Interpreting The History Of Volcanoes And Volcanic Rocks Essay

force from the diagram, the volcanic history is as outlined below and other relevant details atomic number 18 likewise supplied.1.0 Sequential summary of volcanic history the areaa. Andersite porphyritic joggle this is the oldest. Andersite indicates magma of intermediate composition. Its silica guinea pig is halfway between felsic rocks (having high silica pith) and mafic rocks (having low silica content). Its formation is a two-step process where opposed cooling occurs at the initial st shape up within the crust forming large crystals as a result. However, the be magma moves further out and, as a result, cools to form a fine grained texture within which the primarily 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 tufa is finer than breccia but the two are of the selfsame(prenominal) occurrence of formation. The magma forces its way out sha ttering 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 contentc. 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 confine within the forming magma at the onset of eruption. The gases subsequently escapes to form a blithesome, 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 mo de of formation is still violent.f. Basalt with rough blacky surmount 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 include 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.