Lab_03_Igneous_Rocks_Minerals_S21-1

LAB 3: IGNEOUS ROCKS & MINERALS Introduc)on In labs 1 and 2, you were introduced to Plate Tectonics and the paIerns of earthquakes that results from plate movement. Recall that the Lithosphere is roughly 100 km thick and is composed of a layer of upper mantle overlain by either oceanic or conRnental crust. Beneath the lithosphere is the Asthenosphere , a warm, ducRle layer that is mechanically weak and slowly deforms driven by convecRon in the mantle and by the gravitaRonal pull of subducRng lithosphere at convergent plate boundaries. The plates therefore “driT” over the deformable asthenosphere at rates of a few cenRmeters per year. As depicted in Figure 1, mid-ocean ridges ( divergent plate boundaries ) are where oceanic plates are moving apart and new oceanic lithosphere is created. Magma s, generated within the asthenosphere are less dense than the surrounding rock and travel toward the surface through magma conduits, erupt, and cool to form new ocean crust. Igneous Rocks – For the purpose of our invesRgaRon of igneous rocks , we are going to begin our discussion of the rock cycle (Figure 1) with the generaRon of magma and formaRon of new ocean crust at mid-ocean ridges. We will follow the moRon of oceanic lithosphere to convergent boundaries , where the denser oceanic lithosphere is subducted. At subducRon zones, water released from oceanic crust causes parRal melRng in the mantle that results in the formaRon of conRnental crust. Each successive step in this cycle alters the chemical and physical properRes of the rocks that are formed in each tectonic se]ng Figure 1. By NPS - NPS, Public Domain, h>ps://commons.wikimedia.org/w/index.php?curid=3514759. DepicLon of the Rock Cycle within the concept of Plate Tectonics. (Modified from original by adding the Asthenosphere)

Part 1: Igneous Rocks Igneous rocks form from the cooling and solidificaRon of magma or lava . Magmas are formed when rocks deep in the Earth undergo parRal melRng . Most magmas form from melRng in Earth’s mantle at mid-ocean ridges (divergent boundaries). However, some magmas are formed from parRal melRng of the mantle and crust at subducRon zones (convergent boundaries). When magmas cool, they solidify into rocks, which are composed of a variety of minerals . The types of minerals that form and their relaRve proporRons depend mostly upon the chemistry and temperature of the magma. However, factors such as the availability of water or gases in the magma can play an important role in igneous rock composiRon. Before we get into the details of the rock cycle, we will lay out some basic concepts that will be expanded below. • There are four main groups of igneous rocks (Table 1). o Ultramafic o Mafic o Intermediate o Felsic • The upper mantle is composed of the ultramafic rock peridoRte , an igneous rock made up primarily of two minerals , olivine and pyroxene . • At mid-ocean ridges, parRal melRng of peridoLte within the asthenosphere , which is located within the upper mantle , produces magma of mafic composiRon. o Mafic magmas have a composiRon different from the parent peridoRte. They are composed of the minerals olivine , pyroxene , and plagioclase feldspar (Table 1). • Magma is molten rock with a lower density than the surrounding rocks, and it moves upward through conduits in the mantle toward the surface. o These magmas can solidify at depth or reach the surface and erupt. • At subducLon zones , water released from the oceanic crust leads to parLal melLng of mantle and crust producing rocks of mafic and intermediate composiLon . o Intermediate magmas, have a composiRon different from the parent mafic rocks. They are composed of the minerals amphibole , plagioclase , bioRte , and a few other minerals. • Rocks of mafic or intermediate composiRon can be re-melted within subducRon zones producing magmas of felsic composiRon o Felsic magmas are composed of the minerals orthoclase , quartz , and muscovite , among others. • The conRnents comprise rocks of intermediate and felsic composiRon. Classifica)on of Igneous Rocks Mineralogy – As noted above, the predominant minerals that make up igneous rocks change as we move from mid- ocean ridges and the formaRon of oceanic crust to subducRon zones where conRnental crust is produced. Therefore, igneous rocks are classified based on their mineralogy , or mineral composiRon (Tables 1, Figure 2 ). The minerals that form depend on the chemical composiRon of the magma. For example, a magma that contains an abundance of the elements silicon and aluminum will form minerals that are rich in silicon and aluminum, while a magma rich in iron and magnesium will form iron- and magnesium-rich minerals. Rocks that are rich in the elements silicon and aluminum contain an abundance of feldspar and quartz and are called felsic (Tables 1, Figure 2 ). Rocks that are rich in the elements iron and magnesium are called mafic . Rocks that contain moderate amounts of silicon/aluminum and iron/magnesium are called intermediate. Igneous rocks span a conRnuum of composiRons between mafic and felsic.

Igneous Textures – Igneous rocks are further defined by their igneous textures . Magma that crystallizes at depth in the crust cools slowly and the resulRng minerals grow into large crystals . Each individual crystal may be millimeters to even cenRmeters in size. This coarse-grained texture is referred to as phaneriRc ; individual crystals are easily seen with the naked eye . Rocks that cool and solidify at depth are referred to as Intrusive or Plutonic Rocks In contrast, magmas that cool relaRvely quickly, such as lava flows that extrude onto the earth’s surface or onto the sea floor are referred to as Extrusive Rocks . Extrusive rocks do not have Rme for the growth of large crystals. Each individual crystal may be sub-millimeter in size, producing a fine-grained texture is referred to as aphaniRc ; individual crystals are difficult to see or cannot be seen without the aid of magnificaRon . Igneous Rock-Forming Minerals There are eight igneous rock-forming minerals that we will focus on in this exercise, and these are shown in Table 1 and Figure 2. Each of these minerals has a range of mineral composiRons, or chemistry (Table 2). Table 2. Chemical composiRons of common igneous minerals. Note : Parentheses are used to group elements together into atomic structures. When parentheses have commas, it means that any of the listed elements can subsRtute for each other in the mineral. Mineral Proper)es Different minerals exhibit different physical properRes depending on the arrangement of their atoms within their crystalline structures. These properRes are important; however, they are difficult to visualize and measure in the absence of hand samples. Therefore, the properRes are briefly summarized below as background informaRon but will not be part of the exercise. These properRes are cleavage, fracture, color, hardness, and twinning. Cleavage and fracture – When a mineral breaks along flat planes that correspond to the shape of its internal crystalline structure, we call that cleavage — meaning you can imagine cleaving it with a knife. A common mineral you are all familiar with is Halite, common table salt. Halite has cubic cleavage because it breaks to make cubes. If you have a magnifying lens, look at some table salt at home. Each grain of salt is a cube. You can generally idenRfy cleavage planes in minerals because they reflect light and will oTen appear shiny. The mineral Orthoclase has prominent cleavage, which can be seen in the photo in Table 1. Color – Look at the mineral photos in Table 1. You will noRce that they come in many different colors. Color can be a useful property for idenRfying minerals. Color can also be misleading because some single minerals come in mulRple color varieRes. Color in minerals usually comes from the presence of metal elements, like iron that, which oTen gives minerals a red or black color. Mineral Chemical Formula Quartz SiO 2 Muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 Orthoclase KAlSi 3 O 8 BioRte K(Fe,Mg) 3 (AlSi 3 O 10 )(OH) 2 Plagioclase NaAlSi 3 O 8 and CaAl 2 Si 2 O 8 Amphibole (Ca,Fe,Mg) 2 (Fe,Mg,Al) 5 (Si,Al) 8 O 22 (OH) 2 Pyroxene (Ca,Fe,Mg) 2 Si 2 O 6 Olivine (Fe,Mg) 2 SiO 4

Hardness – Hardness is a mineral's ability to resist or inflict abrasion (a scratch) on a reference material. Hardness is directly related to the strength of the chemical bonds in a mineral — the stronger the chemical bonds, the harder the mineral. Hardness is given as a number from 1 through 10, where 1 is the soTest and 10 is the hardest. Hardness of a mineral is determined by comparing it against different reference materials. If you can scratch the material in the leT column with a mineral, then your mineral’s hardness is greater than the number in the right column. For example, if a mineral scratches a penny but will not scratch a knife, then its hardness is greater than 3.5 but less than 5.5. Conversely, if you can scratch a mineral with your fingernail, then its hardness is less than 2.5. For example, the minerals Graphite and Diamond are composed enRrely of the element Carbon . However, their respecRve crystalline structures determine their relaRve hardness. Graphite is one of the soTest minerals with a hardness between 1-2, whereas Diamond is the hardest naturally occurring substance with a hardness of 10. , A. Graphite (By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, hIps://commons.wikimedia.org/w/index.php?curid=10164225 ) B. Diamond (By Unknown author / U.S. Geological Survey - USGS "Minerals in Your World", Public Domain, hIps:// commons.wikimedia.org/w/index.php?curid=110080 ) Material Hardness Fingernail 2.5 Copper penny 3.5 Steel nail/knife or glass 5.5 Streak plate 6.5 Figure 2. Igneous rock classificaLon. Mineral proporLons shown in the upper panel; rock names below. To determine the mineral composiLon of a rock type, draw a verLcal line from anywhere in the rock type to the top of the graph. The percentage of the volume of the rock can be determined from the thickness of its field where the line passes through.

a. The mantle is composed of peridoRte. Using Figure 2, draw a verRcal line from the middle of the PeridoRte field to the top of the graph as illustrated below. i. b. Using a ruler or esRmaRng as closely as you can, determine the proporRons of minerals along this line? That is, what are the proporRons of minerals that make up PeridoRte? List the result in the “Approximate proporRon column”. Use decimals to represent percent (e.g. 0.8 = 80%); the frac)ons should all sum to 1 . c. On Table 1, click on the image hyperlink for PeridoRte. The green rock is a peridoRte, which is composed of Olivine (light green) and two types of Pyroxene , an Orthopyroxene called enstaRte (brown) and a Clinopyroxene called Diopside (dark green). i. Click on the hyperlink for Olivine , and scroll to the informaRon below the image. Record the specific gravity (density) of olivine in in the table above. NOTE: If the references list a range of density for any of the minerals, use an average value . ii. Click on the hyperlink for Diopside , and scroll to the Physical ProperRes. Record the specific gravity (density) of the pyroxene diopside in in the table above. iii. Click on the hyperlink for plagioclase , and scroll to the Physical ProperRes. Record the specific gravity (density) of plagioclase in in the table above. iv. MulRply the proporLon x specific gravity and record the value in the last column. Now sum the values for olivine, pyroxene, and plagioclase in the last row. This will be the average density of the upper mantle . 2. ParRal melRng of the upper mantle produces magma of mafic composiRon, which make up the oceanic crust. a. On Figure 2, draw a line verRcally through the middle of the basalt field, just like the line drawn through the peridoRte field in the example above. Using a ruler or esRmaRng as closely as you can, determine the proporRons of minerals along this line? That is, what are the proporRons of minerals that make up the mafic rocks gabbro and basalt? Record them in the table below. Rock Type Mineral Approximate propor)on Specific Gravity (Density) Propor)on x specific gravity PeridoRte Olivine 0.8 3.6 2.88 Pyroxene .12 3.4 .408 Plagioclase .08 2.7 0.21 Mantle Density Sum= 3.5

b. Record the Specific Gravity (density) of olivine, pyroxene, and plagioclase in the Table above. (hint: these are the same minerals as in the example problem) c. MulRply the proporLon x specific gravity and record the value in the last column. Now sum the values for olivine, pyroxene, and plagioclase in the last row. This will now be the average specific gravity (density) of the ocean crust . 3. ContrasRng textures – On table 1, click on the image hyperlinks for gabbro and basalt. You should be able to zoom in to examine the texture of each sample. Here are two addiRonal images for comparison Gabbro and basalt . a. Describe the texture of each rock type as outlined above in the secRon on Igneous Textures . Enter the textures in the Table below. Indicate whether the rock type is intrusive or extrusive and list its typical occurrence (intrusion or lava flow). B. Subduc)on and the Forma)on of Con)nental Crust Figure 4. GeneraLon of magma at convergent plate boundaries, and the formaLon of conLnental crust. Rock Type Mineral Approximate propor)on Specific Gravity (Density) Propor)on x specific gravity Gabbro/Basalt Olivine 0.4 3.5 1.4 Pyroxene 0.4 3.4 1.36 Plagioclase 0.2 2.7 0.54 Ocean Crust Density Sum= 3.3 Rock Type Texture Intrusive or Extrusive Occurrence Basalt Fine-grained Extrusive Forms from lava flow Gabbro Corse-grained Intrusive In the deep oceanic crust

magma to become richer in Si, Al, Na, and K (that is, more felsic). It also tends to concentrate water (H 2 O) in the magma. 6. ParRal melRng of conRnental crust of intermediate (andesite or diorite) composiRon produces magma of felsic composi=on , which further leads to conLnental crust evoluLon . a. On Figure 2, “slide the line” that you drew verRcally through the intermediate rock (andesite and diorite) field over to the middle of the Felsic Rock field (rhyolite and granite). You may need to draw a new line. Use a ruler or esRmate as closely as you can the proporRons of minerals along this line? That is, what are the proporRons of minerals that make up the felsic rocks rhyolite and granite? Record them in the table below. b. Using the hyperlinks in the table above, navigate to the informaRonal websites, and record the Specific Gravity (density) of Amphibole , BioRte , Muscovite , Plagioclase , Potassium Feldspar , and Quartz in the Table above. If the specific gravity is expressed as a range, use the median value. c. MulRply the proporLon x specific gravity and record the value in the last column. Now sum the values for Amphibole, BioRte, Muscovite, Plagioclase, Potassium Feldspar, and Quartz in the last row. This will now be the average density of the con=nental crust of felsic composi=on . 7. ContrasRng textures – On Table 1, click on the image hyperlinks for Rhyolite and Granite. You should be able to zoom in to examine the texture of each sample. Here are two addiRonal images for comparison Rhyolite and Granite . c. Describe the texture of each rock type as outlined above in the secRon on Igneous Textures . Enter the textures in the Table below. Indicate whether the rock type is intrusive or extrusive and list its typical occurrence (pluton (intrusion) or lava flow). D. Summing up Igneous Rocks Rock Type Mineral Approximate propor)on Specific Gravity (Density) Propor)on x specific gravity Rhyolite/Granite Amphibole 0.1 2.9 0.29 BioRte 0.2 3.3 0.66 Muscovite 0.08 3 0.24 Plagioclase 0.22 2.6 0.572 Potassium Feldspar 0.2 2.6 0.52 Quartz 0.2 2.61 0.522 Felsic ConRnental Crust Density Sum= 2.804 Rock Type Texture Intrusive or Extrusive Occurrence Rhyolite Grainy, smooth (fine-grained) Extrusive Froms from volcanic errupRons Granite Big grained (course grained) Intrusive Slow crystalizaRon of magma just below earths surface

Complete the Table below by summarizing the density of ocean and conRnental crust, the major intrusive and extrusive rock types that comprise each type of crust, and list the major minerals found in each type. Note: the cells in the Table will expand as necessary as you type. 8. Describe how the differences in density of these rocks relate to difference in composiRon (more mafic v. more felsic). 9. How do the differences in density between mantle and crust relate to the posiRoning of those layers in the geosphere? 10. How do differences in density between oceanic crust and conRnental crust help us to understand why oceanic crust subducts, while conRnental crust does not? Geosphere layer Density (or range) Occurrence Major Rock Type Major Mineral Groups Mantle - Perido)te Olivine, Pyroxene, Plagioclase Ocean Crust Intrusive Gabrro Olivine, Pyroxene, Plagioclase Extrusive Basalt Olivine, Pyroxene, Plagioclase I n t e r m e d i a t e ConRnental Crust Intrusive Diorite Pyroxene, Amphibole, Plagioclase Extrusive Andesite Pyroxene, Amphibole, Plagioclase Felsic ConRnental Crust Intrusive Granite Amphilbole, BioRte, M u s c o v i t e , plagioclase, Potassium Felspar, Quartz Extrusive Rhyolite Amphilbole, BioRte, M u s c o v i t e , plagioclase, Potassium Felspar, Quartz More mafic is more dese, the faster they are cooled the harder they are. The denser materials are supposed to “sink” so it makes sense that the mantle is below the earths crust and it makes sense why the dense plate is the one that subducts.

1. Click on the hyperlink images below of three rock types from the Giants Range and the Vermilion District. Examine the textures and mineralogy. You may not be able to determine the mineral make up from the images. However, try to match the samples with the general rock type in Table 1. 2. , , 3. Based on your work and the descripRons above, describe the texture and mineralogy of the samples. 4. Describe the tectonic environment where these types of rocks form. That is, 2.6 billion years ago, what was happening in NE Minnesota? 13. Double-click on NE Minnesota Geology , which will return you to the overview. a. Double-click on LocaRon – Duluth Complex . b. Right-click on Duluth Complex in the places panel and select ProperRes. i. Read the informaRon in the properRes dialog box and the discussion above about the Duluth Complex and North Shore Volcanics . 1. Click on the hyperlink images below of three rock types from the Duluth Complex. Examine the textures and mineralogy. You may not be able to determine the mineral make up from the images. However, try to match the samples with the general rock type in Table 1. 2. , Texture Mineralogy Rock Type 1 Course-grained P y r o x e n e , A m p h i b o l e , Plagioclase Granite 2 Course-grained Amphilbole, BioRte, Muscovite, plagioclase, Potassium Felspar, Quartz Diorite 3 Fine-grained /smooth Amphilbole, BioRte, Muscovite, plagioclase, Potassium Felspar, Quartz Rhyolite Volcanic arcs at convergent plate boundries and with the plates moving and clashing the rocks deposited to minnesota.

3. Based on your work and the descripRons above, describe the texture and mineralogy of the samples. 4. Describe the tectonic environment where these types of rocks form. That is, 1.1 billion years ago, what was happening in NE Minnesota? 14. Double-click on NE Minnesota Geology , which will return you to the overview. a. Double-click on LocaRon – North Shore Volcanics . b. Right-click on North Shore Volcanics in the places panel and select ProperRes. i. Read the informaRon in the properRes dialog box and the discussion above about the Duluth Complex and North Shore Volcanics . 1. Click on the hyperlink image to view the rocks of the North Shore Volcanics close up. 2. Based on your work and the descripRons above, describe the texture and mineralogy of the Duluth Complex and idenRfy the rock type. 3. Describe the tectonic environment where rocks of the Duluth Complex and North Shore Volcanics Group form. That is, 1.1 billion years ago, what was happening in NE Minnesota? Texture Mineralogy Rock Type 1 Course-grained Paligocase feldspar anorthosite 2 Course-grained Olivine, Pyroxene, Plagioclase Gabbro Midcontinet rift system (divergent plate boundary) whiched caused valcanoes to errupt and the lava flowed along lake superior Texture Mineralogy Rock Type North Shore Volcanics Fine-grained O l i v i n e , P y r o x e n e , Plagioclase Basalt Volacanoes erupting along the shore of lake superior

(Gabbro) By Unknown author - hIp://mars.nasa.gov/mer/classroom/schoolhouse/rocklibrary/source/gabbro.html, Public Domain, hIps:// commons.wikimedia.org/w/index.php?curid=48291568 (Diabase) By CrankyScorpion at English Wikipedia, CC BY-SA 3.0, hIps://commons.wikimedia.org/w/index.php?curid=10722179