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LithoDensity-PPT file

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PowerPoint file for Lithosphere Density module

PowerPoint file for Lithosphere Density module

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    LithoDensity-PPT file LithoDensity-PPT file Presentation Transcript

    • SSAChaz.TCJ.3(trad) Oceanic Lithosphere: Sink or Swim The fate of oceanic plates depends on their density—how does it change? The module you are viewing is a Powerpoint slide presentation. •Navigatebar on slide to slide using the up/downQuantitative Issue if available, the scroll from your mouse Core arrow keys, or, Weighted average •Use the mouse to flash animations (underlined, in blue type) orIssues through embedded select hyperlinks Supporting Quantitative to pass • Proportions, percentage When done, use the escape key to exit the presentation. You can and probably should have a spreadsheet open in a separate Core Geoscience Issues window, so you can try out things that are explained in the presentation. Plate tectonics, lithosphere Powerpoint applications use lots of memory, so you may want to exit other Thomas Juster while running this presentation, especially if it starts to act slowly programs or sluggishly. Department of Geology, University of South Florida © 2011 University of South Florida Libraries. All rights reserved. Close this window to proceed with the slide show. 1
    • The paradox of oceanic lithosphereOceanic lithosphere plays two very important roles in plate tectonics. First, it is what the oceanicplates are made of, and thus underlies 70% of Earth’s surface. Second, it is thought to provide themost important driving force for the motion of the plates as it sinks into the asthenosphere atsubduction zones, dragging the rest of the plate along with it (a process called “slab pull”). Figure from USGS web site So how can the density of the lithosphere be both greater and less than the density of the asthenosphere? This is the paradox of the oceanic lithosphere. Here the oceanic lithosphere is less dense than the asthenosphere, causing it float. This is a good thing, because foundering of the oceanic plate would destroy the oceans and all life on Earth! Here in the subduction zone the oceanic lithosphere is denser than the asthenosphere, causing it to sink. This (For a review of density, see Endnote 1) tugging force drags the entire plate along, causing it to move on the surface. 2
    • Objectives of this moduleUpon completion of this module you should be able to:•Explain what the weighted average is, and compare it to the simple non-weighted average;•Compute the weighted average;•Compute the density of the oceanic lithosphere given different proportions of mantle andcrustal rock;•Explain how oceanic lithosphere thickens as it ages, and how its density changes during thisprocess•Explain how oceanic lithosphere can be both less dense than the underlying asthenosphere (inthe ocean basins) and more dense than the underlying asthenosphere (in subduction zones) First, extract the embedded Excel spreadsheet where you will do your homework. Remember to immediately Lith_Density save it under a new, unique name. Q1. Quick review: what kinds of geologic hazards are commonly at subduction zone plate boundaries, like the boundary shown in the diagram to the left? Go directly to End-of-Module Questions 3
    • Math concept: what is a weighted average?But here’s the key point: the weights don’t have to all be equal. And If they’re not allequal, then some terms will get more weight than others in computing the average. 4
    • Weighted average, con’t.When would the weights be unequal? When we’re taking the average of numbers that don’trepresent individual values, but groups of values. For example, suppose we wanted to calculate theaverage age of students at a college given these data: Number of % of college Average age of Q2. If we didn’t use a Class weighted average, students population class (yrs) what would the Freshman 135 33.75% 18.25 weights equal for a Sophomore 107 26.75% 19.37 simple average? Junior 85 21.25% 20.83 (HINT: we would be simply averaging four Senior 73 18.25% 22.09 numbers) Whole college 400 Go directly to End-of-Module QuestionsA simple average wouldn’t be appropriate because it would weigh each class equally in the average,but the classes aren’t equal—there are more freshman, for example, than seniors. To calculate anaccurate average we need to weigh the averages for each class by the fractions of each class in thewhole population, so that classes—like the freshman class—which contain a higher percentage ofthe college’s students, contribute more to the average. 5
    • An example: what is the average asking price of a house in Tampa? Real estate brokers list homes, and you can use these data to compute the average asking price. In many cases the data are broken down by the number of bedrooms (groups of values). For example, here are the data for home listings in Tampa, Florida, in December 2010: Looking at the data, you can see the obvious—larger houses in general cost more than small ones, and the largest houses —those that have five or more bedrooms—can cost millions. You could compute an average of these prices, but what doesData from trulia.com this mean? The average is heavily influenced by the cost of the largest houses, but there were only 500 of them. Instead you calculate a weighted Multiply the average, with weight (W) times the average the weights price for each equal to the size proportion of The sum of these values is the listings. weighed average asking price. Total of all listings using Weight (W) for each size house computed the SUM() function as the number of listings divided by the total of all listings. The weights should sum to one. 6
    • An example: what is the average asking price of a house in Tampa?Here’s what the Excel cell formulas look like. Study them so you can create your ownspreadsheet to calculate a weighted average. This table is also found on the embeddedspreadsheet file. Q3. What does the reference $E$16 mean (it’s found in the formula for cell G11) =E11/$E$16 Go directly to End-of-Module Questions =F11*G11 Copy and paste the formula in cell H11 into these cells =SUM(E11:E15) Copy and paste =SUM(H11:H15 the formula in ) cell G11 into these cells 7
    • Another example: what is the average tuition paid by USF students?Here’s another example: what is the average tuitionpaid by USF students? If you look it up you will findthat the tuition depends on whether a student is in-state or not, and it’s a big difference! In state tuition: $5,124 Out of state tuition: $15,933However, there are far more in-state students thanout-of-staters, so we need to use the weightedaverage to compute the average tuition. Here arethe complete data: Q4. Fill in the rest of the this table, which computes the weighted average of tuition for USF students, both in- state and out-of-state. Note that some of the computed values are revealed so you can check your formulas. Q5. Here is the weighted average. Is it closer to the in-state or out-of- state tuition? Why? Go directly to End-of-Module Questions 8
    • Review: what is lithosphere? Plates: Lithosphere: Rigid rockCrust: Intermediate ormafic rock covered by 100-250 km, 1,300°Csediment, 7-30km thick Recall that the lithosphere is the relatively cool, rigid outer layer of Earth, and is underlain Mantle: Asthenosphere: by the asthenosphere, solid Ultramafic rock Solid rock that can rock that is hot enough to flow flow like a fluid. The lithosphere is not the same thing as the crust, which is the outermost layer of rock on Earth defined on the basis of its chemical composition. Core: Iron (Fe) Liquid Metal The lithosphere consists of two metal compositional layers: the crust and the uppermost part of the mantle. The transition from lithosphere to asthenosphere Layering based Layering based on occurs at ≈ 1,300°C, the on composition style of deformation temperature at which mantle rock begins to flow. 9
    • Review: what is oceanic lithosphere? 140m.y. 70 m.y. 20m.y. 8 Mid- Crust Ocean Mantle Ridge 1,300°C Asthenosp here Click here to see how the oceanic lithosphere thickens as it moves away from the ridge and agesThe oceanic crust is formed at mid-ocean ridges, and consists primarily of mafic volcanic rocks(basalt). Underlying the crust is a small piece of mantle rock cool enough to be rigid, and thesetwo components form the oceanic lithosphere.Typically, about 7 km of volcanic rocks can accumulate at the mid-ocean ridge to form theoceanic crust before the plate moves away from the source of heat and magma.As the lithosphere moves away from the mid-ocean ridge and its source of heat, it cools. As itcools more and more of the mantle rock becomes rigid, and the mantle component of theoceanic lithosphere thickens. Notice that as it thickens, the crust becomes a smaller andsmaller proportion of the lithosphere. 10
    • Average density of oceanic lithosphereThe average density of the mafic crustal rocks in the oceanic lithosphere is 2,800 kg/m3. Theaverage density of the ultramafic mantle rock in the oceanic lithosphere is 3,400 kg/m3.Because it consists of both crust and mantle, the average density of the lithosphere willtherefore be a weighted average of the densities of the crustal and mantle components.When the oceanic lithosphere is approximately 8 million years old it consists of about 13 km ofmantle overlain by 7 km of crust. Q6a. Fill in the orange cells in this table in Excel that will calculate the average density of the oceanic lithosphere when it is about 8 million years old, and consists of 7 km of crust overlying 13 km of rigid mantle. Q6b. The density of the asthenosphere below the lithosphere is about 3,350 kg/m3. Based on the density of the lithosphere you just calculated, will the lithosphere float in this asthenosphere or sink through it? Enter “float” or “sink” in this cell. Go directly to End-of-Module Questions 11
    • Average density of oceanic lithosphere as a function of ageAs shown on Slide 10, the oceanic lithosphere gets thicker with age, as the mantle componentgrows. Now that you know how to compute the average density of the lithosphere using theweighted mean, you can investigate how the density changes with time.Q7a. Fill in the orange cells in this Excel table Q7b. In column N Q7c. Once again,that will calculate the average density of the calculate the density decide whether theoceanic lithosphere at 9 different ages, from 8 difference between lithosphere willto 140 million years old. Note that the the lithosphere and float or sink in thearrangement of the table is a little different the asthenosphere asthenosphere,from the ones you’ve done before, but the [ρ(asth), shown in and enter “float” orequations are all the same—just make sure column M]. “sink” in column O.you enter the cell references properly. I’verevealed the density for 25 m.y. old Go directly to End-of-Module Questionslithosphere so you can check you’re doing itright. 12
    • Density of oceanic lithosphere under subduction zonesMost geologists think that the motion ofthe plates is driven by the sinking ofoceanic lithosphere at subduction zones(the force is indicated with the arrow).What is different about the oceaniclithosphere here as opposed to on thesurface? Basalt Eclogite Q8. Calculate the density of the subducting oceanic lithosphere, and decide whether it floats or sinks. Go directly to End-of-Module Questions Here’s the big difference: basalt, which forms near the surface and is stable there, transforms at depth into a new rock called eclogite. Eclogite is much denser than basalt. Endnote 2 Notice that all the densities are larger because of the greater pressure at 150 km. The greater pressure compresses the minerals so they occupy less volume. 13
    • End-of-Module AssignmentAnswer all questions in the spaces provided in the embedded spreadsheet (Slide 3), which youshould have saved with a different name (e.g., “YourName-density.xls”).2.Answer questions 1-8 on Slides 3, 5, 7, 8, 11, 12, and 13.•How does the density of the subducted oceanic lithosphere change as it warms up? Howwould this change the “slab pull” driving force for plate tectonics?•Continental crust and lithosphere is much thicker than oceanic lithosphere. The averagethickness of the continental crust is 30km, and the average thickness of the continentallithosphere is 200km. Calculate the average density of the continental crust assuming that thecrustal rocks have a density of 2,700 kg/m3. Show all your work in the spreadsheet. 14
    • Endnotes• Density is a measure of the amount of mass per volume. The modern metric unit of density is kilograms-per-square-meter, kg/m3, though many people are more familiar with the older grams-per-cubic-centimeter (g/cm3). Water at normal surface conditions has a density of 1,000 kg/m3 or 1.00 g/cm3. Return to Slide 2.• The difference between basalt and eclogite is the mineral composition. Basalt consists primarily of three minerals: olivine, plagioclase, and pyroxene. When exposed to high pressures, the olivine and plagioclase transform into garnet in the rock eclogite. Here are the densities of the pertinent minerals: Mineral Density (kg/m3) Olivine 3,300 Plagioclase 2,700 Pyroxene 3,400 Garnet 3,500 You can see that a rock made of pyroxene + garnet (eclogite) will be denser than a rock made of olivine, plagioclase, and pyroxene (basalt). Return to Slide 13. 15