Weathering and subsequent alteration processes affect rocks by changing their physical, chemical, mineralogical, petrographical and mechanical properties. Although these changes are important inputs for project designs, especially in engineering geology and civil engineering, their complex nature in rocks makes evaluation difficult for non-specialists. In this paper, we propose a new, simplified method for quantifying the degree of weathering and subsequent alteration in rock materials based on pycnometer measurements of specific gravity and a combination of relevant formulas using porosity, void ratio and dry unit weight. The relationship between these properties and rock material change is defined using a Point Rock Change Value (RCVp) at P-i(x(i),y(i)) coordinates and a point Rock Change Ratio, RCRp. Another numerical value, the Overall Rock Change Value (RCVo) is introduced to characterize specific geological formations in the study area. RCVo uses dry unit weight (gamma(d)) saturated unit weight (gamma(sat)), porosity (n), void ratio (e), degree of saturation (S) and specific gravity (gamma(s)) and optical properties of rock material. These optical properties are quantified from thin-sections of the rocks with a Numerical Image Value (NIV). RCVp and RCVo range from 0 to 1, where I indicates no change by weathering or alteration and 0 indicates completely weathered or altered rocks. Application of this method in the Tuzla geothermal field of northwest Turkey reveals strong similarity in the distribution maps for the physical parameters and the calculated values for RCVp and RCVo. The new method correctly identified greater alteration near hot water sources and along fault planes. (C) 2006 Elsevier B.V All rights reserved.