Volcano Vocab #5: Caldera

Part of my research this summer involves visiting Los Alamos to learn how to work with a computer model; in addition to one of the world's greatest research laboratories, northern New Mexico also hosts the Valles caldera, a major volcanic center north of Albuquerque. (Pretty much everything around me is volcanic, which means that whenever I drive or bike anywhere, I'm always staring at the scenery and going "holy crap, that's amazing!")

Caldera ("cal-dare-uh") is a Spanish word meaning "cauldron", and it describes a type of large, bowl-shaped volcanic structure. Calderas are created by collapse of the roof of a magma chamber after the chamber's contents have been removed, either in effusive or explosive eruptions. They're technically not craters, which are smaller and usually located on the summit of a volcano, but much larger features that form when a volcano expels the contents of a big magma reservoir and then collapses. (If activity continues after this happens, it can even create new stratovolcanoes within the caldera.)

The eruptions that form calderas are big - hundreds to thousands of cubic kilometers of material is involved. Collapse features that form over magma chambers that large are often not recognized as volcanic features until they're seen from the air, because they're simply too large to distinguish from the ground. On the volcanic explosivity index (VEI), caldera-forming eruptions top the chart - and in the case of some, are too big to even show on the chart:

VEI figure from the USGS Volcano Hazards Program Photo Glossary.

To give you an idea of what this translates to in reality, here's part of the Valle Grande in the Valles caldera:

This meadow is only a fraction of the whole caldera, and you can't even see the far walls because they're hidden behind the resurgent and smaller domes. Here's a map of the whole caldera:

View Larger Map

The Valle Grande is the light green patch in the southeast; the hills in the background of the photo are the Redondo Peak resurgent dome and smaller lava domes, which are pretty common post-caldera-eruption features. Resurgent domes are thought to be related to rebounding of the caldera floor, possibly due to new magma intrusion; the lava domes represent later eruptions through fractures. (If you want to know more about the specific geologic history of the Valles Caldera, Garry Hayes over at Geotripper has a great post from last year.) Some well-known examples of calderas in the United States are Yellowstone in Wyoming, Crater Lake in Oregon, Long Valley in California, but there are plenty of others:  Krakatau and Tambora in Indonesia, Santorini in Greece, and Colli Albani and Campi Flegrei in Italy, to name a few. 

I'll leave you with a photo from the rim of the Colli Albani caldera in Italy, with the Faete stratovolcano to the left of center:

Volcano Vocab #4: Lahar

As suggested by a commenter on the last Volcano Vocab post, here's a water-and-volcano-related term for you: Lahar ("lah-haar"). 

Lahar is an Indonesian word for a mudflow of volcanic material - that is, a mass movement of volcanic debris that contains some amount of water. (A dry flow of volcanic material would usually just be called a debris flow or debris avalanche.) The key thing that distinguishes a lahar from a "regular" mudflow is the presence of volcanic material in the flow, which can include tephra, ash, hydrothermal alteration products, blocks of lava flows, and other pyroclastic materials. Some descriptions liken this mixture to a flow of cement, and it's capable of moving house-sized boulders huge distances from their source.

Lahar deposits characteristically show poor sorting (lots of different sizes of material, from boulders to sand), multiple rock types, rounded clasts, and muddy matrix supporting the clasts. (Sometimes in deposit matrices you can find rounded voids where bubbles of air were trapped as the deposit hardened around them!) Lahars are most common on stratovolcanoes, but (as we've seen in Iceland), other types of volcanoes can also create the correct conditions to form a lahar. Lahar formation depends on having lots of loose material, and the addition of lots of water over a short period of time (such as from a melting glacier, a hurricane or storm, or a breached crater lake, among other things). They do not require the volcano to be actively erupting, which is one reason why they are so dangerous.

Here's a video of a lahar from Mount Ruapehu in New Zealand (March 2007):

Lahars are an especially dangerous volcanic hazard because they appear and disappear so quickly. Lahar debris that's deposited in a riverbed can easily be eroded by normal river flow, and lahars that have spread beyond valleys and drainages can easily become reclaimed by vegetation. In fact, one of the most devastating lahars to have come from Mount Rainier in Washington State (the Osceola Mudflow)  is now covered with small towns. Because the people in the area are now aware of the danger a repeat lahar would present, they are required to hold evacuation drills. Lahars are somewhat easier to monitor and avoid; acoustic flow monitors (specially calibrated seismometers) can be placed in source zones, and a timely warning can be sent downstream when lahar signals are detected. Sometimes evacuation can be simply a matter of climbing to a higher elevation, although on a floodplain it could be necessary to travel much greater distances to safety, but it does require advance warning.

Armero, Colombia, destroyed by lahar on November 13, 1985. Photo from the USGS CVO website.

Lahar warnings aren't always heeded, however. It's impossible to mention lahars without also mentioning the town of Armero in Columbia. On November 13, 1985, a small eruption of the nearby volcano Nevado del Ruiz melted part of the snow and ice capping the volcano's summit, and produced a lahar. Volcanologists knew that river valleys on the volcano's flanks could channel lahars toward populated areas, and sent warning to towns in the lahar's path; unfortunately, local officials either received incomplete or conflicting information, and/or decided not to listen to the scientists' warnings. As a result, more than 23,000 people were killed in Armero and nearby villagees, when they could have reached safety by climbing only a short distance up the slopes on the sides of their valley. This tragedy drove a USGS scientist to develop the Acoustic Flow Monitors mentioned above, in hopes that more deaths could be avoided; the system is now used at lahar-prone volcanoes worldwide.

Volcano Vocab #3: Tephra

Today's volcano word is tephra, another term that's directly related to the Eyjafjallajökull- Fimmvörduháls eruptions going on in Iceland at the moment. Tephra ("teff-rah") refers to any fragmented material thrown from a volcanic vent during an explosive eruption. It comes in different sizes, all of which have their own names (just to make things even more difficult!)

Bombs or blocks are large rocks - 64 mm and greater in diameter (cobble to boulder sized; see the photo at left, which is an example of a really big bomb on a scoria cone on Mount Etna). Lapilli are smaller, from 2 mm to 64 mm (the size of the material underneath the bomb at left). Ash is any material smaller than 2 mm, and is one of the main constituents of a volcanic eruption column, such as the one that's disrupting air traffic over northern Europe at the moment. Volcanic ash is composed of fragmented glass, rock, and phenocrysts (crystals), unlike the ash you get from fires (which is mostly carbonized organic material).

The other things that make up an eruption column are typically gases (including water vapor), ambient air that's been entrained and heated, and some lapilli and bomb-sized particles. Some recent news reports have been saying things like "ash and smoke" to describe the Eyjafjallajökull-Fimmvörduháls eruption column, which is incorrect. There is no "smoke" in an eruption column, at least in the sense that most people think of it (as a byproduct of burning materials). The column appears to be smoky, but only because of the presence of the ash, which is generally some shade of gray or black.* (The photo below, from a February 2010 eruption of the Caliente dome at Santiaguito, is quite gray to begin with, but I can guarantee that it's not because something in the vent is burning.)

Tephra is a major hazard associated with volcanoes. Bombs tend to be more of a problem in the vicinity of a volcano, but as many people in northern Europe are finding out, smaller particles like lapilli and ash can travel much higher and farther. Ash from a powerful eruption can reach the upper atmosphere, far higher than airplanes can fly; and because glass makes up a good portion of those ash particles, any plane that does fly through an ash cloud risks sucking glassy particles into its engines, where the glass can melt and re-solidify. This is bad - it could mean total engine failure, which is what happened to a flight over Alaska in 1989. No sane pilot is going to fly a plane into that.

So if you're stuck waiting for a flight to or from Europe, just remember: it's a lot better than risking a plane crash. And you can probably look forward to some spectacular sunsets.


*Okay, maybe some lichen is getting toasted, but that still doesn't mean you can call ash "smoke".

Volcano Vocab #2: Jökulhlaup

Today's obscure volcanologically-related word is jökulhlaup ("yer-kul-hloyp", "YO-kel-yawp" and "yo-kul-h-loip" in varying pronunciations), which is an Icelandic word for glacial outburst floods, both of water and lahars, formed when a subglacial eruption occurs. The water for these floods is formed when heat from those eruptions melts glacial ice, forming lakes that eventually become unstable enough to break through channels in the base of the glacier and flow out from underneath it. (Apparently the word can also refer to flooding caused by geothermal heat rather than a subglacial eruption, but since it's hard to see what's going on under a glacier in the first place, I wouldn't be too picky about the generation mechanism; suffice to say that some sort of volcanic activity is involved.) To give you an idea of what an unstable subglacial lake would look like, here's a diagram from an excellent overview paper:

Figure 3 from Björnsson (2002), showing a stable sub-glacial lake (a) and (b) an unstable lake likely to form jökulhlaups.

How big are these floods? Here's a quote from the same paper, talking about jokulhlaups from formed by the Grímsvötn volcano under the Vatnajökull glacier:

Jökulhlaups from Grímsvötn have occurred at 1– to 10–year intervals, with peak discharges of 600 to 4–5×10⁴ m³s⁻¹ at the glacier margin, a duration of 2 days to 4 weeks and a total volume of 0.5–4.0 km³.

Obviously, this is not a good thing to be in the way of. (By way of comparison, the mean discharge at Niagara Falls is about 1770 m³s⁻¹ , or about a quarter one-thirtieth of the peak discharge during one of those floods.) I don't have any personal or public domain photos of a jökulhlaup, but the Global Volcanism program has some excellent photos from a 1998 event during an eruption of Grímsvötn.

This topic is quite relevant at the moment because of the recent volcanic activity in Iceland. While the fissure that's erupting at Eyjafjallajökull isn't in danger of melting much ice, there are several other volcanoes that are, such as Katla volcano under the Mýrdalsjökull glacier. Since roughly 10% of Iceland is covered in glacial ice, and the country has more than 30 volcanoes that have been active in the last 10,000 years, this is a major concern (see Ole Nielsen's post on jökulhlaups here).

If you're interested in more Icelandic geologic vocab, the USGS has an English-Icelandic glossary here. And here is the full citation for the Björnsson paper:

Björnsson, H., (2002), Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change, v. 35, p. 255–271. http://dx.doi.org/10.1016/S0921-8181(02)00130-3


UPDATE: Whoops! Totally forgot about this page over at Andrew Alden's About.com Geology. Lots more detail there!

Volcano Vocab: Guyot

I don't want to steal the thunder of any of the Skepchicks (especially Evelyn, who's doing a fantastic job on the Geology Word of the Week feature), but I thought I'd start a bi-weekly post on obscure or specialized volcanology words. (Yes, it's really just an easy way for me to post, since I've got the Glossary of Geology sitting here and all I have to do is flip a page to get a post idea, but I'll try to include a little discussion along with the posts.) We'll see if I'm any more successful with this weekly feature than I was the last time.

So what's the first word o' the half-week? By dint of me opening to the glossary of a volcanology textbook and pointing blindly: Guyot!

A guyot ("gee-oh") is basically a flat-topped seamount, or underwater volcano. The glossary in Bardintzeff & McBirney's Volcanology (2000) adds a bit about why it's got a flat top:

A submarine volcano with a flat top produced by wave erosion before the island was submerged. 

A guyot is one of the stages in the life cycle of an ocean island volcano, and the form occurs when a volcano is no longer actively growing and unable to replace what is lost to the erosive force of wind and waves. Here's a diagram to illustrate:

(This came from a powerpoint someone gave me a while back, and I don't know where they nabbed it from - looks like a textbook. If anyone recognizes it, let me know and I'll put in an attribution!)

Unfortunately, it's a bit hard to show off a photo of a guyot, since they're generally underwater. Here's something that looks similar, however, from Hawaii's Big Island. These photos were taken somewhere along Rt. 11 (Mamalahoa Highway) on the way to Punalu'u Black Sand Beach and Ka Lae (South Point).  I remember that there was some discussion going on about guyots, but I'm not sure if these are the real item. Does anyone else know if these are 'stranded' guyots?