Katla Volcano: The Sleeping Giant Under the Ice

If you ask an Icelander which volcano they are truly afraid of, they won’t say Fagradalsfjall, and they won’t say Eyjafjallajökull. They will say Katla.

Located on the southern coast of Iceland, hidden beneath the massive ice cap of Mýrdalsjökull, Katla is one of the most powerful and active volcanic systems in the world. She is the “angry sister” of Eyjafjallajökull, located just 25 kilometers to the west. While her brother made headlines in 2010 by disrupting air travel, Katla has a history of violence that makes that eruption look like a firework display.

For over a century, Katla has remained largely silent. This is the longest pause in her activity since records began in the 12th century. To geologists and locals alike, this silence is not peaceful; it is ominous. The pressure chamber is building, and the question is not if Katla will erupt, but when—and how devastating the consequences will be.

Geological Context: Fire Under Ice

Katla is not a mountain you can easily see. Unlike the classic cone of Mount Fuji, Katla is a massive central volcano dominated by a caldera that is 10 kilometers (6.2 miles) wide and 700 meters (2,300 feet) deep. This caldera is completely filled with ice—the Mýrdalsjökull glacier.

This combination of magma and ice is what makes Katla so deadly. When an eruption begins, the magma doesn’t just flow out as lava. It melts the ice instantly.

• Phreatomagmatic Explosions: The contact between 1,200°C magma and ice creates steam explosions of unimaginable force, shredding the magma into fine, glass-like ash.
• The Ice Lid: The weight of the glacier (hundreds of meters thick) acts as a pressure cooker lid. It holds the gases in until they can no longer be contained, resulting in an explosive release that can shoot ash 20 kilometers into the stratosphere.

The Eruptive Rhythm: Is She Overdue?

Historically, Katla has been remarkably consistent. Since the settlement of Iceland, she has erupted roughly every 40 to 80 years.

• 1625: A catastrophic eruption that darkened the skies over Europe.
• 1755: One of the largest tephra eruptions in Iceland’s history. The floodwater discharge was estimated to be equal to the combined flow of the Amazon, Mississippi, Nile, and Yangtze rivers.
• 1860: A standard explosive eruption.
• 1918: The last undisputed major eruption.

Do the math: It has been over 100 years since 1918. We are currently in the longest repose period in recorded history. While some scientists argue that small, subglacial events occurred in 1955, 1999, and 2011 (which caused a bridge to wash away), a full-scale caldera collapse eruption is statistically overdue.

The Eyjafjallajökull Connection

There is a known tectonic link between Katla and her neighbor, Eyjafjallajökull. Historically, an eruption at Eyjafjallajökull is often followed by an eruption at Katla within months or years.

• When Eyjafjallajökull erupted in 2010, the world held its breath.
• Katla showed signs of unrest—increased earthquakes and conductivity in glacial rivers—but she did not wake up. The link is not a guarantee, but it keeps volcanologists on high alert.

The 1918 Eruption: A Case Study in Destruction

To understand what a future eruption might look like, we look to 1918. It began on October 12th. Within hours, a massive plume of ash rose 14 kilometers into the air. But the ash was not the immediate killer.

The Great Jökulhlaup

As the eruption melted the glacier from below, billions of gallons of water accumulated under the ice cap. Suddenly, the ice barrier burst. A jökulhlaup (glacial outburst flood) of biblical proportions tore down the Mýrdalssandur outwash plain.

• Discharge Rate: The flood peak was estimated at 300,000 cubic meters per second. To put that in perspective, the average flow of the Amazon River is about 209,000 cubic meters per second.
• Land Expansion: The flood carried so much sediment, rock, and ice that it extended the southern coastline of Iceland by 5 kilometers in a single day.
• Ice Bergs: The floodwaters carried icebergs the size of houses, smashing everything in their path.

If such a flood happened today, it would destroy the Ring Road (Route 1), severing the connection between East and West Iceland. The town of Vík í Mýrdal is generally safe from the flood itself (due to topography), but it would be cut off from the world and buried in ash.

Hazards: Why Katla is Different

The danger of Katla is threefold:

1. The Flood (Jökulhlaup)

This is the primary local hazard. The floodwaters can reach the ocean in less than 2 hours. Authorities regularly practice evacuation drills with residents of Vík and the surrounding farms. When the text message alert goes out, people know to run to the high ground immediately (specifically the church hill in Vík).

2. The Ash (Tephra)

Katla produces high-silica, fluorine-rich ash.

• Aviation: A major plume would almost certainly shut down North Atlantic air traffic, potentially for longer than the 2010 disruption.
• Agriculture: The fluorine coating on the grass is toxic to livestock. In past centuries, “famine after the fire” killed more people than the volcano itself because all the sheep and cattle died of poisoning.

3. Lightning

Katla’s plumes are electrically charged. The 1755 eruption descriptions mention “lightning strikes killing people and livestock” miles away from the volcano. The friction of the ash particles creates a massive static charge, turning the eruption column into a thunderstorm.

Modern Monitoring: Listening to the Beast

Today, Katla is one of the most closely monitored volcanoes on Earth. The Icelandic Meteorological Office (IMO) uses advanced technology to detect the slightest sign of waking.

• Seismic Arrays: Thousands of small earthquakes (tremors) are detected every year. Seasonal patterns are observed, where activity increases in summer as the ice melts and reduces pressure on the caldera.
• GPS Deformation: Stations on the exposed nunataks (mountain peaks poking through the ice) measure if the volcano is inflating.
• River Conductivity: Sensors in the Múlakvísl and Jökulsá á Sólheimasandi rivers measure the electrical conductivity of the water. An increase indicates that geothermal gases are leaking into the meltwater, a precursor to an eruption.

In recent years, “unrest” has been high. In 2016 and 2017, large earthquakes (M4.0+) shook the caldera, prompting warnings. Yet, the giant sleeps on.

Katla in Pop Culture

The volcano’s terrifying reputation has made it a star of modern media. The Netflix series “Katla” (2021) depicts a mystery/horror story set in the aftermath of a fictional, year-long eruption. While the “ash changelings” in the show are pure fantasy, the depiction of Vík covered in black ash, the isolation, and the psychological toll of living next to an erupting monster is grounded in the reality of Icelandic history.

Tourism: Walking on the Dragon’s Back

Despite the danger—or perhaps because of it—Katla is a major tourist destination. You cannot “climb” Katla in the traditional sense, as it is a glacier, but you can explore her edges.

The Katla Ice Caves

One of the most popular tours in South Iceland is visiting the “Katla Ice Cave” in the Kötlujökull glacier tongue. Unlike other ice caves that are only safe in winter, some of Katla’s caves are accessible year-round. The ice here is often streaked with bands of black ash from past eruptions, creating a stunning zebra pattern that serves as a geological timeline.

Super Jeep Tours

Specialized 4x4 vehicles take tourists up to the edge of the ice cap. From there, you can see the vast white expanse that hides the caldera. It is a humbling experience to stand there, knowing that just a few kilometers beneath your boots lies a magma chamber capable of altering the global climate.

Safety Note: Never attempt to drive onto the glacier yourself. The terrain is riddled with crevasses and “cauldrons”—sinkholes caused by geothermal melting. Always go with a certified guide.

Photography Guide: Capturing the Beast

For photographers, the Katla region offers some of the most dramatic landscapes on the planet. The contrast between the black volcanic sands and the white glacier creates a monochrome world that looks almost alien.

Best Locations for Photography

• Vík and Reynisfjara: From the famous black sand beach, you can look north towards the glacier. On clear days, the white dome of Mýrdalsjökull looms over the sea stacks of Reynisdrangar. A telephoto lens (200mm+) is essential to compress the distance and show the scale of the volcano relative to the church in Vík.
• Hjörleifshöfði: This isolated promontory on the Mýrdalssandur black desert offers a panoramic view of the entire Katla system. It is also a historical site; legend says the Viking settler Hjörleifur is buried on top. From here, you can see the path of the 1918 flood and visualize the sheer scale of the destruction.
• Thakgil (Þakgil): A hidden canyon located right at the edge of the Katla Geopark. The jagged, moss-covered peaks here are remnants of ancient eruptions. It feels like a scene from Lord of the Rings.

Tips for Drone Pilots

Flying a drone near Katla (outside of national park restrictions) can yield incredible footage of the braided glacial rivers. However, be extremely careful of the wind. The “katabatic winds” falling off the glacier can exceed hurricane force without warning, sending drones crashing into the black sands.