Debris cone. The rod on the image above is 2 m high. Debris cones are common on the Vatnajökull outlet glaciers, especially Skeiðarárjökull. They begin to form when sand accumulates in low areas on the glacier. In the case of Iceland the sand is mainly black basalt ash. The bare ice around the sand is melted and lowers compared with the area of the black sand. Because the sand layer is thick enough and insulates the ice below, this part is preserved and starts to rise relative to the surrounding ice melting part. The sand at the top of the “hill” falls down and spreads a little on the ice beside. Thus the “hill” increases and gradually gets the shape of a cone. Most commonly the cones are up to a meter in size.
In the big 1996 jökulhlaup from Skeiðará river a large number of icebergs were broken up from Skeiðarárjökull and laid to rest on Skeiðarársandur. Once there they began to melt down during the following summer. Such icebergs tend to be deeply rooted and may last for many years in the sand, sometimes not visible because a blanket of sand covers the depression surface.