US's largest volcanic eruptionUS's largest volcanic eruption

Uncovering the Largest Volcanic Eruption in United States History

The United States is a volcanic country. Of course, much of it east of the Rockies hasn’t erupted for tens to hundreds of millions of years, but the western US, Alaska, and Hawaii are full of active or potentially active volcanoes. This made me wonder what the largest eruptions the US has experienced are, and it sent me down a rabbit hole of thinking about what this question actually means.

So, I present to you the largest eruptions in the US in the last 10,000 years. As you will see, this can be a difficult question. To try to normalize something, I took “largest” to mean the largest volume of erupted material, the higher magnitude of the eruption, and then the eruption index of volcanic explosivity (VEI; in the order). I used Smithsonian/USGS Global Volcanism Program and on Project on Global Volcano Risk Identification and Analysis (VOGRIPA) databases of my information.

A few things about measuring volcanic eruptions. First, explosive volcanic eruptions produce tephra, which is common to volcanic debris such as ash, pumice, bombs, etc. Such material is usually full of air pockets, so to put all eruptions on a level playing field since the amount of air can vary, we convert it to a “dense rock equivalent” or DRE. This is a calculation using data on the percentage of air in the deposit, which then removes the volume of air to get how much magma actually erupted. All volumes here are DRE.

Second, I’m leaning toward using flare magnitude above VEI. Unlike VEI, which uses tephra volume to rank eruptions, eruption magnitude uses the logarithm of the mass of erupted material (log10 (erupted mass in kg) − 7), thereby reducing the amount of air in all deposits. So that makes a comparison that’s a bit more apples to apples and as you’ll see in the list, the VEI and flare size aren’t too different.

In the list!

In the Holocene (last ~11,000 years)

Why Holocene? Well, it roughly coincides with the arrival of humans in North America (although that’s a moving target… humans probably arrived in the Americas as early as 30,000 years ago), so it seemed like a good place to start. (Just a quick note: 1 cubic kilometer is about 0.24 cubic miles … which is about the same as 1000 Houston Astrodome).

1. Mazama (Crater Lake): VEI 7, mag 7.1, 5680 BC, 50 cubic kilometers

View of Crater Lake from the Watchman Lookout Tower. Credit: Athleticamps, Wikimedia Commons.

The big one… and the only one in the top 10 outside of Alaska. Crater Lake in Oregon (called Mazama when it was discussed before there was a lake) formed in this cataclysmic eruption, probably close to the largest, if not the largest, recorded in the modern cascade (Rockland tephra from the Lassen Volcanic Center was also ~30-50 cubic kilometers). I have a hard time understanding what the impact of such an eruption would be if it happened today in the Cascade Range.

2. Okmok: VEI 6, mag 6.7, 100 BC, 29 cubic kilometers

Now we begin our tour of Alaska volcanoes. Most on this list are in the Aleutian Islands, although a couple are located further east in the Wrangel Range. The eruption of Okmok in 100 BC leads the group. Okmok has always looked a lot like a dried up crater lake to me, and that’s because it too was formed by a massive eruption that caused the volcano to collapse. Unlike Crater Lake, the 100 BC collapse secondly for Okmok, after one that happened ~12,000 years ago. Also, unlike Crater Lake, Okmok has erupted in the 21st century – a VEI 4 eruption in 2008-09.

3. Aniakchak: VEI 6, mag 6.5, 1645 BC, 27 cubic kilometers

Another major caldera-forming eruption, the 1645 BC eruption of Aniakchak produced pyroclastic flows that flowed 50 kilometers from the volcano. It is also the most active volcano in the Aleutian Islands with dozens of eruptions during the Holocene. We are approaching the 100th anniversary of its last confirmed eruption, a VEI 4 event in 1931.

4. Churchill: VEI 6, mag 6.7; 847 CE, 23 cubic kilometers

How is such a large eruption lost? The source of the White River Ash eruptions (north and east lobe) was disputed for years, but at the turn of the century its source was finally traced to Mount Churchill in the Wrangel Range. It’s not exactly a highly volcanic region, but two of the most explosive eruptions of the Holocene came from remote Churchill. This explosion, only ~1200 years ago, was probably a very dramatic event for the people living in the Yukon during this period.

5. Veniaminof: VEI 6, mag 6.7, 750 BC, 20 cubic kilometers

What you say? Another caldera in Alaska? The 1750 BC eruption formed the 8-by-11-kilometer caldera, which is now filled with glaciers and snow. Veniaminoff continues to be one of the most active Aleutian volcanoes of the 21st century, with nine eruptions, including some as recently as 2021.

6. Novarupta: VEI 6, mag 6.5, 1912 CE, 13.5 cubic kilometers

Now we come to the only eruption that occurred while the US was a country: Novarupta (sometimes incorrectly called Katmai) in 1912. This famous eruption occurred between a number of volcanoes, including Katmai and Trident, the former of which collapsed due to the eruption. Pyroclastic flow deposits created the Valley of 10,000 Smokes, named for the fumaroles (gas vents) formed as the flows cooled.

7. Black Peak: VEI 6, mag 6.5, 1900 BC, 12 cubic kilometers

This Alaska volcano is tied with Aniakchak for #7. I must admit I wasn’t very familiar with Black Peak, but it is another composite volcano in the Aleutian Islands that has undergone caldera-forming eruptions. This ~1900 BC eruption filled the valleys around the volcano with over 100 meters of ash and volcanic debris. Since then, there has been no recorded eruption of this volcano on the Alaska Peninsula

7. Aniakchak: VEI 6, mag 6.5, 6300 BC, 12 cubic kilometers

Aniakchak’s second manifestation on the list, just short of the 1645 BC eruption.

9. Churchill: VEI 6, mag 6.2, 60 CE, 4 cubic kilometers

And Churchill again…still big, but only ~1/5 the volume of the 847 explosion.

10. Fisher: VEI 6, mag 6.2, 7420 BC, 5.5 cubic kilometers

Stop me if you’ve heard this before…but Fisher is a volcano on Unimak Island in the Aleutian Islands that had an eruption that formed the caldera. Fisher Caldera is large, measuring 11 by 18 kilometers. Activities at Fisher have since been at Mount Finch, a volcanic cone inside the caldera, as early as the 1820s and 1930s.

Turns out #11 and 12 aren’t in Alaska. St. Helens (6, 6, 1860 BC, 4 cubic km) in Washington and the precursor to Eruption #1, Llao Rock in Mazama (6, 6, 5900 BC, >2 cu kilometers) just miss.

Well since the founding of the US (so, the last 247 years)

If we’re talking “in US history”, we probably need the US to exist… even though some of this happened before the area became part of the US. That’s OK in my books.

1. Novarupta: VEI 6, mag 6.5, 1912, 13.5 cubic kilometers

You’ve heard of it before.

2. St Helens: VEI 5, mag 5, 1800, 0.4 cubic kilometers

Thunder terrain leading away from Mount St. Helens after the eruption on May 19, 1980. Much of the volcano’s north side collapsed during a massive landslide seconds after the earthquake and before the eruption. US National Archives.

The only entry on this list not in Alaska or the Pacific Ocean, Mount St. Helens has had two significant eruptions since the signing of the Declaration of Independence. The 1800 eruption is slightly larger in magnitude than what comes in #4.

2. VestdalVEI 4, mag 5, 1795, 0.4 cubic kilometers

Another Aleutian volcano. Although the 1795 eruption was larger, it experienced a fissure eruption in 1991.

4. St. Helens, VEI 5, mag 4.8, 1980, 0.4 cubic kilometers

Arguably the most famous eruption in American history, the 1980 Mount St. Helens eruption is small compared to many of the eruptions on this list. However, it gave rise to much of the modern monitoring of volcanoes in the US today.

5. PavlovVEI 4, mag 4.7, 1762, 0.22 cubic kilometers

A beautiful, conical twin volcano (with Pavlof’s sister) … in the Aleutian Islands. She has been very active since 2007.

6. AugustineVEI 4, mag 4.7, 1976, 0.21 cubic km

All alone on an island in Alaska’s Cook Inlet, Augustine is notorious for debris avalanches in the bay.

7. PaganVEI 4, mag 4.7, 1981, 0.20 cubic km

Maybe this is a hoax, but Pagan is located in the Northern Mariana Islands in the Pacific Ocean, which is technically a territory of the United States. It is a complex caldera volcano, but the 1981 eruption was quite large compared to most of its recent activity.

8. Aniakchak, VEI 4, mag. 4.6, 1931, 0.4 cubic kilometers

We talked about Aniakchak above, but its last eruption in 1931 ranks high.

9. Augustine, VEI 4, mag. 4.3, 1883, 0.09 cubic kilometers

Augustine, again.

10. A spurVEI 4, mag. 4.3, 1953, >0.04 cubic kilometers

Spurr is due west across Cook Inlet from Anchorage. So eruptions from this Alaskan volcano can be quite disruptive to air travel to and from the busy cargo hub in Anchorage. The 1992 eruption, which was slightly smaller than the 1953 blast, caused one of the closest near-catastrophes that The 747 went through the ash cloud from the volcano.

Actual eruptions in 50 states (after officially becoming states)

If you’re still not satisfied with this list, how about the top 10 eruptions of a volcano that was officially part of the US when the eruption occurred? Some of these eruptions are starting to get a bit smaller, up to VEI 3, which are fairly common every year around the world.

1. Novarupta

2. Mount St. Helens

We talked about these two above.

3. Kasatochi 2008

4. Spurr 1992

5. Okmok 2008

6. Augustine 1986

7. Redoubt 2009

8. Cleveland 2006

9. Augustine 2005

10. Cleveland 2001

All the others are an eruption of one of the aforementioned Alaskan volcanoes, except for Kasatochi, which is also in Alaska but is abundant in the distant Pacific Aleutian Islands. There hasn’t been a big enough eruption in the Holocene to crack the other lists.

Finally, what doesn’t count Alaska? Well, the only two significant eruptions outside of Alaska and Mount St. Helens that have occurred since the founding of the US and the location in question was a state are as follows. The 1983-2018 eruption Kilauea tops this list, but it was more of a long, effusive eruption than an explosive event. Instead, we should turn our attention to California and the eruptions of 1914-1917. Lassen Peak (Kohm Yah-mah-nee). This eruption ejected a paltry 0.007 cubic kilometers of volcanic debris. Compare that to the 50 cubic kilometers of Mazama/Crater Lake or even the 4 cubic kilometers of Mount St. Helens 1980.

Overall, this shows how few major volcanic eruptions this country has experienced on its territory in the past 247 years. The Founding Fathers probably never thought of volcanoes as something for the nation to contend with, but the reality is that we have many volcanoes that could potentially end up on those lists. Preparedness and research are the keys to making sure we don’t end up with a massive disaster when they erupt again.

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