A Study on Error Sizes and Required Sample Size in Sequential Probability Ratio Test
| dc.contributor.author | Ayalew, Dereje | |
| dc.date.accessioned | 2021-12-13T11:07:31Z | |
| dc.date.accessioned | 2023-11-09T14:29:21Z | |
| dc.date.available | 2021-12-13T11:07:31Z | |
| dc.date.available | 2023-11-09T14:29:21Z | |
| dc.date.issued | 1994-01 | |
| dc.description.abstract | The Gedamsa volcano lies on the floor of the northern sector of the Main Ethiopian Rift. It is characterized by a polygenic caldera resulting from large pyroclastic eruptions. KlAr dating performed by previous studies indicates an age of 0.8 to 0.1 Ma for the exposed volcanic products. Volcanologic and stratigraphic studies allowed recognition of several phases of activity during the evolution of Gedemsa. The lowest exposed products are represented by acidic lavas, which are covered by thick plinian fall pumice deposits. This are followed by an ignimbrite deposit and by intra-caldera lava flows and interbedded pyroctastic products. The caldera, is clearly a composite structure resulting from several collapses which occurred after plinian and ignimbritic eruptions. A separate stage of volcanic activity connected to the Wonji Fault System (basaltic volcanism) formed surge deposits and numerous basaltic cinder cones and lavas, both within and outside the caldera depression. The volCC!nic products from Gedemsa volcano are petrologically and geochemically diverse. Alkaline and peralkaline silicic lavas and pyroclastics (trachytes and rhyolites) are by far the most abundant products. The mafic rocks are only represented by the mafic inclusions occurring within some of the post-caldera products. The younger riftrelated activity is, instead, represented by Na-transitional basalt. On the whole, the rocks occurring in the area have a very marked bimodal distribution, a situation which is typical of almost all the young volcanism of the Ethiopian Rift Valley. Major and trace element variations of peralkaline silicic volcanic rocks from Gedemsa volcano support an origin by crystatlliquid fractionation starting from mafic parental liquids, with separation of olivine, plagioclase, clinopyroxene and opaques during the ear1y to intermediate stages and of alkalifeldspar and a few mafic phases (alkali am;>hiboles and pyroxenes) during the late stages of evolution. These processes generated strong enrichments in incompatible trace elements and depletion in compatible elements in the acidic magmas. Consequently, rhyolites display extremely high values of Zr, Y, Rb and F and low to very low abundances of ferromagnesian trace elements, Sr and 8a. Some rhyolites, howev,e r, have low Na and fluorine, most probably due to interaction with groundwaters. Such a process may represent an explanattion of the high F contents in the groundwaters of the Wonji area and of other zones inside the rift. Although, crystal fractionation best fits the geochemical variations, field and petrographic observations indicate that mixing processes were also active during the magma evolution. A model is presented for the evolution of the intemal structure of the Gedemsa volcano, based on volcanological, stratigraphic and geochemical evidence. According to this model, extensive fractional crystallization of parent mantle-derived basaltic magmas occurred in a huge shallow level magma chamber. This process, possibly accompanied by some mixing and assimilation of wall rocks, generated a zoned magma chamber whose upper part was occupied by silicic magmas. The presence of this silicic magma effectively acted as a density barrier to the mafic magma pending on the bottom of the reservoir. This expla ins why the Gedemsa eruptions were invariably characterized by emission of acid material. Huge plinian and ignimbritic eruptions generated the caldera collapse. This resulted in strong decrease in the size of the magma chamber. The post caldera eruptions tapped a smaller, still zoned reservoir. However, due to the small volumes of acidic magma standing in the upper part of the chamber, in some cases also mafic magma was brought to the surface intermingled with the acid material. The final basaltic eruptions are not related to the Gedemsa volcanic activity but represent liquids arising along regional faults which cut the rift floor, and the Gedemsa volcano itself. | en_US |
| dc.identifier.uri | http://10.90.10.223:4000/handle/123456789/29254 | |
| dc.language.iso | en | en_US |
| dc.publisher | Addis Ababa University | en_US |
| dc.subject | Sequential Probability Ratio Test | en_US |
| dc.title | A Study on Error Sizes and Required Sample Size in Sequential Probability Ratio Test | en_US |
| dc.type | Thesis | en_US |