Re: why are stratovolcanoes so explosive like mount Cleveland?

Dear Kelsey,

First, I am very sorry to take so long to reply to you. My e-mail address changed and it took a while for your question to reach me.

One thing to understand is that there are two separate questions here - what is a stratovolcano and why are some volcanoes explosive and others are not. So let me take them one at a time.

A stratovolcano is a volcano that is built up of layers - almost like a layer cake. These volcanoes will have a layer of lave on top of a layer of volcanic ash - these layers are what help to give the stratovolcano its distinctive shape. So a stratovolcano will have an eruption that sprays ash into the air and that might be followed by a lava flow. Many stratovolcanoes erupt explosively as you mentioned - here is why.

When lava comes to the surface of the Earth it has gas and water trapped inside. When the lava erupts at the Earth's surface it is under a lot less pressure than when it's underground - when that pressure is released the gas and steam want to leave the lava.

Think about a bottle of soda. Soda is liquid that has gas dissolved in it. When you take off the bottle cap the gas dissolved in the soda boils out of the soda and sometimes makes the bottle overflow. This is just like some lavas when the pressure is released during an eruption. In addition to gas many lavas also contain water and when the pressure is reduced the water boils and turns into steam - this also helps make the lava explosive.

Some kinds of lava are thicker than others - like ketchup or molasses instead of like water. These lavas hold the gas and the steam in longer and let pressure build up even higher - when they blow the eruption is much stronger than it is with thinner lavas that release the gas and water more easily.

So it is this combination of things that makes Mt. Cleveland so explosive - it is built of lava that is pretty thick and that contains a lot of water and gas - when this lava erupts from the volcano the water and gas boil out of the lava like a shaken soda coming out of the bottle.