Why Volcanoes Form

Eruptling — Why Volcanoes Form Fumarole Forge

Eruptling here! Welcome to our imaginary Fumarole Forge station.

The Grumblies rearranged our volcano map. I need your help!

Let's compare where volcanoes form. Ooh, I love map mysteries!

Earth's outer rocky shell is divided into moving tectonic plates.

These huge pieces move slowly over long periods.

The edges where plates meet are called plate boundaries.

Many volcanoes form near certain kinds of plate boundaries.

At some boundaries, plates move apart.

Hot mantle rock can rise beneath the separating plates.

Lower pressure can allow some of that rock to melt.

Magma can rise and cool, forming new crust.

Much of this happens along ridges beneath the ocean.

Wow! Volcanoes aren't only mountains we can see on land.

At other boundaries, one plate sinks beneath another.

This process is called subduction.

Water released from the sinking plate helps nearby mantle rock melt.

Some resulting magma can rise toward the surface.

This can form a chain of volcanoes above the sinking plate.

Remember: the mantle is mostly solid, even near volcanic regions.

Some volcanoes form away from plate boundaries, above hotspots.

Hawaiian volcanoes are a well-known example within a plate.

As a plate moves, a hotspot can produce a volcanic chain.

So 'many near boundaries' doesn't mean 'all near boundaries.'

Our simplified map shows most dots near marked boundaries.

A few dots lie within plates. Those matter, too!

The Grumblies hoped we'd ignore them. Let's keep every clue.

Which claim fits a map with some volcanoes inside plates?

All volcanoes sit on boundaries, or many do with exceptions?

Many, with exceptions! The inside dots disprove the 'all' claim.

You checked the whole map. That's thoughtful, brave exploring!

What happens during subduction? One plate sinks beneath another?

Yes! That sinking plate changes conditions deep below the surface.

Plates moving apart can allow mantle rock to rise and melt.

Subduction can also help magma form above a sinking plate.

Hotspots help explain volcanoes away from plate boundaries.

Our map has patterns and exceptions. Both belong in our explanation.

I'll meet you in the adventure. Bring your careful map-reading!

Train more, start this adventure, or explore another map destination.

Free Why Volcanoes Form teacher guide and practice (PDF)