An animation of a tiny volcano trying to impress its larger peers with an eruption seems like a flight of fancy, but it's really a hidden lesson in volcano morphology. The difference parallels the distinction between tiny-yet-enthusiastic cinder cones and intimidating stratovolcanoes .

In the animation, a trio of massive brown volcanoes burble smugly away to themselves, when a tiny blue volcano of the same basic shape pops out to impress them with an exuberant eruption. It's an adorable animation of personified landscapes, but what about the geoscience?

VOLCANO credit: Julien Loth.

The shape and size of a volcano is directly related to its chemical makeup. Specifically, how much silica is in the molten magma (yet-to-be-erupted subterranean lava) directly impacts the fluid viscosity. That viscosity in turn controls how gas will or will not be trapped within the magma, thus how explosive the eruptions will be. The viscosity also impacts how the lava will flow and the shapes it takes as it cools, determining the geomorphology of the entire volcano.


This is going to be a bit hand-wavy oversimplified, but oceanic plates produce low-silica mafic magma, while continental plates produce high-silica felsic magma. Where an oceanic plate subducts under a continental plate, the mafic magma from the melting oceanic plate picks up silica as it rises through the continental plate, and gets classified as a creatively-named intermediate magma.

The low-silica mafic magmas produce hot, runny lavas that don't trap much gas. They have gentle, effusive eruptions, forming broad, rounded shield volcanoes. The classic example is the Hawaiian Island chain, each island a massive volcano built from countless eruptions. Mauna Kea stretches more than 9,000 meters from its base on the sea floor to its summit, yet it certainly doesn't give off the impression of being taller than Everest. Part of that is because all but 4,000 meters hide below the waves, but a large chunk is because the volcano slopes so gently that no one would consider it an epic mountain.

Mauna Kea is a massive shield volcano. Image credit: Nula666

High-silica felsic magmas produce cooler, thicker lavas, a low-viscosity gas-trap that erupts explosively. But if silica-rich magmas are so explosive, why isn't the center of every continent loaded with explosive horrors? Volcanoes don't pop up just anywhere. It's a common misunderstanding that the mantle is an ocean of molten magma, but it isn't. The mantle is plastic, ductile rock, but needs a bit extra to actually melt. This happens at mid-ocean ridges where a reduction in pressure, or in subduction zones where added water content changes the melting temperature.


We already covered mid-ocean ridges, so what about the subduction zones? That's where one tectonic plate dives under another (whether from being dragged by its own weight, or pushed by distant ridges is a question open to debate). The mafic magma of the melting plate picks up silica as it rises through the felsic continental plate, producing intermediate magma. These erupt as violent, explosive stratovolcanoes with a classic conical shape like Mount Saint Helens, Mount Fuji, or Krakatoa.

Mount Fuji is a beautiful stratovolcano. Image credit: Mount Fuji

The massive, chuckling brown volcanoes in the sketch are tall, beautiful conical volcanoes: stratovolcanoes. But what about the tiny blue volcano with so much to prove? It has the conical structure of a stratovolcano, but not the size or explosive power. It's a cinder cone, a small steep hill of volcanic fragments. They cling to the vent systems of bigger volcanoes, building up cones of debris tens to hundreds of meters tall.

Capulin Mountain is a relatively towering cinder cone in New Mexico. Image credit: R.D. Miller/USGS

In the animation, the small blue volcano trying so hard to impress its larger peers is a little cinder volcano, full of fragments and good intentions without the capacity for large-scale eruptions. The towering symmetrical brown cones are stratovolcanoes, monsters capable of deadly, spectacular eruptions. From that perspective, I rather appreciate that it's the cinder cone that politely erupts in effervescent bubbles, not the stratovolcanoes drenching the whole scene in ash, lava, and pyroclastic flows.