The Reykjanes Peninsula in southwest Iceland has been bracing for a volcanic eruption for days. Cracks appeared in the earth over the weekend, and steam is pouring out from deep below ground. The Icelandic Met Office warns of a “significant risk” of an eruption in the days ahead, and it pinpoints the town of Grindavik, which was entirely evacuated Friday, as the most probable location.

Grindavik, with a population of around 3,000, is about 26 miles southwest of Reykjavík, the capital of Iceland. Little has changed since early this week in the overall assessment of the burgeoning volcano, which could erupt on the “time scale of days” according to the Met Office.

Between 12 a.m. and 2 p.m. local time Thursday, around 1,200 earthquakes were recorded. Magma is still believed to be within about 2,500 feet of the surface, but an update from Met Office cautions that “the intrusion is propagating upward slowly.”

The land has sunk by up to 5 feet west of Grindavik and risen by up to 3 feet to the east. There are indications this impending eruption may feature “significantly” more magma than previous similar eruptions in the last two years. For now, officials continue to wait and watch anxiously.

The new developments

Early in the week, specialists installed a pair of sensors to monitor for SO2, or sulfur dioxide, which is the most common gas emitted by volcanoes aside from water vapor and carbon dioxide. (It also is harmful to humans, and can deplete ozone when it oxidizes to form a sulfate aerosol.)

One of the sensors on Monday did detect SO2, which would ordinarily suggest magma within 1,000 feet of the surface. It remains unclear, however, if the SO2 is originating from magma associated with the current incipient eruption or a past eruption of nearby Fagardalsfjall that occurred in July. Not all of that lava has hardened and could still be releasing some gases.

Earthquake activity has subtly increased. The volcanic activity also forced the closure of the popular Blue Lagoon hot spring in Grindavik on Nov. 10, and it will remain closed until at least Nov. 30.

Volcanic activity basics

Why is volcanic activity happening? The Reykjanes Peninsula is located where tectonic plates are diverging. That pulling apart of the North American Plate from the Eurasian Plate allows magma to rise closer to the surface. That is why Iceland is a seismically and geologically active region.

The activity is part of the region in Iceland that produced the Fagradalsfjall volcano, which has erupted several times since 2019, most recently in July.

The magma is probably moving upward near Sundhnúk, a few miles northeast of Grindavik. Since it cannot yet reach the surface, it oozes outward below ground, squeezing in between rocks in the crust of the earth. These are known as dike intrusions.

In some cases, a dike intrusion occurs when magma drives a wedge in between rock and creates a new crack. Other times, the magma takes the path of least resistance and penetrates preexisting cracks or faults.

It appears the magma this time is working through an old crack that ran below the town of Grindavik. Aerial photography from 1957 confirms the existence of such a fault. That is why Grindavik is basically being cracked apart.

The dike intrusion is about 10 miles long and runs from northeast to southwest. It terminates beneath the ocean. If and when an eruption occurs, it will likely be along the dike intrusion. A fissure will most probably open up and magma, which will become lava when it reaches the surface, will ooze out.

This is not the type of volcano that will erupt explosively and pour a plume into the air. Officials at Keflavík International Airport are monitoring the situation, but widespread disruption to air travel is not expected.

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