Mount Spurr, just to the west of the city of Anchorage, is rumbling. The Alaska Volcano Observatory has been monitoring the earthquakes, gas emissions and visible changes at Spurr and think we might be headed towards a new eruption, the volcano’s first since the early 1990s. They’ve placed it at Yellow Alert status thanks to all the unrest since the start of 2025.
More potentially active volcanoes are located in Alaska than any other state in the US thanks to the long chain of the Aleutians that spread out from near Anchorage across the northern Pacific almost to Russia. There are also clusters of volcanoes in the Wrangell Range in eastern Alaska and near Juneau. So, with all these volcanoes, what were the state’s largest eruptions in the past ~10,000 years (also known as the Holocene)?
As with any list like this, there are a few caveats. First, this list is the largest known eruptions. Now, it is hard to completely hide a large eruption. However, volcanic deposits can be easily weathered and eroded, so sometimes we can underestimate just how big an eruption was. Second, this list is based on our current best dates that sometimes turn out to be wrong. Maybe an eruption we think is older could end up in the Holocene and vice versa. That’s okay! That’s science!
I’ll be using the Volcanic Explosivity Index to rank these volcanoes. This index is based on the volume of volcanic debris erupted, usually listed in cubic kilometers. A cubic kilometer is over 264 billion gallons! I’ll also be using the Smithsonian’s Global Volcanism Program database as my source for this information. Rather than try to split them by volume (which can be highly uncertain in places like Alaska), I’ll list them oldest to youngest.
Roundtop – VEI 5 – ~9,600 years ago: Many of these eruptions are on remote islands and Roundtop is no exception. Roundtop is located on Unimak Island, is glacier-covered today but much of the southwestern parts of the Alaska Peninsula are covered with volcanic debris and ash (aka, tephra) from its VEI 5 eruption. However, beyond that event, there is no known evidence of eruptions from Roundtop.
Seguam – VEI 5 – ~9,300 years ago: This eruption came from one of the many islands stretching out into the northern Pacific from the Alaska Peninsula. Seguam produced a giant eruption 9,300 years ago one of the two calderas on this lonely island of the same name. Since then, it has had numerous much smaller eruptions (VEI 0-3) that have slowly healed the scars of this blast. Now, Seguam is literally in the middle of nowhere. According to the GVP, no one lives within 100 kilometers of the volcano. It also happened within a few hundred years of Roundtop. Is this odd? Not at all in a chain of active volcanoes like the Aleutians.
Fisher Caldera – VEI 6 – ~7,420 BCE: The Fisher Caldera shares Unimak Island with Roundtop (and others). Not to be outdone, the volcano exploded over 9,000 years ago and in the process created a caldera that is 11 by 18 kilometers. This eruption sent pyroclastic flows into the Pacific Ocean and Bering Sea. Since then, the volcano also experienced a VEI 5 eruption ~5,100 years ago and erupted in 1826 and 1830.
A view of Aniakchak in Alaska from above, taken by Roy Wood, USGS. Credit: NPS
Aniakchak – VEI 6 – ~6,300 and ~1645 BCE: I’m doing some lumping here. Aniakchak has produced multiple big eruptions. The older of the two explosions from this volcano on the Alaska Peninsula is creatively called “Aniakchak I”. Oddly, although it was huge, it doesn’t seem to have created a volcano. However, pyroclastic flow deposits mantle the valleys around the volcano. We know more about the more recent eruption called “Aniakchak II.” It was also huge, erupting ~70 cubic kilometers of volcanic tephra. It also sent pyroclastic flows 50 kilometers out from the volcano across the Alaska Peninsula and into the sea, generating a tsunami. Since then, not only has it proven to be the most active of the volcanoes in the eastern Aleutians, but it produced two VEI 4 eruptions in ~1540 and 1931, the latter spreading ash across much of southern Alaska.
Black Peak – VEI 6 – 1900 BCE: Black Peak looks like a craggy monster, but the 1900 BCE eruption created a caldera on the volcano and filled some of the valleys leading from the volcano with 100 meters of volcanic tephra. However, unlike Aniakchak and Veniaminof, there isn’t much evidence that Black Peak has had many or any eruptions since this giant blast.
Lava bombs and ash from the 2013 eruption of Veniaminof. AVO/USGS.
Veniaminof – VEI 6 – 1750 BCE: Yet another massive explosion from a volcano on the Alaska Peninsula … and possibly within a ~300 year window (with Aniakchak and Black Peak). This caldera-capped volcano has associated vents that stretch for tens of kilometers away from the main caldera of Veniaminof. It has also been very active in the 21st century, with eruptions, with 9 different eruptive episode as recently as 2021. However, most of these, like many of our other caldera, come from cinder cones within the caldera that are more prone to erupt lava flows that cause another massive explosion.
Okmok – VEI 6 – ~100 BCE: Back out in the Aleutian Islands, Okmok is located on Unmak Island. From the air, the caldera at Okmok looks a lot like Oregon’s Crater Lake (the home of the largest eruption in what is now the US over the past 10,000 years) if it was drained. There may have been a lake in the Okmok caldera in the past as some deposits show evidence of wave action. Okmok last erupted in 2008 where one of the cinder cones inside the caldera produced lava flows as the volcano repairs the scars of that and previous violent blasts.
Churchill – VEI 6 – 847 and 60 CE: I’m lumping again, putting Churchill’s two massive blasts together, but the East and North White River Ash deposits may be the most powerful explosive eruptions of the Holocene. White River Ash material is found in peat bogs in the British Isles. It took until the last 30 years for volcanologists to mostly agree on the source of these deposits — and even today, the remote and enigmatic Mount Churchill in the Wrangell Range is a strange place for such massive eruptions to be sourced.
Novarupta – VEI 6 – 1912 CE: If you’ve heard of a big Alaskan eruption, this is likely the one. Not only was it the largest eruption of the 20th century, it also produced the remarkable “Valley of 10,000 Smokes“. The 1912 eruption was a strange one, forming between known volcanoes and causing one volcano (Katmai) not directly involved in the eruption to collapse (but was originally thought to be the source). The source of the explosion is now capped with a rhyolite dome called Novarupta (above). We’re still feeling impacts of this blast, with the Alaska Volcano Observatory issuing a statement about volcanic ash getting kicked up by strong winds earlier this week.
