glaciers, forbes bands and ogives

Here are a few images of ogives (or Forbes bands) forming on glaciers. These were meant to follow the last blog posted on the 11 May, 2010. The bands are said to represent the annual movement of the ice and represent the volumes of ice shoved out from the cirques at the head of the glacier. The first and particularly the second image show the over-fall from the cirques where the cirques or accumulation areas at the head of the glacier often exhibit very low gradients and hence make it difficult to account for the outward motions based upon gravitational forces alone. Indeed, relic cirques are often found to have concave geometries requiring the ice to climb over the lips of the cirques. This would seem to be difficult to square with any theory of motion that relies upon gravity alone - albeit with a lubricant supplied by melt water at the base of the ice accumulated within the cirque. Interestingly, at the rock faces at the head of the cirques there are often deep crevices formed during the winter period. In the thermal ratchet theory for the motion of the ice (see posting of 4 May, 2010) these and other crevices would be caused by the thermal contractions of the existing ice experience during the winter cooling. The crevices would subsequently be filled with accumulated snow (and water which would turn to ice), so that when the ice experiences increased temperatures during the spring to summer period the now largely integral ice sheet would be subject to thermally induced expansions causing the ice to be shoved out over the lip of the cirque. The bands in this explanation would represent the different properties of ice when compressed during the expansions accompanying temperature rises and the fracturing and desiccation experience during the cooling phase of the seasonal cycle. Downstream these differences in ice properties would be partially preserved so that differential winter fracturing would result in accumulated ice filling the crevices accounting for the widening of the seasonal bands as the ice moves downstream - even though the width of the glacier is normally found to increase.

It is suggested that the existence of the ogives and the clear evidence of pulsatile out-flows from the cirques are both symptoms of motions that are caused by periods of expansion and periods of contraction exhibited by the motion of the glaciers.