Why some volcanoes just keep spewing lava

When the sky turns orange, it's a sight to behold — both spectacular and terrifying. It's a feeling that the people in Iceland's fishing town of Grindavik will find quite normal. They face an almost constant threat from the Fagradalsfjall volcano on the country's Reykjanes peninsula. 

On December 18, 2023, an eruption began west of Fagradalsfjall, about 50 km (30 miles) south of the capital Reykjavik. It spewed lava and smoke more than 100 meters (330 feet) into the air, following weeks of seismic activity.

A month earlier, in November, shifting magma under the Earth's crust triggered hundreds of earthquakes around Grindavik. 

Seismologists warned then that the quakes could be a precursor to a volcanic eruption and Grindavik's 4,000 inhabitants were evacuated.

Iceland is located between the Eurasian and the North American tectonic
plates, which is one of the largest on the planet, and because the two plates move in opposite directions, it is a hotspot for both earthquakes and volcanic activity.

How many volcanic eruptions are there each year?

Data from the Smithsonian Institution's Global Volcanism Program indicates there were 72 confirmed eruptions at some point during 2023 from 69 different volcanoes, and 22 of those were new eruptions started during the year.

But news of volcanic eruptions often only reaches the headlines when the big ones erupt — Etna, Kilauea, Mauna Loa, Merapi, Eyjafjallajökull or Fagradalsfjall — but at any time during a given year, there may be as many as 80 fresh eruptions around the world. The Global Volcanism Program counted 47 active eruptions on December 15, 2023. 

What is a volcano?

The US Geological Survey sums it up nicely: "Volcanoes are openings, or vents where lava, tephra (small rocks), and steam erupt onto the Earth's surface."

Volcanoes can be on land and in the ocean. They are, in part, a result of their own eruptions but also the general formation of our planet, as tectonic plates move.

Mountain ranges like the Andes in South America and the Rockies in North America, as well as volcanoes, formed through the movement and collision of tectonic plates.

There are four main types of volcanoes: cinder cones, composite or stratovolcanoes, shield volcanoes and lava domes.

Their type is determined by how the lava from an eruption flows and how that flow affects the volcano, and, as a result, how it affects its surrounding environment.

How do volcanoes erupt?

Essentially, it's a case of magma, or molten rock, below the surface of the Earth, bubbling up, rising and overflowing, like boiling milk out of a pot on a stove.

The magma finds its way to vents in the volcano and gets spewed across the land and into the atmosphere. When magma erupts from a volcano, it is called lava.

Volcanoes particularly active in Pacific Ring of Fire

Some of the most active volcanoes are located in the Pacific Ring of Fire, which includes New Zealand, Southeast Asia, Japan and the western coast of the Americas. About 90% of all earthquakes worldwide strike within this region.

Can scientists predict volcanic eruptions?

Scientists are capable of predicting volcanic eruptions hours, or sometimes several days, in advance. This isn't the case with earthquakes, which are much harder to predict.

Scientists use seismographic data from earthquakes and other tremors, because those can be a precursor to volcanic eruptions.

They monitor the ground for signs of deformation, which may be caused by the movement of magma. They also take readings of volcanic gas emissions, and changes in gravity and magnetic fields.

What are the most famous volcanic eruptions?

One of the most famous long-term eruptions was Kilauea volcano on Hawaii. Its spewing spree started in 1983 and continued — almost nonstop — for 35 years until 2018, only to start again in 2021. The eruption is still ongoing.

Dukono in Indonesia started erupting in August 1933 and is still continuing. Santa Maria in Guatemala began erupting in June 1922 and continues to this day.

And Yasur in Vanuatu first rumbled to life in about 1270 (± 110 years) and as of June 9, 2023, was still erupting.

Mount Etna is Europe's most active volcano and one of the largest in the world. Its recorded volcanic activity dates back to 1500 B.C. Since then, it has erupted more than 200 times.

Edited by: Fred Schwaller