Hekla

Hekla is a highly active stratovolcano located in southern Iceland.

A stratovolcano is a type of volcano built from mixed layers of lava and tephra, which is volcanic rock and ash. Hekla is one of the most active volcanoes in Iceland. Since the year 1210, there have been over 20 eruptions in and around the mountain. Its unique shape resembles an overturned boat. The keel of this boat is a series of craters. Two of these craters are generally the most active parts of the volcano.

The volcano sits at a complex geological junction. It is located where the South Iceland seismic zone and the eastern volcanic zone meet. This area is part of a mid-ocean ridge, which is a diverging plate boundary. Hekla is part of a volcanic ridge that is 80 kilometers long. The most active part of this ridge is a fissure called Heklugjá. This fissure opens along its entire length during major eruptions. The volcano is fed by a massive magma reservoir located deep underground. This chamber reaches an unusual depth of more than 10 kilometers.

Eruptions at Hekla follow a specific geological process. Many eruptions begin with thick, highly explosive materials. These include rhyolite, dacite, or andesite. These materials create tephra and can cause pyroclastic flows, which are fast-moving clouds of hot gas and rock. As the eruption progresses, the material often changes. The later stages often involve thinner, basaltic magma. This thinner magma forms wide lava fields. Hekla is unique because it is the only Icelandic volcano to produce calc-alkaline lavas.

Throughout history, people have held many different beliefs about Hekla. During the Middle Ages, Icelandic Norse people and much of Europe called it the "Gateway to Hell."

After the eruption of 1104, these stories spread across the continent. Some believed the volcano was a prison for souls. In the Flatey Book Annal, a 1341 eruption was described as having birds flying in the fire. People believed these birds were actually human souls. These legends of Hekla being a gateway to the abyss persisted until the 19th century.

Hekla has a history of massive, prehistoric eruptions. One of the largest was the Hekla 3 eruption around 950 BCE. This event had a Volcanic Explosivity Index, or VEI, of 5. It threw about 30 cubic kilometers of volcanic rock into the atmosphere. This was so large that it likely cooled temperatures in the northern hemisphere for several years. Traces of this eruption have been found in Scottish peat bogs. In Ireland, tree rings show that growth was nearly nonexistent for a decade following this event.

Many recorded eruptions have caused significant destruction. The eruption from 1300 to 1301 was the second largest since Iceland was settled. It covered large areas of land with tephra and expelled over 1 cubic kilometer of lava. This event caused over 500 deaths during a harsh winter. Another destructive eruption occurred in 1693. It produced lahars, which are mudflows, and even caused a tsunami. The 1766 eruption was also massive, producing the second largest lava flow in Iceland's inhabited era.

Hekla is a vital tool for modern scientists. The tephra produced by the volcano is high in fluorine, which can be poisonous to animals. Scientists study the layers of ash to date other volcanic events. In the last thousand years, Hekla has produced about 10% of all tephra in Iceland. This amounts to approximately 28 cubic kilometers of material. Furthermore, one third of the volcanic ash found in Scandinavia, Germany, Ireland, and the United Kingdom originated from Hekla.

Today, researchers monitor Hekla very closely. They study parameters like strain, tilt, and deformation. They also track seismic activity, or earthquakes. When the volcano is dormant, earthquakes are usually below magnitude 2. When it is erupting, they can reach magnitude 3. Scientists have noticed a pattern in the eruptions. Generally, the longer Hekla remains dormant, the larger its next eruption will be. The most recent eruption occurred on February 26, 2000.