Nearly all forms of life that we know of are dependent on water – without water, there is no life. It’s with this thought that there comes a certain irony that water is perhaps the most relevant ingredient that can influence how deadly a volcano is.
Most casual observers would likely think that volcanoes; mountains of rock and fire would have very little to do with water. As many here would know, this couldn’t be further from the truth. Water not only plays a role in how volcanoes behave and form, it also can have profound effects on eruptions themselves, often being the singular difference between a benign eruption and a major disaster.
How Water Helps to Generate Magma
First and foremost, water is one of the biggest reasons we have major volcanic arcs around the world. Without water, volcanoes such as Mt. St. Helens would not exist. Volcanic arcs such as the volcanoes we see scattered across the Pacific rim and Indian Ocean likely would never exist either.
Water acts as a melting catalyst on sunken oceanic sea-floor
When the oceanic floor is pushed deep beneath a tectonic plate, this pushes all the water that is saturated in that plate deep down into the very hot mantle. Believe it or not, the mantle is not made of magma, so even at depths of 200 kilometers deep, the rock is not molten despite extremely hot temperatures.
This is due to the influence of pressure on melting and phase change. Higher pressure acts to prevent melting, even if the temperature is extremely hot. The more pressure rock is under, the more heat you need to get it to melt. But this all changes when you add water into the equation.
Water acts as a catalyst due to some fancy chemical interactions. These chemical interaction help the rock melt at a significantly lower temperature than it normally would melt at. Since the sea-floor that is being subducted is saturated with water, the rock within that slab will start to melt once it reaches the depth range of around 100-120 kilometers below the crust, which results in the production of magma and the birth of a volcanic arc.
How Water Affects the Explosivity of Eruptions
Steam is a uniquely explosive gas. When water transitions from it’s normal state into steam, it will expand by 1700 times its original volume at normal atmospheric pressures. In our day-to-day world, this isn’t so impressive since we rarely get to see that steam expand all at once. When we boil water on the stove, it will slowly evaporate into the air around us. We don’t get to witness the true power of steam until we bottle it up. The video below is a good example of a steam explosion, in this example coming from a critically heated hot water tank.
As we can see, water can be incredibly explosive when confined, and heated to a point that it would flash to steam. This is more or less what happens at many volcanoes that happen to have a lot of water vapor mixed into the magma composition.
The more water a volcano has mixed into its magma, the more likely it is for an eruption to behave in a more explosive fashion.
If you’ve ever wondered why a volcano like Kilauea doesn’t ever seem to erupt in a violent fashion, one of the major reasons is that there is very little water in the magma that comes up from depth to feed the Hawaiian islands. In Hawaii’s example, this is due to the fact that the magma source does not come from a melted sea-floor, but rather from a deep magma plume that is unrelated to water processes.
Magma Interaction with Water Close to the Surface
While we’re on the topic of Hawaii, it may be worth mentioning that there *have* been some explosive eruptions there in the past, they’re just not the normal expectation. But when they do occur, they seem to be heavily related to you guessed it… water.
In the picture provided, the infamous Diamond Head crater came about as a result of non-explosive magma interacting with surface groundwater many thousands of years ago. When the magma worked its way into the crustal water table, this caused the water to flash to steam extremely fast, creating an explosive eruption that created the diamond head crater.
Phreatic Eruptions – Water Influenced Volcanic Explosions
This style of event is not however unique to Hawaii. Known as as phreatic eruption, these types of events occur when water that is trapped within acquifers, channels, and chambers of the bedrock becomes so hot, it turns to steam. Once this happens, the pressure rapidly builds until the pressure can be relieved via explosion.
We see a similar type of action all the time at Yellowstone National Park, where Geysers erupt as the result of supercritically heated water releasing vapor. The only thing that prevents these geysers from becoming significantly larger explosions is the fact that the water system is not confined. With room for the pressure to escape to the surface via the geysers, pressure does not build to the level that would create a large explosion.
Phreatomagmatic vs. Phreatic Eruptions
While it’s a miniscule difference, there is another class of eruption similar to purely phreatic eruptions known as phreatomagmatic eruptions. In a phreatic eruption, there is no direct eruption of magma or lava above ground. The eruption occurs strictly as a result of supercritically heated steam causing an explosion. This explosion will typically mobilize the surrounding rock, but no new magma will reach the surface.
Phreatomagmatic eruptions on the other hand include a steam-induced explosion along with fresh magma being erupted. Phreatomagmatic eruptions are common occurrences when a volcano erupts through a water table. When a volcano erupts to the surface through the shallow ocean, this can be further categorized as a Surtseyan eruption.
Risks with Phreatic Eruptions
Phreatic eruptions may not involve hot magma being erupted, but they are often more dangerous than our more traditional magmatic eruptions. Not only can these eruptions be extremely explosive, they also can occur without much advanced warning, catching those nearby completely off guard.
A recent tragic example of this is the Mt. Ontake eruption tragedy in 2014, which killed 57 hikers who were near the summit of Japan’s Mt. Ontake when it experienced a phreatic eruption that nobody had known would occur. (see video below, taken by hikers during the tragic event)
Can Water be a Catalyst for a Major Eruption?
There is some speculation that water was one of the major reasons the Krakatoa eruption was such a large even in the late 1800’s. Krakatoa. The predominant thought is that during the eruption, at some point, the island that the Krakatoa volcano was sitting on destabilized, allowing water to invade the primary magma chamber of the volcano. This set off a monster of an explosion the likes of which we have never seen in the modern era, resulting in the loudest known noise humans have ever been around to record.
There is now some dispute over whether water was in fact the true reason the magma chamber at Krakatoa was destroyed (it may have been heading that way regardless), but we do know that it probably didn’t make the eruption more benign.
There are other potential examples of water resulting in enormous eruptions. In the far distant geological past, when Iceland was barely peeking out of the the young atlantic ocean, there may have been a series of VEI-8 sized eruptions that were a product of interaction of magma and the shallow oceans mixing together.
The Minoan eruption of Santorini also likely had a water-related component that could have increased the scale of the eruption at that time.
Other Ways Water Can Interact with Volcanic Eruptions to Cause Increased Destruction
First and foremost, lahars are at the top of every volcanologist’s mind when they are considering risk mitigation. Lahars are volcanic mudflows that come when water mixes with volcanic ash and debris, causing massive flash-flooding that can wipe out wide swaths of land.
The biggest volcanic tragedy of the last 50 years occurred due to Lahars that came from an eruption of the Nevado del Ruiz volcano in Colombia, which caused a glacier on top of the volcano to rapidly melt and turn into a gigantic mudflow. This resulted in the burying of the city known as Armero in 1984, killing over 20,000 people.
Lahars can occur at any volcano, but volcanoes that have large glaciers on top or on the flanks along with populated areas in the nearby valleys are especially at risk for tragedy.
Most people know tsunamis can occur from earthquakes, and to a lesser extent landslides. But we haven’t seen volcanically induced tsunamis in the modern era (thankfully). Tsunamis occur when large volumes of water becomes displaced temporarily, resulting in massive waves that travel a far distance.
Volcanoes can do a LOT of different things that can cause the displacement of lots of water in a short period of time. Mt. Unzen in Japan experienced a flank collapse, which caused a landslide into the nearby sea, resulting in a tsunami that killed over 15,000 people in 1792 A.D. Krakatoa’s main killing instrument was not the eruption itself, but rather the tsunami that was generated when the caldera collapse event occurred.
Other volcanoes have formed major tsunamis simply from pyroclastic flows landing in the ocean and displacing the water when the hot ash and material enters the water.
Pyroclastic Flow Enablement
Not as many people realize that water can make pyroclastic flows even more dangerous. Interestingly enough, pyroclastic flows will travel farther and faster over water than they do over dry land.
This can make eruptions near lakes and shores especially dangerous as it increases the radius in which a pyroclastic flow can travel.
Conclusions and Thoughts
Overall, water can have profound effects on volcanoes. It affects how magma is generated, is one of the primary reasons many volcanoes form in the first place, and can greatly contribute to increased destructive potential.
Volcanoes that are close to water sources (including those with overlying glaciers) need extra consideration when thinking about mitigation plans and potential outcomes in the event of an eruption.
One thing that is obvious is that water rarely ever reduces the power of an eruption. The only case where that may happen is during deep under-sea eruptions, where the overlying ocean prevents the eruption from ever reaching the surface.
Please feel free to comment and leave your thoughts. Thanks!
Greg S (Cbus05)
Misc sources and information for further reading