8.5 Explosive Eruptions

Why do we still talk about Pompeii, 79 AD?

Nearly 2,000 years later, the explosive eruption of Mt. Vesuvius remains one of the most talked about eruptions in history. It began with a column of ash that blanketed the area, which was followed by fast-moving, dense, and scorching-hot pyroclastic flows. People suffocated or burnt, and structures in two thriving Roman cities were destroyed. Remains of some of the dead can be seen at Pompeii, where people were entombed in scorching ash. The eruption type was named plinian, after Pliny the Younger, who watched from offshore.

Explosive Eruptions

A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II, in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Explosive eruptions are found at the convergent plate boundaries that line parts of western North America, resulting in the Cascades in the Pacific Northwest and the Aleutians in Alaska.

Causes of the Explosion

Explosive eruptions are caused by gas-rich, felsic magmas that churn within the magma chamber. When the pressure becomes too great the magma breaks through the rock above the chamber and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion (Figure below).

Pyroclastic Material

The erupted rock fragments are called tephra. Ash and gas also explode from the volcano. Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic means fire rock (Figure below).

Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000°C (1,800°F).

Cascades Volcanoes

Prior to the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon (Figure below).

Volcanic Gases

Volcanic gases can form poisonous and invisible clouds in the atmosphere. These gases may contribute to environmental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight (Figure below).

Summary

• Felsic magmas erupt explosively, creating pyroclastic eruptions.
• Pyroclastic eruption types include tephra, ash, and lahars.
• Mt. Mazama blew its top off and then collapsed, creating Crater Lake in Oregon.

Review

1. Why do convergent plate boundaries have explosive eruptions?
2. Why do felsic magmas erupt explosively?
3. How do volcanic gases affect the atmosphere?