What is a Volcano?
A volcano is an opening in the Earth’s crust that allows magma, ash and gasses to erupt from below the surface. It is made up of a magma chamber, a vent, a crater and a cone shaped mountain made of layers of ash and lava. Magma chamber
→ Magma from the Earth’s mantle collects in a large underground pool. The magma in a magma chamber is under great pressure trying to force its way upward to the surface. Vent
→ The magma forces its way up through the vent which is like a chimney for the volcano. There is the main vent but there can also be secondary vents on the side or flank of the volcano. These secondary vents produce secondary cones on the flank of the volcano. Crater
→ The crater or caldera is the bowl shaped feature on top of the volcano that the magma from the vent erupts form. Cone shaped mountain
→ The majority of volcanoes are cone shaped mountains. They are formed of alternating layers of lava and ash from multiple eruptions. As the volcano erupts a layer of lava forms, the ash cloud formed during the eruption later cools and falls, this is known as pyroclastic flow. This forms a layer of ash on top of the lava. This process is repeated each time the volcano erupts. What is the difference between magma and lava?
Magma is molten rock below the surface of the Earth’s crust, when this molten rock reaches the surface of the earth is is then called lava.
Stages of a Volcano’s life.
There are three types of volcano based on the different stages in the volcano’s life. These are Active, Dormant and Extinct.
Active → Active volcanoes erupt regularly examples of active volcanoes are Kīlauea in Hawaii, Mount Etna in Italy and Mount Stromboli also in Italy which has been erupting almost constantly for the last 2000 years.
Dormant → Dormant volcanoes are volcanoes that have not erupted in a long time but are expected to erupt again in the future. Examples of dormant volcanoes are Mount Kilimanjaro, Tanzania, Africa and Mount Fuji in Japan.
Extinct → Extinct volcanoes are those which have not erupted in human history. Examples of extinct volcanoes are Mount Thielsen in Oregon in the US and Mount Slemish in Co. Antrim.
Where do Volcanoes occur?
The majority of volcanoes occur at plate boundaries. They can occur where plates separate, an example of this is Iceland. Here volcanoes are formed by the North American and Eurasian plates pulling apart. (see divergent plate boundaries)
They can also occur where plates collide. Mount Etna is formed by the subduction of the African plate under the Eurasian plate. The melting of the subducted plate causes an increase in pressure which leads to the formation of a magma chamber and in turn a volcano.
Image credit: National Geographic
Volcanoes can also form at areas known as Hot-Spots. Hotspots are caused by magma of increased temperature from a mantle plume. This hot magma melts through the rock of the Earth’s crust and rises through the cracks to form a volcano.
452 of the world’s volcanoes can be found in what is known as the Pacific Ring of Fire. this is an area of intense volcanic activity due to plate tectonics. 75% of the world active and dormant volcanoes can be found here.
What causes volcanoes to erupt?
Rock from subducted plates melts to form molten magma which pushes its way towards the surface forming a magma chamber. Gasses that have been dissolved in the magma expand causing a massive increase in pressure. This increase in pressure causes the magma to rise and force its way through cracks/fissures in the volcano above. As it reaches the surface the pressure is released and an eruption occurs. During an eruption volcanic ash, rock particles, dust, gasses and lava are all ejected.
How violent the eruption is depends on the amount of silica present in the magma. Silica produces a thicker magma that is better at trapping gasses. The more gas present the greater the pressure. Therefore the more silica present in the magma the more violent the eruption will be.
Types of lava and their effect on the volcano structure
There are two types of lava, acidic and basic.
Acidic lava has a high silica content and this makes it thicker. This thick lava doesn’t travel far and due to the high level of dissolved gas it has violent eruptions. Combined these cause the volcano to have a steep sided cone. These are known as Cone Volcanoes.
Basic lava contains less silica, this allows the gasses to escape and gives a runny lava. Eruptions of this type of lava a gentler and this along with it being runny allows the lava to flow further. Volcanoes of this type of lava will have gently sloping sides. They are known as Shield volcanoes.
For more information on the types and shapes of volcanoes see Tulane University website
How can we forecast volcanic activity?
By studying the type of materials and distribution of deposits geologists can learn a lot about the activity of volcanoes.
Eruptions can be predicted in a number of ways:
Tiltmeters are very sensitive devices that are used to identify any bulging of the sides of a volcano. Increased pressure that causes the volcano’s sides to bulge out indicating an eruption may be about to happen.
Gases or steam coming out of vents in the volcano or the appearance of geysers could suggest an eruption will soon follow.
Seismometers are used to detect vibrations in the rock. These could be caused by the movement of the magma or the cracking of rocks due to increased hea both of these would indicate an eruption being imminent.
For real time monitinrg of the worlds volcanoes see the World Organisation of Volcanic Observatories website
Ireland and Volcanoes
Ireland is not known for its high level volcanic activity in recent history but by studying the landscape volcanoes from hundreds of millions of years ago can be still be seen.
There are a number of extinct volcanoes in Ireland these include Slieve Gullion in County Armagh, Croghan Hill in County Offaly, Mount Slemish in County Antrim, Lambay Island in Dublin and Loch Na Fooey in County Galway. These volcanoes are all extinct with the last eruption was approximately 60 million years ago.
Today the Geological Survey continues to identify previously undiscovered volcanoes hidden beneath the surface of Ireland. To learn more about these Geophysical surveys visit the Tellus page.
Other volcanic activity can also be identified here in Ireland. One of the world’s most famous Geoheritage sites The Giant’s Causeway, is a result of volcanic activity. Approximately 60 million years ago Antrim was at the heart of intense volcanic activity, magma from below the Earth’s surface forced its way up through fissures in the rock and formed a huge lava plateau. As this lava rapidly cooled it contracted forming the famous hexagonal columns.
To learn more on the Giant’s causeway website.
Image credit: Ireland.com