Volcanoes


Landscapes from Stone logoVolcanoes are sites where molten rock, called magma, reaches the Earth’s surface. The shape and behaviour of a volcano depends on how the magma erupts…..

Rangitoto, New Zealand
Rangitoto, New Zealand
Photo: Brian McConnell
Low viscosity, or “runny”, magma tends to erupt gently as lava that flows away easily, so runny magma volcanoes are usually low and shield-shaped.


High viscosity, or “sticky” magma tends to get stuck on the way out, until pressure builds up and it gets forced out explosively, scattering rock fragments and volcanic dust over a wide area.



Eruption column caused by a vulcanian-type explosive eruption on 5 October 1998 rises above Tavurvur Volcano in Rabaul Caldera, Papua New Guinea

Eruption column caused by a vulcanian-type explosive eruption on 5 October 1998 rises above Tavurvur Volcano in Rabaul Caldera, Papua New Guinea.
U.S. Geological Survey, photo by J.W. Ewert

1998 eruption, Piton de la Fournaise, Reunion
1998 eruption, Piton de la Fournaise, Reunion
Photo: Brian McConnell

The runny kind of magma is called basalt, and most basalt eruptions produce lava flows.

Some basalt eruptions start with the magma spurting out of the vent to form a fire fountain.
Solidified basalt lavas are of two types, with the Hawaiian names “pahoehoe” and “aa”.

Pahoehoe is the runniest kind of lava, which keeps on flowing more and more slowly until it solidifies in these wrinkly, curved shapes, sometimes called “ropey lava”.

Aa breaks up as it cools to produce rubble, as in the left of the picture.
Fluidal, ropey lava from 1998 eruption, Piton de la Fournaise, Reunion
Fluidal, ropey lava from 1998 eruption,
Piton de la Fournaise, Reunion
Photo: Brian McConnell
Ordovician pillow lava, Lough Nafooey, Co. Galway, Ireland
Ordovician pillow lava, Lough Nafooey, Co. Galway, Ireland
Photo: Brian McConnell

Extrusion of basalt underwater produces these football- to pillow-sized blobs of lava called pillow lavas.

They are very distinctive and we can easily recognise them in these rocks formed 480million years ago in Co. Galway.
There have been events in Earth’s history when huge volumes of basalt magma has been erupted, producing extensive plateaux of lava called flood basalts.

Ireland’s most recent volcanic activity was of this type.

About 60 million years ago much of Northern Ireland was covered in flood basalts to form the Antrim plateau, which includes the world famous geological site of the Giant’s Causeway.

Giants Causeway, Co. Antrim, Northern Ireland
Giants Causeway, Co. Antrim, Northern Ireland
Photo: Sophie Preteseille
Taranaki, New Zealand
Taranaki, New Zealand
Photo: Brian McConnell
In between sticky and runny magma, called andesite, erupts partly as sticky lava that doesn’t flow far and partly as explosive fragments.

Together these build up a more or less conical mountain, the classic volcano shape.

The alternations of lava and fragmental layers give these volcanoes the general name of “composite volcanoes”.

Large explosive eruptions at sticky magma volcanoes send towering columns of volcanic dust and gasses many kilometres into the air.

In some instances, the eruption column collapses under its weight and flows down the slopes of the volcano to form a “pyroclastic flow”.

Such flows are among the most deadly and devastating of volcanic eruptions.

Several pyroclastic flows have been produced during the recent and ongoing eruption of Montserrat, destroying the main town and requiring evacuation of the island.


Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines, during an explosive eruption on 15 September 1984

Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines,
during an explosive eruption on 15 September 1984.
U.S. Geological Survey, photo by C. Newhall
Tarawera, New Zealand
Tarawera, New Zealand
Photo: Brian McConnell
The stickiest magma is called rhyolite, and rhyolite eruptions can be extremely violent.

The force of eruption breaks up the roof of the volcano causing it to collapse, so that rhyolite volcanoes are often depressions, called “calderas”, rather than mountains.

This lake occupies a rhyolite volcano in New Zealand.

This is the Greek island of Santorini. It forms a circular curve with steep inner cliffs.
This is a caldera formed by collapse of the central part of a volcanic island during a large eruption 3500 years ago.

The eruption is implicated in the fall of the Minoan civilization.
Santorini, Greece
Santorini, Greece
Photo: Brian McConnell
Minoan pumice deposit from Santorini, Greece
Minoan pumice deposit from Santorini, Greece
Photo: Brian McConnell
Such explosive rhyolite eruptions produce predominantly volcanic dust and pumice rather than lava.

This white layer is the pumice material deposited on the slopes of the Santorini volcano by the caldera-forming eruption.

Ancient examples of such deposits occur in Ireland, showing that Ireland did have this type of extreme volcanic eruption in the distant past.

This example, in which the large pumices are flattened to give a streaky appearance, is from near Leenane, Co. Galway.
Fiamme in ignimbrite, Ordovician, Co. Galway, Ireland
Fiamme in ignimbrite, Ordovician, Co. Galway, Ireland
Photo: Brian McConnell
Geyser, New Zealand
Geysir, New Zealand
Photo: Brian McConnell
Volcanoes are clearly hazardous things.


However, they do offer some benefits to attract people to live around them.
The magma deep underground between eruptions heats water that bubbles to the surface as hot springs and geysers.


This hot water can be used to produce electricity in geothermal power stations.
A large proportion of Iceland’s electricity, for example, comes from this source.
Another, more widely used benefit is the fertile soil that forms from weathering of volcanic rocks.

It was the fertile volcanic soil that attracted the Romans to settle on the slopes of Vesuvius at Pompeii.

The benevolence of the volcano ended in 79AD when the city was buried by a large explosive eruption.

The town……..
Pompeii, Italy
Pompeii, Italy
Photo: Brian McConnell
Pompeii, Italy
Pompeii, Italy
Photo: Brian McConnell


…..and many of its inhabitants were fossilised and are still being excavated today to give us an excellent insight into the Roman way of life.


Vesuvius continues to be active. The last eruption was in 1944.

Today, the volcano is well overdue an eruption, and the past record shows that a long gap is followed by a violent explosive eruption that generates pyroclastic flows.

Several million people live today in the Bay of Naples, within reach of pyroclastic flows.
It is therefore of great importance to try and predict when a volcano is going to erupt, to give people time to evacuate, not just at Vesuvius but around the world wherever people has settled close to volcanoes.

Vesuvius, seen from Sorento, Italy
Vesuvius, seen from Sorento, Italy
Photo: Brian McConnell
Volcanologists at work on the Piton de la Founaise during the 1998 eruption, Reunion
Volcanologists at work on the Piton de la Fournaise, Reunion
Photo: Brian McConnell

The scientist who tries to predict volcanic eruptions is a volcanologist.

These two are taking the temperature of a lava flow using a wire called a thermocouple.
This array of instruments at the summit of Reunion volcano are constantly monitoring seismic activity, ground deformation, and gases coming out of the ground.

Rapid changes in any or all of these things indicate movement of magma near the surface and probable eruption.

Having detected evidence of a probable eruption, what does a volcanologist do…..?
Summit monitoring station, Piton de la Fournaise, Reunion
Summit monitoring station, Piton de la Fournaise, Reunion
Photo: Brian McConnell


It isn’t possible to stop an eruption, so the important thing is to get people out of the way.
Towns and cities around volcanos have evacuation plans ready for action when the volcanologists say an eruption is imminent.


U.S. Geological Survey photo glossary of volcano terms :
http://volcanoes.usgs.gov/images/pglossary/index.php


Kids corner :
Make your own volcano

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