Volcanism

Introduction

There's a great variety in the shapes and forms of Volcanoes. There are the picture book cones which look like the prototype of a volcano. Hawai'i and Iceland feature gently sloped flat pancakes. And then there are domes like that in Katmai Valley, Alaska, which hardly look like volcanoes at all.

Volcano types seem to vary with the region. On the other hand some regions feature very different volcanoes. As you might have guessed there are scientific explanations for all that. In the next sections we take a closer look.


Shield Volcanoes

Mauna Loa, Hawai'i (1997) Mauna Loa, Hawai'i (1997)

Sea floor spreading and hot spots produce lava from the upper mantle. This basaltic lava (from the rock Basalt) is very hot and fluid and flows a long way from the vent.

Since gas dissolved in the lava can escape easily basalt eruptions are usually calm and without explosions.

This kind of volcanism produces volcanoes which look like a upturned warriors shield, hence the name "shield volcano".


Stratovolcanoes

Ngauruhoe, New Zealand (1995) Ngauruhoe, New Zealand (1995)

In subduction zones material from the oceanic plate gets melted, mixed with material from the continental plate, and rises to the surface. This andesitic lava (which got its name from the Andes mountains) is more viscous. It doesn't travel very far and piles up to steeper mountains.

Gas can not escape that easily so explosions which shatter rock and froth lava are more common. Fluid lava and ash eruptions often occur at the same volcano.

This kind of volcanism produces volcanoes which consist of layers of lava and ash, hence the name "strato volcano" ("strato" = greek for "layer").


Domes

Tarawera, New Zealand (1995) Tarawera, New Zealand (1995)

At some volcanoes the lava is so viscous it pretty much doesn't flow at all. Sometime the lava gets squeezed out of the vent like toothpaste and piles up over the vent.

Since it is difficult for gas to escape from the lava violent explosions and ash eruptions are frequent.

This kind of volcanism produces volcanoes with the lava bulging over the vent, hence the name "dome".


1999 Anita Ford & Christian Treber