Saturday, May 30, 2009

Geological Movie Review of Dante's Peak - Part 4

-Predicting Earthquakes -

0:20:33 - The scientists in the movie use many devices to help them determine that an earthquake is about to occur. These include a robot called "spider legs", as well as some of the information described above. But just because all the signs are there does not mean an eruption is imminent. Most eruptions cannot be determined 100% until they are actually occurring. This is the reason for the debate in the movie, which resembles real life debates surrounding many geologically active volcanoes.

- Costs of Bad Predictions -

There are usually 2 choices with 3 different results for a possible large eruption:

1. Don't do anything --> volcano does not erupt
Result - Nothing, everyone is happy.

2. Don't do anything --> volcano has small eruption
Result - Most people have time to get away and not much damage to property

3. Don't do anything --> volcano has big eruption
Result - Huge loss of life and property because no one knew it was going to happen and so quickly.
One of two worst case scenarios

4. Cause an evacuation --> volcano does not erupt
Result - Cost of everyone evacuating and loss of possible tourists, since now the area is a "threat". Bad politically, and the scientists look really bad too.
The other worst case scenario

5. Cause an evacuation --> volcano has small eruption
Result - Everyone is safe and the town still looks like it has a larger problem than it really had. Not so good politically but better than nothing.

6. Cause an evacuation --> volcano has large eruption
Result - Everyone happy because they are alive although huge loss of property. Best scenario after calling an evacuation for the scientists.

- Previous Predictions and Eruptions-

0:23:35 - Previous bad eruption calls are the main reason that Harry's boss, Paul, is so worried about a wrong call. He mentions that in 1980 he was sure that Mammoth Mountain was going to erupt, which it didn't, but the tourism and everything of the town was destroyed due to the bad call. As previously listed that was one of the worse case scenarios.
Mammoth Mountain is a real volcano located within the Long Valley Caldera of California. It is similar in composition to Mount St. Helens except that it has more of a basaltic magma (Oregon State). Most scientists figured that if the mountain were to erupt that the resulting eruption would be extremely minor. In 1980 the region was hit with four magnitude 6 earthquakes along with 25 cm of dome uplift of the caldera floor. In recent years more activity followed with groups of trees being killed (pictured in the CO2 Gas Levels section above) and more gas emissions (USGS). So it seemed likely that an eruption was possible in 1980, but I can find nothing about any evacuation of the region around that time.

0:34:25 - A comparison of Dante's Peak and Mount St. Helens prompted Harry to say that if the eruption was similar it would produce a blast that could reach the town within a minute. Figuring the Mount St. Helens blast accelerated to about 300 mph, and reached about 17 miles away from the mountain, this is entirely possible if it were to happen again (USGS).

0:45:39 - Another comparison that is made is to Mount Baker in the 1970's where it was thought that Mount Baker was going to erupt and never did. Mount Baker is another stratovolcano located in the North Cascades of Washington. In 1975 activity increased substantially causing much of the glaciers to melt off of the mountain. The activity involved was only a result of heat being released with no earthquakes associated with the event (USGS). So it is likely that most volcanologists never really considered this a threat for eruption back then.

- Gas Levels -

0:25:53 - Measuring the gases is another way to determine the possibility of eruption. Gas is emitted from the volcano in a variety of ways. The first one shown in the movie is gas bubbles in the lake. This is similar to the gas bubbles in the hot spring because there are cracks in the bedrocks that likely lead directly (or indirectly) to the magma. When the magma rises and releases the gas, the gas would be the first thing to reach the surface. Another way that gas is usually released is out of the top of volcano, which is the main vent for the gas.

0:29:15 - As previously mentioned, measuring gas released from the vent is what they are doing by flying over in the helicopter and also with the robot in the volcano. In real life gasses can predict what is going on in a volcano. Usually when magma is deep in the earth the pressure keeps the gas within the magma but the closer to the surface it is, the more CO2 and SO2 is released from the magma out of the volcano. So as magma rises closer to the surface the gas concentrations in the atmosphere should also rise (PBS).
There are 2 ways to measure the gas released from a volcano. The first is what is seen in the movie, by collecting actual gas and measuring the concentrations of the different components (see picture to the right). As in the movie, the SO2 concentrations of Mount St. Helens were measured by a specially outfitted airplane. The second way is using a spectrometer which measures the light absorbed by the gas to determine the concentration of each type of gas (USGS). This method was not used by the movie. So their collection technique of flying in the helicopter should work at get a preliminary estimate, assuming the helicopter does not blow the air away from it. And even then, you still have the readings from the robot.

- Seismicity and Earthquakes -

0:35:43 - One of the machines that is used to determine a possible eruption is a seismograph. This is a machine that measures the size, location and type of earthquakes that are being produced around the mountain and is the best test that a volcanologist can use (PBS). When Harry is using the seismograph he stamps the ground, which produces a mini earthquake that is read on the machine. He does this to determine if the machine is working or not. The resulting image is the same thing a real earthquake would produce just on a far smaller scale. The more seismographs you have surrounding the volcano, the more accurate a reading you can get on the epicenter (location) of each earthquake. There are two typical results produced on the seismograph, both of which are the pictured below. Each result is associated with one of two main types of earthquakes that associated with volcanoes.
The first type is the most common in the world and it is referred to as a tectonic earthquake (USGS). This type of earthquake is the result of two sides of a fault being pushed passed one another (see picture on the right). Now when something gets stuck in between the 2 blocks (red circle) the movement stops. The strain between the two blocks builds and builds (think of trying to break a stick and its not breaking) until suddenly the thing holding the blocks in place breaks (the stick snaps) and the plates slide past each other quickly (see bottom picture on the right). That is why in most earthquakes the initial quake is the largest and following quakes (aftershocks) are smaller (top seismograph image below).

The other type of earthquake typical of volcanoes is called a harmonic tremor (NOVA). This is produced by magma moving through the volcano causing the ground to shake consistently (seismograph below). Kind of like shaking a bowl of Jell-O steadily.

Tectonic earthquakes are common around volcanic areas because the plates that typically produce the volcano in the first place are still moving. Like in the movie tectonic earthquakes are usually no indication of a possible eruption. But it is possible for the tectonic earthquakes to lead to harmonic tremors, or in other words the tectonic earthquake moves something in the earth, which allows the magma to move up through the volcano. So again the movie is faithful to the fact that the tectonic earthquakes could lead to harmonic tremors, which is a precursor to an eruption.

0:44:45 - The depth of the earthquake is also of great importance. The shallower the quake the close to the surface the cause is, and if it is magma, you know what that means (AVO).

- Volcano Robots -

0:36:53 - So since pretty much being able to predict an eruption is a big deal, several tools are used. Paul mentions several different machines in the movie but the main one people notice is "Spider Legs", which is a robot used to go into the crater of the volcano where it is "unsafe" for the volcanologists. The robot is able to measure gas readings, temperature and even produce a video that can be watched back at the lab. In real life NASA has actually developed these things and tested them a couple of years before the movie. The robots were aptly name Dante I and Dante II (pictured on left). (I think the name Dante more refers to the voyage of the poet into Hell rather than the movie but that is just me)
These robots were used for pretty much the same reason that they were in the movie, which was gas and water testing. Since most gas exuded from a volcano is heavier than air, the crater of the volcano could possibly be a toxic swimming pool of gas.
One final question to this section is, what is ELF and what does it stand for? ELF stands for extreme low frequency and we will get into it's capabilities a little later on.

- Earthquake Quantity -

0:37:17 - In the movie, during initial testing, the volcano is producing between 25 and 75 microquakes a day. The question is what are microquakes and are 25-75 really a common occurrence?
Microquake - Any quake that is below a 2 on the Richter Scale and is not detectable by a human on the surface.
Multiple microquakes in a short span of time in an active volcano is not all that uncommon. Volcanoes have been know to have more than 100 microquakes per day and still not be imminent for an eruption. Although it does cause concern and should be considered a strong warning sign (CBC).

- Changes in Size -

0:39:16 - Previously mentioned in the movie are laser beams that measure the amount of growth on the volcano. This can be produced in 2 ways. One is uplift of the crater floor and the other is actually producing what is called a lava dome. The growth of the crater bottom is also called bulging or swelling and lasers are not typically used in measuring this. Usually what is used is Electron Distance Meters (EDM), which uses infrared to bounce a beam off of the ground. This measures the change in distance between the sensor and the crater surface, similar to a laser but not quite. Some other methods include GPS which also can measure the change in height and tiltmeters which measure the change in angle of the surface (AVO).
The material that forms a lava dome is from previous small eruptions. Studying the material can give a good indication of the type of material that might erupt from the volcano in the future. If the lava dome is currently growing that means that the volcano is actively erupting and releasing some of the stored pressure, which would reduce the likelihood of a violent eruption. So studying the dome would actually provide some valuable insight into what is going on in the volcano and what could be possible in the future.

- Contaminated Drinking Water -

0:51:00 - The penultimate event before the eruption is the presence of sulfur in the drinking water. Harry states the same thing happened at Mount Pinatubo in the Philippines before it blew it's top. The eruption that the movie is referring to is the 1991 eruption of the volcano, which is considered one of the most destructive in the 20th century altering global temperatures for years to come. It also produced the largest sulfur dioxide cloud since recording began in 1978. Although I can find no evidence of drinking water contamination for this particular eruption I have no doubt it possibly occurred. Like CO2, sulfur dioxide (SO2) can also be dissolved in the water, especially when it is present in large concentrations. So prior to an eruption there will be an increase in the gases released, which would likely contaminate the local drinking water supply (USGS).

- Final Evacuation -

0:52:55 - Although predicting volcanoes is not an exact science, enough evidence can help make predictions more accurate. After deciding to actually evacuate the town they call several different people. These include the National Guard, the mayor, and the FAA. In real life these are the three groups that should actually be notified in case of an volcanic based evacuation. The mayor is the most obvious choice since she is the one to notify the townspeople. The National Guard is the second most obvious because in the case of any national disaster they are called to help regulate all evacuation procedures (FEMA). This was the case at Mount St. Helens as well (UCDavis).
The last one is the one most people would not realize why. Volcanic ash clouds can be devastating to the propulsion system of airplanes, since ash is essentially microscopic particles of glass. These sucked into an engine can clog it up and tear it apart. Hence the reason the FAA should be one of the first to be notified so they know to adjust all air routes around the region (USGS).

Monday, May 25, 2009

Accretionary Wedge - Time Warp

So the website that I just found (The Accretionary Wedge) is accepting submissions on it's new topic. The full listing is at Outside the Interzone but the gist of the topic is this:

“Where and when would you most like to visit to witness and analyze an event in Earth’s history?” Suppose you have a space-time machine to (safely and comfortably) watch an event unfold; which event would you most like to see? Why? What do we already know or hypothesize about that event that appeals to you, or that you would like to test? What would be the result, the upshot, of knowing more about this event? You do not necessarily need to limit yourself to Earth, nor to the past. You do not need to limit yourself to a particular instant if peeking several times over a period of minutes or ages helps you envision the evolution of something. You do not need to limit yourself to environments that could support life as we know it... imagine being able to take a time-sampling of magmatic composition from 10 miles below the surface as a nascent mid-ocean ridge opens up, or examining the circumference of the vent during one of Yellowstone's mega-eruptions! I'll tell you, this technology is basically magic.


My time and place is this:

I would like to go back, not to the Big Bang (which I would assume several scientists would love to marvel at) but before it (maybe a few billion years, give or take). What came before the Big Bang? Was there just a singularity sitting there for all eternity or was the previous universe collapsing in on itself? This would help answer the current question of whether the universe is going to expand continuously or eventually stop expanding and start collapsing in on itself.
Or was it really just a large guy with a white cloak and big beard thinking what he should do next?
"How bout an incredibly dense piece of matter? I wonder what would happen if I made one of those?"

Dino Pic of the Week

This is a picture of a Tanycolagreus skeleton that my wife took at the Thanksgiving Point Museum of Ancient Life. The skeleton was situated above us on a tree, and with the flash on it gave us this awesome picture.

Friday, May 22, 2009

Geo-Paleo Blogs from around the World (#10)

I just came across this blog - The Accretionary Wedge - which is a really cool idea. Essentially it is a repository of posts from various websites that all contribute to one idea (per post). Then when a couple of months have gone by those posts get copied over to the Archive website which is essentially The Accretionary Wedge main page. I like this a lot because it gets multiple people from multiple disciplines within geology contributing on many different ideas. I wish I had known about it sooner. They also list the current/ future topics that are on the hosting website and haven't been copied over to the archive yet.

Friday, May 15, 2009

What is this?

So I was recently handed this rock with the question of what are these things in the rock. I am unsure and as far as I know anyone who has looked at the rock doesn't know either. So I ask for any possible ideas. The rock is a limestone and that is about all I know about it. I have my ideas but I want to leave this as unbiased as possible.
Front view
Side view

Top view

It appears that the structures go through the rock in a circular/ cylindrical pattern bosed on the side and top views.

Oh Pluto...

So recently I came across this article describing how Pluto has yet again been screwed by the scientific community: As Science Evolves, So Does Pluto. And I thought it pertinent to reminisce about my old friend, the planet, I mean dwarf planet, I mean planetoid, I mean plutoid, what?!?!?.

1930 - 2006 - Pluto was discovered and deemed the 9th planet in the solar system. Essentially completing the popular acronym MVEMJSUNP.

2006 - The IAU (International Astonomical Union) destroyed the ever loving acronym (what the hell is a MVEMJSUN?) by recalssifying Pluto as a "Dwarf Planet". This classification includes Ceres - the largest asteroid in the asteroid belt and some other objects large enough beyond Pluto's orbit in the Kuiper Belt (Like Eris). They determined this because they felt that Pluto did not "clear" its orbit (I think because its moon Charon is roughly the same size).

2008 - And they go and do it again. Pluto is now referred to as a "Plutoid". Why? I don't think there is any good reason other than their definition of a "dwarf planet" sucked so they came up with a less controversial definition and needed a new name with it. Now Plutoids are big objects outside of Neptune's orbit. Abandoning Ceres to be the only "dwarf planet".

And in 2009 the debate continues with the article previously mentioned. As Science Evolves, So Does Pluto.

My take on it is that it is all basically semantics. Nothing has changed since 1930. Pluto is still the same size. It still has the same orbit. And as far as I am aware will be like that for a long long time (until it flies into Neptune). They discovered Charon in 1979 and that didn't change anything. I say put it back to being a planet. It is different enough from the other stuff in the Kuiper Belt and it has a fairly long history as a planet. Just leave it be.

A rose by any other name...

I ranted on this a while back as well HERE.

Thursday, May 14, 2009

Life under glass

Ok, not glass but ice. My wife forwarded this article to me that I found pretty interesting. It seems that life has been found in the frozen region under a glacier with no air or light. The microbes were living off of iron, converting it to energy. Fascinating stuff considering what the implications could be for research of life on other planets.

Ancient microbes discovered alive beneath Antarctic glacier

Sunday, May 10, 2009

Geological Movie Review of Dante's Peak - Part 3

- 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.

Wednesday, May 06, 2009

Geological Movie Review of Dante's Peak - Part 2

- Cascadian Background -
0:06:20 - The location for the movie is a mountain in the Cascade range
monitored by the Cascades Volcano Observatory, specifically, a mountain in the state of Washington. The reason this region has so many volcanoes including Mount St Helens and Mt Rainier is due to the plate tectonics of the region. I will go into this more in the plate tectonics section of the website (under construction) but for now this region is volcanic because of the proximity of a subduction zone. Simply put, this is where one plate goes beneath another plate and starts to melt. The melted rock then rises in the earth and forms the volcanoes (bottom picture to the right).

This type of area along continental edges forms volcanoes with explosive capabilities called "stratovolcanoes." This type of volcano produces the variety of eruption seen in the movie by Dante's Peak (the volcano). The reason the volcanoes are explosive is because of the type of magma that is erupting. Looking at the igneous rocks part of the website you can see there are 3 main types of rocks based on silica content. The less silica in the rock the more "mafic" it is and the hotter the rock is when it erupts. This magma is more common in volcanoes like Hawaii. The magma erupting in the Cascades has a higher silica content (felsic) causing it to cool faster, which results in an eruption consisting of more gaseous and solid material (San Diego State University Geology Department).


We will go in to the type and order of the eruption that is presented in more detail below, but just so you know we are basing this on the typical eruption of the area.


Also so you know the Cascades Volcano Observatory is a real place which was developed following the eruption of Mount St Helens, although it is located in Vancouver not Portland as mentioned.

Monday, May 04, 2009

Geological Movie Review of Dante's Peak - Part 1

Dante's Peak - Overview

This article is an update of an article I wrote for my website. The links and grammar have been updated and all references to Wikipedia have been removed (I have included links to more reputed websites instead.) I soon hope to have the entire article updated on the website but for now this is the only location.

Disclaimer - This is going to be a geological overview of the movie Dante's Peak. I am not going to focus on the plot, the acting, the directing or anything of the sort. This is purely a scientific critique on the movie and one from my own mind, so do not take that into effect on whether or not you are going to like this movie. In "science fiction" movies the role of the science advisors are often outranked by the director or other people in the movie and the science gets left out. This means that the bad science of the movie is often not a result of a bad advisor. So do not take my critique of the science as a direct shot at the advisor.

Of all the Geological Movies that I have seen this is actually the most accurate that I can think of. This critique will be more of a confirmation and helpful understanding of the facts than a ripping apart of the movie to expose the plot holes. Now onto the geological review. I will also make some time notes so that you can coincide when that comment is for in the movie.

- Geological Critique -
- The Story Basis -

The basis for the story is actually the eruption of 1980's Mount Saint Helens, which was used as a backdrop for the erupted mountain at the end of the movie as well for a scene within the crater of the mountain. The current status of several other mountains located within the proximity of Mount Saint Helens also played a part in the science of the movie (University of Maryland).


- Columbian Eruption -
- The Eruptions Itself -

0:01:30 - The first sequence of the movie depicts an eruption in a foreign country. The eruption results in a lot of damage and the girlfriend of the main character, Harry (Pierce Brosnan), dies. There are two main activities that are associated with this eruption, mud rain and lava bombs. Later we find out that these events took place 4 years before the events of the main story and in the country of Columbia. Since the movie was made in 1997, the latest this eruption could be is 1993, but it was likely earlier than that (time for writing the script). Also there are several possibilities for volcanoes in Columbia. The picture on right shows 6 of the major Columbian volcanoes (USGS). Going through them can help narrow down which eruption is depicted in the movie:

Galeras - One of the most active of the volcanoes. The last major eruption was in 1886 but a smaller eruption did occur in 1993 which resulted in the deaths of several scientists. Although, the scientists died because they were sampling in the crater when it erupted. The main town near the volcano is Pasto about 18 km from the volcano, although there is a closer town about 11 km away. The scientists were killed or injured by lava bombs.

Dona Juana - Very little activity of this volcano. The only known historical eruption is an extended eruption from 1897 - 1906. Nothing since.

Purace - This is another of the more active volcanoes in the region with the most recent eruption in 1977. Although, very little information is recorded on this eruption and it was only classified as a VEI 2. This is not likely the eruption in the movie.

Huila - Last known eruption was around 1555. The only activity since is a constant steam column and hot springs.

Nevado del Tolima - Last known eruption was in 1943 with the last major eruption occurring 3600 years ago. The eruptions in the last 2 centuries have all been minor, but explosive.

Nevado del Ruiz - The last known eruption was in 1991, but the eruption in 1985 was one of the most devastating in South American history. The eruption completely buried the nearby town of Armero and cause lahars that traveled more than 100 km. In 1984, the early days of volcanic activity, there was no team of volcanologists that could be sent out, but by the time of the eruption a team was constructed that could go anywhere in the world (USGS).

So the most likely result for the eruption in the movie is the 1985 eruption of Nevado del Ruiz. A second possibility could be Galeras, since several scientist did die and it takes place in the right year, but the scientists that died were in the crater and it does not seem as dramatic or even as active as the volcano is portrayed in the film. That type of drama and activity more closely relates to Nevado del Ruiz.

- Columbian Eruption Products -

So now the question is, since the eruption is likely based on actual events, are mud rain and lava bombs typical of large scale eruptions and what are they anyway? First off, some definitions for volcanic ejecta:

Tephra - Is any air-fall material the gets erupted out of a volcano.
Lava Bomb (AKA Volcanic Bomb) - This is a lump of molten rock that gets shot up into the air and solidifies on its way back down. Typically they form aerodynamic shapes while in flight.
Mud Rain - Is rain that falls through or from an eruption cloud.

First off, the mud rain. When a volcano erupts with a large tephra cloud, the particles in the cloud act as water condensation points. This is essentially the beginnings of a rain cloud. These tephra clouds commonly produce rain with the tephra mixed in causing it to rain mud. So most eruptions, especially in humid climates will often have some if not a lot of mud rain (synapses.co.uk).

Volcanic bombs are named by the shape they produce. The shape is a result of viscosity of the magma, where the more viscous the magma the more defined the shape. Now very few of the bombs are visible in the movie but what we would expect from this type of eruption would be a bread-crust bomb (picture right) which is most of what Mount St Helen's erupted. This is because the outside cools quickly but since there is so much gas and lava still on the inside, the bomb continues to expand, cracking the outer crust, and making it look similar to bread (gc.maricopa.edu).

0:03:51 - So looking back at the movie if you look at the lava bombs that are actually visible, you can see most, if not all, of them have this general shape and appearance.
Now the variety of sizes in the movie include small ones, entirely possible, to ones large enough to destroy the front of the pickup truck. These lava bombs also hit things so hard as to leave major dents in them and even destroy the ground (or the pickup truck) where they land. In real life all these things can occur. Lava bombs can grow to become over 5 meters in diameter and can be shot several kilometers from the vent depending on the force of the eruption (Geotimes). Also, they are usually traveling at such a great rate of speed that they can damage and possibly destroy anything in their path (pictured left are impact holes) (LCC).