[en] Most basaltic magma originates in the mantle. The extrusion of this magma onto the earth's surface produces lava flows. Flow morphology depends on the silica and gas content of the lava and the topography and wetness of the land. An ideal lava flow has a dense, fractured interior with brecciated cooling rinds (interflows) at the base and the top. Ground-water movement through a sequence of basalt flows is controlled by pressure gradients in the flow system; the aperture, spacing, and filling of fractures; and the permeability of interflows and interbeds. The composition of the water is controlled by the original water composition, mixing of different ground waters, rock mineralogy, duration of water-rock contact, and age of the water. All of these factors influence the rate at which ions in solution react with the rock and are able to move through the flow system. Whereas intact basalt is a strong rock, fractures are common and may reduce the in-situ strength. Most mechanical data are from laboratory experiments on intact rock. Data on thermal properties are not always consistent for some parameters because of a lack of standardized testing procedures. 34 figures