6.3A| Types of Volcanic Eruptions

Quiet vs. Explosive Eruptions 

Hawaii's Kilauea Volcano has erupted regularly since it was first observed by Westerners in the early 1800s. These "quiet" and effusive eruptions can be spectacular but rarely result in casualties. On the other hand, explosive eruptions have necessitated the evacuation of millions of people and have killed tens of thousands.  

Instructions

Watch the two videos below. The first is included to demonstrate an effusive or "quiet" eruption. The second is a documentary about Mt. St. Helens, US history's most famous explosive eruption. After watching both, brainstorm a list of differences between effusive and explosive eruptions. Doing so will prepare you for important concepts tied to the learning objectives of this module. 

Read the text, examine the photographs, and study the table. Connect the content to what you observed in the videos. Read and answer the questions that follow. You have three attempts.  

Note: If you find that you need extra support in accessing any items in this quiz, please notify me via email right away so I can offer you an alternative.

Time Needed

1-2 hours.

Learning Objectives

• Students can describe the geologic processes, the relative movement of plates, and the distinguishing landforms associated with each type of plate boundary 
• Students can describe the eruptive and physical characteristics of different types of volcanoes
• Students can relate tectonic stresses to plate boundaries, geologic structures, crustal deformation, and the building of mountains
• Students can describe the evolution of landforms and geologic structures in the context of constructive and/or destructive geologic processes
• Describe the intrusive and extrusive rocks associated with each volcano
• Use texture to interpret the cooling history and origin of rock specimens

Video of Mount Etna erupting.

Note: there is no narration in this video, only the sound of the volcano erupting.   

On the other hand, some volcanoes are capable of explosive eruptions, erupting with force many times that of a nuclear bomb. For example, the recent Tonga volcano eruption exploded with a force of 10 megatons of TNT, or about 500 times more powerful than the atomic bomb dropped on Hiroshima! The most famous explosive eruption in North America is the 1980 eruption of Mount St Helens. Watch the following video for an excellent summary of this eruption.  

Mt. St. Helens Documentary. 

The consequences of explosive eruptions can be total devastation. See the before and after eruption photos of Mt. St. Helens below. 

Before the 1980 eruption of Mount St. Helens

In this photo, Mount St. Helens is a nearly symmetrical cone, surrounded by dense forests.

After the 1980 eruption of Mount St. Helens

In this photo, the upper one-third of the volcano is gone, replaced by a deep, wide crater. The forests have disappeared.

The Importance of Silica and Viscosity

These different eruption styles can have catastrophic consequences. For example, an estimated 36,000 people died from a 100-foot-high tsunami triggered by the 1883 eruption of Krakatoa. If such an eruption were to happen today, with the population exponentially higher, the death toll from such an event would probably be in the 100s of thousands.   So we should ask why some volcanoes explode while others simply erupt. The answer lies primarily with the chemistry or composition of the magma. In particular, the concentration of silica in the magma. Silica (SiO₂) has a direct influence on the viscosity of magma. Viscosity is the resistance to fluid flow: the greater the viscosity, the thicker the fluid will behave. Water, for example, flows easily and therefore has a low viscosity.

In contrast, peanut butter resists flowing and has a high viscosity. Silica is the most common ingredient in any magma. The percentage of silica within a magma body varies from approximately 50% to 75%, depending on the plate tectonic setting that produced the magma and how the magma may have evolved. Another influence on the viscosity is the temperature of the magma: the hotter the magma, the lower the viscosity. You may have noticed how substances become more fluid as they warm up, butter or candle wax, for example. At this point, you might wonder what viscosity has to do with making a volcano explode. Let's consider one point: all magma contains lots of gas dissolved in the magma. This gas creates the internal "gas pressure" within the volcano that causes volcanoes to erupt - Imagine vigorously shaking an unopened bottle of soda and immediately removing the cap. 

As you may have concluded, the greater the viscosity, the greater the gas pressure build-up. Imagine using a straw to blow air into a glass of water. Do your air bubbles quickly rise to the water's surface? How about if you stuck the straw into a thick milkshake? Do your air bubbles rapidly rise to the surface? Using this analogy, we can see that gas has a more difficult time escaping from viscous fluids, so the greater the viscosity of the magma, the greater the build-up of gas pressure, and the more likely it is that the volcano will explode.   

The table below summarizes the relationships among magma composition, temperature, viscosity, explosiveness, the size and shape of a volcano, and common volcano types.

Silica concentration in magma is crucial in controlling the nature of volcanic eruptions, so why does it vary? The answer, once again, is related to plate tectonics. You might recall from the page Origin of Magma in, the previous module, that there are three processes making magma, decompression melting, flux melting, and heat-induced melting. These processes happen in three settings, divergent or convergent boundaries or at hot spots. In general, low-silica lava will erupt along divergent boundaries and hot spots, while intermediate and high-silica lava erupt along convergent boundaries.  

Now demonstrate your understanding of volcanic eruptions by answering the following questions.