Interpreting The History Of Volcanoes And Volcanic Rocks Essay

Drawing from the diagram, the volcanic history is as outlined below and other relevant details are also supplied. 1.0 Sequential summary of volcanic history the area a. Andersite porphyry: this is the oldest. Andersite indicates magma of intermediate composition. Its silica content is midway between felsic rocks (having high silica content) and mafic rocks (having low silica content). Its formation is a two-step process where slow cooling occurs at the initial stage within the crust forming large crystals as a result. However, the remaining magma moves further out and, as a result, cools to form a fine grained texture within which the earlier formed large crystals are embedded. This indicates a slow eruption. b. Turf grading into braccia: these are pyroclastic rocks formed by violent eruption of highly viscous magma. The tuff is finer than breccia but the two are of the same occurrence of formation. The magma forces its way out shattering the surrounding plastic rocks into ashes and block fragments. These consolidate to form tuff and breccia respectively and are likely to be felsic since high viscousity indicates a high silica content c. Dacite porphyry: this is of similar composition and formation style as andersite porphyry though it has higher silica content than andersite. d. Pumice Breccia: pumice is fine textured, filled with porous holes containing gases that were trapped within the forming magma at the onset of eruption. The gases later escapes to form a light, porous glassy rock. The breccia was likely to have resulted from the shattering of the rocks as the magma erupts out. Pumice is light colured which indicates a felsic composition (felsic rocks are lightly coloured than mafic rocks) e. Polymictic breccia: this indicates a mixture of breccia of varying silica content which indicates the onset of the formation of an interrupting basaltic formation which is to follow after. However, the mode of formation is still violent. f. Basalt with rough blacky top: this is a fine grained mafic rock. The rough top indicates that the forming magma was gently erupted and the viscosity low enough to cause the included gas to just escape leaving the bubble spaces at the top. g. Andersite: this has been seen as of intermediate silica composition. The absence of porphyritic material may indicate a less quiet eruption. h. Obsidian with pumice streak: the youngest rock; obsidian resembles pumice but without the bubble holes. The presence of pumice streak indicates violent eruption since some of the gases were trapped as a result of the violence. It is felsic. 2.0 Since the last eruption is about 19 years earlier (the age of the youngest layer), the volcano is still active. The earthquakes and the likes are likely pointers to another volcanic eruption. So evacuation is imminent. 3.0 This is an example of a composite or cinder-cone volcano. The seldom occurrence of pyroclastic materials and rocks of intermediate composition suggests this. These, in addition to the fact that the rocks are from highly viscous magma, also suggests that the volcano is located at a volcanic arc.