When thinking of Antarctica, active volcanoes are typically not the first things that come to mind. But the continent has 18 active volcanoes (https://33science.com/2025/12/01/active-volcanoes-in-antarctica/), two of which have erupted during the past century—see locations on map below. We visited Isla Decepción, which last erupted in 1970, on 23 February 2026—the last full day of our Antarctica expedition. This post briefly describes the island’s geologic setting and remnants of its eruptive and human history that we could see.
I previously published posts about Antarctica on 21, 25, 28 February, and 2 April, 2026. You can also find those posts by clicking on “Antarctica” under Categories. I’ve been home for 2 months, but feel compelled to share some additional observations.
The geographic and geologic setting
A polar projection of Antarctica, which is nearly twice as large as the Australian continent. East Antarctica is an ancient continental block that has been geologically inactive for a very long time. In contrast, West Antarctica remains active—this is where the active volcanoes are located. The two volcanoes that have erupted during the past century are Mt. Erebus, located on Ross Island just offshore from McMurdo Station (U.S. base), and Isla Decepción, located in the South Shetland Islands—see more detailed map below. Thwaites glacier, Antarctica’s fastest melting glacier, and the focus of a research expedition in January 2026, flows into the Amundsen Sea.The main map shows the interacting tectonic plates between southern Patagonia and the Antarctica Peninsula. For millions of years the Phoenix Plate was subducting beneath the Antarctica Peninsula and creating a line of volcanoes similar to the Cascade Range in the Pacific NW. As the Phoenix Plate was consumed beneath the Antarctica continent, subduction ceased progressively from south to north. Now the Phoenix Plate is largely fixed and subduction has mostly ceased, even off the coast of the South Shetland Islands (area colored red).
The insert map shows details of the South Shetland islands, including Deception Island. The South Shetland Trough is the trench produced when the Phoenix Plate was still descending beneath the Antarctica continent. On the main map it is marked by a toothed black line between the Phoenix and Antarctica plates. The area between the South Shetland Islands and the Antarctica Peninsula is the Bransfield Strait, shown in the map below. The figure above is from Bastias et al., 2023—see reference below.This 3-D image of the Bransfield Strait shows the location of Isla Decepción near the south end of the strait. The labeled islands of the South Shetlands bound the strait on the west. We flew into Chile’s Frei Station (label at upper left) and transited the strait to reach the Antarctica Peninsula, the land mass that bounds the strait on the east. This figure is from a book in the library of Antarctica21’s facility in Punta Arenas.
Since the Phoenix Plate has ceased subducting beneath the Antarctica continent, it has begun to roll back and cause extension that pulled the South Shetland Islands away from the Antarctica Peninsula and created the Bransfield Strait rift basin. The N-S arrows on the map above show the direction of extension. Note that Isla Decepción is located where the rift has opened to provide a conduit for magma to rise to the seafloor. Orca is another volcanic island in the rift, but it has not erupted recently.
Characteristics of the Isla Decepción volcano
Isla Decepción is a stratovolcano that “blew its top” about 10,000 years ago, collapsing the volcanic cone and creating a horseshoe-shaped caldera—see map below. There have been many eruptions since then, mostly recently in 1967–70. These eruptions tend to be pyroclastic flows and air falls—consequently, most of the island’s glaciers are covered with volcanic ash.
This map of Isla Decepción shows how the caldera center—Port Foster—provides a natural harbor for ships. This is similar to Oregon’s Crater Lake (actually a caldera) that has a lake in its center. The hot magma beneath the island creates geothermal activity and there are photos of people soaking in natural hot pools.
We entered the caldera through Neptunes Bellows and disembarked in Whalers Bay. There we walked along the beach, hiked to a volcanic edge, and viewed remnants of a violent human past.
A photo of some of our “science group” members on a volcanic edge with the caldera to the left and the strait to the right.
Human interactions with nature—both positive and negative
This is a view of our ship in Whalers Cove (see map above) with a broad plane of volcanic materials leading to the beach.This view to the east shows Neptune’s Window (see map above) and ruins of tanks that held whale oil when this bay was a bustling hub for early 20th-century whaling operations. At times, up to 13 ships operated simultaneously. It’s a haunting view—to imagine the relatively pristine Antarctica environment as a killing field for thousands of sentient whales. We thought they should remove these rusting hulks, but maybe it’s instructive to retain these reminders of humankind’s capacity for cruelty.These ruined buildings remain a testament to humanity’s often degrading impact on the natural environmental. The ridge in the background is part of the volcanic ridge that surrounds the caldera.Yet nature is thriving today. We saw a large group of monk seals on the beach—remember that they have external ears and are therefore not true seals. We experienced our worst weather of the trip at this location. Those white streaks are falling snow flakes in front of my phone. All of the rocks on the ridge are volcanic rocks that built up the edifice of the cone before it collapsed.A few chinstrap penguins posed for our photos. They really did not seem bothered by human visitors.
Half Moon Island
Our “worst weather” morning at Deception Island was followed by our “best weather” afternoon on Half Moon Island, a South Shetland Island that is not an active volcano. In addition to rare sunny skies, we were treated to views of a large chinstrap penguin colony and beautiful mountain and glacier views.
Our scenic landing spot was populated by a few chinstrap penguins who were oblivious to the rotting boat—another remnant of the region’s whaling past.Left-side photo: a chinstrap penguin on the rocks—that’s where they make their nests. Right-side photo: Beautiful rock pinnacle in front of the Tangra Mountain range.On the other side of Moon Bay is the spectacular backdrop of the Tangra Mountains, named for the supreme, mythical god in Bulgaria. It was the Bulgarians who mapped much of this area in detail in the early 20th century.
I’ll end this post with another photo of this remarkable island.
A Half Moon Island view of penguins, rocks, ice bergs and a distant glacier.
In my final Antarctica post, I’ll summarize our itinerary and the science our group managed to accomplish.
Thanks, Kären. Fascinating.
Thanks for reading Marisa!
Thanks for my continuing education. 😃
Thanks for being a loyal reader Keeley!
As usual, beautiful photos and learning more about this location.
Thanks, Karen & Jay
Thank YOU, Joan, for reading!
Thanks for the fascinating photos and detailed explanation!
Thanks Linda!