Krakatau Volcano

Krakatau Volcano erupting

A volcano is a special type of mountain that contains one or more holes that lead deep below the Earth’s surface. Every now and again a liquid material, called lava, reaches the surface of the mountain and erupts, sending out massive amounts of molten rock, ashes, dust and rocks high in the sky.

Volcanos can be found on the land as well as under water. When an underwater volcano erupts close to the surface of the water, it may form an island. Hawaii in the North Pacific Ocean is an example of a island that formed from volcanoes.

How volcanoes work

All volcanos start forming deep in the underground. Before we continue, we need to understand how the structure of the Earth.

Earth structure

Let’s examine the structure of our planet. Look at the diagram below:

  • The centre of the Earth is a big ball of extremely hot metal, which is over 6’000 km in diameter. It is called the core. The temperature can reach up to 5’500 °C. The core is made of two layers – the centre is a solid metal, whereas the outer core is liquid metal.
  • On top of the core is a 3’000 km layer of hot, molten rock, which is called the mantle. The temperature of the mantle is 4’000 °C near the core, but it cools down to approximately 900 °C close to the surface. The molten rock is called magma.
  • And finally, there is a thin layer of the cold rock and soil called the crust. It is only 40-50 km thick, with the thinnest parts hidden below the water, where it can be only 9 km thick. The crust can get as hot as 200 °C on it’s border with the mantle, but it cools down to the air temperature at the surface.
Earth Layers

Earth Layers

Magma reaching the surface

Hot molten rock is lighter than cold rock, so it begins to raise, just like oil raises in water. When magma raises, it heats and melts the rock around it. This allows it to move slowly towards the surface. This process is extremely long, and can last million of years, before it reaches the surface. Once is has reached the surface, the magma will try to find its way out though the cracks in weak rocks. Once it reaches the surface, a volcano erupts.

The raising magma “bubbles” are called magma chambers and can be very large, reaching tens of kilometres in diameter.

The picture below shows the whole process of magma “bubbling up” and forming a volcano:

Magma Chambers

Magma Chambers

  • A magma chamber starts forming (A)
  • The colder rock is melting and allowing the chamber to raise (B)
  • The hot magma finds its way though crack, called a vent, in the rocks and erupts (C)

Volcanic mountains

When magma erupts it flows away from the vent and starts to cool down. Once it’s completely cold, it becomes as solid as a rock. As you already know, the magma is just an extremely hot, melted rock. Next time the volcano erupts, it’ll spit out more magma, which will cool down and for another layer of rocks. If you repeat this many times adding layer after layer, eventually you will build a mountain. At the top of the mountain is a crater, which looks like a chimney. This type of mountain is called a volcanic mountain.

Volcano Structure

Volcano Structure (formation of a volcanic mountain)


Lava is liquid rock, which flows across the ground and slowly cools. When it cools down, it forms hard rocks. Lava flow is very slow, because lava is very sticky, and you can actually run away from the flow. Depending on the size of the eruption, lava rivers might run for tens of kilometres.

There can be two types of lava: very sticky or very runny.

Sticky lava moves very slowly. The surface cools down quicker, but the flow pushes it further, so it starts to break up. This type of lava makes a rough, bouldery type of surface. This lava is called Aa Lava (pronounced aaah-aaah lava).

Sticky lava

Sticky lava (click for larger picture)

Runny lava flows very quickly, and the surface has no time to cool down and form rocks. This thin layer of lava stops running once the surface cools down and becomes a solid rock all at the same time, forming a nice and smooth surface. This lava is called Pahoehoe Lava (pronounced pa-howie-howie lava).

Runny lava

Runny lava (click for larger picture)

Lifetime of a volcano

Most volcanoes go through a cycle of events. When magma chamber reaches the surface of the Earth a volcano is formed and it erupts. It does so because of the great pressure in the magma chamber, which pushes out all lava with enormous force. The volcano is called active, because it erupts lava.

Once the chamber is runs out of hot magma, the volcano stops erupting. A plug of hard rock is formed in the volcano “chimney” (the vent). Magma slowly fills up the emptied chamber. The volcano is called to be in the dormant state. Dormant means sleeping. This is very dangerous state. When enough pressure is built in the chamber, the magma blows out the plug and the volcano becomes active again. If the plug was very thick and hard to push through, the eruption usually is extremely violent. Sometimes a side of the volcano is blown out if the plug cannot be pushed out.

This cycle can repeat many times, until the plug becomes really big and reaches the chamber. At this point there is no enough pressure to push the plug out and the whole chamber might cool down. Once this happens the volcano becomes extinct, because the will be way for new magma to enter the chamber, which is filled in with hard rock.

Lifecycle of a Volcano

Lifecycle of a Volcano

Extinct volcanoes

Once volcano is completely extinct the softer rocks on the flanks (sides) of the volcano soon wear away, leaving just the plug standing. The plug is made of an extremely hard rock, because it is formed under very high pressure and was cooling down very slowly.

In the picture below you can see Agathla Peak in Arizona, which is a plug of an old volcano. The shading indicates what the original volcano might have looked like.

Agathla Peak volcano in Arizona

Agathla Peak volcano in Arizona

 In the past these plugs made an ideal place for building defence castles and towers. In France there is an area near the town of Le Puy where there aremany small plugs, most of the with a castle or church on it. Another good example is Edinburgh Castle, which is also built on a plug of volcanic rock.

Edinburgh Castle on volcano plug

Edinburgh Castle built on volcano plug (click for larger image)

Sometimes the old volcano plugs make tiny islands in the seas or oceans. One example is Ball’s Pyramid – a 7 million years old volcanic rock. This rock is a home to one of the largest stick insects in the world, with some species growing as large as 15 cm.

Ball's Pyramid

Ball’s Pyramid

Famous volcanoes

One of the most famous volcanoes is Mount Vesuvius in Southern Italy. It has erupted over 30 times, but the most famous eruption was in the year 79 (yes, nearly 2000 years ago). The volcanic ash covered the surrounding area including the Pompeii city nearby, killing over 1000 people.

In 1883, the volcano on the Indonesian island of Krakatau erupted with 13,000 times the power of an atomic bomb. The Krakatau volcano is still active, occasionally erupting and spiting hot lava. You can see the picture of Krakatau volcano eruption at the beginning of this article.

Mount St. Helens in America was getting ready to burst for nearly two months before it exploded, after more than 120 years it lay dormant. While the eruption was anticipated, the manner in which it occurred was completely unprecedented. An earthquake triggered a sideways blast that swept the mountain’s north face away into a cascading landslide that shot hot ash and stone out some 25 kilometres at speeds of at least 500 km/h. At the same time, a mushroom-shaped plume of ash shot 20 km into the air, eventually covering three states. Complete darkness blanketed Spokane, Wash., a city about 400 km northeast of the volcano. When the ash came down it fell in the form of black rain that literally coated the residents of Washington, Idaho and parts of Montana with a fine gray powder. Fifty-seven people and thousands of animals were killed.

Eyjafjallajokull, whose name means “Island Mountain Glacier” in Icelandic, first erupted on March 20, 2010. It threw a large amount of volcanic ash high into the sky, causing a massive disruption to the airline services.

Mount Etna is located on the east coast of Sicily, Italy. It is the tallest active volcano in Europe. It erupts quite often, therefore the eruptions are not very violent, because it has no time to form the hard and big plug. In 2011 it has made nearly 20 eruptions. You can see some of them in the video below: