Volcanoes

In January 2022 (very recent!), an underwater volcano erupted near Tonga in the South Pacific Ocean. This is one of the most significant recorded eruptions in over 30 years, and the volcano caused a series of tsunami waves on the shores of Tonga. But what exactly is a volcano? How are they formed? What different types of volcanoes are there? Let’s find out.

How are volcanoes formed?

Convergent plate margins

The majority of volcanoes are formed at convergent plate margins where subduction occurs. As the denser plate sinks beneath the less dense plate, the temperature and pressure increase. The increase in temperature and pressure causes rocks to release water, resulting in a reduction in the melting point of the rock above. This creates magma, which rises to the surface, forming a volcano.

Hot spots from mantle plumes

Certain volcanoes are attributed to hot spots from mantle plumes instead of convergent plate margins. A mantle plume is an area under the crust of the Earth where the magma is hotter than the surrounding magma. The volcano is formed as the hotter magma melts the crust and causes magma to rise to the surface. The mantle plume remains stationary whilst the tectonic plate continues to move (this can cause a chain of volcanoes). Some examples include the Hawaiian Islands that are located far from plate boundaries.

What are the states of volcanoes?

The different states of volcanoes are active, dormant, and extinct.

• Active volcanos are prone to eruptions. This means that they have erupted or could still erupt.
• Dormant volcanoes are those that have not erupted for over 2000 years.
• Extinct volcanos will never erupt again and are no longer a danger.

What are the different types of volcanoes?

The different types of volcanoes include cinder cones, composite volcanoes, shield volcanoes, and lava domes.

Cinder cones

Cinder cones are formed by the particles from lava that erupt from a single opening. When the gaseous lava is ejected into the air, small fragments solidify and form cinder. This cinder falls around the opening and forms a circular or oval cone.

Example: The Parícutin volcano in Mexico.

Parícutin volcano, Mexico, Jim Luhr, Smithsonian Institution, Public Domain

Composite volcanoes

Composite volcanoes are formed by alternating layers of lava, volcanic ash, cinders, blocks, and bombs. They are often steep-sided and have symmetrical cones. The cone is often destroyed when the volcano becomes inactive and hardened magma blocks the opening.

Example: Mount Fuji in Japan.

Mount Fuji, Japan, pixabay

Shield volcanoes

Shield volcanoes consist mostly of fluid lava flow and are shaped (unsurprisingly!) like a shield. Basaltic lava pours out in different directions from the opening or a group of openings. This builds a gently sloping surface. Shield volcanoes are more common in oceanic settings rather than continental.

Example: Mauna Loa, Hawaii, is the world’s largest shield volcano.

Lava domes

Lava domes are formed by lava that is too viscous to flow far. The lava often remains around the vent, building the volcano from within. This process includes the outer surface hardening, breaking, and falling down the volcano’s sides. Lava domes are sometimes formed within the crater of a previous volcanic eruption.

Example: Mount Pelée in Martinique.

Mount Pelée, Martinique, lava dome, pixabay

How do physical processes impact the magnitude and type of volcanic eruption?

Let’s see how different plate margins affect the magnitude and type of volcanic eruption.

Volcanoes at divergent plate margins

Volcanoes are generally small at divergent margins because basalt lava has a high viscosity and low gas content. Currently, there are around 100 volcanoes at divergent plate margins. Some of these are extinct, but over 30 are still active.

Volcanoes at convergent plate margins

Volcanoes at convergent plate margins tend to erupt frequently and with high force. This is because the magma from the process of subduction has high gas and silica content. Ocean-ocean convergent plate margins tend to consist of volcanic islands.

The volcanoes formed at the ocean-ocean convergent plate margins are called island-arc volcanoes. The volcanoes formed at the ocean-continent convergent plate margins are called Andean-type volcanoes.

What are the effects of volcanoes?

The harmful effects of volcanoes include lava flows, pyroclastic flows, ash falls, gas eruptions, and further secondary hazards.

Lava flows

Volcanoes can erupt with lava flow (molten rock) and damage nearby infrastructure. However, these lava flows tend to travel slowly, which gives time for evacuation. The lava will destroy anything in its path by burning, burying, or knocking it down.

Pyroclastic flows

Pyroclastic flows are eruptions of rock, ash, and gas that are superheated. They can travel at 80km/h, significantly faster than lava flows. This means that they can travel a long way with little warning. They can threaten nearby people and cause widespread death and destruction through burning.

Ash falls

Ash falls are eruptions of ash clouds caused by the expansion of gas in the volcano. Ash falls do not tend to endanger human life but can cause issues with aviation, infrastructure, and contamination to water and agriculture.

Gas eruptions

Gas eruptions escape through soil, lava, or vents and include gases such as carbon dioxide, sulphur dioxide, hydrogen, hydrogen sulphide, and carbon monoxide. These can impact the climate and cause irritations, poisoning, breathing issues, and acidic rain.

Secondary hazards

Let’s assess two types of secondary hazards from volcanoes: lahars and Jökulhlaups.

Lahars

Lahars are secondary hazards that are caused by the mixtures of volcanic material and water. They are mud and debris flows that create threats nearby the volcanoes, but they can also travel far from volcanoes along rivers and cause issues further away. They can bury or destroy natural habitats, settlements, and infrastructure. They can also generate tsunamis when they enter the sea.

Jökulhlaups

Jökulhlaups are sudden floods of water from glaciers, which can be caused by volcanic eruptions under a glacier. Therefore, they are potential secondary hazards caused by volcanic eruptions.