Geothermal Potential of Several Small Basaltic Islands
Surtsey
Much of what has been learned during the last 20 years concerning phreatomagmatic volcanism began with the mid-1960s submarine eruptions south of Iceland that eventually formed Surtsey, one of the Westmann Islands (Thorarinsson, 1965; 1966; 1967). The eruption, first noted on November 14, 1963, lasted more than 4 years and ended on June 5, 1967; this eruption formed an island of 2.8 km2 and an elevation of 174 m, which
is shown in Fig. 6.19. The early phases of activity were phreatomagmatic; much of the heat was lost in magma/sea water interactions that generated very energetic steam eruptions. The tephra deposited by these eruptions was barely warm to touch when was deposited as fallout and surges.
In later phases of activity, after a growing tuff ring denied the sea access to the vent, the main activity was lava fountaining, and lava flows that moved across the edges of the cone toward the sea. After activity ceased entirely, Stefánsson et al . (1985) drilled a 181-m-deep borehole on the cone at an elevation of 58 m. The tuff above sea level has a permeability of 1.2 × 10-10 m2 and the system within it is vapor-dominated. Below sea level, the estimated permeability of the basaltic tuffs is 4.1 × 10-13 m2 , and the tuffs are altered to palagonite (a mixture of smectite clays, zeolites, and iron oxides). No pillow lavas were encountered in this borehole, and the maximum temperature was 140°C at a depth of 104 m. Below this depth, near the contact between the sea floor and the base of the Surtsey cone, temperatures dropped to 40°C within altered tuffs that were cooled by sea water.
During the drilling operation, the borehole crossed a 13-m-thick dike at a depth of 80 m (Stefánsson et al ., 1985; Fig. 6.20). Most likely emplaced during the lava fountaining episodes late in the history of the cone, the dike can account for high heat flow within the cone. Fluids and heat from a tuff ring such as this one could be used for heating water but not for producing electricity; therefore, it is a limited, short-term resource unless the heat source were to be replenished by a new eruption.
Heimaey
Within sight of the new island of Surtsey is the small, populated island of Heimaey, which is an important Icelandic fishing community. On January 23, 1973, a north-north-east-trending fissure, located only 1 km east
Fig. 6.18
Interaction of the Krafla hydrothermal system with rising magma. Changes in the CO2
concentration and pH within wells KG-3 and KG-4 correspond to renewed eruptive activity.
(Adapted from Stefánsson, 1981.)
Fig. 6.19
Map of Surtsey volcano indicates the location of the 181-m-deep borehole.
(Adapted from Jakobsson and Moore, 1982.)
of the town center, opened over a length of 1200 m. Lava fountains occurred along the length of the fissure but were soon concentrated at one point (Williams and Moore, 1983). Within 2 days the lava fountaining had covered the island with ash and had constructed a 120-m-high scoria and spatter cone. Over the subsequent 2 weeks, lava fountaining decreased in intensity and a thick lava flow moved toward the edge of town. The 43- to 120-m-thick basaltic lava flow, at temperatures of 1030 to 1055°C, moved into the town and also threatened the harbor entrance.
To save the town and harbor, Icelandic officials were determined to stop the lava flow. Their method was to increase the lava's viscosity by spraying it with cold seawater and to construct a barrier along the flow margin. Seawater was sprayed onto the flow front and distributed across the flow surface at a rate of 1.7 m3 /s, cooling the flow to well below its solidus temperature. Barriers within the flow, which were formed by cooling, caused the flow to thicken. Over a 6-month period, the ~10 × 106 m3 of water sprayed onto the lava flows converted ~6.5 × 106 m3 of molten lava into hot, but solid rock (Jonsson and Matthiasson, 1974; Williams and Moore, 1983).
The eruption ceased June 23, 1973, leaving a lava flow ~1.5 by 1.5 km and ~100 m thick that was overlain by ~5 m of scoria. The residents of Heimaey immediately began to examine ways to take advantage of this heat source. A district heating system was created and after a successful prototype system was tested, construction of a geothermal heating system began. Four 100- by 100-m areas were developed, each consisting of steamwells in the unconsolidated scoria overlying the lava flow and an overlying network of pipes that spray water onto the ground surface. The water seeps into the scoria and the lava flow, is converted to steam, and rises to the steamwell collectors (Fig. 6.21). Each well produces 2.5 MWthermal during normal operations (Björnsson, 1980; Williams and Moore, 1983). By 1982, the entire town was heated by steam from the lava flow.