- Volcanic Geothermal Systems can involve both shallow and deep heat resources. In either case, heat rises up from the Mantle in Magmatic Conduits. For Volcanic Island Arcs as shown in the sketch at left, an oceanic tectonic plate (L) is shown subducting beneath another oceanic tectonic plate (R). As the subducting plate descends, increasing pressure and heat causes the release of water and other volatile materials that cause melting of rock and the formation of magma that rises upwards due to being buoyant (less dense than the surrounding rock). Faults and fractures help provide pathways called Magmatic Conduits as shown in the lower sketch at left.
- These pathways involve dikes (vertical intrusions) and sills (horizontal intrusions). As magma rises through these intrusive conduits it may find upward flow restricted at times, so it can accumulate in chambers where gas and pressure builds up as magma continues to enter until continued upward movement is once again possible. Series of chambers at different depths throughout the crust are possible. In some cases different magma compositions may be possible over time. Finally at some point, if the magma pressure is sufficient, it may breach the surface and successive eruptions begin to form layers of volcanic material including lava, ash, and pyroclastic material.
- Magmatic flow to the surface may decline and eventually cease for a variety of reasons including magma supply being depleted or cut off, changes in tectonic activity (e.g., subduction rate), less degassing of magma at depth reducing buoyant flow, blockage of magmatic conduits, or collapse of volcanic edifice (large explosion and caldera formation) sealing off magmatic pathways.
- Even without active magmatic flow to surface, substantial heat pathways could still exist to enable conductive and convective heat to provide sufficient heat for shallow hydrothermal reservoirs to exist utilising meteoric water.
