Multifaceted incremental growth of a geyser discharge apron – Evidence from Geysir, Haukadalur, Iceland

Abstract

Located in southern Iceland, Geysir was first named in 1647 and thereafter became the eponym for geysers that are known from geothermal areas throughout the world. As noted by many early explorers, Geysir has a long history of erratic eruptions with periods of activity alternating with periods of dormancy. Vertical sections through the discharge apron, exposed in the valley cut by Geysisgil along its western and northern margins, reveal complex sequences of interbedded spring deposits, volcanic ashes, and soils that provide a historical record of the discharge apron development. The basal beds are formed of opal-CT whereas the higher beds are formed of opal-A. Before Geysir became active the area was covered with volcanic ashes that resulted from the eruptions of Katla and Hekla ~3300 to 2850 years ago. Once Geysir became active, precipitation of opal-A from the geothermal waters that flowed across the area around the geyser led to development of siliceous sinters. Today the triangular shaped discharge apron around Geysir is ~200 m long and ~200 m wide with the vent pool located in the southwest corner. Its surface is divided into a southern sinter area, northwest vegetated area, and northeast broken sinter area. Precipitation of the opal-A is controlled by the large volumes of water that are episodically ejected from the geyser and water flow patterns across the discharge apron. Such waters cool rapidly, especially during the winter when air temperatures may be as low as −15 °C. Facies evident in the siliceous sinter that formed on the surface of the discharge apron include thinly laminated sinter, microbial sinter, flakey sinter, and plant sinter. Microbial sinters are generally rare in proximal areas but common in distal parts of the apron. Silicified birch leaves and wood are common, especially in areas where the water flows through the vegetated areas of the discharge apron. The unique facies architecture and topography of the discharge apron around Geysir, which developed in response to the interaction between many different variables, provide a depositional model that can be used in the interpretation of ancient geothermal deposits.