What Is A Composite Volcano: The Truth About Composite Volcanoes

Volcanoes are a constant threat to humanity. Even the smallest eruption can have devastating consequences, so it’s important for people to understand how volcanoes work and what to look out for. There is one type of volcano that has recently received some attention from the media – Composite Volcanoes. 

These geologic formations are made primarily of non-volcanic material (mostly limestone or sandstone) instead of molten magma like traditional volcanoes. They appear much less threatening than their explosive counterparts, but there are many rumors about composite volcanoes that aren’t exactly true. In this article, we will explore what is a composite volcano and discuss everything you need to know about the unique features of these intriguing natural wonders.

What Is A Composite Volcano?

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At the most basic level, composite volcanoes are a type of geologic formation that is made entirely of non-volcanic material. The “composite” part of the name refers to the fact that these mountains are generally comprised of multiple different types of rock. Most composite volcanoes are built from a combination of sedimentary rock types including limestone, sandstone, and shale. 

The molten magma found in traditional volcanoes is usually absent, although some composite volcanoes do have pockets of molten rock in their composition. There are several different types of composite volcanoes, including: 

Crater volcanoes: These are the most common type of composite volcano. Crater volcanoes are typically conical in shape and often have an identifiable central crater (hence the name).

Shield volcanoes: These massive formations occur when thick layers of lava repeatedly flow out of a central vent. The lava cools as it travels, forming a thin, gently sloping plateau. 

Composite volcanoes: These are generally made of alternating layers of lava and ash topped with a thick cap of sedimentary rock. 

Central volcanoes: When multiple volcanoes erupt simultaneously within a shared caldera, the result is a central volcano.

The Basics of a Composite Volcano

The most important thing to remember about composite volcanoes is that a majority of them are not volcanoes at all. The term “volcano” is commonly used to describe any area of the planet where molten rock is present, but the majority of these formations aren’t active enough to be considered volcanoes. This can be confusing for many because the geologic definition of a volcano only applies to active volcanoes, but popular terminology often lumps everything together. 

A composite volcano can refer to either a geological feature made up mostly of non-volcanic material or an active caldera where multiple volcanic eruptions occur simultaneously. A caldera is formed when the roof of a magma chamber collapses, creating a large depression (also known as a volcanic crater). Calderas can be formed by a single volcano or multiple volcanoes erupting simultaneously (such as in a central volcano).

Why Are They Called Composite Volcanoes?

As we discussed earlier in the article, most composite volcanoes aren’t traditional volcanoes at all. This raises the question, why are these non-volcanic geologic formations called volcanoes at all? The simple answer is that we call these formations “volcanoes” because they look like volcanoes. 

Even though they aren’t made with molten magma, the majority of composite volcanoes are conical in shape with a central crater. In short, they resemble the traditional volcanoes we are all familiar with. Scientists simply call these formations volcanoes because they assume they function like volcanoes. They aren’t wrong, but it isn’t exactly accurate either.

Things You Should Know About Composite Volcanoes

1. They Are Not Explosive 

One of the most common misconceptions about composite volcanoes is that they are explosive. This isn’t true for the vast majority of these geologic formations, but it is often assumed that the non-volcanic material in their composition will cause them to erupt violently. 

Scientists use the term “dormant” to describe a volcano that is unlikely to erupt in the near future but could become active again at any time. This can make it seem as though a volcano is harmless when it is actually extremely dangerous.

2. They Aren’t Always Active 

Just because these formations are called “volcanoes” doesn’t mean they are always active. Some composite volcanoes do have pockets of molten rock in their composition, but these areas are usually dormant and don’t pose a significant threat to humans. 

The non-volcanic material in a composite volcano doesn’t change the fact that the volcano’s caldera is made of porous rock that allows molten magma to pass through. Because of this, there are many instances where composite volcanoes have erupted with as much force as traditional volcanoes. 

3. Their Ash Clouds Tend to Be Very Fine 

Since the ash clouds emitted by composite volcanoes aren’t formed by molten magma, they are much finer than those of traditional volcanoes. This has important implications for the fallout of an eruption. Traditional volcanoes produce ash clouds that are heavy enough to collect on the ground nearby. 

If a traditional volcano is close enough to your home, the results can be devastating. With the lighter ash of a composite volcano, the consequences of an eruption are much less serious. The ash is easily dispersed by the wind, so it is unlikely to impact a populated area. Although it is important to recognize the dangers of ash clouds, they aren’t as dangerous as people often assume.

4. They Typically Erupt With Shifting Phases

Another interesting aspect of composite volcanoes is that they typically erupt with shifting phases. Because the material is a composite volcano and isn’t molten magma, the composition of each eruption is likely to be different. A single eruption may consist primarily of fine ash, but the next eruption could be comprised of layers of lava. 

And the eruption after that may be a combination of both materials. The abundance of each type of material could also change over time, making it difficult to predict what will happen next. The variation in composition is likely due to the fact that the porous rock within the volcano is pressurized. As new magma flows into the volcano, it pushes the existing materials out of the way and makes room for itself.


Composite volcanoes are fascinating geological formations that are often misunderstood. The non-volcanic composition of these mountains makes them less likely to erupt violently, but it also means that they won’t produce as much dramatic lava. These formations are still extremely dangerous, but they have a fascinating story to tell.

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