Fjaðrárgljúfur Canyon

Fjaðrárgljúfur is a canyon in south east Iceland which is up to 100 m deep and about 2 kilometres long, with the Fjaðrá river flowing through it. It is located near the Ring Road, not far from the village of Kirkjubæjarklaustur. The canyon was created by progressive erosion by flowing water from glaciers through the rocks and palagonite over millennia. "When the ice cap retreated during the last glacial period, a deep lake was formed in a valley above the present gorge, the water being retained behind a threshold of rock, with overflow water running down the slope now cut by the gorge. As rivers fed by the edge of the ice cap deposited a lot of sediment in the lake, the spill-water eroded the rock threshold and the hyaloclastite in front of it. The result was a powerful(answer B) that cut the gorge we see today. Eventually it dwindled as the lake filled up and the river cut its way lower and lower into the sediment in the valley. It is still doing this, but the flow rate is now feeble and it has little influence on the deep channel in the hyaloclastite. Hyaloclastite is a hydrated tuff-like breccia rich in black volcanic glass, formed during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It has the appearance of angular flat fragments sized between a millimeter to few centimeters. The fragmentation occurs by the force of the volcanic explosion, or by thermal shock during rapid cooling. Several minerals are found in hyaloclastite masses. Sideromelane is a basalt glass rapidly quenched in water. It is transparent and pure, lacking the iron oxide crystals dispersed in the more commonly occurring tachylite. Fragments of these glasses are usually surrounded by a yellow waxy layer of palagonite, formed by reaction of sideromelane with water."