Lake Okaro : explosions and erosion : a study into erosion on the hills to the north of Lake Okaro and the 0.7 ka phreatic and hydrothermal eruptions at Lake Okaro to help understand the current geomorphology

Hardy, Lyndon Winter.

Lake Okaro was formed approximately 700 years ago, simultaneous with a late stage of the Kaharoa eruptive phase at Tarawera, in a phreatic eruption that induced a number of secondary hydrothermal eruptions. The phreatic eruption was initiated within or beneath the welded Rangitaiki Ignimbrite and involved the excavation of a crater at least 80-metres deep at the south end of the present lake, with the resulting ejecta creating a ‘cap’ over an area of previous hydrothermal activity now occupied by the north end of the lake. I infer that increases in pressure as a result of the hydrothermal system being buried by material from the phreatic eruption led to a number of shallowly focused (<60 metres depth) hydrothermal eruptions, which helped fill the phreatic eruption crater, making the lake its present shape. Slumping into the craters, in addition to redeposition of Kaharoa tephra and AD1886 Rotomahana Mud from the surrounding hills, filled the lake to its present observed depth, with deep points indicating the position of original craters.
Erosion rills scarring hillsides to the north of Lake Okaro occur entirely within the Rotomahana Mud and represent part of the immediate sedimentary response to the AD1886 eruption. Increased sediment flow from erosion of the Rotomahana Mud over a low-permeability soil horizon developed in the Okaro Deposit initially caused degradation of valley floors, but once the rills stabilised and the sediment flow decreased, aggradation rates of approximately 0.013 metres/year occurred raising the valley floors 0.10 metres higher than their pre-eruption level by 1917, when the Frying Pan Flat hydrothermal eruption at Waimangu deposited a layer of material over the area.

66 leaves : ill., maps ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2005Hardy