Arenal volcano is deforming the basement under −20 × 103 kPa, and affects it for several kilometers below the surface and about 5 km around the volcano base. The total settlement below the present (1968–2009) lava field (0.75 km2; 0.6 km3) is 2 m o more, but it represent at the moment only 20% of the consolidation, so its deformation will be continuous for years. The volcano grew up on the top of weathered volcanic rocks (weak and plastic portion) conditions that are ideal for deforming the basement (subsidence, folding or faulting) and generate instability on the cone according to structural and volcanic models. The results of numerical models show that Arenal is at an incipient deformation stage by spreading of the basement. The overall effect generates stability at the interior of the volcano and its foundation. The twin edifice (cones C and D) can generate rock slides (cold or hot) as well as debris avalanches (0.03–0.75 km3).

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