What is a Volcano/Types of Eruptions

🌋 What is a Volcano?

A volcano is an opening, or vent where magma pushes up from deep inside the Earth and erupts onto the surface. When magma reaches the surface, it is called lava. Along with lava, volcanoes can release ash, gases, and rock fragments during eruptions.

Volcanoes usually form along tectonic plate boundaries, where plates are either pulling apart (divergent boundaries) or colliding (convergent boundaries). The vast majority of volcanoes are located around the Pacific Ocean subduction zone known as the Ring of Fire. They can also appear in the middle of plates at hot spots, such as the Hawaiian Islands.

Although they can be dangerous, volcanoes are also important. They build new land, release gases that helped form Earth’s atmosphere, and create fertile soils that support life.

Lava and Magma Viscosity

Viscosity is how thick or sticky a liquid is and how easily it flows. For example, water has low viscosity (it flows easily), while honey has high viscosity (it flows slowly). The same idea applies to magma and lava. The viscosity of magma is important because it controls how a volcano erupts—whether the lava flows gently or explodes violently.

Several key factors affect magma viscosity:

Silicon Dioxide (Silica)

Magma with a high silica (SiO₂) content, like rhyolitic magma, is very sticky and has high viscosity.

Magma with low silica, like basaltic magma, is runnier and flows more easily.

2. Temperature

Hotter magma is less viscous and flows more easily.

Cooler magma becomes thicker and stickier, increasing its viscosity.

3. Gas Content (Volatiles)

When gases are dissolved in magma (under high pressure deep underground):
Gases like water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂) act a bit like lubricants. Water, in particular, breaks up strong bonds between silica molecules in the magma. This makes the magma less viscous (more fluid).

When pressure drops and gases come out of solution (closer to the surface):
Gas bubbles form and expand. This increases the magma’s apparent viscosity, because the bubbles resist flow. The magma becomes stickier and harder to escape smoothly.

High Viscosity Magma

Low Viscosity Magma

How Gases Affect the Type of Volcanic Eruption

The amount of gas inside magma is one of the main factors that controls how a volcano erupts. Magma contains dissolved gases, such as water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). Water vapor is the most abundant gas released during volcanic eruptions.

Deep underground, these gases stay dissolved because of the high pressure. As magma rises toward the surface, pressure decreases and the gases begin to escape, forming bubbles. If the magma is thin and fluid (low viscosity), the gas can escape easily. This leads to quiet eruptions known as effusive eruptions, like those in Hawaii, where lava flows smoothly.

If the magma is thick and sticky (high viscosity), the gas cannot escape as easily. Pressure builds up inside the magma until it is released suddenly, much like when you open a can or bottle of soda after it’s been shaken. This results in explosive eruptions, such as those at Mount St. Helens or Mount Vesuvius.

In short, the way gases escape from magma determines whether a volcanic eruption will be gentle and flowing or violent and explosive.