Study of a 3-km-thick section of Tertiary basalts in eastern Iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the middle-Atlantic Ridge. Deep drilling at Reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3.5 km. Three major stratigraphic units composed largely of basalts are recognized, each characterized by distinctive major and trace element patterns. Each of these units is believed to be the product of a separate episode of mantle melting and fractionization and probably reflect accumulation on the flanks of a central volcano. Extensive fractionation in shallow magma chambers led to intermediate and silicic lavas, which mark the cores of the central volcanic complexes. Dike density does not increase downward below about 1.5-km crustal depth, and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes. Secondary mineral assemblages are dominated by smectic and zeolites near the top of the section and by epidote and quartz near the base. In general, the temperature of alteration increases downward, but the distribution of individual minerals is irregular. Locally, earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins. The upper surface of greenschist facies metamorphism crops out at the surface in Thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the Reydarfjordur borehole, located only 3 km east of the edge of Thingmuli. Thus, the metamorphic zones crosscut the lava stratigraphy. Physical property profiles correlate well with variations in secondary mineralogy, and the top of layer 3 velocities coresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth. Magnetic susceptibility decreases with depth and is extrapolated to zero at about 4.3-km crustal depth close to the estimated upper limit of greenschist facies metabasalts. A much greater crustal thickness, leading to the emplacement of high level magma chambers within the extrusive part of the crust, is thought to account for many of the differences in crustal structure and evolution between Iceland and the rest of Mid-Atlantic Ridge.