MS 20 Introduction To Oceanography. Lab 3: Density, Specific Gravity, Archimedes and Isostasy

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1 Grade 10/10 MS 20 Introduction To Oceanography Lab 3: Density, Specific Gravity, Archimedes and Isostasy Team Number: 1 Team Leader: Team Members MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 1 of 7

2 MS20 Laboratory: Density, Specific Gravity, Archimedes and Isostasy Answer sheet: record all data in the appropriate metric units (centimeters, grams, etc.). Remember to use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g, your answer is not 13.4, but 13.4 g (or 13.4 grams). A. Demonstrating Archimedes Principle Pine Block Oak Block Mass of block <grams> grams grams Weight of water in pan <grams> Displaced volume <cm 3 or ml> (volume of water in pan) grams grams ml ml Block thickness <cm> 2.5 cm 1cm Length of side <cm> 6cm 6 cm V block (equation 1) <cm 3 > 90 cm² 36 cm² R hole <cm> (½ diameter).85 cm.75 cm V hole (equation 2) <cm 3 > 5.67 cm³ 1.77 cm³ V b - h (equation 3) <cm 3 > cm³ cm³ ρ wood (equation 4) <g/cm 3 >.35 g/cm³.68 g/cm³ Archimedes' Principle states that a floating body will displace its own weight of fluid. Are the masses of the blocks the same as the masses of water each displaces? If there is a difference, what are the likely sources of error? Should be the same. Errors could be: Not filling the beakers up with enough water so part of the water was not displaced in the pan. Spilling water. MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 2 of 7

3 B. Modeling the Continental and Oceanic Lithosphere Pine Block Oak Block Depth of water <cm> 9.125cm 9.32cm Volume of water <cm 3 or ml> cm³ 9.32cm³ Mass of water <g> 9.125g 9.32g Mass of block <g> 0.875g 0.68g Combined masses <g> (water column + wood block) 10.0g 10.0g Depth of water / no. blocks <g> 10 cm 10cm Mass of water / no. blocks <g> 10g 10g Again, according to Archimedes' Principle the pressure (or the total weight) acting on the bottom of the tank (or at some depth in the asthenosphere) should not change as more floating masses are added; i.e., the combined masses of each of the wood blocks and the water columns beneath them should be the same as the total mass of the open water. Is this true here? Again, try to account for likely sources of error. Yes. Possible error: Water absorbed by the blocks changes their mass from beginning to end of the experiment. MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 3 of 7

4 Figure 5 shows our model of the earth's crust using two different types of wood of to represent the two different types of crust. Assume you shaved some wood off the top of the pine block (representing continental erosion). What would happen to the pine block? Explain. The pine block would rise in the water because it is decreasing in mass, the water will fill in under the block to replace the shaved off wood. Depth of water under two pine blocks <cm> Volume of water column <cm 3 or ml> 8.25 cm 8.25 cm³ Mass of water column <g> 8.25g Total mass of two pine blocks <g> Combined masses of blocks and water column <g> 1.75 g 10g Is the total mass acting on the bottom of the aquarium approximately the same as in the previous calculations? Again, try to account for likely sources of error. As mass is added (the second block) water depth (and therefore water column mass) decreases. The total mass should be about 10g. MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 4 of 7

5 Gravity and time, aided by various processes of physical and chemical weathering, removes rock from higher continental elevations and transports it to lower elevations, ultimately to the ocean floor. For every meter of rock removed from a mountain range, would you expect the elevation to decrease by one meter? Explain. No. Although each meter of rock removed will lessen the elevation, it will not be decreased by a full meter. Why? Because the lower density mountain range will now have smaller roots to rebound. What happens to the oceanic crust as water, ice and wind continuously deliver and deposit continental sediments? Explain. The ocean crust will sink as more sediments are delivered to it. Because the additional weight will push down on the ocean floor. C. Density and Specific Gravity of Rocks GRANITE BASALT PERIDOTITE Mass in air 65.2g 55.4g 87.6g Mass in water 40.3g 36.1g 61.7g Mass air - Mass water 24.9g 19.3g 25.9g Rock volume (equation 5) 24.9g/cm³ 19.3 g/cm³ 25.9 g/cm³ Rock density (equation 4) 2.62 g/cm³ 2.87 g/cm³ 3.38 g/cm³ MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 5 of 7

6 Can you think of another way to measure the volume of the rock specimens? Fill a graduated cylinder filled with a known quantity of water. Record how much the water rises (is displaced) when adding the specimen. The rise in water should equal the volume of the rock. D. Determination of total mass exerted at a fixed depth within the asthenosphere weight under ocean crust 5.0 x 10 5 cm x 1 500,000 g A. Depth of water Water density Length (L) Width (W) 10 x 10 5 cm x ,875,000 g B. Depth of ocean crust Basalt density Length (L) Width (W) 135 x 10 5 cm x ,697,500g C. Depth of mantle Peridotite density Length (L) Width (W) 49,072,500g Total weight under ocean (A + B + C) = weight under continental crust x 10 5 cm x D. Depth of crust Granite density Length (L) Width (W) x 10 5 cm x E. Depth of mantle Peridotite density Length (L) Width (W) Total weight under continent (D + E) = 7,875,000g 40,560,000g 48,435,000g weight under mountains ,400,000g 55 x 10 5 cm x F. Depth of crust Granite density Length (L) Width (W) ,800,000g 100 x 10 5 cm x G. Depth of mantle Peridotite density Length (L) Width (W) 48,200,00g Total weight under continental mountains (F + G) = MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 6 of 7

7 During ice ages slightly cooler temperatures at mid-to-high latitudes cause less snow to melt in spring and summer than accumulates in winter. Over many years this ice piles up to form continental glaciers that can exceed 2 kilometers in thickness. Glacial ice has a density of about 0.9 g/cm 3, just below that of water, and about 1/3 that of granite. What would be the effect of a 2000 meter thick ice sheet on the continent? What would happen when the ice melted? The continent would rise (about 2000m/3 or 666m). The total weight under the continents should precisely equal the weight under the ocean basin. The calculations, although close, don't really match. What does this tell us about our simple three-rock model? Explain. These rocks are a little to simple to model the earth because the earth is much more Complex that just 3 rocks. There are much more than 3 rocks in the under the oceans Basin. The last ice age ended in northern Europe and in North America approximately 10,000 years ago, yet there is excellent evidence that the Scandinavian peninsula (which includes Norway, Sweden and Finland) is still uplifting rapidly. From this information, what can we conclude about the physical properties of the asthenosphere? Although the asthenosphere acts as a silly putty and conforms to the amount of weight that displaces it, it takes awhile to react to the reduced weight. MS 20 Laboratory Density, Specific Gravity, Archimedes and Isostasy Page 7 of 7