Picture a volcano, what do you imagine? Many people would visualise a conical mountain of rock spewing ash or lava. Indeed, this is one type of volcano, but it may surprise you to know that there are many other types of volcano too! In this article we will see what makes a ‘volcano’, by investigating some of the different types and briefly discussing how each one forms.
Defining a volcano
In simplistic terms a volcano is a natural structure which has built up in concentric layers and has the potential to release fluids, rocks and gas in one-off or multiple eruptive events. The most well-known and widely publicised type of volcano is magmatic in origin and erupts lava, ash and gas (it is this type of volcano that volcanologist study). These volcanoes provide a wealth of construction material, fertile soils and shelter to thousands of people and animals alike. However, these are also the most destructive volcanoes that can wipe out an entire population in seconds.
These are not the only type of volcano though…
In the depths of winter parts of America and Canada are completely frozen. In New York it is not uncommon for geysers to build 5-storey high ice volcanoes. These form due to the gradual build-up of frozen water around the geyser which forms layers of ice around a central vent. Although, this feature is still a geyser, its conical shape and open vent issuing hot water and steam likens it more to a true ‘volcano’.
In Antarctica ice volcanoes can be found protruding from the slopes of a much larger magmatic volcano called Erebus. Here the ice volcanoes form where hot gases and material are emitted from fractures in the ground known as fumeroles. The hot gases emitted include water which cools when it escapes from the hot fumerole into the cold Antarctic air. Like when a kettle is boiled next to a window forming a layer of condensed water on the glass, the water vapour quickly condenses on contact with the cold air. This forms droplets which freeze and build-up an ice volcano.
Mud & sand volcanoes
Mud and sand are terms used to describe the size of particles that make up a sediment or rock. Grains of sand have a size range of 187µm – 1500µm, which are less than 2mm. Any particles that are smaller than 187µm are classed as mud. As with ice volcanoes, mud and sand volcanoes form when fractures in the ground erupt mud or sand building up a protruding structure.
In 2006 the largest eruption of a mud volcano began in eastern Java, Indonesia. The cause of this eruption is highly debated and may have been triggered by a 6.3 magnitude earthquake or by drilling of a borehole into a pressurised aquifer (underground reservoir). Both of these scenarios would have released built up pressure beneath the ground allowing a mud slurry to travel upwards and erupt at the surface.
Mud and sand volcanoes often form during earthquakes. This is because shaking of material during an earthquake can lead to liquifaction which occurs when water rises to the surface of water-logged sediments. For example, if you jump on a dry looking patch of sand on the beach close to the sea you will eventually sink as the sediment moves and water escapes between the grains. When you stop jumping the sand and water eventually return to their original state before the ‘quake’. In an earthquake this can happen below the grounds surface, the liquid-mud slurry doesn’t have anywhere to go so exploits fractures – particularly those opened up in the ground during an earthquake. The erupted material then forms a mud or sand volcano depending on the size of the grains erupted. Footage of erupting mud and sand volcanoes was widely publicised following the 2010, 2011 Christchurch, New Zealand earthquakes.
Volcanoes can take on many forms and do not just form from the eruption of lava and volcanic rocks formed from molten rock! Some form in freezing conditions where hot gases and water are emitted from the ground and condense to form an ice volcano. Whereas, others are created out of mud and sand that erupt onto the surface from fractures due to the release of underground pressure, such as, during earthquakes.