Canakkale district, Tuzla region in Turkey houses one of the major geothermal fields of Turkey. Tuzla geothermal field, with its 173 degrees C temperature is also an important site of weathering and alteration occurrence. Weathering and alteration develops in many different irregular forms and stages in the field. The regional geology consist of volcanics, such as rhyolites, latites, andesites and pyroclastics. Volcanics in the field had been weathered and altered in various stages. Gradual weathering from the outer walls of the rocks due to atmospherical effects are seen together with extensively fractured rocks that are weathered to almost completely fall apart. The outlined alteration also shows different characteristics in relation to weathering. Gradual alteration developing from outer wall is possible as well as alteration developing from inner rock bodies and pockets of alteration products of irregular sizes are seen in the field. It is possible to see large pockets of alteration development just as it is possible to find very small pockets of extensively altered zones both of which develop from likewise rocks. In terms of types of alteration products, Silicified zones, montmorillonite, kaolinite and Al-Fe oxide zones were encountered in the field. The aim of this study is to reveal the formation mechanisms of the weathering and alteration development in the Tuzla geothermal field and to investigate the process by petrographical and geochemical points of view. During polarized microscope studies, weathering and alteration development patterns were classified. Different forms of mineral alteromorphs were investigated together with different weathering patterns. Also 16 rock samples from altered volcanic rocks in the geothermal field were gathered and chemically analyzed by means of ICP-AES method for major elements. Aluminum behavior is important in weathering studies. The Al(2)O(3) content of the samples change from %13.72 to %20.30 likewise SiO(2) change from %57.39 to %70.08. The variation in the results are better seen on elementary correlation diagrams. These evaluations together with petrographical data indicate that, multi stage discontinuous development of tectonism with formation of widespread network of structural discontinuities in Tuzla field caused the irregular weathering and alteration occurrences by enabling thermal fluids percolate extensively in the massively fractured rocks.