How do scientists detect that a volcano is swelling before it erupts?

When magma rises or shifts, it can push the ground upward or outward by centimeters or even meters. GPS stations and tiltmeters measure these changes directly, while satellite radar interferometry can map deformation over large areas. Scientists also watch volcanic gases such as sulfur dioxide, which may increase as magma nears the surface. In some cases, inflation begins weeks to months before an eruption, although the timing is not always predictable. Monitoring is especially important at active volcanoes such as Mount Etna in Italy and Kīlauea in Hawaii.