Time-Climate Matrix (Hawaii)
Islands offer a unique opportunity to study processes of soil formation.
First, the Hawaiian Islands result from the movement of the Pacific plate
over a stationary convective plume ("hot-spot'') in the mantle; distance
from the currently active volcanoes closely corresponds with parent material
age. Developmental sequences provide a useful constraint on studies of the
sources and fates of elements; the net effects of inputs and outputs can
be determined straightforwardly. Such developmental sequences can be characterized
very well from geological, pedological, and ecological perspectives.
A second useful feature is well-defined precipitation gradients. The quantity of precipitation affects the quantity and pathways of element inputs, but it also affects weathering and element losses. As with age sequences, a well-defined precipitation gradient provides an important constraint on determinations of the sources and fates of elements and the mineralogical changes associated with leaching intensity. Precipitation in the Hawaiian Islands varies as a function of elevation and aspect (exposure to the prevailing northeast trade winds) extreme variation in annual precipitation can be found within relatively small areas.
Finally, both the age and the precipitation sequences are remarkably little-compromised by confounding variation in other factors. The parent material in Hawaii is all basaltic lava, with relatively little chemical variation in space or time. Topographic position can be kept constant by selecting sites on the constructional surface of the shield volcanoes, which persist as remnants into the oldest sites. The dominant organisms are also nearly constant; the Hawaiian Islands are the most isolated archipelago on Earth, and the few species that colonized naturally have radiated evolutionarily over an extremely broad range of environments. One tree species, Metrosideros polymorpha, dominates developmental sequences from 300 to 4.1 million y; it also dominates sites that receive from 400 to 10,000 mm of annual precipitation. The constancy of these factors makes it possible to approach the ideal pedological research condition of a gradient of sites that differs in one and only one underlying factor.