Volcanologist: Why Guatemala's volcano is so lethal

Four years ago, during my very first field season as a volcanologist, I was at the bottom of Barranca Ceniza (Ash Val...

Posted: Jun 7, 2018 8:02 AM
Updated: Jun 7, 2018 8:02 AM

Four years ago, during my very first field season as a volcanologist, I was at the bottom of Barranca Ceniza (Ash Valley) in Guatemala. My colleagues and I had lowered ourselves into the valley with ropes and walked around looking for an ideal sampling site. It was muggy and overcast, and we couldn't see the summit of Volc-n de Fuego, the Guatemalan volcano that has been erupting since Sunday. But we could hear it rumbling above the clouds.

Earlier that morning, from the street in front of the local observatory in Panimache, we had seen some volcanic explosions -- small puffs of ash, spectacular to me as a newbie, yet not uncharacteristic of this volcano. But now, down on the floor of the "barranca," the surrounding scenery reminded us of how differently things could go here: we were walking amid layer upon layer of consolidated ash, which had made its way all the way down there, miles away from the summit, certainly during a much bigger eruption than those we had just witnessed from the observatory.

I worked nervously, knowing that if a big one came -- and it could have come at any time from this volcano, one of the most active in Latin America -- it would have found us on a preferential path, one that the ash had already taken many times before. A few hours later we made our way up safely and with plenty of samples. I felt very lucky that no major eruption had caught us there.

Right now, though, things are very different.

At least 75 people have died since Fuego began erupting four days ago, and that number may well grow as the eruption continues. Yet Kilauea -- the other volcano that has captured our attention with the slow, rolling devastation of its lava flow -- hasn't claimed a single life over the month since it started erupting on the big island of Hawaii.

And that's not likely to change. Why is that?

The two major eruptions we are witnessing, one in Guatemala and the other in Hawaii, are profoundly different. Unsurprisingly, not all volcanoes erupt in the same way. Some, like Kilauea, produce lava. Others, like Fuego, produce ash. What's the difference between lava and ash? Lava is, basically, a relatively tiny amount of gas in a lot of partially molten rock; ash is a relatively small amount of solid rock in a lot of hot gas, instead.

Whereas flowing lava moves slowly, destroying infrastructure but giving people the chance to escape, flowing ash moves very fast, making outrunning it impossible.

Pinning down one factor that makes a volcano erupt in an effusive (lava-making) or explosive (ash-making) way is tricky, and the same volcano can behave differently at different times. But for Kilauea and Fuego, the key is, once again, water: very little of it at Kilauea -- just enough to help its fluid magma rise to the surface -- and lots of it at Fuego, so much that its stickier magma can't keep up with it when it rises. I've written a little about how this works here.

Where does Fuego's water come from? Fuego is located on the Caribbean Plate, at what geologists call a subduction zone. At a subduction zone, one tectonic plate (in this case the Cocos Plate) goes beneath another (in this case the Caribbean Plate) -- we say it subducts. Now, the subducting plate has been sitting at the bottom of the ocean for millions of years, and it has soaked up a lot of water over that time. As it subducts beneath the Caribbean Plate, the Cocos Plate warms up, and that water gets released and prompts the upper plate to melt (much like salt prompts ice to melt on the roads), eventually ending up in the newly formed magma.

All is well as long as the magma sits deep in the magma chamber. But as it rises toward the surface, it is under less pressure, and the water contained within begins exsolving (or separating) from the magma, forming bubbles that grow bigger and bigger and move faster and faster, just like when you open a bottle of soda. The molten rock all around it can't quite keep up, and it breaks (or fragments, in volcanology jargon) into a billion pieces. So by the time magma reaches the vent, it isn't glowing, flowing lava coming out, but, rather, solid ash -- fine fragments of incandescent but solid rock.

That ash forms an eruptive column. Initially, the ash column rises because it leaves the vent at the top of the volcano really fast, propelled by that expanding water vapor. At that point, it rises because it is really hot, and hence not very dense compared with the surrounding atmosphere. But eventually, it becomes unstable and collapses under its own weight in a pyroclastic density current (PDC). And this is where it becomes lethal.

A PDC is a hot mixture of ash and gas traveling at speeds up to 450 miles/hour. Most PDCs separate into a pyroclastic flow and a surge soon after forming. The pyroclastic flow -- dense and rocky -- travels at the bottom. The surge -- gassy and buoyant -- travels above it. The pyroclastic flow follows the topography it encounters, quickly becoming channeled in pre-existing valleys and deepening them through erosion. Conversely, the surge can easily overcome valley sides, hills, and infrastructures; it travels in a straight line away from the volcano, and nothing can stop it, making it all the more dangerous.

Fuego's many barrancas, radiating from its vent outward, are the scars left by its own explosive activity, and serve us as a powerful reminder of its destructive potential. I thought about that potential very much while I was working at the bottom of a barranca in 2014, hoping I would never see it unleashed, the way we now have in this destructive eruption.

Mississippi Coronavirus Cases

Data is updated nightly.

Confirmed Cases: 115763

Reported Deaths: 3263
CountyConfirmedDeaths
Hinds7973177
DeSoto703979
Harrison522384
Jackson457884
Rankin394086
Madison383194
Lee357380
Forrest304678
Jones292484
Washington258399
Lafayette250443
Lauderdale2478135
Lamar225538
Oktibbeha202454
Bolivar201677
Neshoba1849111
Lowndes179962
Panola170040
Leflore167187
Sunflower162349
Warren154855
Monroe150673
Pontotoc147220
Marshall143129
Lincoln140157
Pike138456
Copiah137536
Scott125429
Coahoma124937
Grenada122638
Yazoo122234
Simpson121549
Union118825
Tate116839
Leake115041
Holmes114760
Itawamba113925
Pearl River113660
Adams108544
Prentiss106120
Wayne101722
Alcorn100112
George99218
Covington97527
Marion95042
Tippah90322
Newton86627
Chickasaw85526
Tallahatchie84526
Winston84121
Hancock84028
Tishomingo81241
Attala79426
Clarke75851
Clay69321
Jasper68717
Walthall63927
Calhoun62612
Noxubee59817
Smith59416
Montgomery54923
Yalobusha54514
Claiborne53716
Tunica53517
Lawrence51814
Perry49423
Carroll49312
Greene47818
Stone47514
Humphreys43816
Amite42513
Quitman4206
Jefferson Davis41011
Webster37613
Benton3416
Wilkinson33820
Kemper32615
Sharkey28514
Jefferson27610
Franklin2423
Choctaw2086
Issaquena1074
Unassigned00

Alabama Coronavirus Cases

Confirmed Cases: 158701

Reported Deaths: 2680
CountyConfirmedDeaths
Jefferson23292377
Mobile16916315
Tuscaloosa10345140
Montgomery10250197
Madison935096
Shelby739063
Baldwin665869
Lee654665
Calhoun459961
Marshall439550
Etowah428551
Houston417034
Morgan416435
DeKalb342629
Elmore320853
St. Clair295542
Limestone287230
Walker279492
Talladega266435
Cullman248024
Lauderdale229442
Jackson215915
Autauga205931
Franklin205531
Colbert202132
Russell19493
Blount193225
Chilton188432
Dallas186627
Coffee177111
Dale176351
Covington174729
Escambia172730
Clarke135217
Chambers135044
Pike134113
Tallapoosa132987
Marion108129
Barbour10339
Marengo101922
Butler101140
Winston92913
Geneva9067
Lawrence85832
Pickens85218
Bibb84014
Randolph82716
Hale76830
Washington74912
Clay74412
Cherokee73814
Henry7176
Lowndes71328
Bullock64917
Monroe64610
Crenshaw60830
Perry5926
Fayette57713
Cleburne5698
Wilcox56812
Conecuh56113
Macon53620
Lamar4965
Sumter47221
Choctaw39212
Greene34216
Coosa2043
Out of AL00
Unassigned00
Tupelo
Clear
62° wxIcon
Hi: 68° Lo: 54°
Feels Like: 62°
Columbus
Clear
64° wxIcon
Hi: 73° Lo: 57°
Feels Like: 64°
Oxford
Overcast
57° wxIcon
Hi: 62° Lo: 50°
Feels Like: 57°
Starkville
Overcast
59° wxIcon
Hi: 67° Lo: 53°
Feels Like: 59°
WTVA Radar
WTVA Temperatures
WTVA Severe Weather