Lecture 05 - Igneous Rocks Question: In lecture on Tuesday, 9/3, we talked about silicate structures. You said the more complex structures are part of the crust and less complex structures are part of the mantle. Why are the more complex structures nearer to the surface? (9-4-96). Answer: The complexity of the silicate structure is a function of the temperature of crystallization. The hotter, more refractive minerals (i.e., those with a higher melting temperature and, hence, higher crystallization temperature) are more loosely bonded to large cations with and share less oxygen atoms between the silica tetrahedra. Olivine has a relatively simple structure with independent tetrahedra which crystallizes very high temperature found deep within the earth. Quartz has a complex crystalline structure which favors crystallization and melting at a much lower temperature. Crystals with a complex structures form nearer the surface because the temperature there is cooler. (9-4-96) Question from Student #1: If an intrusive rock is defined as a rock derived from magma that solidified in the mantle OR the crust, how do we determine its origin? Furthermore, if peridotite escapes onto the surface with ease, can we not assign this fact to it being found in highly reactive areas, or areas under in the mantle where it can more easily rise through the crust? Question from Student #2: I understand that the continental crust is formed of mostly granite, and the oceanic crust is formed of mostly basalt, but why is this? If the mantle that forms all of the crust comes from the same location, why is it different in those areas? Is it because of chemical reactions with water and air? Answer: Peridotite is the name of the rock found in the mantle and it is brought to the surface through tectonic faulting but not through melting as a magma. Most magmas originate deep in the crust or upper mantle. It the magma develops by melting peridotite then it will most likely have the composition of a basalt, the type of rock commonly found in oceanic crust. If the magma forms by melting in the lower crust, it is more likely to have the composition ranging from diorite to granite. In the lower crust it is the presence of water the lowers the melting temperature so that a silica-rich magma forms. Question: My father is a jeweler, and the diamonds that he works with are very fine, smooth, glassy-textured rocks. This is characteristic of extrusive rocks, those that cool on earth's surface pretty rapidly. However, text page 164 states that many diamonds are extracted from diatremes in the mantle, and that some are as deep as 240 km ( p 125). These conditions way down in the mantle would provide very hot temps. and pressures, and would form rocks that are coarse-grained in structure. How do diamonds come out so fine and smooth among these conditions? Answer: It is true that some igneous rocks like obsidian have a shiny surface after cooling quickly. Glass also has a highly reflective surface. However, crystals which grow very slowly deep within the crust or upper mantle also have highly reflective and shiny surfaces. These surfaces are crystal faces forming as a consequence of the regular arrangement of atoms. You will find that the surface of obsidian is nonplanar whereas crystalline faces are planar (9-9-96). Question: During class one day, you said that the more complex silicates are in the crust, closer to the Earth's surface, and the more common minerals, like the simple silicates, are in the mantle. Why are the minerals set up like that? Answer: In defining the silicate structure I equated structural complexity with the number of shared oxygen in the crystalline lattice. I equated simple silicates to those without shared oxygen and therefore made of isolated tetrahedra. The most complex are those silicates with the most shared oxygen such as quartz. The simple silicates such as olivine with isolated tetrahedra form at high temperature by bonding with cations such as iron and magnesium. The composition of magma deep within the earth favors the formation of olivine at high temperature. The composition of magma near the surface of the earth favors the formation of quartz at lower temperature (9-9-96). Question: Today in class you were explaining chemical and mechanical weathering about rocks and rock formations. My question is: when I go camping and get a blazing fire going, why and how do rocks that have been in or near the fire explode of shatter? This always happens and it always scares me. Answer: Neat question because it will remind students that I started lecture #5 with a simple presentation of the ideal gas law: PV = nRT. V is volume, P is pressure, and T is temperature. I can use the ideal gas law to explain exploding rocks. The rocks around your campfire often have pore space that is filled with water trapped within. As the rock is heated by the campfire T increases, the pore acts like a pressure cooker where V is constant. Because T goes up, the gas law equation is balanced only if pressure (P) increases. Eventually the pressure within pore space gets so large that the rock will explode (9-13-96). Question: In my readings about igneous rock classification, I noticed that all of the rock families have both a phaneritic(coarse-grained) texture and a corresponding aphanitic(fine-grained) texture, except for Peridotite. Why is it that Peridotite can only be found with a phaneritic texture? Answer: Aphanitic rocks evolve from magma that cools very quickly so that grains do not have time to grow by slow crystallization. Aphanitic rocks are those whose magma flows on the surface of the earth. Conditions the that would yield an aphanitic rock of pure peridotite composition at the surface are not found at the surface. Basalt is the aphanitic rock that comes closest to being a peridotite is composition (9-16-96).