The physical geography of earthquakes and volcanoes has been studied in a multiplicity of ways. For example, improvements in the collection of atmospheric data and their analysis as time-series have enabled the climatic impact of volcanic eruptions to be assessed accurately (Handler 1989). At the same time, improvements in remote sensing have led to more comprehensive and accurate assessments of the extent and composition of volcanic aerosols. Erupting and passively degassing volcanoes are estimated to emit the following quantities of gases into the atmosphere each year: 100-200 million tonnes (Mt) of carbon dioxide, 18.7 Mt of sulphur dioxide, 0.4-11 Mt of hydrochloric acid and 0.06-6 Mt of hydrogen fluoride. For the most part, tropospheric gases are easily swept out by precipitation, and CO2 emissions are dwarfed by those from anthropogenic sources. Hence the main effect of eruptions is to inject large amounts of SO2 into the stratosphere (e.g. 17 Mt by Mount Pinatubo in 1991), and these may oxidise photochemically to sulphuric acid aerosols, which both add to the acidity of precipitation and reduce global temperatures by up to 0.5 C for a few years by reflecting some insolation back into space. Quantities of hydrogen fluoride (HF) are also significant and may gradually help to change the composition of the Earth's atmosphere.

There has been increasing interest in the hazards to aviation associated with suspended particulates and gas plumes from volcanic eruptions. Acidic aerosols can etch the exterior surfaces of aircraft, especially cockpit wind-screens. Thus in the early 1980s the Mexican volcano El Chichón injected 0.5-0.6 km3 of sulphur-rich products into the stratosphere and one airline found that it had to replace up to forty-two aircraft cockpit windows in a single month at a total cost of US$6.8 million (Bernard and Rose 1990). Moreover, glass pyroclasts can remelt when sucked into jet engines, which increases the operating pressure ratio of turbine compressors and may cause the engines to stall or flame out. Eruptions of Redoubt Volcano in Alaska and Galunggung in Indonesia led to in-flight emergencies and severe damage to aircraft, although fortunately not to casualties. Hence steps have been taken to utilise real-time vulcanological information in flight control (Casadevall 1991). Of particular use are AVHRR images from, for example, the NOAA 10 and 11 satellites, which can help to map the height, distribution, temperature and concentration of plumes and indicate their paths of movement.

About one-tenth of the world's 450-500 active volcanoes may erupt in any single year. Although relatively few are intensively monitored, fifteen are considered to offer such a high risk of disaster that they are the subject of a special monitoring initiative under the auspices of the International Decade for Natural Disaster Reduction (Figure 5.1), and all potentially active sites of vulcanism have been catalogued (Bullard 1984). Vulcanologists were among the first to make intensive use of the Internet for the exchange of data and information, and the Global Volcanism Network is now one of the most active and extensive of such operations. Vulcanologists make considerable use of electronic communications and satellite monitoring, with special emphasis on Total Ozone Mapping Spectrometry and Advanced Very High Resolution Radiometry for the height and composition of atmospheric plumes derived from eruptions, infrared data from the Thematic Mapper for the volume and temperature of emitted radiation, and radar and global positioning system (GPS) data for monitoring the deformation of a volcano's surface.