Lab 5

Name: Lab #5: Dikes and volcanic threat in Costa Rica A) Purpose of the assignment: This lab has two parts and should be somewhat easier than the last few ones you have completed. The first part is about dikes, how they form in the crust depending on the regional tectonic stresses and what you can learn about the latter based on dikes orientation as observed in the field. The second part is about the volcanic threat in Costa Rica. Using a map of some volcanic deposits in Costa Rica and tables with the composition of these deposits, you will determine where the volcanic threat in Costa Rica is located. B) Learning objectives and skills • carefully read instructions and a table with data • be able to understand dike propagation based on regional tectonic • apply theory to real data C) Tasks Part I: Dikes Consider any location below the surface, within the Earth. The stress is different in different directions. The up-down stress equals the weight of the rock above that place (remember the lab on isostasy). The stress in the horizontal directions, typically, is a little more or less than this. See an example in Figure 1. Figure 1: different stresses applied to a cube within the Earth. The East-West stress (red) is less than the up-down stress, and the greatest stress is in the North-South direction (blue). Now imagine that this cube of Earth happens to contain a volume of magma, and the pressure in the magma is increasing because, for instance, magma is supplied at depth. If the pressure continues to increase, eventually it will crack the rock. Questions: 1) If the pressure exerted as part of the regional tectonic setting is too high, part of the crust may crack. Sketch the orientation of such crack on the cube in Figure 1 given the

intensity of the stress depicted by the blue, red, and black arrows (red=lowest, blue=highest, black=intermediate). Explain why the crack is oriented in this direction (4 points). Once cracked, magma will flow into the crack. As we have seen in class, if the crack is oblique or (sub)vertical and cuts across the bedding or foliation of the older rock through which it has intruded, we will call it a dike . The magma will flow up the dike toward the surface. Sometimes the magma will make it all the way to the surface, which it did during the Columbia River flood basalt eruptions. The dikes in and near NE Oregon are called the Chief Joseph dike swarm (see Figure 2). 2) Based on that, what can you say about the stress in NE Oregon at the time when the Chief Joseph dike swarm as active (red)? (3 points) 3) How was the stress different at the Steens dike swarm (blue)? (3 points)

Figure 3: Geology of Costa Rica and location of Sites listed in Table 2. A B C D E Geology of Costa Rica

Table 2. SiO 2 content of volcanic rocks at five locations in Costa Rica. SITE SAMPLE PERCENT SILICA COMPOSITION A A-1 51.4 A A-2 51.2 A A-3 50.8 A A-4 50.6 A A-5 50.5 A A-6 50.5 A A-7 50.2 A A-8 50.1 A A-9 49.6 A A-10 49.4 B B-1 56.0 B B-2 55.2 B B-3 54.7 B B-4 54.6 B B-5 54.5 C C-1 52.9 C C-2 49.8 C C-3 47.1 C C-4 46.1 D D-1 55.7 D D-2 55.1 D D-3 55.0 D D-4 54.9 D D-5 54.4 D D-6 54.1 D D-7 54.0 D D-8 53.5 D D-9 51.3 E E-1 52.5 E E-2 52.3 E E-3 51.7 E E-4 51.6 Questions: 4) Use Table 1 to assign a composition to each rock sample in Table 2. In the COMPOSITION column of Table 2, write “U” for ultramafic, “M” for mafic and so on. (3 points) 5) At which site are all samples intermediate? (1.5 points) 6) At which site are all samples mafic? (1.5 points)