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Names:________________ & _________________ Circle Section: 2=Thurs th GEOS 100 Lab 1 - Based on AGI manual 10 edition F-2014 Thinking like a Geologist: Observing and Measuring Earth Materials and Processes This lab is worth ~2% of the course total. Use and hand in this lab handout and do not tear pages from your lab manual to preserve it for later use. Work in pairs (or in larger groups for some of the “experiments”) and hand in one lab for each pair, by the beginning of the following lab period. The written part of the lab is based on the lab manual. In addition, familiarize yourself with F300, the maps and diagrams on the walls, and minerals and rock specimens. These are the materials that you will be dealing with during the course (and the rest of your life). You will become aware of the geology of western Canada and how the rocks and processes around Victoria are related to larger scale processes over space and time. Turn to Lab 1 of the lab manual and work your way through the readings and exercises. Answer the questions from the manual in the corresponding spaces provided. Activity 1.1 Geological Inquiry (Read and examine: pp 4, 9 & 25, Fig 1.1) Part A 1.1A Aster Image 1,2 & 3: What are the dominant false colours shown and wavelengths do they represent?______________________________________________________________________ ____________________________________________________________________________(3) Image Bands 4, 6 & 8: What are the dominant false colours shown and wavelengths do they represent: ___________________________________________________________________ (3) 1.1B Ground View – Visible Light Photo. Note the trucks have 3 m tires and carry 13 Tons of ore. What physical materials appear as Brown___________, Tan __________ & Green ________? (3) 1.1C Ore rich rocks blasted from the walls/benches of the open pit mine and their minerals. How large are the ore rich rocks carried away by the trucks? __________________________ metres (1) 1.1 D Name 3 copper bearing ore minerals mined at La Escondida Mine ____________________, ______________________________________, and _________________________________. (3) What is the richest copper ore of the 3 minerals? ____________________________________ (1) 1.1 E What is the density of native (metallic) Copper ______________________________ g/cm3 and how much does a penny weigh? ____________________________________________ g (2) 1.1 F Approximately how much copper is needed per square cm of circuit board? ________g (2) 1.1 G Approximately how big (diameter) is a single copper atom in metallic Cu? ________nm (1) Part C 1.1A Compare the remote sensing images and note the location and size of the active and older inactive mine pits. The false colours come from pixels that are several meters across and certainly cannot detect either single copper atoms nor directly detect the minerals which contain the copper. Heat in the infrared bands actually measures differences in soil and rock and vegetated surfaces 1 related to: moisture content, albedo, heat capacity and chemical reactions like the oxidation of pyrite (fool’s gold, FeS2) which is actually more common than the ore minerals we are interested in mining. Explain in a sentence or so how geologists might use these Aster Infrared satellite images to discover new ore deposits in this same region? ______________________________ ____________________________________________________________________________ __________________________________________________________________________ (2) 1.1B Closely examine the ground view photo (B) and explain how geologists can locate enriched zones of Cu ore minerals at this scale and use that information to profitably run the mine? ____ ____________________________________________________________________________ __________________________________________________________________________ (2) 1.1C Copper iron disulphide (CuFeS2) is the most common primary copper bearing ore mineral and the Cu-Carbonates (azurite and malachite form in shallow weathered zones from the introduction or rain and groundwater. What must be done to these ore bearing rocks and minerals to provide pure copper metal? ____________________________________________________ ___________________________________________________________________________ (2) Part D 1.0 Presuming Escondida Mine wishes to extend its fiscal lifetime by finding new deposits, how you as a keen exploration geologist use the satellite imagery to decide between, and purchase the rights for just one site from: A, B, and C which are offered for tenure. Consider what the mine sites look like on these images and the possible significance of the false colours. Which site do you recommend and why? _______________________________________________________ ___________________________________________________________________________ (2) 2.0 Having chosen one of the 3 sites, what activities and tests should your exploration plan include to test whether there is copper present and if so whether it is in profitable enough concentrations to call it a new ore body? __________________________________________________________ ____________________________________________________________________________ ___________________________________________________________________________ (2) Skip Activity 1.2 Activity 1.3 Basketball Model of Earth Label in the correct relative position the spheres of the Earth on the following page using the scale provided, wherein the position of the Inner Core at 1196 km is represented by a circle at 22.3 mm radius. The ratio (scale factor) of km in the real Earth per mm in your diagram is given by 6371 km per 119 mm. 1. Write the numerical value of this ratio here after converting both numbers to meters:____ m/m. _____________________________________________________________________ (2 points) 2. What is the fractional value of each radius in the table, if the basketball’s radius is 1.00 for the top of the Crust? If you do this correctly, the top of the crust should be the surface of the basketball and the ocean and atmosphere sit above that. 2 Hint: the thickness of each layer is the difference between its top and its base. The fractional radius needs to count the total thickness of all layers below that level. We want the top of the crust to be the total radius, the oceans and atmosphere sit above that so their fraction is >1.000. Complete the numerical values in the table below for 11 points. (11) Sphere Atmosphere Hydrosphere Crust UM Lithosphere UM Asthenosphere Lower Mantle Outer Core Inner Core Distance to top/base (km) Thickness of layer Fraction of Earth Radius 97.2 0 3.79 35 35 1.000 100 650 2890 5150 6378 1228 0.193 CARTOON EARTH NOT TO SCALE 3 Activity 1.4 (Skip A through E) Measurement and derivation of liquid density ρH2O (deionized water) in g/mL: F-1. Explain how you can derive the density of deionized water using a graduated cylinder and a balance. _____________________________________________________________________ ___________________________________________________________________________ (2) F-2. Obtain a thermometer and note the temperature of the water in the lab today then look up its ideal density for this temperature using the chart on the board at the front of the lab. Note the temperature and published density for pure water. ____________________________________ ___________________________________________________________________________ (2) F-3. Perform this measurement for > 50 ml of deionized water and perform the calculation to turn your mass and volume measurements into the derived quantity density. Do not use an empty graduated cylinder but start with a noted amount (eg. 5.00 mL) to get an accurate initial reading as the scale is always better relatively than the exact zero position. Show your math and calculate the answer to 2 decimal places. If you are not within 5% of the correct value repeat the exercise to eliminate your sources of error. Discuss your sources of error. _________________________ (4) Measuring the density of an irregular solid (plasticine modelling clay), ρCLAY in units of g/mL. G. Explain the procedure for a lump of clay, a graduated cylinder and water (for displacement). Think about Archimedes in the bathtub! Perform the calculation. ______________________ (4) H-1. Explain why the lump of modelling clay sinks in water? _________________________ __________________________________________________________________________ (1) H-2. Describe what you can do to make the same clay float and draw what you did to make this happen. ___________________________________________________________________ (2) I. Look at the table you filled out and the cartoon on Page 3 of this handout and explain the reason for the order and arrangement of the spherical shells within and above the Earth. ____________ _____________________________________________________________________________ _________________________________________________________________________ (2) 4 J. Calculating rates and gradients: Grand Canyon of Colorado, flat on top of plateau. Flat lying strata uplifted. Incised Horseshoe Bend meander from a former lower gradient river prior to uplift and incision. J-1-a. Most terrain on earth is relatively flat (low relief). Erosion cuts down because of gravity and slopes to the water table (energy minimum and cohesion). Even uplifted plateaux were former basin floors including the Colorado Plateau above the Grand Canyon of the Colorado River (Arizona, New Mexico, Colorado, Utah). Geologists have been puzzling over why flat lying ancient rocks of the Colorado Plateau (Late Paleozoic) should have become uplifted at a much younger time (Later Tertiary). Judging from the oldest sediments from the Colorado River filling the downstream regions of the Colorado River (5-6 Ma) and canyon depth 1.60 km, depth (= height uplifted) divided by time span (oldest materials) to provide an uplift and incision rate for cutting the canyon and forming the high standing plateau. Geologists have believed the time span to be on the order of 6.0 Ma (mega-annum, million years). Calculate the uplift rate in __________ km/Ma and in ________________________ cm/a (cm per annum, year). _________________ (4) J-1-b. Recent work (Polyak et al., 2008, Science V.319, p.1377-1380) from the western plateaucanyon has found vadose zone (above the water table) stalactites in 9 cave sites near the bottom of the canyon with 7 to 17 Ma dates from disequilibrium U/Pb (Radium) dating of the calcite. The argument is that canyon cutting dropped the water table enough to form cave systems and adorn them with speleothems, and that the formation of those calcite deposits and their elevation marks the river depth at that time. The depth in the west canyon realm is about 1615 m. Assume this downcutting (= uplift) occurred over 17 Ma. What is the rate in _____________________ km/Ma and _______________________________________ cm/yr? __________________________ (4) Other researchers emphasize pieces of Cretaceous and Eocene strata and argue parts of the Grand Canyon are reused older river systems from 70 Ma (Cretaceous, Dinosaur era)! In geoscience, we measure or observe facts. How we view or interpret those facts is more of an art and a skill than a concrete science. Getting the truth out of the incomplete geological record requires precisely measuring certain facts, cleverly relating them to fundamental constraints from regional studies and just as cleverly learning to recognize to ignore bad data and shaky assumptions. For further discussion of this continental scale, time spanning, uplift history problem see Rebecca Flower’s article in Geology http://geology.gsapubs.org/content/38/7/671.full . The geologically recent uplift 5 history and topographic invertion (basins became uplands) extends along the western Cordillera of North America from Mexico to the Chilcotin Plateau north of Kamloops, B.C. The Colorado plateau is just a big puzzle piece in the middle of this problem. J-2. The geothermal gradient sets a limit on how deep underground mining can be conducted. The deepest mine, Anglo Gold Ashanti’s Tau Tona Mine in Carletonville South Africa had rock face temperatures of 60° C at 3.9 km depth in 2008. Most of the heat in the rock is internally generated by the radioactive decay of U, K and Th in the crustal rocks of the South African Shield. The mean annual surface temperature there is 14.75°C at an elevation of 1541 metres above sea level. Air temperatures in the mine without air conditioning reach 55°C and need to be lowered to 28°C to carry on with the mining. Even so up to 14 miners died in one year. Show your work and calculate the geothermal gradient in this region within the upper lithosphere. ___________________________________________________________________________ (4) Skip activity 1.4-K Activity 1.5 Density, Gravity and Isostasy Omit C and D. A-1. Measure the dimensions to 2 decimal places and calculate the volume of a wooden block. List the dimensions in cm (H, W, L) and calculate the volume as their product. __________ (4) A-2. Weigh your wooden block while it is dry and calculate its density in ρwood in g/cm3 showing all of your work. ____________________________________________________________ (3) B-1. Measure the height of your wooden block and record this as Htotal here _________ cm. (1) B-2. Gently place your wooden block in a water basin at the end of the lab bench on its widest side. Quickly remove it and mark the wet/dry position with a pencil or pen. If you block is uneven, you might wish to do this on 2 or 4 edges and average them. Record Hbelow _________ cm. (1) B-3. Draw your wooden block showing the Htotal, Hbelow and record the Habove ________ cm. (4) Labelled Drawing showing “plimsoll line” = water line: C-1. Write the Isostasy Equation for Hbelow in terms of Htotal and the densities ρwood in g/cm3 and a ρwater in g/mL. Recall that an equation is a general mathematical formula which allows us to measure or observe something and calculate something else. It is more useful and applicable than just performing a calculation for a particular set of numbers. It is a model relating all such values. Hbelow Equation: (5) 6 C-2a. Test your Hbelow equation by substituting your Htotal, and the densities ρwood in g/cm3 and a ρwater in g/mL for your block and our deionized water in the lab today. If your measurements are precise and your math is correct you should predict a value for Hbelow that is very close to the real wetted depth you measured on your block. This is a test of your work and of the validity of your equation (predictive model). Calcualte Hbelow here ___________________________________ (3) C2-b. Calculate your % error in your model or calculated value using the difference between your measured value on the block and your predicted value. Your value should be within 5% otherwise explain the reason for the discrepancy here. Use the following equation: %Error = 100 x (Hbelow measured – Hbelow calculated) / Hbelow measured (4) D-1. Isostasy is the geological concept that lighter things like wooden blocks in water, icebergs in the polar seas or mountains and crust in denser peridotite mantle float, and float only to one particular level or elevation. Higher floating things tend to be lighter or thicker or both and the converse is true as well. While it is easy for us to pluck a wooden block out of the tub and measure the wet part, it is not so easy to do this for an iceberg (Ask Captain Smith!) or a mountain. It would be much more useful as we can easily see the Habove of these things to rewrite your isostasy equation in terms of Habove . As a hint, recall that we may substitute equal things for one another and still keep an equation valid. I this case, you need to replace your Hbelow values with their equivalent in terms of Htotal and Habove, then rearrange and solve the equation for Habove . Do this substitution and derivation here: ______________________________________________ (5) D-2a. Test your Habove equation to see if your measurements and math are correct like we did for step C above. Calculate your Habove and write it here. ___________________________ (3) D-2b. Calculate your % error in your model or calculated value using the difference between your measured value on the block and your predicted value. Your value should be within 5% otherwise explain the reason for the discrepancy here. Use the following equation: %Error = 100 x (Habove measured – Habove calculated) / Habove measured (4) 7 E-1a. Unlike our rectangular wooden block, most geological objects we’d like to know about are irregular. Sea ice has bubbles and cracks and an average density of about 0.917 g/cm3 . Unlike our deionized water or fresh Victoria tap water, the seas have lots of dissolved salts and are colder as well. Examine figure 1.10 on p 21 with the iceberg and the grid. Use the Hbelow isostasy equation and the total height (thickness) of the iceberg to calculate how much of it should be below the waterline. Calculate and show your work to predict what Hbelow should be here: (4) E-1b. Now calculate the %error for Hbelow as you did in step C-2b. Show your calculation, your %error and discuss why the error is greater here that it was for the wooden block. (5) E-2a. Use the isostasy equation for Habove and predict what Habove should be for your iceberg show your work and write the answer here. (4) E-2b. Now calculate the %error for Habove as you did in step C-2b. Show your calculation, your %error and discuss why the error is very much greater here that it was for the wooden block or even for your Habove in the previous step. (5) E-3a Real world objects like icebergs, mountains, continents, ocean basins etc., are rather irregular and using a single (1 dimensional) thickness as an analog of the volume or mass is poor. Due to this irregular shape, the cross sectional area is probably a better estimate of mass and buoyancy than height alone. This time instead of height, count all of the squares and fractional squares and sum them to obtain the total cross sectional area and the dry area and wetted area above and below the plimsoll line. This takes about 5 minutes if you split the work between 2 students to count whole and partial squares. List your Areas here: Areabelow = ________ , Areaabove = ________ and Areatotal = ___________ . (3) E-3b. Use your Hbelow equation substituting your total Areatotal and densities for sea ice and sea water to predict your Areabelow . Show your work and put your calculated area below here. Now use this to calculate the %error in your area as before. This time you should be much closer (within 2%). Discuss your error and give reasons for the improvement. (6) 8 E-3c. Use your Habove equation substituting your total Areatotal and densities for sea ice and sea water to predict your Areaabove . Show your work and put your calculated area above here. (4) E-3d. Use your Habove equation substituting your total Areatotal and densities for sea ice and sea water to predict your Areaabove . Show your work and put your calculated area below here. Now use this to calculate the %error in your area as before. This time you should be much closer (within 5%). Discuss your error and give reasons for the improvement. (6) E-4. Examine Figure 1.10 of the iceberg again and note the tilted ledge that marks the old waterline. Using what you know about isostasy and floatation, what will happen to the position of the current water line if the iceberg loses mass by either melting from above or having a chunk break off below. Discuss this and suggest a general rule concerning how much of an iceberg shows to aid future Captains! (2) F-1. Clarence Dutton was the first to propose the “Isostasy Hypothesis” to explain how ancient shorelines, beaches, wave cut cliffs, surge channels and other coastal features came to be elevated up to several hundred metres above sea level. Explain how blocks of rocky crust might behave like icebergs over geological spans of time. (2) F-2. Suppose you could find something that could be dated on one of these uplifted shorelines, extinct fossils, carbon in plant or shells etc. How could you use this new information to calculate an uplift rate such as post glacial rebound or tectonic strain? (2) Activity 1.6: Isostasy and Earth’s Global Topography Measure the density of 1 real rock sample for each type of crust (Continental and Oceanic) and use this and the mantle density to predict and account for high standing continents and continental mountain ranges versus and low lying oceanic basins. A. As you did for the lump of clay, obtain and weigh piece of basalt and use a plastic graduated cylinder to measure its volume by displacement and determine the density of a piece of basalt. My basalt sample: mass _______ g , volume ______ basalt = _________g/cm3 (3) Average density of basalt (yours plus 9 in table A p.36) =___________ g/cm3 9 (1) If your basalt is out of range of the other 9, reject it and average only the other 9 for step C. B. As you did for the basalt, obtain and weigh piece of granite and use a plastic graduated cylinder to measure its displaced volume and determine the density of the granite. My granite sample: mass _______ g , volume ______ basalt = _________g/cm3 (3) Average density of granite (yours plus 9 in table B p.36) =___________ g/cm3 (1) If your granite is out of range of the other 9, reject it and average only the other 9 for step C. C. From seismic wave velocities and peridotite samples taken from ophiolites and volcanic inclusions we know the average density of peridotite and upper mantle it comprises to be 3.30 g/cm3 . Both high standing granitic continental crust and low lying basaltic ocean crust “float” on this denser solid mantle peridotite, like wood does on water. When seen from space the Earth presents 2 different crustal elevations (a bimodal distribution of heights) and not much in between. The real object of today’s lab is to use your familiar isostasy model equations for Habove and Hbelow to explain and account for global topography and bathymetry. The densities come from your lab measurements while the crustal thicknesses (depths to the mantle) come from seismic soundings. 1. For 5 km thick seafloor basaltic crust in peridotite mantle calculate: H basalt above = km to obtain how high the basalt would “float”. Show your work using the 1 dimensional height or thickness. (3) 2. For 30 km thick continental granite in peridotite mantle calculate: H granite above = km to obtain how high the granite would “float”. Show your work using the 1 dimensional height or thickness. (3) 3. If there were such a block of basalt floating next to such a block of granite, calculate how much higher the granite would float. For this net elevation difference = Continental Crust above mantle minus Oceanic crust above mantle: (H granite above - H basalt above) = km or _____m. (2) 4-1. The hypsographic curve in Figure 1.11 on p.23 comes from plotting a histogram of all land elevations and seafloor depths. (The project that made this global map took 5 years and tens of people to digitize and integrate into a single database at the national geophysical data centre in Boulder Colorado.) Subtract the 2 modes on this curve to obtain the “true” average difference in the height of land and depth of seabed measured relative to sealevel as a datum (reference elevation). How close are your calculated isostasy H above difference results compared to the true elevation differences? Write down the difference between the average continental land elevation and average ocean basin depth in terms of distances ______________ (metres) and percent errors ________________ % using your 3 rock, 1 dimensional isostasy model. Assume the topographic averages to be the true value for this error calculation. (3) 10 4-2. Reflect on the errors from 4-1 when you compared your isostatic prediction to the actual difference between the 2 main predominant elevations. If your error is less than or equal to ~5% you should find this amazingly good. If it is more where did you go wrong in your calculations or assumptions. Finally comment on how powerful (or not) and accurate (or not) it is to use this isostatic model and a 3 rock mantle and crust. This is the true power of science in that it can make simplifications and generalizations that explain most of the variation we observe. Explain in your own words what conclusions you can draw and what reservations or qualifications you might have after this exercise. (5) F. Pratt was perplexed at a survey near Hyderabad in northern British India of the day because surveying a closed triangle around 3 cities and back failed to close by an amazing 70 km. He was forced to conclude the surveyor’s plumb bob was off from the vertical and deflected by the mass of the Himalayas. He interpreted this to be due primarily to rocks of different density beneath adjacent regions traversed by the survey with substantially lower density but somewhat thicker crust beneath the mountains compared to the plains before them. George Airy used astronomical zenith and transit data from La Condamine’s Peruvian survey to conclude similar mass problems in the Andes Mountains. He interpreted this to be due to substantially thicker roots beneath mountain ranges, but rocks of essentially the same density. Both of these hypotheses predate the science of seismology and any real knowledge of crustal thicknesses or mantle depths. Use the data provided in the manual p21-38 and your calculations, to make an inference concerning the Pratt (variable density) versus Airy (variable thickness) hypotheses for crustal elevation and isostasy’s driving forces. Which model best explains continents and continental mountain ranges or between those and ocean basins? Which model best explains the elevation differences between mid ocean ridges and ocean basins? Is one model more correct than the other or does the real Earth require that we understand and use a combination of both models to account for different features? Feel free to make and label your own sketches here. (4) 11