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One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.

The Nesjavellir high temperature geothermal field is a part of the Hengill geothermal area in SW-Iceland. The Nesjavellir field has been under exploration and development during the past 25 years. This paper summarizes the main exploration results, i.e. temperature and pressure distribution in the reservoir and the discharge parameters of the production wells. A conceptual model of the Nesjavellir field is described, along with a numerical simulation model. The calculation of the thermal power of district heating schemes in Iceland normally assumes cooling of the network fluid down to 40°C. The numerical model results indicate a power potential large enough to supply a 300 MW thermal power plant at Nesjavellir for 30 years without re-injection into the reservoir. Hitaveita Reykjavikur began the construction of the Nesjavellir geothermal co-generation power plant in 1987. The plant utilizes high pressure steam for electricity generation and low pressure steam and the separated water to heat fresh ground water for district heating in the Reykjavik area. The first stage, a 100 MW, thermal power plant, was commissioned in September 1990. The fully developed plant, using a re-injection system, is planned to deliver 400 MW, for district heating and 80–90 MWc for the national grid.