Lab 3, due before the end of lecture 4/28/15
Transcription
Lab 3, due before the end of lecture 4/28/15
Name (10 pts):___________________ GSC307: Global Geophysics Lab 3 (100 pts) Due before end of next Lecture 4/28/15 PROBLEM 1 (30 pts) Consider the geometries shown to scale below for Plates A, B and C. For plate geometry II, the relative motion of plate C with respect to Plate B is 2.2 cm/yr in a direction of 206 degrees (so: clockwise from North). For both geometries: create a velocity space diagram (one for each). Place and label all three plates, and place and label all the plate boundary velocity lines in these diagrams. Use a different color for each of the plate boundary velocity lines. Then, if possible, mark the triple junction in velocity space and label it with TJ. Determine whether the triple junction is stable or not, and clearly label the diagram as stable or unstable. (I) (II) PROBLEM 2 (30 pts) A velocity diagram of a triple junction not only indicates whether it is stable or not, the location of the triple junction in the velocity diagram also indicates the velocity and direction with which the triple junction will migrate (move), just like the plates themselves. You will use this fact to answer the following questions. 1. (24 pts) After the Pacific plate made contact with the North American plate, triple junctions started to evolve. Analyze the two triple junctions on either end of the San Andreas Fault in the bottom map by drawing two velocity diagrams for the two triple junctions (shown as circles on the bottom map). Create these diagrams to the same specifications as in problem 1, but here hold the North American (N) plate fixed. The velocity on the San Andreas Fault is 50 mm/yr and the velocity between the Pacific and Farallon plates is 60 mm/yr. Also explain in a few short sentences what these diagrams show about the stability of these two triple junctions and then explain in a few sentences what these diagrams show in terms of the direction of migration of each of these two triple junctions. 2. (6 pts) Due to its continued migration (the nature of which you determined in 1.) and the limited size of the plates, the southern triple F junction will eventually change in type (try to visualize the future of the plate boundary using the answer to the previous question and the bottom map). Write down the three-letter combination of the original configuration and that of the future type of triple junction. Once the southern triple junction changes its type, will it subsequently cease to migrate, or change its migration direction, or maintain its migration direction, and most importantly: why? (Hint: you do not need to create an additional velocity diagram to answer this question.) PROBLEM 3 (30 pts) CALCULATING ISOSTATIC SUPPORT OF MOUNTAINS Isostasy is based upon Archimedes principle—that the weight of a floating solid is supported by the weight of the fluid that it displaces. Archimedes was the ancient Greek scientist who is said to have made his insights about fluid displacements getting into the bathtub. The reason that an iceberg with a mass of 100,000 kg does not sink to the bottom of the ocean is that it displaces exactly 100,000 kg of water. The reason that nine-tenths of the iceberg is below the waterline is that ice is only nine-tenths as dense as water—100,000 kg of water are displaced by 90,000 kg of ice, and the remaining 10,000 kg of the iceberg stand above the surface. A Airy isostasy: B Pratt isostasy: 1 H Crust 2 1 H Crust T ρc 2 ρ c1 T ρc2 T ρc1 R Mantle ρm Da Da Mantle ρm Depth of compensation The calculation of isostatic support is based upon the principle that at any given depth within a fluid, called the depth of compensation, the pressures generated by overlying material are everywhere equal (see Figure 1). This means that the weights of any two columns of material, measured down to the depth of compensation, are equal. Given an area of uniform crustal density (ρ c) and thickness (T), a mountain block with elevation H above the surface, and an arbitrary distance down to the depth of compensation (Da), we can calculate the mountain-root depth (R) beneath the base of the crust for Airy isostasy (Figure 1A). Weight (W) equals mass (volume × density) times gravity (g). For column 1, through the mountain block W 1 = (Hρ c + Tρ c + Rρ c + D ρ m) g (1) The weight of column 2 equals W 2 = (Tρ c + Rρ m + D ρ m) g (2) The weights of the two columns equal each other, therefore W 1 = W2 (3) (Hρ c + Tρ c + Rρ c + D ρ m) g = (Tρ c + Rρ m + D ρ m) g (4) Hρ c + Tρ c + Rρ c + D ρ m = Tρ c + Rρ m + D ρ m (5) Hρ c + Tρ c + Rρ c = Tρ c + Rρ m (6) Hρ c + Rρ c = Rρ m (7) Rρ m – Rρ c = Hρ c (8) R = Hρ c / (ρ m – ρ c) (9) 1) An iceberg is floating in the ocean as illustrated to the right. Use the information provided to calculate the height that the iceberg stands above the water line. H - ice density = 800 kg/m3 34 m water density = 900 kg/m3 2) A simple iceberg is floating in water separated by a thermocline, which results in a density difference. Use the information shown to find the density of the water beneath the thermocline (ρ). water density = 34 m water density = 900 kg/m3 ice density = 800 kg/m3 13 m 7.0 m ρ 3) A stratified iceberg is floating in the ocean as illustrated below. Use the strata thicknesses and the densities shown to calculate H, which is the height of the iceberg above the water. H 900 kg/m 3 10 m 700 kg/m 3 5m 800 kg/m 3 20 m water density = 1000 kg/m 3