Explosive eruptions are favored by high gas content & high viscosity magmas (andesitic to rhyolitic magmas). The explosive bursting of bubbles fragments the magma into clots of liquid that cool as they fall through the air. These solid particles become pyroclasts or volcanic ash.

If the magma is too viscous to rise to the surface it will become stuck in the crust to create intrusive igneous rocks.

Composite volcanoes are some of the most dangerous volcanoes on the planet. They tend to occur along oceanic-to-oceanic or oceanic-to-continental boundaries because of subduction zones. They tend to be made of felsic to intermediate rock and the viscosity of the lava means that eruptions tend to be explosive.

Magma can rise when pieces of Earth’s crust called tectonic plates slowly move away from each other. Magma also rises when these tectonic plates move toward each other. When this happens, part of Earth’s crust can be forced deep into its interior. The high heat and pressure cause the crust to melt and rise as magma.

As magma rises toward the surface, what happens to the gases in it? The gases begin to expand, forming bubbles because the pressure decreases as the magma rises. Contrast the silica content of magma in quiet and explosive eruptions. A volcano erupts quietly if its magma is low in silica.

As the oceanic plate sinks, fluid (shown in purple) is squeezed out of it. The fluid flows up into the mantle rock above and changes its chemistry, causing it to melt. This forms magma (molten rock). The magma rises and collects in chambers within the crust.

Oxygen, the most abundant element in magma, comprises a little less than half the total, followed by silicon at just over one-quarter. The remaining elements make up the other one-quarter. Magmas derived from crustal material are dominated by oxygen, silicon, aluminum, sodium, and potassium.

Volcano Vocabulary Review

Close all windows, doors, and fireplace or woodstove dampers. Turn off all fans and heating and air conditioning systems. Bring pets and livestock into closed shelters.

Survival during an eruption

1. Close all windows, doors, and fireplace or woodstove dampers.
2. Turn off all fans and heating and air conditioning systems.
3. Bring pets and livestock into closed shelters.
4. Keep necessities on hand.
5. Listen for emergency alerts and do what they say.
6. Stay inside until you hear that it’s safe to come out.

5kms

In the end, your average eruption is releasing millions to trillions cubic meters of ash into the atmosphere. Most of it falls near the volcano (within tens of km), but a significant portion can travel far away, drifting in the atmosphere for hundreds, thousands, tens of thousands of kilometers around the globe.

Fluid basalt flows can extend tens of kilometers from an erupting vent. The leading edges of basalt flows can travel as fast as 10 km/h (6 mph) on steep slopes but they typically advance less than 1 km/h (0.27 m/s or about 1 ft/s) on gentle slopes.

Volcanoes can be dangerous even when they’re not erupting, but there are different levels of risk depending on the state of the volcano. While extinct volcanoes pose virtually no threat, the others might not be so safe.

The length of dormancy is tricky as well. A volcano could be dormant for tens to maybe even hundreds of thousands of years, provided there are some bouts of unrest that suggest it has the potential to erupt again.

How can we tell when a volcano will erupt?

• An increase in the frequency and intensity of felt earthquakes.
• Noticeable steaming or fumarolic activity and new or enlarged areas of hot ground.
• Subtle swelling of the ground surface.
• Small changes in heat flow.
• Changes in the composition or relative abundances of fumarolic gases.

A dormant volcano is one that is “sleeping” but could awaken in the future, such as Mount Rainier and Mount Fuji. An extinct volcano is “dead” — it hasn’t erupted in the past 10,000 years and is not expected to ever erupt again. Kohala on the Big Island of Hawaii could be considered an extinct volcano.

A dormant volcano is an active volcano that is not erupting, but supposed to erupt again. An extinct volcano has not had an eruption for at least 10,000 years and is not expected to erupt again in a comparable time scale of the future.

Tamu Massif