Very little can be done to control a volcanic eruption. Lava flows are the only  primary hazard which people have attempted to control with any success. Two methods have been used – water sprays and explosions. Sea water sprays were used successfully to cool the lava flows during the 1973 eruption of Eldfell on Heimaey, Iceland, to protect the harbour of Vestmannaeyjar. Explosives were used with some success in the 1983 eruption of Mount Etna when 30% of the slow moving lava flow was diverted from its course.

Artificial barriers have been proposed to protect Hilo Hawaii from future lava flows. Barriers have also been used to protect against the secondary effect of lahars, which tend to follow well defined routes. In Indonesia some villages have artificial mounds to enable villagers to escape to higher ground, although adequate warning is needed if this is to be effective e.g. tsunami warning systems. A notch was built through the crater wall of Mount Pinatubo to drain a lake which developed in the years after its big eruption. http://news.bbc.co.uk/1/hi/world/asia-pacific/1530182.stm

Friday, 7 September, 2001, 21:25 GMT 22:25 UK
Filipinos return as volcano lake drains
The massive lake was filled by heavy rainfall

Hazard-resistant design

Building and structure design can do little to resist lava, pyroclastic flows and lahars since they destroy any structure in their path. Ash fallout has the largest spatial impact and design may help to reduce its impact. The weight of ash on roofs, especially if its wet, can be enough to cause roof collapse. Roofs need to be strong and designed to shed ash with steep sloping sides. The long axis of the roof needs to be kept short: http://www.geogonline.org.uk/pinatubo_effects_question.htm. In Hawaii timber houses in high risk areas could be moved if necessary.