Alaska’s Tanaga Complex is Restless (March 16 2331 UTC)

March 16, 2023, 4:31 p.m., Pacific: AVO has lowered the status of both Tanaga and Takawhanga volcanoes to Yellow, as seismicity has dropped a bit.

March 13, 2023, 10:54 a.m., Pacific: AVO went to Orange aviation code on the 9th, soon after the rise to Yellow. That’s fast, and it must have been some intense seismicity to get them to do that.

However, nothing has changed since then, and per the current update (March 12th), the seismicity has backed off a little bit. Orange code is still in effect.

They name two volcanoes: Tanaga and Takawangha, which are separately monitored but not clearly (to this layperson anyway) in two very different locations.

The eruptive history for that area confuses me, too. Apparently it’s a little uncertain which volcano in the complex has sourced which eruption in the past.

Anyway, until there is an eruption or until very different updates are presented for Tanaga and Takawangha, I’ll just call this the “Tanaga Complex” from now on, changing this post’s title accordingly.

Original post

I haven’t been following this much, other than noting some strong earthquakes have occurred in that general area, as well as elsewhere in the Aleutians, down through the years.

But on Tuesday, AVO raised Tanaga’s alert level to Yellow, noting:

Earthquake activity beneath Tanaga Volcano began to increase slowly starting at about 1:30 PM AKST today. At roughly 8:45 PM AKST this evening, the activity escalated with earthquakes occurring as often as 2 or 3 each minute. Initial locations of these earthquakes place them at shallow depths beneath the summit of Tanaga Volcano, and the largest of these earthquakes have magnitudes between 2 and 3. In response to this increase in seismicity, we are raising the Aviation Color Code to YELLOW and the Volcano Alert Level to ADVISORY.

Tanaga Volcano is monitored with a local seismic and infrasound network, regional infrasound and lightning detection networks, and satellite data.

Tanaga Island lies in the Andreanof Islands approximately 100 km (62 miles) west of the community of Adak and 2025 km (1260 miles) SW of Anchorage. The northern half of the island is home to the Tanaga volcanic complex, comprising three main volcanic edifices. Tanaga Volcano is the tallest of these (1,806 m or 5,925 ft) and lies in the center of the complex. The last reported eruption of Tanaga occurred in 1914 and earlier eruptions were reported in 1763-1770, 1791, and 1829. Reports of these eruptions are vague, but deposits on the flanks of the volcano show that typical eruptions produce blocky lava flows and occasional ash clouds. Eruptions have occurred both from the summit vent and a 1,584 m (5,197 ft)-high satellite vent on the volcano’s northeast flank. Immediately west of Tanaga volcano lies Sajaka, a 1,354 m (4,443 ft)-high compound edifice with an older cone to the east that collapsed into the sea within the last few thousand years, and a new cone that has grown in the breach. The new cone is 1,312 m (4,305 ft) high and consists of steeply dipping, interbedded cinders and thin, spatter-fed lava flows. To the east of Tanaga lies Takawangha, which is separated from the other active volcanic vents by a ridge of older rock. Takawangha’s 1,449 m (4,754 ft)-high summit is mostly ice-covered, except for four young craters that have erupted ash and lava flows in the last few thousand years. Parts of Takawangha’s edifice are hydrothermally altered and may be unstable, and could produce localized debris avalanches. No historical eruptions are known from Sajaka or Takawangha; however, field work shows that recent eruptions have occurred and it is possible that historic eruptions attributed only to Tanaga may instead have come from these other vents.

Thus far, swarming continues, per the observatory’s activity page.

According to the GVP’S Tanaga page, there was restlessness in 2005, too.

If this escalates into an eruption, it might be problematical — certainly for international air traffic; I haven’t read up on other hazards yet, but the general mention of hydrothermal alteration on a flank, as well as a VEI 6 in the GVP eruptions section, although thousands of years ago, make this worth following.

I’m not saying anything major is looming — 1914’s eruption didn’t shake the world, and the Anchorage Daily News noted yesterday:

The volcano is on an uninhabited island in the western Aleutians. There are no communities or structures there, but Adak, a city of about 170 residents on another island, is about 65 miles away and could see ashfall.

It’s just an interesting set of facts.

I didn’t see anything helpful on YouTube and haven’t checked Google Scholar yet.

Unless and until they go to Aviation Code Orange (or higher), will just pin this and get back to DV eBook revision work; once the Merapi chapter is done, it will be just formatting stuff and I’ll be able to do a closer look at Tanaga.

Featured image: Tanaga volcanoes (there’s a whole complex on that island), by M. W. Loewen/AVO/USGS

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