You need this background to the recent news that, at Havre Seamount near New Zealand, scientists have found the largest deep-water silicic eruption in history. Some cool videos of eruptions are included, too.

Thanks to plate tectonics, almost three-quarters of Earth’s lava is erupted in the oceans, at mid-ocean ridges, not on the land where we can see it.

These spreading ridges, as they are called, are really a high volcanic mountain range rising from the abyssal depths and running down the middle of just about every ocean on the planet.

Most of that lava and rock is mafic – a word coined from the chemical symbols for magnesium and iron, which enrich this type of magma and make the resulting rock dark.

This runny red stuff usually erupts quietly as pillow lava, but in relatively shallow water, mixing molten rock and good old H₂O can be spectacularly explosive.

That is the 1963 Surtsey eruption, off Iceland’s southern coast. This volcano sits in the middle of the Atlantic, but since it’s on a coastal shelf (and therefore atop the Mid-Atlantic Ridge, along with Iceland), ocean depth there is measured in hundreds of feet, not miles.

Even so, scientists would have had a hard time studying the eruption if it hadn’t broken through to the air.

Surtsey is made out of mafic rock. Another type of dramatic volcano comes from silicic magma – it contains a lot of silica instead of magnesium and iron.

Usually you find silicic volcanoes near the coast, as in the High Cascades of the US Pacific Northwest. They form in such places because, offshore, a seafloor plate is sinking down into the depths of the Earth and the continental plate is riding over it.

Of course, it’s hot down there in the mantle. Not only that, seawater is mixed in with the sinking rock – this lowers the melting temperature and causes chemical reactions that, among other things, make the silicic magma very sticky.

Volcanoes form wherever this magma reaches the surface near the trench made by the sinking plate edge.

The eruption may be explosive, as when gases build up a lot of pressure. Without so much gas, it can be nonexplosive but gooey, as in the Mount St. Helens eruption that began in 2004.

Here is a USGS closeup view of how Mount St. Helens rebuilt itself between 2004 and 2008:

Sometimes subduction (one tectonic plate sinking underneath another) happens far from land.

Volcanologists know this, but they haven’t been able to learn much about the resulting silicic volcanoes, since it all happens in deep water, which is technically defined as more than 500 meters, or over a third of a mile, below the surface.

The pressure of water at those depths is enough to prevent Surtsey-like blow-ups, so evidence of these eruptions usually goes unseen.

In 2012, though, volcanologists were able to trace a raft of pumice that had been sighted by satellite – see the image at the top of the page – back to Havre Seamount in the general vicinity of the Kermadec Islands.

That in itself was big news, but now technology has also enabled volcanologists to visit the volcano.

That is what all the fuss is about today!

Here is the original University of Tasmania fly-through used by Scientific American in the above video:

As of this writing, the Smithsonian doesn’t have a picture of Havre for the volcano’s GVP page. That’s going to change soon, thanks to this major volcanological success!

Sources:
Carey, R.; Adam-Soule, S.; Manga, M.; White, J. D. L.; and others. 2018. The largest deep-ocean silicic volcanic eruption of the past century. Science Advances. 4(1):e1701121.

Jakobsson, S. P.; Thors, K.; Vésteinsson, A. T.; and Ásbjörnsdóttir. 2009. Some aspects of the seafloor morphology at Surtsey volcano: The new multibeam bathymetric survey of 2007. Surtsey Research. 12:9-20.