This post continues in the series analysing the volcanology of the 1997 disaster film Dante’s Peak. In the last post we looked at the consequences of acting quickly on little evidence – after Harry Dalton (played by Piers Brosnan) called an emergency meeting with the town council after concluding that Dante’s Peak could be about to erupt, based on only a few hours of data collection at the volcano. It is true that some volcanoes around the world can give less than an hours warning before they erupt, for example Hekla in Iceland has previously given about 40-45 minutes warning! However, it is still important to base a prediction on as much scientific evidence as possible before causing panic in a town. In this post we will take a look at some of the eruption precursors that were highlighted in the film.
1 Earthquakes/volcanic tremor
Dante’s Peak is first brought to the attention of the USGS by a series of small earthquakes in the region of the volcano. It is not clear at first where the epicentre of these earthquakes is or whether they are tectonic earthquakes, caused by movement of the earths crust, or volcanic tremors caused by the movement of magma beneath the volcano.
Small earthquakes or volcanic tremors often occur at a volcano when magma is on the move. As magma forces its way into solid rock the rock will shift and fracture, it will be displaced and faulting will occur. As this happens energy is released in the form of small earthquakes which can be detected by seismometers.
An increasing amount of these small earthquakes can accumulate into an earthquake swarm, which is often indicative of moving magma. This could be one warning sign that magma might be moving closer to the surface and might be able to breach the surface and erupt. It is a common warning sign witnessed at volcanoes, for example preceding the 1980 eruption of Mount St Helens and the 1991 eruption of Mount Pinatubo.
2 Dying trees
One of the next things that is observed in the film is the death of vegetation in the forests on the slopes of the volcano. This is another feature that can imply that magma and other fluids have begun to move within the volcano. When magma moves it releases gases and other fluids, and may cause other fluids around it to heat up and move around, these are hydrothermal fluids. These fluids and gases may find their way into the soil or escape out into the atmosphere, for example at fumeroles. These fluids can contain high concentrations of substances that are poisonous to vegetation, therefore killing patches of trees within a forest.
3 Boiling hotsprings and acidic lakes
One of the most dramatic changes at Dante’s peak highlighted by the horrific death of a couple of tourists, is a change in the temperature of natural hot springs. As magma moves closer to the surface and groundwater gets heated up it is possible that natural geothermal features, such as hotsprings and fumeroles, may change in temperature and/or acidity. An increase in temperature and acidity could indicate that magma has moved closer to the surface, and acts as another eruption precursor that Harry Dalton can check off of his list in the film.
4 Sulfurous drinking water
For a town with a generally drinkable and clear-running water supply the sudden change to brown sulfuous water comes as a shock to Major Wando. Harry Dalton decides to investigate the cause of this dirty water and heads straight up to the main source of the towns water supply. Like that witnessed at water supplies near Mount Vesuvius in AD 79, Harry Dalton finds the water source to be hot and steaming and stinking of sulfur. And, just like at Mount Vesuvius this is taken as a sure sign that something strange is going on, and in this case, is a cherry on the cake indicating that an eruption of Dante’s Peak is imminent.
Eruption precursors: Ground deformation – article on this site all about ground deformation as an eruption precursor and how this is monitored
https://volcanoes.usgs.gov/vhp/forecast.html – USGS Volcano Hazards Programme: information about eruption precursors and how these are monitored by the USGS.