I Lava You
Following the tradition of Disney films produced by Pixar Animation Studios, the recent 2015 release of Inside Out was accompanied by a short video simply called Lava. Here it is:
This video is a (rather corny yet incredibly cute) song telling the story of a male pacific island volcano singing songs to a female underwater island volcano. This video caused quite a controversy regarding the depiction of male versus female characters and related matters. However, not enough attention was given to the actual background story here. Those are volcanoes! Science! Geology! Exploding stuff! Surprisingly, this video is rather scientifically accurate.
The majority of Earth’s volcanic activity occurs in the oceans, underwater. A significant portion of that activity occurs in the Pacific Ocean (ironically, as pacific means peaceful). Where is all of the magma coming from? Contrary to the common conception of the general public, most of the Earth’s interior is solid rock rather than molten. There are several types of anomalies deep in the Earth that cause rock to melt. The two types of volcanoes that form in the Pacific Ocean can be divided into two: hot-spot volcanoes and arc volcanoes. This terminology fits their geographical distribution. Here’s a map of the distribution of active subaerial (that is, not underwater) volcanoes:
Notice the “ring” of volcanoes around the Pacific Ocean. Each segment of the ring is arcuate, thus the name arc volcanoes. Also notice the several “spots” of volcanoes far from the arcs – those are usually the hot-spot volcanoes. Hawaii is a wonderful example of such hot-spot volcanism. Hot-spot volcanoes form rather simply: a thermal anomaly deep in the Earth causes rocks to heat up and melt. This molten rock is less dense than the surrounding solid rock, so it flows upward to the surface. Since most of the Earth’s surface is covered by oceans, it turns out that a rather large proportion of hot-spot volcanoes erupt underwater. Arc volcanism is slightly more complex. Basically, wet ocean floor is being subducted (that is, pushed down in the Earth) usually together with devastating earthquakes. Once down, the water is released from the ocean floor into the surrounding rocks and melts them. This may be counter-intuitive: we usually think of water as something that puts fire out, not something that melts rocks. Think about it like that: melting pure water ice requires 0 °C and melting pure table salt requires more than 800 °C. However, add one to another and the mixture melts at a temperature lower than 0 °C. This is why authorities add salt to ice on roads during winter – to melt it away even though it’s not actually hotter than 0 °C. The same principle works in the deep Earth. Let’s say that melting a rock requires 1200 °C but the ambient temperature is only 900 °C. By adding water to the rock, the melting point of the mixture goes down below 900 °C and you get magma. The trick here is that it happens deep in the Earth – the water can’t escape or evaporate. For more information on this and some beautiful diagrams to help you visualise this process you might want to check out this excellent post on Georneys.
Back to our video now. The entire video is rather Hawaiian-themed and the volcanic process that explains it is appropriately of the hot-spot type and not the arc-type. The video starts with an erupting (and singing) volcano above the ocean. One of the amazing things about such volcanoes that more often than not gets under-appreciated is the scale of this. Those things are HUGE. Think of Hawaii’s big island, hosting Mauna Loa and Mauna Kea. That mountain is about 4 km high above the sea. That’s high, but not too impressive. But considering that the sea floor is more than 5 km deep in this area, you end up having a mountain that’s over 9 km high. That’s higher than Mount Everest! Here’s a simple illustration of that:
Here’s the key point: while the oceanic crust slowly moves (imagine ice floating on the ocean), the hot-spot is mostly fixed. Eventually, the volcano will move away from the hot spot and go extinct. This is exactly what happens at around 1:35 in the video. The volcano is not as high as it used to be though. There are two reasons for that. Firstly, once the volcano is not above the hot-spot, the crust below is cooler. You all know that when things cool down, they contract. When those “things” are hundreds of cubic kilometres of rock, the contraction is sufficient to lower the elevation of entire volcanoes. The second reason is erosion: ocean waves and wind break down the rocks and the extinct volcano crumbles into the sea around it. Again, the Hawaii island chain is an excellent example of it:
Another prominent feature of the extinct volcano is the ring of little islets around it. As usual in this post, there are two reasons why would that could happen. The first one is the formation of a caldera. Some eruptions are extremely explosive and erupt such an enormous amount of magma that the end result is a huge hole in the Earth. Subsequent formation of a smaller volcano in the middle of that hole and filling everything with water results in something that looks like this:
If you make the inner island a bit higher and the cliffs around it slightly lower, this will not look too different from the appearance of the volcano in the video. Even though impressive, this is unlikely to form in hot spot volcano islands. A less explosive process is the formation of a coral reef. Here’s how it looks like:
The formation of this is rather simple. The shallow water just off the beach are particularly suitable for the formation of coral reefs that keep growing while the volcano subsides. The Wikipedia page on coral reefs has excellent diagrams and animations to illustrate the process.
At around the 2:05 mark of the video, we see an underwater volcano. Remember our moving plate? This is what happens when deep sea floor moves over a hot spot. A new volcano grows! Although this process was known for quite a while, only recently scientists have been able to actually observe this process. Here’s an amazing video, the first ever recorded of such an underwater eruption:
A similar process is occurring now in Hawaii (where else?).
Loihi is an underwater volcano (called a seamount in the know), feeding from the hot spot. It may be the case that soon (geologically speaking), Mauna Loa will go extinct and Loihi will rise above the sea to form a new island.
This is where the (somewhat) scientifically accurate bit of the video ends. Unfortunately, an extinct volcano that’s so old it is completely underwater (also known as a guyot) is not going to magically erupt again above the sea. However, since this is a Disney video, it has to have a happy end or else the kids will cry.