Kirishimayama Volcano Group


 

For most of us, the words “Japan” and “volcano” summon up an image of Mt. Fuji, near Tokyo. But this country has many more volcanoes, some of them not as simple in appearance as Fuji-san.

Take Mount Kirishima, for instance. It is a study in contrasts.

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Not least because it rises out of townlands and has a lot of vents. (Jun Seita, CC BY 2.0)

On the one hand, Kirishima frequently erupts; on the other, it’s a great place for a walk through the park.

A violent history

Starting some 600,000 years ago, huge caldera-forming eruptions began here, at the head of Kagoshima Bay on the island of Kyushu. Ever since then, volcanic events have been explosive in nature (not showing much runny “Hawaiian-style” lava).

About 330,000 years ago the style changed to building stratovolcanoes instead of big holes in the ground (which is what a caldera is, basically). Ever since, this complex volcano has built more than 25 relatively small peaks and cones. Two vents are currently erupting: Shinmoedake and Ioyama.

Occasionally magma interacts with ground water, forming a maar with the resulting steam-driven eruption.

The park

Here is someone’s video of a walk up three volcanoes in the area; the first two–Takachiho (crowned with a legendary spear, per Wikipedia) and Karakuni–are part of the Kirishima center; the last one, beautiful Kaimon, belongs to the Ata caldera underneath Kagoshima Bay.

That certainly looks peaceful. Volcanoes sculpt some of the most beautiful landscapes in the world. But as you can see, a lot of people live near Mount Kirishima.

This volcanic group is slowly becoming more active and its magma rate is increasing. Volcanologists are watching Kirishimayama closely to better understand how it works so they can most accurately predict its future course.


Featured image: NASA Earth Observatory.



Sources:

Global Volcanism Program. 2018. Kirishimayama.

Nagaoka, S., & Okuno, M. 2011. Tephrochronology and eruptive history of Kirishima volcano in southern Japan. Quaternary International, 246(1-2): 260-269.


Mount Rainier


This was first published at my other blog on May 3, 2014.


There is a king in the Pacific Northwest, his brow crowned in glittering ice.

Mount Rainier starts to rise only about 25 miles from the Seattle-Tacoma metropolitan area. Today this beautiful Cascades stratovolcano, towering 14,410 feet above Puget Sound, dominates the skyline of towns and cities that sit on material that once made Rainier almost 2000 feet taller.

Flank Collapse

About 5600 years ago, around the time when the ancient Egyptians were getting organized, Mount Rainier’s northeast flank and summit collapsed. It was dramatic even as described by scientists, who say that a cubic mile (4 cubic kilometers) of flank and summit material (now called the Osceola Mudflow):

…washed across Steamboat Prow and Glacier Basin and then ran up to about the 6400-foot level of Goat Island Mountain and Sunrise Ridge. It then descended the White River valley 80 to 150 m (260- 490 ft) deep, spread out over 210 km2 (82 mi2) of Puget Sound Lowland 70-100 km (44-62 mi) from source, and flowed into Puget Sound, moving underwater up to 20 km (12.4 mi) to the present sites of Tacoma and the Seattle suburb of Kent. The contemporaneous phreatic and phreatomagmatic explosive eruptions blew hydrothermal clay and mud northeastward across Sunrise Ridge and spread pumice across an arc from south to northeast of the volcano. The Osceola edifice collapse left a horseshoe-shaped crater open to the northeast at Mount Rainier, much like the open crater formed at Mount St. Helens in 1980.

Mount St. Helens composite image by Ewen Roberts

Mount St. Helens composite image by Ewen Roberts.

Geologists believe that this collapse happened because the rock had been weakened by the circulation of hot, acidic water inside the volcanic structure. Over time, through many eruptions, Rainier built itself back up into the majestic but dangerous structure everyone today knows and loves.

The USGS says it has seen no change in the pattern and expects Rainier to continue growing, erupting and collapsing.

Volcanic Hazards

Today, about 80,000 people are at risk from a potential mudflow, also known as a lahar, from Mount Rainier, say experts at the United States Geological Survey (PDF). This could be triggered by the sort of volcanic activity that the USGS monitoring network would pick up, but it might also happen without warning as another flank collapse. Such a collapse, say the geologists, could reach Orting, Washington, in as little as 40 minutes.

For this urgent need, an acoustic network now surrounds Rainier. Pierce County, Washington, also is developing a specific volcanic hazard plan (PDF) for Mount Rainier.

No one wants to live in fear when there is so much beauty and wonder about this monarch of the Cascades. Having recognized its dangers, people are working to minimize them so that everybody can continue to enjoy this beautiful mountain.. This requires a lot of work but, as shown in Jayson Yogi’s video of a 2011 Rainier summit climb via the Emmons Glacier, difficult struggles have their own special rewards.
 

 
Update, July 17, 2014: “Detailed imaging of Mount Rainier shows subduction zone in glorious detail.” Scott Johnson, Ars Technica.
 


Front Page Image of Mount Rainier is by Michael Lehenbauer.

Sources:

Mount Rainier,” United States Geological Survey: Volcano Hazards Program.

“Mount Rainier – Living Safely With A Volcano In Your Back Yard.” (PDF) USGS Fact Sheet 2008-3062

Timeline — B.C.” Air War College: Contents of 12,000 Year Timeline.

“Volcanoes of the Cascades: Their Rise and Their Risks.” Richard L. Hill. Globe Pequot Press, Guilford, Connecticut. 2004.


Guest Video: New Mexico’s Volcanoes


Rifting in this region happened because of a complex series of events, called the Mid-Tertiary Ignimbrite Flareup, many tens of millions of years ago. During this tumultuous time volcanism was extreme throughout what are now the western United States and northern Mexico.

Things obviously have quieted down since then, but geoscientists are still keeping an eye on the southwestern US. Researchers study a magma body miles below the city of Socorro, while the US Geological Survey is aware of such potential problem sites as the Zuni-Bandera volcanic field and Valles Caldera.


Guest Video: Villarrica Volcano


Almost sixty eruptions have been documented at this Chilean volcano since Europeans first saw it erupting in 1558.

Villarrica is a classic stratovolcano–the pointy kind – and is one of a group of Southern Andes volcanoes.

Per the volcano’s GVP page, the lava lake has been there intermittently since the mid-1980s.

Since the eruption style has been Strombolian (occasional brief “shots” rather than a sustained lava flow or explosive phase), it’s a little more approachable than some active volcanoes.

Despite the obvious dangers, Villarrica attracts daredevils who bungee a tolerable distance into the crater from a helicopter or do this —

Villarrica is monitored by both Chile (Spanish) and a private group. (Spanish)

Lahars (mud flows) from this volcano killed a hundred people during the 20th century. Other dangers include projectiles – something people soaring or hanging over the crater are especially at risk for in addition to the poisonous gases, heat, and chance eruption cloud. Lava flows also happen at Villarrica and, less frequently, pyroclastic flows.


Featured image: Cristian Gonzalez G. CC BY-ND 2.0.


Caribbean Plate Volcanoes: The Central American Arc

The Caribbean tectonic plate gets little respect. This is probably why it lashes out so violently at times in earthquakes and volcanic explosions.

First, most of this relatively small chunk of Earth’s lithosphere is underwater, so hardly any of us even know it’s there.

Then North America jostles it from the north, messing up its boundaries, while South America does the same thing to the south. East and west, seafloors on other plates dive underneath the Caribbean plate’s edge in subduction zones and then melt upward through it to form explosive volcanic arcs.

These are the Lesser Antilles island arc in the east and the land-bound Central American Volcanic Arc to the west.

This post is about those Central American volcanoes–well, not all of them. There are hundreds of stratovolcanoes, lava domes, and cinder cones in this group.

Let’s break it down by country.

Guatemalan volcanoes

Here are a just a few of the 300+ vents in this small nation.

Monitoring: INSIVUMEH. (Spanish)

Risk: High, obviously. In fact, only Indonesia has more people living near active volcanoes. (Ewert and Harpel) According to one source:

The highest risk is located around the vents of Almolonga and Santa Maria, due to the very large population living within 10 km of the volcanoes and also the high VEI 6 figure for Santa Maria as designated by the Global Volcanism Program. The areas to the south and east of Fuego are also designated high risk due to the numerous hazards that combine and overlap within these areas, from hazards generated by Atitlan and Acatenango.

Biggest known eruption: That 1902 VEI 6 event at Santa Maria, in which thousands of people died, was the second largest eruption of the 20th century (the largest one, in 1912, happened in Alaska).

And then there was the VEI 7 caldera eruption at Atitlán during the last ice age some 85,000 years ago. Today a lake fills the caldera left by this “Los Chocoyos eruption,” and “normal”-sized eruptions at one of the three stratovolcanoes on the calderas rim are the norm.

El Salvador

This is the country where cameras caught the start of an eruption at Chapparastique Volcano, the most active of some 20 or so El Salvadoran volcanoes. You might have seen the video:

Monitoring: SNET. (Spanish)

Risk: High.

Biggest known eruption: A few high-end VEI 6 eruptions and one VEI 7 are listed.

Also, a VEI 6 caldera eruption at Ilopango, which sits right next to the country’s capital city, happened in the fifth or sixth century AD.

Besides devastating the Maya politically and economically, as well as socially, this “Tierra Blanca Joven eruption” may have caused climate effects and hardship elsewhere in the world, and possibly even contributed to the Justinian Plague! (Oppenheimer)

Nicaragua

There are at least 19 volcanoes in this country, and some of them are popular tourist destinations.

Monitoring: Hard to say. The link given at WOVO doesn’t work.

Risk: Per this abstract, there are a variety of risks.

They don’t mention gas, probably because that is a common (and very dangerous) volcanic hazard everywhere:

Biggest eruption: Both Apoyeque and Masaya have had VEI 6 eruptions, though not any time recently; Masaya’s eruptions are usually less intense.

Costa Rica

This country’s beautiful string of volcanoes draws a lot of tourists.

It is quite a beautiful country.

Monitoring: OVSICORI-UNA. (Spanish)

Risk: High.

Biggest eruption: An unexpected VEI 3 eruption at Arenal in 1968 killed 87 people, per Wikipedia. Also, a few Costa Rica volcanoes, including the Barva complex, Miravalles, and Poás have VEI 6 or 7 eruptions listed in the distant past.

Panamá

Wait, Panamá has volcanoes? Yes, at least three of them.

Monitoring: The National Civil Protection System, apparently. They responded to what turned out to be false concerns in 2015 that Barú was going to erupt.

Risk: Barú is the youngest and most recently active volcano, so most attention focuses on it.

Biggest eruption: El Valle had a VEI 4 ignimbrite eruption about 56,000 years ago, though it has been quiet ever since.

By now, you have probably forgotten the poor Caribbean plate–no respect–but it’s still there, underneath all these Central American lands, interacting with various microplates that lay between it and the huge Pacific tectonic plate.

Let’s close by standing atop Barú volcano – the only place in the world where, thanks to the narrowness of the Panama isthmus, you can see the Atlantic (on our right) and the Pacific (left) oceans at the same time.


Featured image: Central America volcanic front. CrazyKnight. CC BY-SA 3.0.



Sources:

Bachmann, R. 2001. The Caribbean plate and the question of its formation. Institute of Geology, University of Mining and Technology Freiberg Department of Tectonophysics http://www.redciencia.cu/geobiblio/paper/2001_Raik%20Bachmann_THE%20QUESTION%20OF%20ITS%20FORMATION.pdf.

Ewert, J. W., and Harpel, C. J. 2004. In harm’s way: Population and volcanic risk. Geotimes. http://www.geotimes.org/apr04/feature_VPI.html Last accessed March 18, 2018.

Giunta, G. and Orioli, S. 2011. The Caribbean Plate Evolution: Trying to Resolve a Very Complicated Tectonic Puzzle, New Frontiers in Tectonic Research – General Problems, Sedimentary Basins and Island Arcs, ed, Sharkov, E. InTech, DOI: 10.5772/18723. Available from: https://www.intechopen.com/books/new-frontiers-in-tectonic-research-general-problems-sedimentary-basins-and-island-arcs/the-caribbean-plate-evolution-trying-to-resolve-a-very-complicated-tectonic-puzzle

Oppenheimer, C. 2011. Eruptions That Shook The World. Cambridge: Cambridge University Press. Retrieved from
https://play.google.com/store/books/details?id=qW1UNwhuhnUC

Caribbean Plate Volcanoes: The Lesser Antilles


It’s not hard to grasp the basic idea of plate tectonics: There’s a crack in the middle of the ocean floor where magma rises from deep within the planet, forming two seafloor halves that spread apart.

And if we were flat-landers, that would be that. However, the whole thing happens on round and curvy Earth, so plate interactions can get complicated–but also very interesting.

Take the Caribbean plate, for instance–the floor of the Caribbean Sea, south of the Gulf of Mexico, and the land masses around it (including much of Central America).

No one is quite sure how this minor plate formed up through the end of the dinosaur age, but watch how it moves in this elegant, multi-million-year-old dance:

Certainly the Atlantic fits our stereotype of “big crack,” two sides spreading apart for many tens of millions of years in the past (and this likely will continue for far longer into the future than any of us need to consider). But there is a lot more going on in the Caribbean.

And it involves volcanism and earthquakes, both of which are capable of kicking up a tsunami in this part of the world.

The eastern and western ends of the Caribbean plate are currently the most active. In honor of this week’s change in the status of Kick-’em-Jenny volcano near Grenada, let’s start with the eastern side – the Lesser Antilles island arc.

Why are there volcanoes, and accompanying earthquakes, there? The University of the West Indies Seismic Research Centre knows.

Sunday Morning Volcano posts at my other blog on some of these volcanoes include:

That’s only a few of the many Caribbean volcanoes.

Next time we’ll look at volcanoes on the western edge of the Caribbean plate, including one whose eruption may have affected the fate of the Eastern Roman Empire (and certainly affected the nearby Mayan Empire) and another whose Ice-Age eruption may have been big enough to qualify as “super.”


Featured image: Eastern Lesser Antilles (Barbuda to Grenada), Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC, via Wikimedia.



Sources:

Allen, R. W.; Berry, C.; Henstock, T. J.; Collier, J. S.; and others. 2018. 30 Years in the Life of an Active Submarine Volcano: A Time‐Lapse Bathymetry Study of the Kick‐‘em‐Jenny Volcano, Lesser Antilles. Geochemistry, Geophysics, Geosystems.

Bachmann, R. 2001. The Caribbean plate and the question of its formation. Institute of Geology, University of Mining and Technology Freiberg Department of Tectonophysics http://www.redciencia.cu/geobiblio/paper/2001_Raik%20Bachmann_THE%20QUESTION%20OF%20ITS%20FORMATION.pdf.

Giunta, G. and Orioli, S. 2011. The Caribbean Plate Evolution: Trying to Resolve a Very Complicated Tectonic Puzzle, New Frontiers in Tectonic Research – General Problems, Sedimentary Basins and Island Arcs, ed, Sharkov, E. InTech, DOI: 10.5772/18723. Available from: https://www.intechopen.com/books/new-frontiers-in-tectonic-research-general-problems-sedimentary-basins-and-island-arcs/the-caribbean-plate-evolution-trying-to-resolve-a-very-complicated-tectonic-puzzle

Macdonald, R.; Hawkesworth, C. J.; and Heath, E. 2000. The Lesser Antilles volcanic chain: a study in arc magmatism. (Abstract only) Earth-Science Reviews. 49(1-4): 1-76.

Wikipedia:


Guest Videos: Mount Erebus


We don’t usually think of Antarctica as “fire and ice” country, but it has many volcanoes, including Mount Erebus, which erupts fairly frequently and hosts a major lava lake (shown above from space).

Volcanologists also study the volcano’s gases and ejecta from its explosions.


Mount Erebus Smithsonian GVP page.

More information about research at Mount Erebus. (I can’t find a working link to the Mount Erebus Volcano Observatory.)


Featured image: Mount Erebus. Source.


Guest Video: The Biggest Eruption of the 20th Century

This 2012 presentation tells the story of the VEI 6 eruption of Katmai/Novarupta in 1912.

Alaskans are still experiencing ashfall from that eruption’s deposits whenever the wind is right, most recently in November 2017 according to the Smithsonian.

This huge eruption was unusual in that it was associated with a caldera collapse in nearby Katmai Volcano.

The Internet Archive has the 1922 National Geographic Society report on their expedition to the volcano here.

The Alaska Volcano Observatory monitors Katmai around the clock, but the only trouble the volcano has caused lately has been remobilized ash from the 1912 eruption.


Featured image: NASA


Fuego and Acatenango


This is a post from my other blog, Clear Sight, published on August 30, 2015. Since then, Fuego (Guatemala) has been very active; Acatenango, not at all.


There are two frequently active volcanoes in Latin America called Fuego (“fire,” in Spanish). English-speakers tend to refer to the one in Mexico simply as Colima, but Spanish-speakers also call it Volcán de Fuego, or just Fuego.

Today we’re going to look at the other famous Volcán de Fuego – the one in Guatemala – and its neighbor Acatenango, both of which are southwest of Atitlan and close to urban areas around Antigua and Guatemala City.

This Fuego is exciting, too!
 

(I’m pretty sure this video, taken at sunset, also captures the astronomical phenomenon known as the Belt of Venus…basically, Earth’s shadow as the Sun disappears beyond its limb!)
 

Those people are camping on the Fuego-Acatenango massif, a string of multiple volcanic vents. The massif runs perpendicular to the coast-hugging Central American volcanic front that’s related to the subduction of the Cocos tectonic plate underneath the Caribbean plate.

Indeed, thanks to the complicated plate-tectonic picture in this region, Central American volcanism happens in a number of distinct segments.

The volcanoes in this massif are at least 17,000 years old, fairly young in geological terms. Together with Agua, a third nearby colossus, they are the stereotypical pointy type of volcanoes (stratovolcanoes or composite cones). The ones that concern volcanologists and emergency planners are Fuego and Acatenango. In terms of hazard, briefly, besides the usual eruption effects they also tend to collapse unpredictably over geologic time.

1974 eruption (Image: Paul Newton, Smithsonian)

A subplinian VEI 4 eruption at Fuego in 1974. (Image: Paul Newton, Smithsonian)

Volcán de Fuego

Almost 4-kilometer-high Fuego Volcano was erupting in 1524 when the conquistador Pedro de Alvarado first saw it. There are many legends and historical reports (Spanish language) about Fuego.

Experts say that Fuego spends many years in basically an open-vent condition. The eruption captured on video above was one of its many small events, but this volcano has also had 60 subplinian explosive eruptions during historic times, most recently in October 1974.

Fuego’s last activity was a month ago, as of this writing, when per the Smithsonian Global Volcanism Program website:

Based on INSIVUMEH notices, CONRED reported that for a 30-hour period during 30 June-1 July activity at Fuego was at a high level, characterized by explosions, high-temperature pyroclastic flows (that began on 1 July), and ashfall. Ash plumes rose 4.8 km above the crater and drifted 25 km W and NW, producing ashfall in 22 local communities. The majority of material deposited by pyroclastic flows was in the Las Lajas drainage. Activity decreased later that day. During 4-6 July, INSIVUMEH reported that explosions produced ash plumes that rose as high as 800 m above the crater and drifted 8-10 km SW and W. Incandescent material was ejected 100 m high, and avalanches descended the Santa Teresa and other nearby drainages.

Michigan Tech has an incredible Fuego web page – check it out!

Although the two volcanoes Fuego and Acatenango are physically joined together in an area called La Horqueta (“the fork”), their magma is very different. Fuego’s magmatic products have become more mafic (like basalt) over time, scientists report.

Fuego (left) and double-peaked Acatenango tower over nearby Guatemalan fields.  (USGS)

Fuego (left) and double-peaked Acatenango tower over nearby Guatemalan fields. (USGS)

Acatenango

This volcano sits a little to the north of Fuego and appears like Fuego’s twin, but its magma is the grey explosive type known as andesite. The ridge between Acatenango and Fuego is actually all that remains of a much older volcano called Meseta that may have begun forming some 230,000 years ago and remained active until as late as the last Ice Age.

From what I’ve read, I think Acatenango is a little older than Fuego, but don’t quote me on it. At any rate, both volcanoes are younger than the Los Chocoyos ash that Lake Atitlan laid down some 85,000 years ago. The current edifice called Acatenango is actually the second volcano to form there – the first one collapsed some 40,000 years ago. Twenty thousand years after that catastrophe Yepocapa, the northernmost of the current double summits, was in place. Next came the summit that’s called Pico Central or Pico Mayor.

This volcano’s eruptive history isn’t as well known as Fuego’s. Certainly it has had several prehistoric eruptions, but the first documented one was in December 1924, a VEI 3, on the north slope of Pico Central. Pico Central itself had a VEI 2 deruption from August 1926 to May 1927. The last eruption at Acatengo was on the Pico Central-Yepocapa saddle in late 1972 (VEI 1).
 

Left:  Debris avalanche remnants at modern Acatenango.  (Smithsonian)  On the right is the ashfall map for a major eruption at Fuego-Acatenango - note Guatemala City and Antigua are both at risk.  (USGS)

Left: Debris avalanche remnants at modern Acatenango. (Jim Vallance at MTU/Smithsonian) On the right is the ashfall map for a major eruption at Fuego-Acatenango – note Guatemala City and Antigua are both at risk. (USGS)

Volcanic hazards at Fuego and Acatenango

Landslides triggered by heavy rain or earthquakes are as much a hazard at the Fuego-Acatenango massif as are eruptions. So is the possibility of a major collapse on the scale of the flank failure at Mount St. Helens in 1980, which might or might not be accompanied by an eruption.

Magma breaks rock as it moves inside a mountain. This causes seismic activity that seismographs detect. Other eruption precursors include ground deformation and increased gassing. Guatemalan geoscientists are monitoring Fuego for these.

However, landslides and debris avalanches happen without warning. Landslides essentially happen anywhere on sloped ground when downward-moving forces are stronger than forces holding the soil in place. It doesn’t take much to trigger a landslide when the soil is saturated with water from rain or earthquake-induced liquefaction.

Volcanic landslides are much bigger in scale. They were first recognized at Mount St. Helens in 1980 (hard to miss, that!). They are absolutely terrifying both because of their size and their speed (over 60 mph/100 kph).

Such a collapse happened to the first edifice at Acatengo, when some 4 cubic miles (15 cubic kilometers) of material suddenly sped down to the Pacific coast in a huge flow that was over 30 feet (10 meters) deep. It moved so fast that it traveled easily over 19 miles (30 km) of essentially flat ground at the end of its run-out.

Events like this have happened at least twice at Fuego-Acatenango. Now 100,000 people live in the path of the next one.

I’d like to say that everybody is prepared for the volcanic hazards. Unfortunately, I can’t.

Not yet.
 
 


Featured image: Fuego, February 2017, from Acatenango, by Arden at Flickr. CC BY-SA 2.0.


 
More information:

The High Cascades


The Pacific Northwest is full of surprises.

For one thing, the train from Eugene, Oregon, to Portland is sometimes a bus.

For another, from the top of the last big hill on I-5 before the bus reaches the Portland metro area you will see the white shoulders of Mount St. Helens, surprisingly close to this city (at least it elicits a gasp from those of us who aren’t from the area).

2018-02-15-17-35-34

Here is what Mount St. Helens looks like from the aerial tram in town. Luckily for Portland, 70 miles away, the lateral blast in May 1980 went north instead of south. (Jamidwyer, CC BY-SA 2.0)

But perhaps the biggest surprise for someone traveling up the Willamette Valley towards Portland is that most of what you see from the highway or railroad are rolling mountains, heavily forested, only a few thousand feet high.

Those are the Western Cascades.

This was unexpected. As a former Easterner, I thought the whole Cascade Range would look like Switzerland, or at least the Rockies.

Many people probably share this misconception, thanks to what makes the news–Mount St. Helens in 1980, for example, or Mount Hood in 2018 (the stranded climbers tragedy).

In reality, those are just part of the High Cascades–some individual stratovolcanoes that include (but aren’t limited to):

The much less impressive-looking Western Cascades, sitting at the feet of these majestic fire mountains, are volcanoes, too. They are remnants, going back many millions of years–long before the young giants arose.

The Pacific Northwest has very complex geology, and volcanic structures and rocks reflect that. It’s a geologist’s ideal outdoor lab, but the rest of us just wonder how the same volcanic zone can form both High and “low” Cascades.

The vast Willamette Valley, which separates the Cascades from the coastal mountains, is a big clue.

It didn’t form the way many vallies do. Instead of being underlain by soft rock that has eroded faster than the mountain rocks on either side, the Willamette Valley is a forearc basin.

The sea once flowed here (and still does in Puget Sound, a separate northern section of the same basin).

The clearest video I could find on forearc basins was made from a petroleum industry standpoint, but don’t worry: there’s no exploration going on in the Willamette, as far as I know. The farmers would protest.

Here in the Pacific Northwest, that downgoing slab is called the Juan de Fuca plate. It’s made of basalt (runny red lava, frozen into stone, basically). The granitic North American plate is overriding it.

This regional subduction zone is also why there is going to be another Really Big Quake here one of these days (the last one was in the 18th century).

The Cascades are volcanoes, not merely mountains like the Coast Range, because that is where, miles underground, the basaltic Juan de Fuca plate is hot enough to melt.

Hot matter always rises when it’s denser than its surroundings, so some of that molten basalt heads upward. Where it has reached the surface, there are now “low” Cascades volcanoes (and a few High ones). The basalt interacts with the country rock (the overlying granite rock of the North American plate) along the way, so the erupted lava is quite varied.

Like any seafloor, the Juan de Fuca plate also has a lot of water. This facilitates melting of basalt. It can also help melt the silica-rich rocks of the overlying North American plate as the magma and fluid rise. In that case, geochemical processes change the magma’s characteristics.

This type of rising magma becomes more sticky, for one thing, and it tends to squeeze up into a taller shape than a basalt formation.

Here is Mount St. Helens, rebuilding itself in a less harmful eruption that happened between 2004 and 2008. (Portland is about 70 miles away on the other side of that back crater.)

That’s definitely not Hawaiian-style lava!

Eruptions of this type of “gray” lava are more explosive, too, because the gases that you can see (and others you can’t see) in that video sometimes get locked into the sticky lava dome.

And since what goes up must come down, explosively erupted material piles up close to the vent, building over time those beautiful High Cascade volcanoes.

The person who made the video below concentrated on the volcano’s glaciers, but look carefully and you will also see that the ice has carved into the flank, exposing the various layers that were deposited during previous eruptions.

The “low” Cascades, in between the giants, on the other hand, have been dissected by water and wind, and sometimes winter temperatures; they are beautiful, too.

Together, the High and “low” Cascades offer some of the most scenic views in the world.


Featured image: Screen cap from Steve Smith’s video (above).