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Particle Settling Time Introduction Materials that are transported during erosion and dropped during deposition are bits and pieces of weathered rock. These pieces of rock may be of almost any size from ions on up to boulders. You can use your Earth Science Reference Table to determine the size of any particle. Whatever their size, they are called sediments. The process of deposition is therefore also called sedimentation. Size of the particle is one important factor in determining the rate at which particles settle out of water. Layers of sediment can result from different sized particles settling out of the water. If we measure the time it takes for particles of the same material but of various sizes to settle through the same depth of water, our data will show a definite relationship between particle size and settling time. Materials & Methods Plastic Tube With Water Stop Watch Scoop 3 Different Sized Sediments Samples Calculator Graph Paper 1. Time how long it takes for a scoop-tip full of each of the 3 pure sediments to fall through the column. Start the stopwatch as the sediment is poured, and stop it when the middle of the sediment cloud reaches the bottom. 2. Create the following data table in your “Observations & Data” section. Conduct three trials for each sediment size and record the times in your table. Calculate the average time for each sediment. Sediment Average Size Trial One Trial Two Trial Three Time Pebbles 10 Mesh Coarse Sand 40 Mesh Fine Sand 60 Mesh 3. Return any extra sediment to the proper container. 4. Obtain a small cup full of the mixture, pour all of it into the water column. Record your observations regarding what happens to the different sizes as they fall through the water. How are the sediments arranged on the bottom? Record observations in the appropriate section. It is not necessary to use the stop watch for this part. 5. Make a graphical representation of your data that illustrates the relationship between particle size and settling time. Important Note: Pebbles are assigned a mesh size of 10 because that sized particle can not fall through a sieve that has 10 wires per inch. Mesh size of 40 means that 40 wires per inch are used and coarse sand cannot fall through the sieve. More wires means smaller holes in the sieve, therefore, fine sand will be trapped in a mesh size of 60. 60 Mesh = Small Particles 10 Mesh = Larger Particles a. Put the variable mesh size on the horizontal axis. To have the particle size increase from left to right, mesh size must decrease from left to right. Therefore, number this axis from 100 to 0. b. Label the y-axis average settling time. c. Plot your data, draw the line of best fit. Use a dashed line (---------) to continue the graph line from 60 mesh to 100 mesh and from 10 mesh to 0 mesh. 6. Use all your resources to answer the following questions in your “Conclusion” section. Make sure you use complete sentences. 7. What is meant by the term deposition? 8. If all other factors are equal, what effect does particle size have on settling rate? 9. Clay size particles have a mesh size of 225. How would their settling time compare to sand? 10. According to your graph, how long would it take for a particle of mesh size 35 to settle? 11. Particles of what mesh size would take 7 seconds to settle? 12. According to your reference tables what size range are pebbles?
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