Active volcanoes

Volcanic activity in the last 30 days

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How a volcano is formed

The most common type of volcano is a stratovolcano, also known as a stratovolcano. It is formed when magma - molten rock - rises through the Earth's crust and collects in a chamber below the surface. As the magma continues to build pressure, the Earth's surface rises and it can eventually erupt through weak spots in the Earth's crust.

During an eruption, magma and gas are ejected from the volcano along with ash and other volcanic material. The type of eruption depends on the viscosity of the magma as well as the amount of gas trapped in the magma.

In some cases, the magma may flow relatively smoothly from the volcano. In this case, no large explosions occur, but a lava flow is formed that emerges at the surface and cools after some time. However, if the magma is more viscous, it accumulates in the vent of the volcano and builds up a large pressure there. This pressure finally discharges explosively and during the eruption magma masses are hurled into great heights.

Volcanoes often form at subduction zones, where two tectonic plates collide and one is pushed under the other. The subducting plate may melt and rise as magma, creating a chain of volcanoes along the plate boundary called a volcanic arc. This is most notable in the Pacific Ring of Fire.

The strongest volcanic eruptions of the past 2,000 years

The strength of volcanoes is measured in a volcanic explosivity index (VEI). Volcanoes at a VEI of 8 or above are called super volcanoes, which occur extremely rarely. The last outbreak of a super volcano took place around 26,500 years ago in New Zealand (volcano Taupo). The strongest values calculated for the last 2,000 years achieved a VEI of 7, which is nevertheless enormous. Tephramasses of up to 1,000 km³ are thrown at heights above 25 km.

The Pacific Ring of Fire

The Pacific Ring of Fire runs in a horseshoe shape around the Pacific Ocean. Its edges are subject to exceptionally strong seismic and therefore volcanic activity. The ring runs along the west coast of South and North America and the Bering Strait and turns from there to the south over Japan, the Philippines and Indonesia. It ends near New Zealand.

The reason for the high occurrence of volcanoes and earthquakes along this line are the boundaries of numerous tectonic plates. Since the Earth's crust, as the outermost layer, is always moving slightly, these plates rub against each other at the boundaries. Due to the collision and jarring discharge of seismic energy, this zone is characterized by frequent and powerful eruptions, often occurring as volcanoes.

Over 75% of all volcanoes are located along the Pacific Ring of Fire, and about 90% of the world's earthquakes occur there. The figure shows the course of the ring of fire around the Pacific continental plate. (Fig.: National Oceanic and Atmospheric Administration, NOAA)

Active, dormant and extinct volcanoes

It is difficult to distinguish between active and extinct volcanoes. There is no uniform time period from which a volcano is considered extinct after its last eruption. To distinguish between the two, it is often assumed that a volcano is considered extinct when it has not been active for a period of 10,000 years. For normal household use and in relation to our own lifetime of only a few decades, this may be a sufficient criterion. From a scientific point of view, however, this is not sufficient, because there are numerous volcanoes that may well still show underground activity and could erupt again. While some volcanoes only have rest periods of a few years between their eruptions, others (e.g. on Gran Canaria) even have rest periods of several million years.

Therefore, one examines the specific volcanic system in each case, i.e. the branching and activities of underground magma flows, in order to come to a conclusion. Depending on the activity that can still be measured in the magma chambers, a probability for a further eruption is calculated. If there is no or only very little activity, the volcano is considered to be extinct. Otherwise only as dormant.

In total, there are still about 1400 to 1900 volcanoes that are considered active.