When I was young, like many children, I used to love building sandcastles and sand structures on the beach. One of my favorite games I played was to build a sand pyramid and see how long I could get it to stay up in the face of a rising tide. Typically, I would devise a complex system of moats and barriers to try to prevent the waves from washing away my creation, although I was never able to win against nature.
I mention this because it is a close analogy to how many volcanoes work. Volcanoes have a tendency to be somewhat cyclical. If you build them high enough, after a certain point, they will come back down.
Volcanoes tend to be somewhat cylical. That cycle often starts small, but the end of each cycle often involves the destruction or partial collapse of a volcanic cone. We will be discussing how height and size influence collapse in this post.
How Volcanoes Collapse
Believe it or not, when a volcano “comes down”, It’s not always violent (although most of the largest geologically recent eruptions involve some form of collapse). That’s because one of the major causes of volcanoes collapsing is a result of simple erosion, which occurs at an extremely gradual rate.
Nonetheless, volcanoes can collapse in a few different ways:
- Caldera Collapse: Caldera Collapse occurs when a volcano empties out it’s magma chamber to such a large degree, that the roof above collapses into the space vacated after an eruption.
- Flank Collapse: Flank Collapse occurs when a volcano’s cone becomes destabilized, causing a portion to break off from the slope, leading to a potentially massive landslide.
- Slumping: Slumping occurs when a volcano slowly compresses back into the ground, growing shorter and wider due to gravity.
- Erosional Weathering: Erosion is constantly occurring at every volcano, but the erosional rate can increase through glaciation, heavy rain or vegetation, or other factors.
How Height and Size Influence Flank Collapse Events
Much like the sandcastles I would build when growing up, the height and size of a volcano has a strong influence on a potential collapse event. In short, the bigger a volcano is, the more structurally unstable it tends to become.
How Height and Size Influence Flank Collapse Events:
Flank collapse events are the most straightforward. Using our sand example, If you were trying to make a gigantic sand-mountain out of wet sand, you would shovel it into a pile and keep going. At some point or another the pile would start to either slump, or one of the sides would shear off and flow down to the base of the pile.
You would eventually find that there is a limit to how high it could be built, as the repeated collapses would limit the total height. Many volcanoes behave in a similar way, except on a much larger scale.
As a volcano grows taller, its mass grows at a roughly exponential rate. The sheer size and weight of the cone itself actually can cause the surrounding land to compress and deform slightly, especially if there is a large magma chamber below. The growing mass of the edifice eventually puts too much strain on the sides of a volcano, causing one of the flanks to give out.
Other Influences in Volcanic Flank Collapse and Landslide Events
It’s not all about the height. If height or prominence were the only factors, Kilamanjaro at its elevation of 5,895 meters should have had a major edifice collapse event. While there may be some minor collapses that have occurred here, the fact remains that Kilamanjaro is still absolutely enormous, and has not come crumbling down.
Slope is a Major Factor
This is also fairly easy to understand: a volcano’s slope is a big factor in whether it can stay upright for a long period of time. The steeper a volcano is, the less likely it will be able to grow significantly tall before crashing back to earth.
Shield volcanoes don’t seem to collapse as often as stratovolcanoes, because their slopes can be extremely gradual, leading to a stronger overall edifice. Despite this, even shield volcanoes will experience flank collapse events. The shield volcanoes of the Canary Islands are riddled with collapse scarps, Etna had a major collapse into the mediterranean at one point in the distant past, and the Hawaiian islands have experienced multiple collapse events of varying sizes over their very long history.
For a more detailed analysis on slope, I suggest reading the Wikipedia article on Angle of Repose
How Magma Composition Affects Volcanic Slope
Since slope is so relevant to whether a collapse event can occur, it’s important to understand the relationship between slope and the type of magma erupted at a given volcano. The more sticky, or viscous a magma is, the steeper an edifice will tend to be, and the more explosive eruptions tend to be (although there are many exceptions to these rules).
- Basaltic magmas, which tends to be very runny typically creates shield volcanoes with a very low profile (think Hawaii or Iceland).
- Andesitic magmas, which are common in many subduction arcs seem to create very tall volcanoes with a traditional cone profile that are prone to larger collapse events.
- Dacitic Magmas tend to create steep blocky volcanoes that are prone to mutliple flank collapse events (such as Mt. St. Helens). Many dacitic volcanoes never experience proper flank collapses since the eruptions themselves are explosive enough that a proper cone never truly forms.
- Rhyolitic Magmas rarely form traditional cones since they are simply too viscious to flow out of the ground in a manner that would form a cone. The explosive nature of Rhyoltiic magma makes it more closely associated with caldera collapse.
Of these types of volcanoes, Basaltic volcanoes tend to have the highest potential to grow to large heights, although they often grow extinct before ever doing so. Kilamanjaro may be a current example of this. This is because the volume of magma to grow taller becomes greater and greater the larger the volcano becomes, so eventually the volcano will die, or the deep magma source will move on to a nearby location before the old volcano can grow any taller.
Another relevant factor is that at a certain point, the amount of erosion on a volcano will exceed the height gained by adding new magma, which sets a limit to the size a volcano can grow if no collapse event were to ever occur.
Structural Strength Matters For Volcanic Landslides
A stronger structure can grow taller before it topples over. For volcanoes, there are a few factors that can influence strength.
- Water Saturation: Water causes strength issues in a volcano, and the more that a volcano is affected by the water table or aquifers, the more there is potential for structural weakness. Water also flashes to steam when it is heated, so any new magma rising can cause increased damage. Alternatively, in cold environments, water freezing can cause expansion, which leads to more structural problems.
- Erosion and Glaciation Create Weakness: A big glacier digging a steep valley right through a flank of a volcano will not only slowly eat away at the size of said volcano, it will also increase the weakness by removing vital supporting rock. If that glacier melts, this becomes an even bigger problem as there will be nothing to support the surrounding rock.
- Bedrock Matters: If a volcano is built on a bed of silt or loose rock, this will make it easier for the above-ground cone to slip out from underneath. Think of this as the Leaning Tower of Piza principle – don’t a tall structure on unstable ground.
Edifice Size and Caldera Collapse Events
Caldera collapse events can be rather complex, and while there is a lot of research on them, we have studied precious few actual caldera collapse events in written history.
With that said, there are some ideas out there that mention influence of edifice size on a potential caldera event. These events likely do not apply for Rhyolitic volcanoes such as Taupo, since they never build much of an edifice in the first place.
Potential Influences of Height and Mass on Caldera Formation
Increased Edifice Size Leads to Increased Pressure in a Magma Chamber: As a volcano erupts over time, a central conduit tends to form within the developing cone. Conduits are not particularly wide, and typically the magma will harden between eruptions. In a small cone, it’s easy for the magma to clear out the hardened old magma and push out the top. But the larger a volcano becomes, the longer the conduit grows, making it more and more difficult for magma to reach the surface between eruptions.
This process of increased difficulty for magmas to access the surface can lead to increased growth in the magma chamber and a higher tolerance for high pressure. These factors lead to larger and more explosive eruptions, and also lead to magma trying to find alternative pathways to the surface. This can potentially lead to weakening and faulting of the edifice, which can further influence a collapse or caldera type event.
The big takeaway here, is that additional risk should be considered for volcanoes that are especially tall and steep for collapse type events. This is especially true if many surrounding volcanoes experienced similar events in their history.
While not all collapse events are violent, the majority of them that occur can cause a lot of problems, leading to big eruptions or massive landslides.
Some Sources I Used: