Quirks and Quarks·Analysis: Bob's blog

What makes Iceland so volcanically active? Shifting plates below the island nation

Iceland is part of the longest mountain range in the world where plate tectonics is violently creating a growing, massive rift along the Mid-Atlantic Ridge.

One town in Iceland successfully stopped a lava flow from a nearby volcano back in 1973

A police officer stands in front of a huge crack in the middle of an intersection in a deserted town.
A police officer stands by the crack in a road in the fishing town of Grindavik on November 15, 2023, which was evacuated due to volcanic activity. (Marko Djurica/Reuters)

As residents of Grindavik evacuate over fears of a volcanic eruption that could destroy their town, that looming threat is likely one they've been preparing for all their lives, given that their island nation sits on top of one of our planet's most active volcanic hot spots.

This type of activity has been going on for millions of years and is likely to continue far into the future.

The entire island nation of Iceland straddles a crack in the Earth's crust known as the Mid-Atlantic Ridge, a boundary between the North and South American and the Eurasian and African plates.

These two continent-sized pieces of crust are pulling apart from each other allowing new magma from within the Earth to fill in the gap, forming a line of volcanoes along the way. 

Unknown until the 1950s, the Mid-Atlantic Ridge is part of the longest mountain range on Earth, running 65,000 kilometres from just below the North Pole, all the way down the centre of the North and South Atlantic Oceans to the southern tip of Africa in the Antarctic Ocean. The ridge connects under the Horn of Africa to the Indian Ridge system. All of the ridges together wrap like the seams around a baseball.  

A footbridge connects two sides of a major rift in the flat barren rocky landscape in Iceland.
Aerial view taken in 2021 shows the footbridge, known as the Bridge Between Continents, linking the Eurasian and North American tectonic plates on the Mid-Atlantic Ridge near the town of Grindavik, Iceland. (Halldor Kolbeins/AFP/Getty Images)

Most of the Atlantic Ocean ridge is totally hidden underwater, but Iceland is one of the few places where the mountain peaks rise above the ocean surface.  

This spreading action along the ridge pushes the plates apart by a few centimetres a year, which doesn't sound like much, but over the last 200 million years it created a rift as deep and wide as the Grand Canyon that's also thought to have opened up the entire Atlantic Ocean from the Pangea supercontinent.

There is no indication this movement will stop any time soon, so volcanic eruptions will continue and Iceland will gradually become wider. You could say that Iceland is one place where they are making more land.

A see-through view of Earth showing two red blobs at the base of the mantle as the planet spins.
Earth's mantle blobs, illustrated in red, are the likely reservoir for Iceland volcanoes, scientist Qian Yuan told Quirks & Quarks. (Caltech)

But there is more to the story.

Underneath Iceland lurks a plume of molten material that rises up from deep within the Earth forming a hot spot under the crust and provides a source of magma for the multitude of volcanoes on the island.

These plumes are believed to run all the way down through the mantle to the boundary with the Earth's core.

LISTEN | Primordial planet that created Earth's moon is 'likely' reservoir for Iceland's lava:

Major hot spots around the world also lie under volcanic regions such as Hawaii and Yellowstone. The combination of Iceland's being located above a mantle plume and straddling a plate boundary makes it one of the most volcanically active places on Earth.

Icelanders are well aware that they live on top of an active volcano. They take advantage of the geothermal heat to generate clean electricity, and their natural hot springs are world famous. 

People are in steaming blue water coming up from the rocky Earth beneath it with a mountain with snow in the background. It looks like heaven.
In this 2020 photo, bathers enjoy the warm volcanic hot springs of the Blue Lagoon in Grindavik. (Hannah McKay/Reuters)

There was even an occasion where they fought back against a volcano and won.

This year marks the 50th anniversary of an eruption in the Vestmannaeyjar archipelago, an island chain off of Iceland's south coast, from the Eldfell volcano on the island of Heimaey — the only island in this important fishing region which remains inhabited year round — that covered it in ash.

The island was evacuated and the eruption continued over the following months, burying many homes in black ash. But when lava began flowing down toward the town, threatening to cut off the entrance to their harbour, the town took action.

A plane's wing is in the foreground of a black and white aerial view of a small town on the left hand side with an encroaching stream of lava and smoke billowing out.
The eruption of the Eldfell volcano on Heimaey, in the Vestmannaeyjar island chain off the southwest coast of Iceland, as seen from an aircraft in January 1973. (Terry Disney/Express/Hulton Archive/Getty Images)

A network of pipes and pumps were laid across the land and seawater was sprayed for weeks over the leading edge of the lava flow. The lava hardened and formed a natural dam that, at one point, almost broke before a dredging boat off the coast got involved in the seawater spraying effort to reinforce the lava wall.

From there, with a few more powerful pumps, they halted the lava flow altogether to save the harbour.

Today, Vestmannaeyjar is still an active fishing region and a popular tourist destination because of its dramatic volcanic landforms.

Volcanic eruptions are a fact of life in Iceland. Will the residents of Grindavik take similar action to protect their town?  


Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.