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The
dyke of Red Crater, on Tongariro
Photo NC
| "Sometimes magma rises but does not
reach the surface. It may harden in the walls of a composite cone
to form horizontal sheets called sills, or vertical sheets called
dykes. Sills and dykes help to reinforce the structure of the volcano,
but may, in time, be exposed by erosion. This explains the unusual feature in Red Crater. At some stage, magma rose up through a crack in the wall then sank again without reaching the surface. Molten lava cooled and hardened on the sides of the fissure, but retreated from the centre, leaving it hollow. The dyke was revealed when the Red Crater cone was eroded." Excerpt from Anderson: Tongariro, A Volcanic Enviromnent |
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