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Why do volcanoes erupt?

Updated Monday, 27th November 2017

Bali is bracing itself for an eruption of the Mount Agung earthquake. What forces create these geological timebombs?

The main factor controlling the nature of an explosive eruption is the viscosity of the magma. If viscosity is low, vesicles may coalesce as they rise and merge into large bubbles a metre or more across before reaching the top of the conduit.

These arrive at irregular intervals and on bursting throw out a shower of ejecta. If the bubble bursts gently, the ejecta flops out around the rim of the conduit.

Strombolian activity

More violent bubble bursting throws ejecta to greater heights, and this sort of episodic activity is described as strombolian, after the active Mediterranean volcano Stromboli.

Sometimes the expansion of vesicles in a basaltic conduit can cause magma and bubbles to accelerate upwards together, forcing them out of the vent at speeds of the order of 100ms−1 as a fountain of incandescent lava,graphically described as a fire fountain.

Stromboli [Image: sabrina.MILLOT under CC-BY-NC-SA licence] Creative commons image Icon Sabrian.MILLOT via Flickr under Creative-Commons license
Stromboli [Image: sabrina.MILLOT under CC-BY-NC-SA licence]

What happens to the ejecta in a strombolian eruption?

Most of the material thrown up in this way falls to the ground nearby, although some finer ash may be carried away by the wind. If the large chunks are still molten when they hit the ground, they can feed a lava flow.

However, if they are sufficiently chilled in flight, they hit the ground as clinkery pieces called scoria (millimetres to centimetres in size).

A toppled Moai figure atop a pile of scoria on Easter Island [Image: stevesheriw under CC-BY-NC licence]  Creative commons image Icon stevesheriw via Flickr under Creative-Commons license
A toppled Moai figure atop a pile of scoria on Easter Island [Image: stevesheriw under CC-BY-NC licence]

Bombs and cowpats

The larger pieces flung out by an explosive eruption are described as volcanic bombs. In the case of a scoria-producing fire fountain, the bombs chill less (because of their large size) and are typically soft and gooey so that they splat on the ground in a distinctive manner.

Bombs thrown out more violently may take on aerodynamic shapes.

 

Cowpat bomb Copyrighted  image Icon Copyright: The Open University
This picture shows a "cow pat bomb" of about 300 centimetres across.

 

Plinian eruptions

Intermediate and felsic magmas are too viscous to allow vesicles to merge or to rise independently of the magma. Instead, vesicles remain small (less than 11cm).

As exsolution proceeds, gas pressure forces the vesiculated magma up the conduit at high speed, emerging at the vent at several hundred metres per second.

This usually gives rise to a type of eruption known as plinian, because the first detailed account of such an eruption (Vesuvius in AD 79) was written by Pliny the Younger.

The eruption destroyed the cities of Pompeii and Herculaneum, and killed over 3500 people (one of whom was Pliny the Elder).

Vesuvius [Image: Alexandra Svatikova under CC-BY-SA licence] Creative commons image Icon AlexandraSvatikova via Flickr under Creative-Commons license
Vesuvius [Image: Alexandra Svatikova under CC-BY-SA licence]

A plinian eruption may last for hours or days, and is marked by a grey column of ash known as an eruption column rising to kilometres or even tens of kilometres above the vent.

The physical processes occurring within a plinian eruption column make it convenient to describe it in three parts.

 

Ejecta Copyrighted  image Icon Copyright: The Open University
A plinian eruption column, showing the gas thrust region, the region of convective ascent, and the umbrella cloud.

In the lower part of the column, fragments are driven upwards by the force of expanding gas in the conduit. This is called the gas thrust region, and is equivalent to the entire eruption column of a fire fountain.

However, in a plinian eruption column a lot of air is drawn in and heated by contact with the hot ejecta, causing the air to expand.

Mixed with the hot volcanic gases (and despite the weight of the ash particles within it), this is buoyant relative to the cold surrounding air. It rises convectively above the gas thrust region and is called the convective ascent region of the eruption column.

Eventually, the column reaches a height where it is neutrally buoyant, and spreads out to form an umbrella cloud. Because the umbrella cloud is not rising, all the ash is now able to fall out, and the extent of the airfall deposit therefore depends on how far the umbrella cloud spreads.

What happens to the ejecta in a plinian eruption?

A plinian eruption creates an airfall deposit with clearly recognizable characteristics.

The larger ejecta falls too rapidly to be carried by the convective ascent region, so it hits the ground relatively close to the vent, although some bombs can travel several kilometres.

Slightly smaller material falls out of the sides of the convective ascent region, and is dispersed further than the majority of the bombs.

Ash reaching the umbrella cloud falls at a speed that decreases with decreasing particle size, so the finest material settles out last and is dispersed furthest from the volcano.

Dispersal of ejecta

This is an appropriate time to introduce the term tephra (from the Greek word for ‘ash’) which refers to airfall material in general; hence the term tephrachronology applied to the use of widespread airfall deposits that have been radiometrically dated as stratigraphic marker horizons. This table shows the accepted grain size terminology for tephra:

 

Grain size/mm

Pyroclastic fragments (tephra)

More than 256 coarse bombs and blocks
64 - 256 fine bombs and blocks
2 - 64 lapilli
1/16 - 2 coarse ash
less than 1/16 fine ash

Particularly in felsic eruptions, many of the fragments in the lapilli size range are lumps of glassy froth described as pumice. Most ash particles are sharp glassy fragments representing the walls of bubbles, though some consist of isolated phenocrysts or are fragments of phenocrysts.

Not only does coarser ejecta fall closer to the vent, but the total amount falling from the umbrella cloud decreases outwards.

Plinian airfall deposits thus have a tendency to become both thinner and finer-grained away from the source. However, there is a very important external factor that has a considerable influence on the shape of this pattern.

Can you think what this is?

The dispersal of the umbrella cloud is controlled by the wind.

When there is a wind, the umbrella region is blown off to one side so tephra is deposited asymmetrically. When the thickness of an airfall deposit is contoured, the pattern is consistent with the wind direction during the eruption.

Vulcanian eruptions

Vulcano, a volcanic island in the Med [Image:LucaLuca in  Public domain] Copyrighted  image Icon Copyright: via Wikimedia
Rocks on Vulcano, a volcanic island in the Med [Image:LucaLuca in Public domain]

Other styles of explosive eruption are determined by the relationship between volatiles and magma. Sometimes a volcanic conduit is choked by debris from earlier eruptions, and this is cleared out by explosive release of gas from an underlying magma chamber.

Little or no fresh magma is erupted, and the tephra consists largely of redistributed fragments of older volcanic rocks.

This style of eruption is known as vulcanian, after the Mediterranean island volcano of Vulcano, whose most recent large eruptions exemplify this style of activity.

Phreatic eruptions

Alternatively, a violent explosion can be triggered when magma suddenly encounters water.

Phreaticec eruption [Image: USGS] Copyrighted  image Icon Copyright: via Wikimedia
A USGS image of a phreatic eruption at Mount St Helens

 

This can simply be groundwater, or be caused by eruption below less than a few hundred metres of water, or when lava flows into a sea or lake. This kind of explosive eruption is described as phreatic, and is driven by the sudden conversion of water to steam.

Find out more

Study with The Open University: Volcanoes, earthquakes and tsunamis.

This course extract is adapted from Block 3, Internal Processes, of the course Geology (S260) which ceased presentation in 2009. Good news: There are still lots of ways to study geology at the University

It was originally made available more widely as part of the Rough Science: Colorado content on Open2.net

 

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