Sunday, June 07, 2009

Geological Movie Review of Dante's Peak - Part 5

Dante's Peak - Overview

- Eruption!!! -

- Types of Eruptions -
0:56:02 - First a little background on eruptions before we delve into the eruption itself. There are 5 main types of eruptions. Each type is indicative with a different level of activity and different eruptive materials (

1. Fissure eruptions - Fissure eruptions come from cracks in the ground. The lava erupted usually is basaltic with a very low viscosity.

2. Hawaiian eruptions - Similar to fissure eruptions just on a larger scale. It produces very gentle eruptions with low viscosity lava that usually contains a sustained flow.

3. Strombolian eruptions - These are short lived eruptions that are usually more active than Hawaiian eruptions. The eruptions contains built up air, which causes the lava to be more viscous and louder but no more dangerous.

4. Vulcanian eruptions - These eruptions are more explosive then strombolian eruptions and involve a series of cannon like explosions followed by more subdued eruptions. The eruptive columns can reach about 5-10 km in height and but they produce only a small quantity of tephra (erupted material).

5. Plinian eruptions - These eruptions produce an eruption with qualities that are similar to both Hawaiian and Vulcanian combined. This is because it produces an eruptive column like the Vulcanian eruption but it is sustained like the Hawaiian eruption. The lava that is erupted is usually felsic. The felsic lava cools quickly and usually erupts in solid form like ash and pyroclastic debris. These eruptions usually produce large quantities of tephra that blankets the surrounding region in pumice and ash.

- Analyzing the Eruption -
0:56:02 - Based on the main type of eruptions it is obvious that Dante's Peak produces a Plinian eruption (lots of material ejected and very very active). Since we know that, we can now analyze the Dante's Peak eruption in depth, to determine if the eruption seen is what would be seen in real life.

First the order of eruption at Dante's Peak:
Tectonic Earthquakes
Harmonic Tremors
Vertical Eruptive Cloud
Spreading of the Eruptive Cloud and Ash Fall
Lava Flow
Relative Calm
The Next Day
Pyroclastic Cloud
Relative Calm

- Tectonic Earthquakes and Harmonic Tremors -
0:56:02 - These have already been discussed above so I will not delve into them too much again. Just remember tectonic quakes occur first, followed by harmonic tremors. The main question for this part is: do earthquakes continually occur with the eruption and if they do how large can they possible get. Study of the Mount Pinatubo eruption showed continuous seismic activity until either the signal was lost or the station was destroyed, indicating that yes activity does occur consistently with the eruption (USGS). The reason that the earthquakes are continual is because a variety of things are happening at the same time. Initially with an explosive eruption the explosion itself is usually powerful enough the shake the earth. Also the magma moving through the magma chamber and the neck of the volcano at an accelerated rate, which will cause stress on the surrounding rocks, producing earthquakes. So the combination of all that causes the entire region to shake, usually (MSNucleus).
With all this going on it does not seem unlikely that the eruption is capable of destroying a town with just the earthquakes alone, especially a town not designed to withstand such earthquakes. So what is the size of the eruption that is associated with a particular earthquake? The eruption of Mount St. Helens was associated with a 5.1 earthquake, which is generally large enough to be felt but does not cause much damage (USGS). This is hardly large enough to cause the damage seen in the movie even close to the mountain. But Mount Pinatubo's eruption, the 2nd largest in the 20th century was associated with a magnitude 7.8 earthquake, which falls in the range of major destruction (About). So a relatively small eruption on the scale of things, Mount St. Helens, would not produce the damage seen but when you increase the destructive power to even halfway between Mount St Helens and Mount Pinatubo you get in the range of Dante's Peak destruction (UNR). So yes, town destruction by earthquake is possible.

- Vertical Eruptive Cloud -
0:56:30 - The eruption of Dante's Peak can be thought of in two parts with the first part occurring here (0:56:30) in the movie and the second part occurring on the second day of eruption. This is a similar pattern as was seen at Mount Pinatubo which contained 4 eruptive columns on the first three days then a sustained 9 hour eruptive column the 4th day (USGS).
Most stratovolcanoes are thought to begin an eruption with a vertical eruptive column. This is similar to firing off a gun. The volcano finally releases all that steam that has been building up with blowing the top off the mountain, so that all the pent up pressure is going to push the contents of the mountain straight up. The picture of Mount St. Helens to the left shows this vertical plume cloud, while the picture on the right shows the Mount Pinatubo eruption which forms less of a plume and more like a reverse funnel. The Mount Pinatubo picture is from the first eruption of the 4. But still it contains the basic premise that all the material is going to be directed upwards. This first eruption is, as they put it in the movie, "just clearing it's throat."

- Spreading of the Cloud and Ash Fall -
1:01:13 - After the initial plume, the plume in the movie starts to spread out into an all encompassing cloud, which causes ash to fall like snow everywhere. Typical eruptions will start to form a mushroom cloud with the plume still pushing up from the middle. The ash cloud that forms is the result of this mushroom cloud, which often blankets the neighboring region in ash as is seen in the movie. Larger eruptions would not have resulted in an ash cloud forming that quickly after the eruption, but assuming that this first eruption is smaller, it could produce a faster forming ash cloud, since there is not as much force pushing up on the cloud. So the closer you get to the peak the heavier the ash fall is going to be, hence the reason when the kids go up the mountain they are always in a thicker ash cloud than Harry, who is further down.

1:04:48 - See calling the FAA (previously mentioned in - Final Evacuation - section) on the results of flying through an ash cloud.

1:07:07 - See the mud storm (previously mentioned in the - Columbian Eruption Products - section) to see the effects of putting all that ash into the atmosphere. Volcanic eruptions are almost always associated with their own generated thunderstorms, including lightning.

- Lava Flow -
1:09:12 - This is when things start to stray from what might actually happen. As mentioned above there are several different types of eruptions but they can be grouped into two main ones. Eruptions that produce lava and eruptions that produce ash and pyroclastic debris. Most eruptions will not produce both because there are 2 different mechanisms for doing that. Some pyroclastic eruptions are capable of producing lava, although the lava that they produce never would flow as fast as seen in the movie (UCCS). The fast flowing lava is basaltic lava while this volcano would probably have felsic lava, which would flow more like thick molasses down the mountain. People could walk away from felsic lava and still be ok.
So this is the one major thing I can find wrong with the movie.

- Mirror Lake -
1:11:04 - We have already discussed the initial concept of acidic lakes above with the measurements Harry did at the beginning of the movie (see the - Acidic Lake Levels - section). Now we will go into the extent of how acidic is it possible for a lake to get and is this strong enough to melt a metal boat?
The chart to the right shows how acidic a lake can get in an volcanically active region. The pH ranges from 3 down to almost -1 which is highly acidic ( Since the most common gasses in volcanic regions are CO2 and SO2 the likely acids in the lake are sulfuric acid and hydrochloric acid. 33% sulfuric acid is used as battery acid and contains a pH of about 0.5 while 90% sulfuric acid has a pH of about 0.1 (NIOSH). Both of these acids are commonly used to clean rust off iron and steel and other things (called pickling) although this does not result in the metal melting (Encyclopedia). So if this boat was to dissolve we would have to assume that the boat is not made of iron or steel, which is entirely possible. So the most common boat material, which is still metal, is aluminum. Luckily for us aluminum is able to be dissolved by hydrochloric acid, especially acid with a very low pH (
So it is entirely possible to melt an aluminum boat in the acid of a volcanic lake. The only problems we have here have more to do with dramatic effect than with the actual scientific content of the movie. This involves why the fish don't dissolve. Also why does the metal on the propeller melts before the plastic bearings, especially since the motor was agitating the water. This would have caused the whole thing to dissolving much faster then the rest of the boat. And finally, why when Harry's hand is splashing the acid everywhere while he is paddling does less damage occur to his arm than when Granny jumps in the water. Although she did end up with the appropriate amount of burn as she should have been.

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