Volcanoes deform before they erupt. Magma rising through the crust inflates the edifice — the surface rises, tilts, cracks. Monitoring this deformation with InSAR and GPS is a primary tool for eruption forecasting. But deformation is ambiguous: some volcanoes inflate and erupt; others inflate and subside without erupting; still others erupt without measurable inflation.
The thermal state of the volcano controls which pattern occurs. Hot volcanoes — those with active hydrothermal systems, recent eruptions, or shallow magma bodies — behave simply: they uplift before eruptions and don't cycle. Cold volcanoes — long-dormant systems with solidified shallow plumbing — exhibit complex deformation: inflation-deflation cycles, lateral migration of deformation centers, and apparent unrest that doesn't lead to eruption.
The difference is rheology. In a hot volcanic system, the surrounding rock is warm, weak, and viscous. Magma ascending through warm country rock pushes it aside plastically. The deformation is monotonic — up, then eruption. In a cold system, the surrounding rock is brittle and elastic. Magma ascending through cold rock pressurizes it elastically, but the rock can also relax, fracture, redistribute stress, and channel fluids laterally. The same magma input produces complex, oscillating surface signals.
The forecasting implication is asymmetric. Simple uplift at a hot volcano is highly predictive. Complex deformation at a cold volcano is not — it might indicate impending eruption, or it might indicate the system absorbing and redistributing the intrusion without erupting. The cold volcanoes are harder to read precisely because their brittleness creates more degrees of freedom in how they respond.
The most dangerous volcanoes — the long-dormant ones capable of the largest eruptions — are the ones whose signals are hardest to interpret. Difficulty of prediction scales with severity of consequence.