HOW DO LAVA TUBES FORM?

Lava tubes such as those seen at La Cueva de los Verdes (pictured) can form in two ways

Lava tubes such as those seen at La Cueva de los Verdes can form in two ways.

'Overcrusted' tubes form when thinner lava flows fairly close to the surface, developing a hard crust that thickens to create a roof above the moving lava stream.

When the eruptions end, the channel created by the lava is drained leaving a tunnel a few metres beneath the surface.

'Inflated' tubes are complex and deep structures that form when lava is injected into existing fissures between layers of rock or cavities from previous flows.

The lava expands and leaves a huge network of connected galleries as it forces its way to the surface.