The observed large variations in petrophysical properties in Hole 990A are caused by interplay of primary lava flow emplacement structures and subsequent alteration. The initial flow emplacement generates large, primarily subhorizontal variations in textures, porosity, and permeability structure. Subsequent subaerial alteration transforms the upper flow into a clay-rich red soil layer. After burial and submergence, salt water may preferentially flow in fractures below this upper layer, providing a different alteration regime as suggested by more potassium-rich clay minerals and higher concentration of magnetic minerals. High eruption frequency will inhibit the development of a soil horizon and a less heterogeneous lava pile. The very dense lava core will remain less affected by both alteration episodes, whereas the lava base may experience an alteration development similar to the upper crust.

Modeling based on sand/clay clastic lithologies suggests a prominent softening of the rocks when the clay proportion increases above 30%-40% (i.e., when clay minerals become interlocked). In accord with this, we observe that seismic velocities within basaltic lava flows are significantly lowered when the clay proportion exceeds 50%.

Drying and wetting of only slightly altered basalts severely changes the elastic properties of the rock. This is related to micro-crack development induced by the swelling properties of the alteration clay smectite.