WEATHERING, SOIL AND HYDROLOGICAL PROPERTIES OF LAVA FLOWS

Lava Flows create dark, bare rock surface that is inhospitable to colonization by plants, however their inherent aerodynamic roughness allows them to trap fines in voids relatively quickly. Initial soil formation is fueled by accumulation of fines derived external to the flow itself and therefore the rate of soil formation is directly dependent on the flux of organic and inorganic dust. In dry regions, dust accumulates in void spaces tens of cm below the surface providing the water-holding capacity necessary for seed germination. The young plants are shielded from harsh thermal conditions by the surface rock rubble. In humid regions, plant growth is less dependent on external inputs as roots can exploit nutrients released from rapid surface weathering of mafic minerals. Young lavas with large void spaces feed groundwater tables as opposed to fostering lateral flows and integrated drainage; it can take thousands of years for flow lines to change from vertical to following local contours. The prevalence of dust accumulated in lava flows creates confusion over protolith-, time- and climate-dependence of soil mineral formation in mafic lavas. In dry regions, mineral patterns often show little influence of the host rock, and even when mafic material is shown to be weathering smectite is often not the end product of silicate decomposition. An allied observation is that it is not uncommon to find pedogenic dolomite as a carbonate phase on olivine-rich basalts. Soil development patterns show that allophane and nano-crystalline Fe phases predominate for as much as a million years on basalt in humid climates whereas in dry climates it takes <400 ky for these metastable phases to “ripen” into more crystalline oxides. Oxide formation and strong anisotropic horizon development directs water along lateral flow lines leading stream channel formation.