sayhan hed honogiin omno duussan reportoo tawilaa hund hereg bolj magad.Group part iin haslaa husvel tavij bolno.
Bismarck Volcanic Arc
The Bismarck Volcanic arc was formed due to tectonic interactions between two converging Indo-Australian plate and the Pacific plate. More specifically from the group work it is acknowledged that Solomon sea plate is subducting under South Bismarck plate. As an example for one of the islands, Manam has been chosen. Manam is part of an island group called Schouten Islands within the Bismarck volcanic arc. The island is a 2800-3000 metres high conical stratovolcano and is one of the most active volcanoes in the arc. The morphology is dominated by four, steep walled valleys arranged regularly, radiating 90 degrees apart from the area of the two craters at the summit. The valleys channel lava flows and pyroclastic avalanches erupted from the crater. It can be deduced that Manam grew out of seafloor due to tectonic processesThe subducting slab of oceanic plate The Northern boundary of the South Bismarck Plate is a major tectonic feature which passes through Schouten Islands, forming a triple junction with the Indo-Australian plate due west. The northern part of the bismark plate is a segmented ridge transform feature and the segment running through Schouten Islands is a zone of left lateral strike slip movement (C O McKee, 1981). More specifically, during subduction the molten material, lava rises through the overlying plate and interacts with water to form pillow lavas which can build up to ocean surface. As it reaches the ocean surface pressure drops to allow water to seep thought cracks which in turn fragments explosively forming hyaloclastites. This explosive process occurs repeatedly until finally a volcano is formed (lecture notes). From group work, a number of erosional processes affect island volcanoes. These include debris and mudflow, Aeolian erosion, fluvial and wave action. Even though these erosional processes do contribute to the destruction of a volcano, they are not the main trigger which leads to a sudden sector collapse which can result in landslides, flank collapse, which also results in generating tsunamis. it is assumed that the these disasters are associated with sector collapse, like that which occurred on Ritter island. Volcanic sector collapse is attributed to the instabilities; water may seep through cracks in the flanks, leaving it porous and weak, generated at their flanks or base over a period of time, which is then suddenly triggered into collapse due to eruption, fault movements, and seismic events. The flanks fail due to hydrothermal activities that occur in the magma chamber where plutons heat the surrounding water which lowers the density of its surrounding rock leaving pores and cracks. From the group work it was established that the dredge haul samples were mainly basalts, andesites, rhyolites and dacites. Many of the samples contained plagioclase phenocrysts within an aphanitic groundmass. Theolitic basalts and low silica andesites which are main rock types of Manam are charecteristics of Island arc chains. Also since the arc is not underlaid by Benioff zone the primary magmas were probably partial melting of upper mantle peridotite.More petrologically speaking, the rocks found on Manam are highly porhyritic, particularly plagioclase and clinopyroxene which constitue 30-40% of the rocks.Phenocrysts of olivine, orthopyroxene and opaque minirelas are much less abundant and most groundmass of Manam lava is hyalopilitic,glassy, dark rich with needles of clinopyroxene . New lava flow evidence can be found in eastern part of the island suggesting frequent eruption and also histori pyroclastic avalanche deposits are found indicating the islands treacherous history. The island Manam erupts frequently producing lava and airfall tephra. The longest repose period in the last 100 years is 9 years therefore the island was equipped with seismographs and tiltmetres in 1965 for the safety of its 5000 or so inhabitants.(C O McKee, 1981)
Tsunamis are a water waves triggered by impulsive geologic events such as sea floor deformation , landslides , slumps and volcanic eruptions. According to Yalcnir tsunamis can inflict catastrophic damage and casualties to both nearfield and after evolving long distances and impacting distant coastlines (Yalcnir, 2003).Tsunami occurred on the island of Bali in December 2004 is a grim reminder of the potential of damage tsunamis can inflict. It is established that most tsunami generation is due to submarine earthquakes, vertical sea-floor deformation. Even volcanic action due to sector collapse like that of Ritter Island do generate a great magnitude of tsunami, it is not as frequent. In case of Manam historically there have not been any recorded sector collapse unlike Ritter, and the flanks are very well supported with constant renewal of lava with its frequent eruption. The seismographs and tilt meters as well as the volcanological observatory installed by G.A.M Taylor in the island will be essential in keeping the inhabitants on the island as well as the nearby islanders. Only major concern is the islands history of producing violent Pyroclastic avalanches. Compared to the pyroclastic flow deposits which occur on nearby islands like Karkar and Long island, these deposits are volumetrically large and could be potentially dangerous (CO McKee, 1981).
C.O. McKee, 1981, Cooke-Ravian Volume of Volcanological Papers, Geological Survey of Papua New Guinea Memoir 10, p 23- 28.
Yalcnir, 2003, Submarine Landslides and Tsunamis, Kluwer Academic Publishers, London.
Sedimentary processes at ocean island volcanoes- lecture notes.
Buk Lau loves Tyrone.