The Iceland Eruption and A Few Magma Facts

Things got a little tribal out at the volcano, Friday night, after experts had stated that this was one of those rare eruptions from the planet’s mantle.

Night viewing through the national broadcaster’s live webcam showed pinpoints of artificial light all around the fiery cones and lava field in Geldingadalir Valley.

Up there on hilltops, hidden in darkness, were thousands of Icelanders, most individuals still wearing the head lamp that had lighted their two-hour walk in to this unique site from the nearest road.

The little white “fireflies” were moving around. People were probably shouting and singing (though you couldn’t hear it on the cam over the roar of the icy wind and the volcano’s explosions), and laughing, too, while hanging out at Nature’s gigantic bonfire.

The on-scene point of view.

It also must have been a great release, after all of those tens of thousands of earthquakes these people have patiently endured on the Reykjanes Peninsula over the past weeks and months.

And it was also a good way to celebrate their native land, birthed in fire and scarred by it, but alive and growing right in front of them.

There was good news to party about, too: while magma has invaded the peninsula in other places, experts announced this week that there would be no other eruptions for a while.

Edit, April 1st: Here is a time-lapse of that week, ending with the “fireflies.”

Meanwhile, all the world is fascinated by the fires of Geldingadalir.

Of course. We all take a primitive delight in this terrifyingly beautiful stuff.

But what exactly is magma? Here are a few simple facts.

1. “Magma” versus “lava.”

As you might already have heard, magma is the stuff when it is in the ground: lava is its name when the same material is at the surface as a dome or flow.

I don’t know why geologists chose the word magma in 1859, though its 15th-century meaning of “mixture” is certainly appropriate, as we’ll soon see.

But we have Mount Vesuvius and its frequent eruptions to thank for the word lava, which goes back to the 1750s and means “torrent, flow” in the Neapolitan or Calabrian dialects of Italian.

Back then, people were more interested in surviving an eruption than in figuring out what was going on underground!

2. It’s beautiful under a microscope

Magma is more than just molten rock. It contains an assortment of minerals — including olivine, pyroxene, and plagioclase — as well as dissolved volatiles like water, carbon dioxide, and sulfur.

This topic is of key importance in volcanology, but it gets very complicated very quickly, so let’s just look at some cooled (and ancient) magma samples through a polarizing microscope. Some of the links lead to more information.

Two melt inclusions within a zircon crystal, by Kathryn Watts/U.S. Geological Survey, public domain

Magma sometimes gets trapped inside a crystal, which delights geologists who are trying to understand what this molten mixture was like before the chemistry changed as it erupted (and afterwards).

Analysis of these melt inclusions can supply clues about possible climate effects of large ancient eruptions.

Here is what rock fresh from Geldingadalir looks like:

That light green crystal, upper left, is an olivine xenolith that the rapidly ascending magma broke off from surrounding rocks as it roared upwards.

It’s early days, as far as publicly available microscopic views, but there is this:

3. Magma doesn’t form from direct heat.

Yes, Earth’s mantle is very hot, and yes, that heat does come from Earth’s core.

But the core is almost two thousand miles down. In Iceland, we’re looking at stuff from a depth of “only” ten miles or so. And the scientists are excited because that’s such a deep source.

Two mantle processes form magma, and one of those is unique, as far as I know, to subduction zones. Let’s skip it for now, since Iceland is on a spreading center: the Mid-Atlantic Ridge.

This is where the other process occurs: partial melting. Actually, this occurs everywhere. Here’s how it works.

Imagine Earth’s mantle as a simmering pot of soup (this analogy is Oppenheimer’s). The stove burner is the Earth’s core.

Hot soup rises lazily, cooling off a bit along the way. Since the cooler stuff is denser, it eventually sinks back down towards the burner, where it’s heated up and the cycle repeats itself over and over as a convection current.

Per Oppenheimer, solid rock does this, too, under certain conditions of pressure and chemistry. It is theorized that the mantle convects this way.

There is more than one idea about how mantle convection works. (Image: Ewalde1 via Wikimedia, CC BY-SA 3.0

But the mantle is composed of a variety of minerals, each of which has a different melting point.

Those that make up basalt have a lower melting point, and they do turn liquid thanks to decompression melting when their convection current gets within about 190 miles of the surface.

This is still far deeper than any volcano can go, but a combination of overburden pressure and buoyancy keeps the melt moving upwards.

It eventually pools under the outer crust. Sometimes it keeps rising through weak spots in this crust.

It’s a herky-jerky process. More often than not, the magma stalls and slowly “freezes,” i.e., crystallizes underground as a sill, dike, or pluton. Some of it does reach the surface and there is an eruption.

This can take a long time, up to many thousands of years. Along the way, magma typically interacts with the rock its moving through, as well as changing its own chemistry as it slowly cools.

Every now and then, though, a path from that magma pool at the bottom of the crust all the way to the surface opens up, and fresh, relatively unadulterated melt shoots up quickly.

This is happening right now on the Reykjanes Peninsula. It’s one of those rare combinations of spectacle for us laypeople and a unique scientific opportunity for the boffins.


Well, there are many more interesting facts about magma, but I got carried away by the sight of all those people gathered around the eruption Friday night. And this has gone on long enough — will save it for another post.

Featured image: Department of Civil Protection and Emergency Management of Iceland via Wikimedia, CC BY-SA 4.0.


Etymonline. 2021. Lava.

___. 2021. Magma.

Oppenheimer, C. 2011. Eruptions That Shook the World. Cambridge: Cambridge University Press. Retrieved from

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