Eruptions Newsletter #7 for April 25, 2025

Activity News

Kīlauea, Hawai`i

Talk about spectacular. The 18th episode of the current summit eruption at Kīlauea produced twin lava fountains. One of these lava fountains topped out at 200 meters (600 feet) while the other produced copious lava flows from a much smaller fountain. In all, the

estimates that this episode that ended on April 22 dumped 5 million cubic meters of new lava onto the floor of the summit crater.

We are lucky that webcams like the one I grabbed a series of screenshots from (above) exist around the summit of Kīlauea. These webcams allow for remote viewing of potentially hazardous volcanic activity. And sometimes a webcam will be lost in the line of duty, much like this one (B2Cam) that melted in the 18th episode eruption. The lava flows got too close and that was the end of that cam. Let this be a reminder: lava is hot.

Etna, Italy

Not to be outdone, Etna’s busy 2025 continued this week. The activity there is much more Strombolian than Hawaiian, meaning that giant bubbles rising up through the magma will pop near the surface in the crater. This is what drives much of the eruptions at the Sicilian volcano as well as the “booming” noises that accompany the activity. You can check out some of this explosive activity in the video posted by Dr. Boris Behncke (below).

Whakaari (White Island), New Zealand

GNS Science in New Zealand raised the Alert Status for an eruption to Level 3 (of 5) and Orange Alert for aviation hazard at Whakaari in the Bay of Plenty. More ash has been observed in the steam and gas plumes coming from the mostly-submarine volcano. The presence of ash means that magma is likely close to the surface, increasing the chances that an eruption could occur. Right now the only hazard to people on the north coast of the North Island is sulfur odors, but if any eruption were to occur, that might change.

Merapi and Marapi, Indonesia

I bring up these two restless Indonesia volcanoes because they tend to cause a lot of confusion. Yes, Merapi and Marapi are two different volcanoes. The former (MErapi) is one of the most active volcanoes in Indonesia and is located on the island of Java with over 24 million people live within 100 kilometers. The latter (MArapi) is on the island of Sumatra and is that island’s most active volcano. It “only” has 4 million people living within 100 kilometers.

Both volcanoes go through cycles of lava dome growth and explosive eruptions. This type of activity produces pyroclastic flows and volcanic mudflows (lahars) during the rainy season.

Currently, both volcanoes are erupting. Marapi on Sumatra has generated explosions that they shook windows in towns 11 kilometers away. Over at Merapi, the lava dome continues to grow and Indonesia authorities has warned people to stay at least 3 kilometers away from the volcano’s summit.

Volcano Word of the Week

Accretionary Lapilli

So, technically, two words. Let’s look past that. Accretionary lapilli are great because of their weirdness. The second word “lapilli” really just refers to the size of the material. Volcanic debris between 2-64 millimeters is classified as lapilli, with ash being smaller and bombs being larger.

“Accretionary” refers to how these blebs of volcanic material are born. Accretion is the process of progressive growth through addition, kind of like rolling a snow ball to make it larger. The Earth, early in its history, grew by accretion as asteroids bombarded the surface, adding material to the planet. Accretionary lapilli growth by adding fine ash either in layers or as particles.

More or less, accretionary lapilli look like little balls of mud. The 3D video I included here shows some accretionary lapilli from the Perry Creek Formation on Vinalhaven Island off the coast of Maine. You can see these ~5-10 mm blebs in a matrix of ash and those are the lapilli. If you can believe it, these features are preserved in rocks that are over 350 million years old. They look almost showroom new!

In modern lapilli, you can sometimes see concentric layering of ash starting from a tiny nucleus. They might accumulate as a discrete layer or be mixed in with fine ash as well.

So, there are two theories for how they form: the wet option and the charged option.

The Wet Option: This might also be called the Hailstone model. The tiny fragments of volcanic ash are great places for water to form in a cloud. This means moisture from the eruption or ambient humidity will start a raindrop on a fragment of ash and then more ash sticks to that raindrop, with adds more moisture and more ash layer after layer. In the end, you get small sphere of ash that accumulate ash as they fall through the ash cloud.

The Charged Option: This model is related to the idea that volcanic ash has an electrical charge. Most people associate that with volcanic lightning, but it also causes fine ash to sit to other bits of fine ash, building loose spheres of ash that usually disintegrate when they hit the ground. Remember, they are just being held together with static charge!

The Wet Option means that most people, when they first learned about accretionary lapilli, heard it described as an eruption on a rainy day. This is true for some accretionary lapilli, although that rain might have come from the water in the erupting volcanic debris.

One weird side effect of accretionary lapilli is that they can make volcanic ash deposits get thicker further from the volcano, contrary to the usual rule of thumb. More ash will rain out close to the volcano during most explosive eruptions, but if fine ash is carried away and then forms accretionary lapilli, it will begin to fall from the ash plume. This could lead to “premature fallout”. So, in the 1980 eruption of Mount St. Helens, the ash thickness was ~1 centimeter at 180 kilometers from the volcano, but thanks to accretionary lapilli, it was ~4 centimeters thick in some places ~300 kilometers from St. Helens.

Odds & Ends

So, I realize the internet loves to recycle content and usually the more it is recycled, the more inaccuracies tend to accrue. I ran across this video on the Geology subreddit and it does a lava ball from the 2021 eruption of Cumbre Vieja in the Canary Islands (I think). The problem is that the video refers to the lava ball as “a giant boulder from beneath the Earth’s crust …” in its title.’

This is, of course, wrong. Lava balls are not boulders from beneath the Earth’s crust. They form at the Earth’s surface as lava cools and then gets tumbled down an active lava flow, growing a bit like a snowball. They are very cool, but volcanoes don’t tend to erupt “giant boulders”.

I did debate whether I should correct this in the comments of the post, but when I do get that feeling, I just remember the best XKCD comic and think better of it.

Sounds of the Week

In honor of accretionary lapilli, this week’s songs are rain focused. However, as great a song as “Purple Rain” might be, it is too easy a pick.

Sometimes bands just miss becoming big. I hear This Picture and I wonder if they could have been the Verve. Yet, they weren’t for whatever combination of circumstances.

A weird thing about getting older is changing musical tastes. For me, this isn’t necessarily different taste but maybe it is better described as expanded taste. Anyway, long story short, I find myself liking Phil Collins. Oddly enough, “I Wish It Would Rain Down” has always been my favorite Collins song and this has only strengthened with time. Eric Clapton’s role probably is a big reason … which makes it even sadder that he ended up being such a mostly terrible person.

Speaking of youthful tastes, I will freely (well, almost freely) admit that when Milli Vanilli was big, I was into them. I think it reflected my internal predilection for the descendants of disco. I think nearly everyone over age 35 knows the Milli Vanilli story, so I won’t rehash it here, but check out Tom Breihan’s excellent take on this song and remember that the scandal didn’t need to happen.

I’m not saying I added this for one of my readers, but you know who you are!

Many times students ask me about my favorite concert. Although the easiest answer in terms of “kids these days” having any idea of the bands would be Nirvana, the true answer is the Beta Band at the WOW Hall in Eugene, Oregon.

KBO, my friends.

Questions? Comments? Thoughts? Feel free to send me a note or follow me on Bluesky (@erikklemetti.bsky.social).