2006 Final Section 7&8 Q&A

EPSC201 2006 Final Exam Review Section 5&6 1) Draw a structural cross-section through the Andes. Identify two types of faults on the section and explain their origin. Himalayas are continent-continent collision formed. The Rockies and Andes are the result of subduction of an oceanic plate under a continental plate. In extension environments, there is also mountain formation. Grabens and horsts are formed, and can be up to a thousand meters high. There is a thinning of the continental crust when it is pulled apart, but there is still ridge formation. The fault lines do not go straight downward in extension environments. They seem to go down on an angle. The material responds to stretching differently depending on how ductile it is. If very ductile, the continent will stretch and thin. However, if it is brittle, there will be fault formation. 2) Name and explain three criteria for establishing relative ages of rocks. Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past and apply everywhere in the universe. It has included the concept that "the present is the key to the past" and is functioning at the same rates. Uniformitarianism has been a key principle of geology and virtually all fields of science. So this Danish bishop, Nicholas Steno, developed the principle of superposition. He proposed that the top layer of rocks were the youngest, and the deepest rocks were the oldest. We now refer to this as stratigraphy. This was his first principle. Steno’s second principle was that all sedimentary layers are formed in horizontal layers. He noticed that canyons contained strata that traveled for long distances. Steno’s third principle was the principle of original continuity. This states that if some amount of sediment is worn away, you can pick up the same sediments some distance away, because at some time, the sedimentary layer was continuous. Based on cross cutting relationships, we know the vertical crosscut is the youngest. Using fossil records can help with the determination of age. This can help us form a global age. Any rocks with trilobites are about 400 million years old. Due to the progress of evolution, we see tiny changes in fossil records, that can accurately determine the age of the rock. Looking at an outcrop with fossils, and other rock layers, can help us determine rock layers a few thousand kilometers away, with the exact same fossil records. Fossils are good for relative ages, while radioactive dating is good for absolute aging. Combining these two techniques makes it easy to date rocks. 3) What is an unconformity? Discuss the origin of two types of unconformity. An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. Angular unconformity - An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The flat top layers have an angular relationship with the rocks layers below. A nonconformity between sedimentary rocks and metamorphic or igneous rocks occurs when sedimentary rock has been deposited above pre-existing (eroded) metamorphic or igneous rock, indicating an environmental alteration in mode of formation of strata. 4) How are fossil assemblages helpful in identifying disconformities? The sea level has changed levels. When the sea level was low, walking animals were able to walk on it, and form fossils. Then the sea level rose back up, and there are more marine fossils, in the more recent layer of sediment. So we get layers of marine to land to marine fossils. You can identify this gap in deposition by noting gabs in fossil records. If the land got covered by water, no land fossils would be able to form. Then it got re-exposed, and more fossils will be able to deposit. However, there will be a gap in the fossil record, showing that no deposition occurred when the land was covered by the sea. There will be a gap in rock layers. Disconformities mark the difference in age of layers next to each other. So there must have been a time when no deposition occurred. This can be represented by a gap in the fossil record. Disconformities can be the result of erosion, where the top layer has been transported away, or an era of non-deposition. 5) Using equations and examples, explain the principles of radiometric age dating. Radiometric dating is based on the decay of unstable isotopes. Uranium is unstable and will decay at a constant rate. Uranium-238 will eventually form lead-106. Uranium-235 will decay down and form lead-107. The key is that they decay by emitting alpha radiation, which consisted of two protons and two neutrons. If there are a large number of uranium atoms in the sample, we can measure the constant decay rate to determine how old the rock is. If we know the rate of decay, concentration of uranium and lead present, we can determine how old the rock sample. Only trace amounts of uranium (ppm) need to be present for this to be applicable. It can be Uranium-238 or 235. In reality, uranium breaks down into many different atoms long its path to lead. So we can measure all the intermediates to help determine the original concentration of uranium, and therefore the age of the rock. Natural lead has a mass of 207, which is different then radioactively produced lead. So we can distinguish b