CRYSTALLIZATION AND GRAVITATIONALLY CONTROLLED PONDING DURING THE FORMATION OF MOUND SPRINGS, TERRACES, AND BLACK SMOKER FLANGES

We present a series of laboratory experiments in which hot, dense aque ous solutions were cooled and crystallized by contact with cold ambien t solutions as they flowed on horizontal, sloping, or nearly vertical boundaries. In each case, the crystals grew to form ''dams'' on the bo undary, behind which the input fluid ponded before overflowing the rim . On a sloping boundary, simultaneous crystallization of a number of d ams led to the growth of terraces that are reminiscent of natural trav ertine and sinter terraces. This phenomenon is not critically dependen t on the composition, concentration, or flow rate of the fluid. If the fluid overflowing the rim becomes undersaturated, uniform crystalliza tion is replaced by dissolution that soon focuses at one point of the rim, leading to a ''breakout'' of the ponded fluid, In an appendix, we also examine quantitatively the flow of water over the rim of mound s prings and terraces and show that surface tension can determine both t he two-dimensional volume flow rate and the length of rim overflowed.