Färoe‐Iceland Ridge Experiment 2. Crustal structure of the Krafla central volcano

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Abstract
The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high‐velocity dome extends from the lower crust (11–14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2–0.3 s compressional wave delays and shear wave shadowing to be 2–3 km N‐S, 8–10 km E‐W, and 0.7–1.8 km thick. The near‐surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat‐lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near‐solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high‐velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.