Geological Movie Review of Dante's Peak - Part 3

Dante's Peak - Overview

- Pre - Eruptive Activity -

Several things happened in the movie before the actual eruption took place. This includes the lava eruption in the hot spring, the CO2 levels in the air, different types of earthquakes, and sulfur dioxide (SO2) in the water supply. Looking at these individually will help us to understand these processes a little better. This starts off with the reason that Harry is actually being brought out to Dante's Peak in the first place. These includes earthquakes which are located 10-20 km below the earth surface, prompting Harry to speculate that the chances for an eruption of the mountain are about 10,000:1.

- Initial Disturbances -

0:06:35 - The piece of paper that Harry is looking at is called a seismograph. I will go into that in a little bit but the main purpose is to show that there are earthquakes taking place under the volcano. They state that the earthquakes are about 10-20 km in depth. Earthquakes that deep are not usually a cause for alarm, but they do put up warning bells. The reason the great range could be one of two things. First there are several earthquakes which are occurring at about the same time, starting around 10km down and going down to about 20km. The other possibility is that since this volcano had such a high improbability of erupting (10,000:1) they did not have many seismographs in the region, which would prevent a highly accurate reading. The reason that the probability is 10,000:1 is likely due to the location of the volcano as well as previously recorded activity of the volcano.

- Hot Springs -

0:10:40 - So 2 young lovers decide to go skinny dipping in Twonset Hot Springs on the side of the volcano. Well apparently the volcano gods disapproved of this and caused an eruption to happen directly in the springs essentially boiling the couple alive instantly. The question is, do eruptions happen in hot springs before surrounding areas and what is the result of an eruption in a spring?
First off, how does a hot spring form? There are two types hot springs, ones in volcanically inactive regions and ones in volcanically active areas. In volcanically inactive areas, the springwater travels deep into the ground where the rocks are warmer (or radioactive) and then it travels back up into pools. This type of hot spring is present in Hot Springs National Park in Arkansas. In volcanically active areas, the water is warmed by the nearby magma (see diagram). Typically the water will obtain chemicals and minerals from the magma in the water causing the water to be: A. superheated from the magma and B. acidic from the minerals (USGS). Usually springs of this type are impossible for human use because of those two reasons. But if the water is not in direct contact with the magma it is possible for it to be heated from the rocks but not too hot for human use. And since the area where the water is being heated is close to the magma it is possible that the eruption would happen there before anywhere else.
So this scenario is possible, though it requires a rather shallow earthquake (which does not seem to be felt) to cause the magma to move upward into the water stream, which we did not see in the movie. Although it could have happened just previously. Also the magma should not have moved through the water that fast since the water would cause the magma to solidify on contact. This could be overcome if the magma had a lot of pressure behind it, which is possible since it is on the verge of eruption. Also afterwards the water seems to be highly acidic, which would cause the damage seen to the bodies. Though the likely cause of death was being boiled alive and the acidic concentration of the water was altered afterwards.

- Acidic Lake Levels -

0:16:00 - When driving around the mountain Harry samples some of the pH levels of the lake. The readings come back at 3.48, which he then says he needs to compare to previous readings from that lake and Mount St. Helens. So the main question is, are acidic lakes common in volcanically active areas and how acidic is normal?
To understand what the pH of volcanic lake water should be we need to understand the pH levels of normal lake water. "Normal" pH levels should be around neutral (7) ranging about 6.5-7 (K12Science). In comparison, volcanic lakes are far from usual lakes since they are effected by all the chemicals and gases being released from the magma (picture to left). Also lakes with an active volcano would have an even lower pH because the gases that are released cause the pH to drop. This pH drop produces a downward trend where with the more active the volcano, the lower the pH level (pH graph below under "Active" crater lakes). The pH of a lake in a volcanically active region is commonly below 3 while the pH of a quiescent volcano ranges 2.5 and up (Pasternack.ucdavis.edu).
So according to this study the pH of the Dante's Peak lake is initially within normal limits of a quiescent volcano but it is too high for an active volcano. This is what the volcano actually should be at that moment in the movie. The only thing that would make this abnormal is if the normal readings for the lake were far higher previously, hence the reason he wanted to compare them to previous measurements.

- CO2 Gas Levels -

0:16:28 - In the movie several trees were killed off near a lake prompting the geologist to wonder how long they have been dead. He concludes that they were possibly killed due to high levels of CO2 released from the volcano. Does this happen in real life?
For a real life example lets look at the volcano Mammoth Mountain, which was an eruption false alarm (meaning it never erupted as expected). One of the reasons that scientists thought it was going to erupt was because of the dead trees in the area of the mountain (see picture). The reason the trees died was because high levels of CO2 in the soil killed off the roots of the trees, leaving the trees with nothing to feed with. The gas escapes from the magma in the volcano through cracks in the rocks explaining the large distance between the dead trees and the peak of the volcano in the picture. Since the CO2 is heavier than air it commonly gets trapped in snow banks, basements, and the soil surrounding a volcano (USGS). Just looking at the picture gives me an idea where the writers came up with their material for this scene. So in essence, yes this is also possible.

- Previous Activity -

0:17:40 - Another thing that can tell a geologist just when an eruption is to be expected is the previous activity of the volcano. When Harry is driving around he is taking pictures and looking at the rocks on the flanks of the volcano. He then says that the last eruption occurred about 7,000 years ago. Of course he says this after looking at a rock on the volcano's flank, kind of giving the impression that he gained this knowledge from the rock, but that is information he would have known prior to driving out to the volcano in the first place.
The picture to the left shows a cross section of a volcano. Scientists can determine how often a volcano usually erupts by dating each of these layers using a variety of methods. Now assume this is Dante's Peak, the top layer would be 7,000 years old. Now lets say the layers below that are 14,000, 21,000, and 28,000 years old respectively. Since each flow is about 7,000 years apart, a scientist would predict that a new eruption would be imminent. But since the eruption is a 10,000:1 likelihood (previously mentioned), we can assume that the next eruption would be predicted at several thousand to hundred thousand year from now.

- Current Activity -

0:20:33 - There are several classifications for the activity of a volcano. There are active, dormant, and extinct volcanoes, which all rely on when the most recent eruption was. Any volcano with an eruption within the last few hundred years is considered active, the last few thousand years is dormant, and nothing within the last few thousand years is extinct. Now a volcano can go from extinct to active anytime assuming the right conditions, but usually extinct volcanoes have a very little likelihood of erupting (OregonState). Calling this volcano dormant is correct since, as previously mentioned, the last eruption was only 7,000 years ago.