Volcano – a mountain that spits fire? Volcano basics part 2

How do the insides of a volcano look like?

Illustration of a volcano.

Illustration of a volcano.

Most volcanoes form a cone, as you can see in the graphics above. During an eruption the volcano flings out material which builds up, layer upon layer, and often forms this form of landscape. There are different types of conesstratoconesspatter conesash conestuff cones, and cinder cones.

What a volcano needs as a kind of “driving  force” is a magma chamber deep below the ground. The depth and size of such chambers varies for each volcano. Some have more than one chamber in different depts.

If new magma arrives in a magma chamber, the pressure rises. The magma starts to rise through cracks. When this stress is high enough, it starts cracking rocks. This causes volcanic earthquakes. Those earthquakes are rather weak most and often hardly felt by the people living above such a magma chamber. The strongest volcanic quakes are around magnitude 4. Such volcanic quakes come in swarms. There were thousands of quakes during the month before the submarine eruption in El Hierro.
The magma may eventually start a dyke intrusion. Very very often there is not enough magma or no new magma is arriving and an earthquake swarm dies down and just stops without an eruption happening.

The magma rises through dykes. If the vent is not open but obstructed, additionnal dykes and sills may form. Dykes are (more or less) vertical, sills are horizontal (again, more or less).

Dykes. Image Wikimedia Commons.

Dykes. Image Wikimedia Commons.

Sometimes ancient dykes or sills are clearly visible in rock formations.

Image Wikimedia Commons http://en.wikipedia.org/wiki/File:Horton_Bluff_mid-Carboniferous_sill.JPG

Image Wikimedia Commons http://en.wikipedia.org/wiki/File:Horton_Bluff_mid-Carboniferous_sill.JPG The image shows a sill in a rocky wall in Nova Scocia.

The most interesting feature on an active volcano is probably the crater which is formed where the vent reaches the surface.

Cierro Negro Crater. Image Wikimedia Commons

Cierro Negro Crater. Image Wikimedia Commons

Haelema´umaú Crater Hawaii. Image Wikimedia Commons.

Halema´uma ´u Crater Hawaii. Image Wikimedia Commons.

In some craters you might find a dome. A volcanic dome or lava dome is created when magma pushes upward but the pressure is not strong enough to start an eruption.

Dome at Chaiten . Image Wikimedia Commons.

Dome at Chaiten . Image Wikimedia Commons.

Dome of Mount Saint Helens 2005. Image Wikimedia Commons.

Dome of Mount Saint Helens 2005. Image Wikimedia Commons.

Recently the dome of Gunung Sinabung collapsed and created pyroclastic flows.

USGS on Lava Domes with many examples: http://vulcan.wr.usgs.gov/Glossary/Domes/description_lava_dome.html

Other features you may find on volcanoes and read mentioned in the comments here:

Side vents.


There might be more than one crater at the top of a volcano or side vents can be created by flank eruptions.

This happened recently on Etna (Dr. Boris Behncke has a wonderful image on his Flickr page https://www.flickr.com/photos/etnaboris/4259183831/in/set-72157625829221508 )
and here you can see what developed over 3 years. -> https://www.flickr.com/photos/etnaboris/8704101819/in/set-72157633480142061 The former flank eruption / side vent  developed into Etna´s new Southeast crater.

Lava Stream

This image shows the red glowing lava in motion on Hawaii while the next image shows an ancient version on Teneriffa.

Image by author

Image by author



Caldera formation shown at the example of Crater Lake, Oregon.

We like to disuss calderas on VC. 😉  A caldera is the result of the collapse of a magma chamber of the volcano. Calderas are much bigger than craters: Sometimes new volcanoes grow out of a caldera. The formation of a caldera can have disastrous effects especially when a submarine caldera is formed and water streams into the hot, new caldera, like it has happened at Krakatau 1883.

Crater Lake oregon. Image Wikimedia Commons

Crater Lake oregon. Image Wikimedia Commons

Other features which you hopefully never get to see live but they are discussed here and one might stumble over the results climbing a volcano.

Pyroclastic flow. A Pyroclastic flow happens when the ash column breaks down under its own weight. But they can also happen when a dome collapses as we saw recently at Sinabung (video above) or like at Mount Saint Helens when a directional blast happened. The resulting ignimbrites can be found on many volcanoes.

The greenish layer is made of ignimbrites. Image Wikimedia Commons.

The greenish layer is made of ignimbrites. Image Wikimedia Commons.

A Lahar is a mixture of volcanic materials with water and is a Javanese (Indonesia) word that describes mudflows. Lahars are called Jökullhlaups in Iceland.

A lahar may happen during a volcanic eruption when some kind of interaction between the volcano and
water. Either a glacier is being melted or a lake is involved or it is raining heavily. Many eruptives from a volcano contain a lot of water/steam so rain often comes automatically with volcanic ash clouds.

Mount Saint Helens 1982- Image Wikimedia Commons.

Mount Saint Helens 1982- Image Wikimedia Commons.

Lahars have the consistency of liquid concrete and may endanger people far away from the erupting vent.

Lahars may also occur long after an eruption when water remobilizes volcanic material. We started a series on volcanic hazards https://volcanocafe.wordpress.com/2013/03/20/volcanic-hazards-lahars/ with lahars a while ago and will try to cover other dangers like volcanic ashes soon.

While trying to write this post I discovered how difficult it is to describe volcanic terms with very little words just to explain the terms we on VC use so frequently. All newcomers to this blog please note that this only the very basic about volcanoes written by a non-expert volcanophile. Again… Every volcano is different. No 2 craters look exactly alike, neither do all the other volcanic features. I would like to continue with the different kinds of eruptions next time.
Part 1 of this series: https://volcanocafe.wordpress.com/2014/04/29/volcano-a-mountain-that-spits-fire-volcano-basics/


Friday emergency riddles: 

1) What is brown and sticky? (The answer is a certain cinder cone) Solved (Sissel, 2 pts)! The word ʻŌʻō also means digging stick, because in Hawaiian legends the volcano goddess Pele uses her magic rod pāoa to create volcanic pits.

2) All the colour had gone from her life, so after breaking bread Big Red set her kids free. (Thanks to Kelda!)

Cheers! chryphia


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