Sunday, November 9, 2014

Some Questions About Volcanoes

Some questions are just fun to get. Perhaps it’s the teacher in me that likes to see young eyes light up with intellectual excitement. I infer from the following that volcanoes first get talked about seriously in the 5th Grade. It's beyond the soda, vinegar, and food coloring lesson.

Q: Question for my 5th grade class!
My students have some questions,
-           Megan A
1. Why do volcanoes erupt?
A: Pressure builds up from rising low-density magma below the earth. The low density is caused by the heat from the Mantle and Core of the Earth convecting upwards, sorta like a pot of Cream of Wheat cooking, or a lava lamp. The path of least resistance is to break out through the Earth's Crust at its weakest point. Where are those weakest points? Well, where you now see volcanoes is a pretty good hint. Some geologists have speculated that when tectonic events leave faults, and two faults happen to cross, that may make the intersection a “target of opportunity” for rising magma. However, there are a number of other factors involved, including where is the magma rising, and from what source, is there some under-plating of the crust happening, are there some gross compositional differences in the crust, etc. 

2. What are volcanoes like?
Some volcanoes look like cones (Mount Hood in Oregon, Mount Fuji in Japan). Some look like giant bulges (Mauna Loa in Hawai'i). Some volcanoes don't look like much of anything. You just see black-gray lava that has broken out of fissures, then poured out and run across the land in all directions – but generally the "pouring" goes downhill. There are vast, nearly impassable volcanic fields in western Saudi Arabia. There are huge obsidian flows (volcanic glass, caused by lava emerging in water and cooling too rapidly to form mineral grains) at Medicine Lake volcano in California. These look like a giant painted the ground with swirling green-black glass.

3. What is lava like?
Lava is very hot initially when it first reaches the air - it glows yellow-red from incandescence in cracks and at the flow-fronts. You can walk on it, because it is denser than a human body, but it is pretty rough on your boots. It melts boot-soles while hot, and cuts them up when cold because lava (e.g., in Hawai'i) is really just black glass. As lava cools, it sounds like a bowl of Rice Crispies crackling. As the flow-front reaches trees and houses, it engulfs them and the very high heat sets them on fire. This often forms tree molds - molds of where trees once were before being engulfed by the lava, for instance in HAwai'i and at Newberry Volcano in central Oregon. On Mauna Loa, a fast moving flow-front in the 1950's burst out of a fissure high on the volcano's west flank. I talked with a man who watched the flow run down the volcano's flank and onto a forest. It clipped off the trees at the base, then stack them vertically like bunched toothpicks at the front of the flow as the whole thing raced downhill at 60 kilometers per hour into the Pacific Ocean. 

4. Have you seen a volcano erupt?
I was inside Mutnovskiy volcano in Kamchatka when it started venting. I watched Mount St Helens erupt several times in 2004-2005. I've walked over active (evolving, moving) lava fields from Kilauea volcano, tracking the growing flow-front using a GPS device.

5. Is your job dangerous?

Not any more dangerous than driving a car on a Friday night when there are drunks on the road. Most volcanologists know someone, a friend or a colleague, who has died while working on a volcano, so yes, volcanoes ARE dangerous, and must be treated with respect. Because volcanoes are so dangerous, we take extra precautions when working on one that is restive, and generally stay well away of they are erupting. It's sort of like wearing seatbelts when you drive in a car. If you don't you are being deliberately careless - and statistically you have a much higher chance of dying. 

Sunday, November 2, 2014

NEGATIVE Earthquake Magnitudes?

Some questions require an explanation of a different kind of number that some students haven't yet seen before. These different ways of expressing numbers were developed to help explain very large things, very many things, very small things, or very complex things, among others.

Q: Hi, My name is Anthony. I was wondering how negative magnitudes can be recorded for earthquakes, and what is the smallest earthquake ever measured? Thanks
- Anthony N

A: Earthquake magnitudes are actually exponentials, so a negative exponential doesn't mean a "negative" value in the usual sense of the word. I'm hoping you've already had exponentials in school - or at least you can go ask a teacher what they are.

For instance,
10(exp)+2 = 10^+2 = 100.0
   The exponent here is +2 and it means one hundred. This is 10 to the second power.
10(exp)+1 = 10^+1 =   10.0
   The exponent here is +1 and it gives ten - ten to the first power.
10(exp) 0  =  10^0   =     1.0
   The exponent here is 0 and it means one - ten to the zeroth power.
10(exp)-1  =  10^-1  =     0.1
   The exponent here is a negative number, but it refers just to a SMALLER value than a non-negative exponent would. Here ten to the minus first power means one tenth.

The smallest earthquake ever recorded is a bit more difficult to answer. There are three parts to the answer:

1. It depends on the sensitivity of the instrument, and how close the hypocenter of the earthquake (the actual rupture point) is to the instrument. There are a lot of sensitive seismometers set up around the world as part of the global seismic network - they are designed to look for earthquakes in the magnitude 2 range or higher. There are also some really, really sensitive seismometers positioned on and around volcanoes. These are set up to look for earthquakes so tiny that earthquake people wouldn't really be interested in them - events so small that only one or two of the nearby instruments may even detect them, and no human would likely feel them.

2. I believe that the smallest recorded events are probably in the M= -2 range (negative two magnitude) for a very clean, noise-free station. That's also what two seismologists in my office tell me (independently!).

3. When you are looking at magnitudes this small, you are also dealing with a lot of noise: cars driving by on a nearby highway, people or animals walking nearby, wind vibrating trees and buildings, etc. In a sense, the smallest earthquake ever recorded is sort of meaningless, because it becomes harder and harder to even know if it's real - or just noise. Also, the smaller the seismic events, the more common they are. As an example, the US Geological Survey estimates that there were about 1,300,000 earthquakes worldwide in the 2.0 - 2.9 magnitude range. There are MANY more as you get to ever smaller magnitudes. See an earlier chapter on how many earthquakes are detected each year in each magnitude range (

No one is really interested in most of the wiggles you see in these two examples:
This is an instrument set up on Veniaminof volcano in the Aleutians. At 8:30am PDT on 22 October, I can see a few distant teleseismic events (distant earthquakes) and a lot of tiny events that may or may not be small volcanic earthquakes, or in some cases just small rock-falls from the crater walls. I can also see some large swings of the recorder that are instrument noise - probably electrical noise, either human-caused or natural, like distant lightning.

Whereas, if you look at Augustine volcano's webicorder for that same day, you see only a huge amount of wind noise:
This is an instrument set up on Augustine volcano in Cook Inlet in Alaska. At 8:30am PDT on 22 October I could only see masses of blue "ink" on the plot that indicate a lot of wind noise on this station. There is so much noise on this seismometer record at this point in time that any "real" earthquake would be impossible to see.