There are volcanoes all over the world, but while some trickle out the sides, some burst high into the sky. To find out why, you can do a demonstration in your own kitchen. If you have a mop.
There are a lot of factors that can contribute to making a full-scale eruption of a volcano instead of a slow trickle. The degree of pressure and the suddenness of the release contribute, of course, as does any other force applied to the eruption. One of the main contributors to the height of an eruption, though, is the consistency of the lava. The way it contributes may be counter-intuitive. It seems that a thin liquid will shoot out of a container higher and faster than a thicker and more viscous one, since the viscous one has molecules that will hold together and resist movement by a force. Compress a tube of water and the water will squirt far. Compress a tube of molasses with the same amount of force and you won't see the same amount of splatter.
But neither water nor molasses have compressed gasses inside them, and liquid that's been trapped inside the earth for a long time does. That makes a huge difference, which we can see with a simple experiment. Grab two bottles of soda and stick them both in the freezer for about a half hour. You want to get them extremely cold, but never cold enough to freeze. Next you'll need a packet of wallpaper paste, or some other material that turns soda extremely viscous. Open the soda bottles, and pour out a little soda from one - just enough to have space to pour the wallpaper paste in. Close both. Try to let as little carbon dioxide as you can out of each. (You only open the 'control' bottle in order to make the experience of both bottles as similar as possible.) Once the lid is on tight, it's time to shake 'em both up. Shake for a good long time, until the paste is evenly distributed in the first bottle, and the liquid inside seems thick and gluey.
Now go out to you back yard, wearing something you don't care about, and let the bottles warm up a little. Once they're warm, take the top off of both bottles. The regular soda will shoot out of the bottle. The gluey soda, though, will go much higher.
Why? It seems like it's harder to move glue than it is soda. And it is harder. That's why the trapped bubbles of carbon dioxide are under so much more pressure inside the gluey bottle. The thickness of the liquid, combined with the cramped conditions, compresses them into tiny points, much smaller than they would be in the regular soda. Once the liquid is free of the bottle the pressure difference is so intense, and the balls have been compressed to be so energetic, that they expand massively, shooting liquid high into the air. The regular soda bubbles can't compete with that kind of pressure difference, and so they don't force the liquid to go up as high.
In volcanoes natural gasses fill the magma under the surface of the earth. The thicker the magma, the more the gasses are compressed, and the more trouble they'll cause when the pressure of the ground above them is released. Thin, runny magma will flow, often quite fast, but thicker magma will make it more likely for a volcano to shoot lava into the air.
Top Image: Alaska Volcano Observatory
Second Image: Wolfgang Beyer
Via The Naked Scientists.