Types of Volcanic Eruptions

At a Glance

• Quiet Hawaiian and fissure eruptions
• Moderate Strombolian and Vulcanian
• Violent Plinian and Peléan
• Scale Volcanic Explosivity Index, 0 to 8
• Control Viscosity, gas content, water
• Exam link UPSC Prelims, Geography: Mountains, Volcanoes, Earthquakes and Tsunami

Where Does It Fit?

This is the fifth topic of the category on mountains, volcanoes, earthquakes and tsunami. The previous topic described volcanic landforms, and this one covers how eruptions differ in style and size. The next topic covers hotspots, geysers and hot springs. Previous topic: Volcanic Landforms – Extrusive and Intrusive. Next topic: Hotspots, Mantle Plumes, Geysers and Hot Springs.

What Decides the Style of an Eruption?

In short: thick, gassy magma explodes, and thin, runny magma flows.

Three factors decide how a volcano erupts. The first is the viscosity of the magma, which depends on its silica content, as described in Volcanism – Causes and Lava Types. The second is the amount of dissolved gas, since gas expands rapidly as pressure falls, like a shaken bottle that is opened. The third is the presence of water, whether groundwater, a lake or the sea, which flashes to steam when it meets magma and makes eruptions more violent. Eruptions are also divided by the form of the vent. A central eruption takes place through a pipe-like vent and builds a cone. A fissure eruption occurs along a long crack, often many km, and spreads vast sheets of lava, as in Iceland and, in the geological past, in the Deccan.

Which Are the Main Types of Eruption?

How Do the Extremes Compare?

A Hawaiian eruption is the quietest and produces flowing lava and fountains, a Strombolian eruption throws out glowing fragments at regular intervals, and a Plinian eruption is the most violent, sending ash many km up into the stratosphere. A Plinian eruption is named after Pliny the Younger, who described the eruption of Vesuvius in 79 CE, which buried Pompeii and Herculaneum.

How Is the Size of an Eruption Measured?

The Volcanic Explosivity Index, VEI, is a scale from 0 to 8 that measures the volume of erupted material and the height of the eruption column. Each step is about ten times larger than the one before, so it is a logarithmic scale like the Richter scale for earthquakes.

The Tambora eruption of 1815 in Indonesia was the largest in recorded history, and its sulphur aerosols cooled the climate, leading to the “year without a summer” in 1816 in Europe and North America. The Krakatoa eruption of 1883 produced a tsunami and a sound that was heard thousands of km away. A supervolcano is a volcano capable of a VEI 8 eruption, which has happened only a few times in the last 100,000 years.

Which Recent Eruptions Are Important?

The eruption of Eyjafjallajokull in Iceland in 2010 sent an ash cloud over Europe that grounded thousands of flights. The January 2022 eruption of Hunga Tonga-Hunga Haapai in Tonga, a submarine volcano, produced one of the most powerful atmospheric explosions in modern times, a global pressure wave and tsunamis across the Pacific. Mauna Loa in Hawaii erupted in November 2022 for the first time in about 38 years. Mount Semeru and Mount Merapi in Indonesia erupt repeatedly. In India, Barren Island in the Andaman Sea has had intermittent eruptions of ash and lava in recent years. As an analytical point, the global pattern of eruptions follows plate boundaries, and effusive eruptions are common at divergent boundaries and hotspots, while the more explosive ones occur at subduction zones, as shown in Distribution of Volcanoes and Earthquakes – Ring of Fire.

How Are Eruptions Monitored and Predicted?

Volcanologists watch for signs before an eruption: small earthquakes caused by the rising magma, swelling or tilting of the ground measured by GPS and satellites, a change in gas emissions, particularly sulphur dioxide, and rising temperature. These signs allow warnings and evacuations, and they save lives, although the exact time and size of an eruption are hard to predict. Hazard maps and exclusion zones around active volcanoes depend on knowledge of past eruption styles. The effects of eruptions on people and climate are in Effects of Volcanoes.

Why Is This Topic Important?

• Classification Types asked in Prelims
• Case studies Krakatoa, Pinatubo, Hunga Tonga
• Climate Eruptions alter climate

What Came Next?

The next topic describes hotspots, mantle plumes, geysers and hot springs. Next topic: Hotspots, Mantle Plumes, Geysers and Hot Springs.

Key People and Terms

• Fissure eruption Eruption along a crack
• Plinian Violent column eruption
• Nuee ardente Glowing avalanche
• VEI Volcanic Explosivity Index
• Supervolcano VEI 8 volcano
• Phreatomagmatic Magma plus water eruption

Exam Corner

Mains Angle

Questions link eruption style to magma and tectonic setting. These points add depth.

• Control Viscosity, gas, water
• Setting Effusive at ridges, explosive at arcs
• Risk Monitoring and warning