Gravity and elevation changes caused by magma movement beneath the Krafla Caldera, Northeast Iceland

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G.Y. Johnsen
A. Bjornsson
S. Sigurdsson


Prior to the present activity of the Krafla volcano, which started in 1975, levelling and gravity surveys had been carried out in the area. The network has since been extended, and measurements are carried out about every second month. The measurements reveal a quasi-periodic behaviour of the tectonic activity, characterized by slow inflation of the caldera for several weeks or months at a rate of 6–10 mm/d, interrupted by sudden subsidence events lasting for one or a few days. Elevation changes within the caldera are described by a deflation-inflation swelling with its apex near the center of the caldera. Calculations. using a model of a spherical chamber with varying pressure at 3 km depth, show good agreement with the measured elevation values. Comparison of gravity and levelling data, using a Bouguer type relationship, suggests that inflation and deflation of the floor of the caldera is caused entirely by in- and outflow of magma. Some discrepancies between the levelling and gravity data immediately after subsidence events can be explained by additional mass flow of groundwater.

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Johnsen, G., Bjornsson, A., & Sigurdsson, S. (1980). Gravity and elevation changes caused by magma movement beneath the Krafla Caldera, Northeast Iceland. Journal of Geophysics, 47(1), 132-140. Retrieved from


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