Cross Sections

Schematic cross-section of a Hawaiian volcano. Vertical exaggeration = 2x. Magma, liquid rock, forms by melting some of the minerals in the mantle at depths of 40-100 miles (60-170 km). The melting is in the Hawaiian hot spot. Magma ascends due to its low density relative to the surrounding rocks, and follows sets of more or less continuous conduits towards the surface. Pressure from the magma causes overlying rocks to fracture, forming the conduits. Enough conduits are beneath the volcano to provide a steady supply of magma to a shallow reservoir 2-5 miles (3-7 km) beneath the summit caldera. As more magma fills the reservoir, pressure builds. Schematic cross-section modified from Eaton and Murata (1960).

Hypothetical cross-section beneath the summit of Kilauea Caldera. The numerous eruptions at Halemaumau Crater suggests a magma conduit between the reservoir and the crater.

When the pressure exceeds the strength of the surrounding rocks, the rocks fracture. Magma moves into the fractures, making dikes or sills (Eaton and Murata, 1960; Decker, 1987). Dikes are tabular-shaped bodies of magma that cut across rock layers. Sills are tabular-shaped bodies of magma that intrude parallel to rock layers. After the magma solidifies, the tabular-shaped rock bodies are still called dikes and sills. Summit eruptions result from the upward growth of dikes from the shallow reservoir. Dikes can grow laterally and feed magma into the rift zones of the volcano.

Cross-section of magma conduits to the summit reservoir and east rift zone of Kilauea Volcano. Conduits were identified by the distribution of earthquakes. Modified from Ryan and others (1981).

The current eruption of Kilauea is fed through a dike that extends almost 12 miles (20 km) from the summit reservoir into the flank of the volcano.

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