Volcanic Eruption

The Operational Land Imager (OLI) on the Landsat 8 satellite acquired this top image of Mount Etna on December 28, 2018. The image highlights the active vent and thermal infrared signature from lava flows, which can be seen near the newly formed fissure on the southeastern side of the volcano. The image was created with data from OLI (bands 4-3-2) and the Thermal Infrared Sensor (TIRS) on Landsat 8. Image: NASA.

Definition

A type of volcanic event near an opening/vent in the Earth’s surface including volcanic eruptions of lava, ash, hot vapour, gas, and pyroclastic material (IRDR Glossary).

The majority of volcanoes in the world form along the boundaries of Earth's tectonic plates. When tectonic plates collide, one often plunges deep below the other in what's known as a subduction zone. Not all volcanoes are related to subduction: another way volcanoes can form is what's known as hotspot volcanism. In this situation, a zone of magmatic activity—or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano. Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface (National Geographic).

A volcano is currently active if it is erupting lava, releasing gas or generating seismic activity. An active volcano is labelled dormant if it has not erupted for a long time but could erupt again in the future. When a volcano has been dormant for more than 10 000 years, it is considered extinct. Volcanoes can remain inactive, or dormant, for hundreds or thousands of years before erupting again. During this time, they can become covered by vegetation, making them difficult to identify.

How explosive a volcanic eruption is depends on how easily magma can flow or trap gas. If magma is able to trap a large amount of gas, it can produce explosive eruptions. (Australian Government).

Facts and figures

Over the last 11,500 years, more than 1,500 major eruptions have occurred, with approximately 500 in the Pacific "Ring of Fire" alone (PreventionWeb).

There are volcanoes on every continent, even Antarctica. Some 1,500 volcanoes are still considered potentially active around the world today; 161 of those—over 10 percent—sit within the boundaries of the United States (National Geographic).

There are different types of eruptive events. We can distinguish between primary and secondary events.

Primary events are:

Pyroclastic explosions
Hot ash releases
Lava flows
Gas emissions
Glowing avalanches (gas and ash releases)

Secondary events are:

Melting ice, snow and rain accompanying eruptions are likely to provoke floods and hot mudflows (or lahars)
Hot ash releases can start fires (WHO).

Volcanoes can have many different appearances. The shape of a volcano provides clues to the type and size of eruption that occurred. Eruption types and sizes depend on what the magma is made up of. Three common volcano forms are:

Shield volcano: have a broad, flattened dome-like shape created by layers of hot and runny lava flowing over its surface and cooling.
Composite volcano : also known as stratovolcanoes, they are formed from explosive eruptions. These eruptions create steep sided cones.
Caldera volcano: these volcanoes erupt so explosively that little material builds up near the vent. Eruptions partly or entirely empty the underlying magma chamber which leaves the region around the vent unsupported, causing it to sink or collapse under its own weight. The resulting basin-shaped depression is roughly circular and is usually several kilometres or more in diameter (Australian Government).