Hotspot volcanoes

the internal “magmatic filter” that causes the eruption of oceanic island volcanoes

The volcanoes we see on the Earth’s surface are just the tip of the iceberg. Below the surface, they are fed by a complex network of conduits and reservoirs that bring molten rock, called magma, back to the surface.

When the magma erupts, it can generate lava flows that cool down to become volcanic rocks. These rocks contain key clues to the inner workings of volcanoes and what triggered them in the past. But decoding these clues is a confusing task.

Our new research, published in the journal Géologie, reveals information previously hidden in the chemistry of erupting lavas. Oddly enough, we have found that many volcanoes have an internal “filter” that causes them to erupt.

If we can detect the magma at this crucial tipping point inside the volcano, it might even help us detect when an eruption is imminent.

Hotspot volcanoes

Most volcanoes, such as those in the Pacific Ring of Fire and the Mid Atlantic, lie on the border between tectonic plates. But some volcanoes, including those that created the Hawaiian Islands, occur where hot plumes from the depths of the Earth reach the surface. These are known as “hotspot” volcanoes.

Australia hosts the longest track hotspot volcanoes in a continental setting. For tens of millions of years, volcanoes such as the Glass House Mountains in Queensland, or Wollumbin (Mount Warning) in New South Wales, have followed the movement of the Australian mainland over a stationary hot spot.

In the oceans, hotspots build chains of paradisiacal islands like Hawaii, the Galapagos or the Canary Islands. These oceanic island volcanoes were previously thought to be made up of magma from tens of kilometers below the surface, deep in the Earth’s mantle.

But our new research suggests that volcanoes on oceanic islands may erupt from magma that has been filtered and altered to a shallower depth.

Rich in crystal, not crystal clear

Volcanic lava often contains crystals from the interior of the volcano, which have been mixed with the erupting magma. Crystals tell us a lot about the interior of the volcano, but they can also disguise the chemistry of the lava itself.

Read More: Volcano Crystals Could Help Predict Eruptions

Think of it like chocolate from the rocky road. If we want to analyze the ingredients of the chocolate itself, we must first ignore the marshmallows and nuts.

Microscopic image of crystals in magma.
Author provided

We can do this by analyzing rocks made from lava without crystals. In our study, we compared the crystal-free, crystal-rich magmas of El Hierro volcano in the Canary Islands, which last erupted in 2011.

It turns out that the crystalless magma of these volcanoes is very similar across millions of years of volcanic activity and across many oceanic volcanoes around the world, including the Canary Islands and Hawaii. This is how we realized that the magma was not virgin and came directly from great depths, but rather filtered at shallower depths.

And if the magma of the hotspot’s island volcanoes is so similar, it’s likely that their eruptions are triggered by common mechanisms as well.

The “secret volcano filter”

When crystals form inside the volcano, it “steals” chemical elements from the magma. In turn, this changes the composition of the remaining magma, almost as if it had been passed through a sieve.

This filtering process makes the magma less dense and increases its gas content. This gas can then bubble and propel the magma to the surface, just like the cork of a bottle of champagne.

In the volcanoes of oceanic islands, magma can reach this “tipping point” at the base of the earth’s crust, just a few miles below the surface, rather than at depth. This means that if we detect magma at that depth using earthquake monitoring equipment, an eruption could follow. This is exactly what happened when El Hierro erupted in 2011.

El Hierro underwater eruption
Volcanic activity under the sea during the eruption of El Hierro in 2011.
Spanish Civil Guard / AAP

Does this make it easier to predict rashes?

If we could open a volcano like a dollhouse, we would be able to follow the movement of magma towards the surface. It is a pity that we cannot, although we may try to “see” this trip indirectly, by surveillance earthquakes, deformations and gas emissions, all of which can indicate the rise of magma inside a volcano.

But to assess whether a volcano is likely to erupt, or if a dormant volcano is awakening, we also need to compare current observations with information about what triggered the eruptions in the pass.

This is where our new discovery could prove particularly useful. While eruption triggers occur at similar depths in volcanoes in oceanic islands around the world, warning signs of those depths can be especially important to watch out for and consider as early warning signs of an eruption.

Read more: Australia’s volcanic history is much more recent than you might think

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