What follows is A VERY elementary description of what volcanoes are and do. Some of the information and illustrations below drawn from online site of the magazine Arqueología Mexicana which, believe it or not, I have seen in our local Chedraui supermarket. I've put links to the other sources I've used and they are all worth exploring, and none is overwhelmed by jargon.
There are three important and intertwined factors used to describe volcanoes:
Spatial distribution of volcanoes
Nature and intensity of volcanic activity
Volcanoes are not dispersed arbitrarily over the surface of the earth but rather are distributed in different regions which are defined by global tectonic activities -- that is, shifts in the earth's plates. México is a region that is tectonically active, especially in the band from Nayarit to Veracruz. The map below shows this belt and labels the major volcanoes in it as well as a couple in Baja and one at the very top of the Sea of Cortez (also known as the Gulf of California). Where we live in the state of Veracruz a little north and west of Veracruz, the city, we can see a number of small cones that are no longer active and probably haven't been for a huge amount of years, if ever. Directly opposite our house sits one called Acamalín.
I took the picture below on a hike. This is a spot sort of between Xico and Xico Viejo. Acamalín is the bump between the two tree-covered hills.
My father-in-law witnessed some of the growth of Paricutín in West Mexico, number 10 on the map below, in the late 1940s. .
As everyone who has come across it knows, it literally sprang up in a cornfield and buried everything in the town of Paricutín except the church.
Below is a picture Jim took during a trip we made to the state of Morelia. The volcano Paricutin is the small dark caldera outlined by the mountains behind it.
Well, maybe a little about plate tectonics. There are a whole bunch of good sites with well-illustrated information if you want to know more.The earth's outer layer, the crust, is divided into a set of large moving plates. The lines where they meet are called plate boundaries.
The illusltration below shows the different kinds of plate boundaries and hotspots.
From left on the picture below, a divergent boundary, a hotspot, a convergent boundary and another hot spot.
HOTSPOTS are stationary plumes of magma. This is a good brief discussion of them. Volcanoes in Hawaii are the result of hotspots.
The illustration below shows a hotspot forming an island in the Hawaiian Emperor chain.
This is the second factor considered important in evaluating volcanoes. It refers to the frequency of eruptions, especially of the biggest and most destructive ones. Timing is related to the intensity of tectonic processes. Figuring out timing remains a puzzle. No one can exactly say when a volcano will erupt. Furthermore, eruptions don't happen in any predictable cyclical way though the past does provide some guidance about what might happen in the future.
The nature and intensity of volcanic activity
Temporal distribution and spacial distribution affect the nature and intensity of volcanic activity although they are not the only factors. And volcanic activity is not necessarily harmful. For instance, volcanoes enrich the soil.
Kinds of volcanoes
In a very interesting and easy to read article from Oregon State University, the author explains that though people like to characterize things in strict ways, really, nature chooses its own ways. People miss a lot by holding to strict categories, especially the three that have long been used for categorizing volcanoes. The unnamed Oregon State author (see link immediately above) proposes six and hopes you won't be limited by the brief explanations he gives.
Popocatepetl, sort of scientifically speaking
Lies at latitude 19.02 and longitude -98.62, about 43 miles to the east of Mexico City and roughly 28 miles to the west of the city of Puebla. It is similar distances from Tlaxcala and Cuernavaca. It occupies parts of the states of Morelos, Mexico and Puebla. So clearly people in the Valley of Mexico and near it have a personal interest in Popo and in how it might affect them.
Briefly, Popo is a stratovolcano which spews out lava characterized as andesite and dacite. These are lavas that are cooler and more viscous than basalt, Popo is also known as a composite volcano.
"Below is a schematic diagram of a stratovolcano intended to illustrate the different layers of different materials that comprise them. The purple colors are meant to represent layers, either the products of fall-out from big eruption clouds or the products of pyroclastic flows. Notice that these ash layers tend to be thin but widespread. The orange colors represent lava flows, and note that some of them have cinder cones associated with them at the vent. The green colors are meant to represent lava domes, and notice that they do not flow very far. Each eruption, regardless of what it produces, is fed from the the magma chamber by a dike. Most dikes come up through the center of the volcano and therefore most eruptions occur from at or near the summit. However, some dikes head off sideways to feed eruptions on the flanks."
Popo has been rather active of late, and so there are many questions about what it might do. The past few days it seems to have quieted down a bit. You can read the official daily report and see the related charts at the Cenapred website here. Although Popo is the second tallest volcano in Mexico and one of the most active, it hasn't had a big eruption for many years. It is not considered extremely explosive, either, rating a 1-2 on the Volcanic Explositivity Index where the highest score is 8 and all the values after 1 (i.e. 2-10) increase from number to number by a factor of ten the way numbers on the Richter scale for measuring the intensity of earthquakes do.
Here people are aware that Popo has been a more active than usual. What can they expect? Currently the warning level is yellow. That's right in the middle with red indicating it's time to move nearby people and green meaning things are very quiet.
This is what Popo might do in the near future:
- Continue explosive activity at a low to intermediate level.
- Cause rains of ashes from a light to moderate level in nearby populated areas
- Possibly there will be pyroclastic flows which are "fast-moving currents of hot gas and rock" which can reach "speeds moving away from a volcano of up to 700/km/h (450 mph).
Below is a kind of scary picture of pyroclastic flows "sweeping down the flanks of Mayon Volcano, Philippines in 1984." This is NOT Popocateptl. The Mayon volcano has a Volcanic Explosivity Index of 4.
Volcanoes' eruptions can't be predicted over the long term, but over hours and days, it becomes more possible. Cenapred and other government offices have things well-covered. They are experienced and can be depended on.
Coincidentally, today the NY Times has a short video on what volcanoes are and how they work. It is on the front page of the digital edition at the moment. If it is moved from there, you can find it in the science section.