Part 4, From Saprolite to Sediment You might do this experiment, or your instruc
ID: 299879 • Letter: P
Question
Part 4, From Saprolite to Sediment You might do this experiment, or your instructor might dem Crumble some of the saprolite into a jar or empty plastic water bottle. Fill the jar or bottle with more than enough water to cover the saprolite. Put the lid on and shake the jar vigorously until the saprolite is totally onstrate it for you, separated into individual grains and no lumps remain. following questions. 1. How long does it take for the larger grains to settle? 2. How long does it take for the finest grains to settle? 3. After all of he particles have setled and the water is clear, examine the lager. answer the *Set the jar or bottle down and observe the settling of the sediment, then ing you see. (Or examine the photo below.) Describe the characteristics of each layer you can observe. Use Wentworth Scale terms to describe the ribe the grain sizes Example of saprolite that has been allowed to settle in a water bottle about 3 inches in diameter. 4. Write a hypothesis about how the various grain sizes settle, to explain your observations. WEB EXERCISE: THE VIRTUAL SAND COLLECTION The questions below are adapted with permission of Dr. Dave Douglass, from the Virtual Sand Collection website at Pasadena City College in California: For each question, go to the web links given to see the sand sample as viewe Click on ZOOM IN several times to increase the magnificalii can get a closer look at individual sand grains under higher magnificat ation the microscope. higher magnificationExplanation / Answer
Answer: This experiment is best understood when actually performed. Saprolite is a heavily weathered rock which can be easily broken down. This rock will have rock fragments of every size- from coarse, gravel sized to sand-sized particles to even clay particles. Also these particles will have different masses, different densities and hence when they are placed in water, they react differently.
Answer 1: The coarsest grains (gravel) settle down at the base of the jar almost as soon as you set the jar down on a horizontal surface. (Please do the experiment for the precise time. You can use a stop watch for the same)
Answer 2: The finest grain size (which in this case can be clay) will take the longest to settle down and will form the topmost layer of the vertical gradation.
Answer 3: The heaviest of all particles (gravel) will be pulled by gravity and will form the bottommost layer of rock particles followed by particles that have a lighter mass- sand. The layer of sand will be followed by a slayer of even finer particles. This layer will be of silt- a term given for grains that are finer that sand but bigger than clay particles.
The clay particles will first dissolve in water (when the jar was shaken) and after all the particles settle down with respect to their mass, clay particles will form the uppermost layer of the profile. To use the Wentworth scale, you have to actually do the experiment and classify the rock fragments as either gravel, sand, silt or clay. Since the sample of saprolite that I will use will have different content that what you will use, the result will be slightly different.
Answer 4: The phenomenon that you see here is called ‘fining upwards’. When the water is in motion (when you are shaking the jar with the saprolite and water in it), all the particles of saprolite move randomly. But as soon as the jar is put down on the table, gravity acts on the particles. The particles with the heaviest mass will react first to gravity and will settle down first. Hence, ‘gravels’ will form the base layer followed by the less coarse particles of ‘sand’. This layer will be followed by the more fine grained ‘silt’ which will be topped off with the finest of all rock particles- clay.
Note: please understand that even though we use terms like ‘coarse’ and ‘grave;’, so much of the ‘fining upwards’ phenomenon has to do with ‘mass’ and ‘density’ and with the fact that this gradation can only be seen where the water is quiet and not turbulent. In real scenario, this condition can be seen where the rock fragments have enough time to settle and rest- e.g. in deep water spaces, lakes, ponds etc.