The geological history of Geysir, Iceland: A tephrochronological approach to the dating of sinter

The vertical succession through the discharge apron of Geysir, up to 3.5 to thick, is formed of five lithological units, A-E. Units A, B, C, and E are formed of siliceous sinters whereas Unit D is a brown ash bed. Tephra grains in units C, D, and E came from Katla, Hekla, and Veioivötn. Katla tephra in Unit D has been largely altered to palagonite. Integration of tephrochronological ages, historical records, and geological and geomorphological relationships shows that the Geysir succession developed in four main phases. During Phase 1, c. 10 000-4000 years ago, laminated sinters formed from hot spring waters. Phase II, initiated c. 3300 years ago, saw the termination of sinter formation and blanketing of the area with tephra from Katla and Hekla. Phase III, c. 3000-900 years ago, represented by the boundary between units D and E, was characterized by weathering and little sinter deposition. Phase IV was initiated with the (re)birth of Geysir 800-900 years ago. Sinter deposited from the discharged waters has, however, been largely confined to the southern part of the discharge apron. Only limited opal-A precipitation has occurred since 1916 because of a decline in Geysir's eruptions.