In November of 1963 an oceanic volcano produced an island, Surtsey, just off the southern coast of Iceland. The volcanic crater was often flooded with sea water. Vigorous eruptions of steam and tephra were accompanied by an enhancement of the normal fine-weather potential gradient, and lightning was often observed. Measurements of atmospheric electricity and visual and photographic observations lead us to believe that the electrical activity is caused by the ejection from the volcano into the atmosphere of material carrying a large positive charge. The concentration of charge in the eruption plume as it issued from the orifice of the volcano is estimated to be of the order of 10(5) or 10(6) elementary charges per cubic centimeter.

More than 2100 microearthquakes were recorded and crudely located by using data from portable seismographs operated in Iceland during the summer of 1968. Another 600 events were located more precisely in three areas by using data from tripartite arrays. The earthquakes recorded are largely confined to 13 regions that are generally less than 100 km2 in area. Most of the well‐located events are at depths of 2 to 6 km but some less well located events may be as deep as 13 km. The microearthquakes are largely confined to the upper few kilometers of the oceanic layer, or layer 3 (Vp ≈ 6.5 km/sec in Iceland). Geothermal areas in Iceland that are structurally related to a large number of faults and fissures generally have high microearthquake activity. Geothermal areas that have few fissures and appear to be structurally related to acidic intrusions contain little or no microearthquake activity. The distribution of zones of microearthquake activity generally supports the hypothesis of a transform fault in southern Iceland. It appears that the stress along this fault is being relieved in geothermal areas by numerous microearthquake swarms occurring more or less continuously. Outside the geothermal areas, mainshock‐aftershock sequences seem to be the dominant mode of stress release. The swarms can be attributed to weakening of the crust by fluids or fluid pressure.

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