Half of the jellybeans are white, half are colored. In case you are wondering, the white jellybeans represent silica, the colored jellybeans represent other elements (iron, magnesium, aluminum, etc.) I periodically remove a handful of jellybeans from the jar. With each handful I remove a few more colored jellybeans than white ones. How will the proportion of white jellybeans change over time? Graph the proportion of white jellybeans with time. 50 A jellybean graph 51 Proportion of White JB s 50% Time (Handfuls removed) A B C 17
Bowen s s Reaction Series and Crystal Removal 52 Olivine Pyroxene Amphibole Silica-Rich Magma How Different Igneous Rocks Form 53 Bowen s s reaction series Magma crystallizes over a temperature range of several hundred degrees Therefore, minerals crystallize in a predictable order Last minerals to crystallize are very different in composition from the earlier formed minerals Bowen s s Reaction Series 54 Figure 2.9 18
Bowen s s Reaction Series & Jellybeans 55 From an Earlier Question About Quartz 56 In groups: Speculate about how minerals might get separated from magma in nature? 57 19
Separation of Minerals By Crystal Settling 58 Figure 2.9 Crystal Settling 59 20