The Gedamsa volcano lies on the floor of the northern sector of the Main EthiopianrnRift. It is characterized by a polygenic caldera resulting from large pyroclasticrneruptions. KlAr dating performed by previous studies indicates an age of 0.8 to 0.1 Marnfor the exposed volcanic products.rnVolcanologic and stratigraphic studies allowed recognition of several phases ofrnactivity during the evolution of Gedemsa. The lowest exposed products arernrepresented by acidic lavas, which are covered by thick plinian fall pumice deposits.rnThis are followed by an ignimbrite deposit and by intra-caldera lava flows andrninterbedded pyroctastic products. The caldera, is clearly a composite structurernresulting from several collapses which occurred after plinian and ignimbritic eruptions.rnA separate stage of volcanic activity connected to the Wonji Fault System (basalticrnvolcanism) formed surge deposits and numerous basaltic cinder cones and lavas,rnboth within and outside the caldera depression.rnThe volCC!nic products from Gedemsa volcano are petrologically and geochemicallyrndiverse. Alkaline and peralkaline silicic lavas and pyroclastics (trachytes and rhyolites)rnare by far the most abundant products. The mafic rocks are only represented by thernmafic inclusions occurring within some of the post-caldera products. The younger riftrelatedrnactivity is, instead, represented by Na-transitional basalt. On the whole, thernrocks occurring in the area have a very marked bimodal distribution, a situation whichrnis typical of almost all the young volcanism of the Ethiopian Rift Valley.rnMajor and trace element variations of peralkaline silicic volcanic rocks fromrnGedemsa volcano support an origin by crystatlliquid fractionation starting from maficrnparental liquids, with separation of olivine, plagioclase, clinopyroxene and opaquesrnduring the ear1y to intermediate stages and of alkalifeldspar and a few mafic phasesrn(alkali am;>hiboles and pyroxenes) during the late stages of evolution. Thesernprocesses generated strong enrichments in incompatible trace elements andrndepletion in compatible elements in the acidic magmas. Consequently, rhyolitesrndisplay extremely high values of Zr, Y, Rb and F and low to very low abundances ofrnferromagnesian trace elements, Sr and 8a. Some rhyolites, howev,e r, have low Narnand fluorine, most probably due to interaction with groundwaters. Such a process mayrnrepresent an explanattion of the high F contents in the groundwaters of the Wonjirnarea and of other zones inside the rift. Although, crystal fractionation best fits therngeochemical variations, field and petrographic observations indicate that mixingrnprocesses were also active during the magma evolution.rnA model is presented for the evolution of the intemal structure of the Gedemsarnvolcano, based on volcanological, stratigraphic and geochemical evidence. Accordingrnto this model, extensive fractional crystallization of parent mantle-derived basalticrnmagmas occurred in a huge shallow level magma chamber. This process, possiblyrnaccompanied by some mixing and assimilation of wall rocks, generated a zonedrnmagma chamber whose upper part was occupied by silicic magmas. The presence ofrnthis silicic magma effectively acted as a density barrier to the mafic magma pendingrnon the bottom of the reservoir. This expla ins why the Gedemsa eruptions wererninvariably characterized by emission of acid material. Huge plinian and ignimbriticrneruptions generated the caldera collapse. This resulted in strong decrease in the sizernof the magma chamber. The post caldera eruptions tapped a smaller, still zonedrnreservoir. However, due to the small volumes of acidic magma standing in the upperrnpart of the chamber, in some cases also mafic magma was brought to the surfacernintermingled with the acid material. The final basaltic eruptions are not related to thernGedemsa volcanic activity but represent liquids arising along regional faults which cutrnthe rift floor, and the Gedemsa volcano itself.