Subduction Zone Volcanism

Subduction Zone Volcanism

Subduction zone volcanism occurs when one plate is shoved under another. In the case of the Cascades subduction zone that would be the Juan de Fuca plate sliding underneath the North American plate. To see more about it take a look at Cascadia Subduction Zone at wikipedia.

This has been the focus of some of the most violent geology on the planet since the beginning of the planet. The mega-quakes, the volcanic eruptions and lava flows are that we modern humans have seen are child's play in comparison with some of the earth shattering geological calamities that this region, and others, have seen in the past. Juan de Fuca Plate

Subduction zone volcanoes are quite different than hot spot volcanoes (which we will discuss tomorrow). As the plate subducts, it melts creating silica rich lavas like rhyolite- the kind so common around Mount St Helens. Rhyolite

There are many subduction zones in the world. One runs through Japan. Another through Indonesia. Another through New Zealand. There is one that runs down Italy of which the famous Mount Vesuvius is a part. Another one runs through Greece. If you notice the volcanoes all have several things in common.They build huge, beautiful but explosive stratovolcanoes like Mount St Helens, Krakatoa, Mount Pinatubo. They are explosive because their silicate rich lava it thick and doesn't flow very well. They get all stopped up sometimes for hundreds of thousands of years (or thousands or even hundreds of years)  and explode. There is some evidence that when groundwater gets in the magma chamber, flash to steam and provides some of the violence in these explosions.

These volcanoes are killers and can devastate hundreds of square miles when they blow. 

Why are other obviously active volcanoes almost gentle? 

Tomorrow we'll talk about hot spot volcanism, how it is different from subduction zone volcanism and how these massive basaltic flows have formed much of the earths surface.

Mount Hood ~50 miles east of Portland Oregon is a stratovolcano. It hasn't been active since 1805 but it still burps and gurgles. There have been microquakes underneath it since the seventies so there is lava in the magma chamber. It's known for great skying and snowboarding but there is a beast underneath.