GEYSERS AND HOT SPRINGS 109 Bioherms and Stromatolites The mounds around hot springs may accommodate living creatures, forming bioherms Stromatolites are layered, built-up structures found near hot springs; they are composed of populations of unicellular cyanobacteria and algae, organisms that have been dated to as far back as 3500 million years ago (see Biosediments and Biofilms, Minerals: Carbonates) At Lake Bogoria, Kenya, there are intermittent developments of stromatolites in the highly saline shallows of the lake close to the cluster of hot springs, pools, and geyser Such stromatolites and algal fossils are commonly preserved in travertine that is used for ornamental building stone or cladding Exploitation of Volcanic-Related Geothermal Heat In Reykjavı´k, Iceland, the volcanic-sourced heat is used to meet energy needs for general urban purposes This local utilization is similar to that applied to non-volcanic-related geothermal heat in the Paris Basin (see later) The derived energy is Figure (A) Temperature requirements of geothermal fluids for various uses Reproduced with permission from Arnorsson S (2000) Geothermal energy In: Hancock PL and Skinner PJ (eds.) Oxford Companion to the Earth, pp 437 440 New York: Oxford University Press (B) Diagram showing a vapour dominated geothermal field Reproduced with permission from Cargo N and Mallory BF (1977) Man and His Geologic Environment Reading, MA: Addison Wesley