Volcanoes at a Plate Boundary
Volcanism has it origins deep within the Earth’s interior. Vast convection cells in the upper mantle generate basaltic magma at mid-ocean ridges and drive cold basaltic oceanic crust away from the ridge crests and down into the mantle at subduction zones.
The active volcanoes in New Zealand are part of the long line of volcanic and earthquake activity that extends around the Pacific Ocean as the Ring of Fire.
The Pacific and Australian tectonic plates that meet in New Zealand push against each other along a curving boundary. At the southern end of the South Island, the Australian Plate dives down (subducts) below the Pacific Plate whilst in the North Island the opposite situation occurs with the Pacific Plate being pushed under by the Australian Plate. In between, through most of the South Island, the two plates grind past each other along the Alpine Fault.
The Hikurangi Trough marks the collision boundary to the east of the North Island, and is where oceanic lithosphere (the Pacific Plate) descends into the Earth’s interior as a huge inclined slab.
At depths down to several hundred kilometres, a zone of intense earthquake activity marks the top surface of the descending plate (a). Along this zone hot liquid basaltic magmas are generated by partial melting in the overlying wedge shaped mantle layer (b). They rise through the crust and may interact with it to generate andesitic, dacitic or rhyolitic magmas, each with progressively higher silica contents (c). These magmas either crystallise below the surface, forming granitic plutons (large bodies of intrusive igneous rock), or are erupted at the surface.
See the Glossary for volcanic terms.