ABSTRACT
Volcanoes with a more viscous lava tend to erupt only sporadically, since the vent can easily become blocked. In volcanoes where periodic violent eruptions occur, the shape of the volcano may be determined as much by explosions as by the accumulation of lava or pyroclastic material. The Hawaiian Islands are formed by the peaks of a line of basalt volcanoes, which become progressively older to the northwest. Major earthquakes are much the same wherever they occur, unlike volcanoes. A small but significant number of earthquakes is recorded from along the spreading oceanic ridges. The more general fracture zones extending beyond the ridge crest are less likely to generate earthquakes, since the surface is moving in a common direction. The intensity, or magnitude, of an earthquake is measured on the Richter scale, which takes as its value the logarithm of the maximum amplitude recorded on a seismograph trace at a set distance from the epicentre.
