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5) Ground Cracks and Settling

Ground cracks and settling are commonly associated with volcanic activity; both generally occur neat active or recently active volcanic vents as the result of shallow underground movement of magma. The beginning of an eruption at a new site is preceded by cracking of the ground as magma is forcefully injected into the area. The cracks may be as much as 6 feet wide and over a mile long; typically they form within a period of hours. The Kapoho area on Kilauea's lower east rift zone experienced such ground breakage prior to eruptions in 1924, 1955, and 1960.

Before the 1960 eruption, a large ground crack opened in the village of Kapoho,
which was built on Kilauea's east rift zone.
The village was later completely destroyed by the eruption.

Ground settling may occur near a vent at the end of an eruption; magma is draining away from beneath the vent area. This process produces both small depressions and large collapse features, such as the pit craters and summit calderas of Kilauea and Mauna Loa. In either case, the subsidence may be gradual or abrupt.

The hazard presented by ground cracks and settling associated with eruptions is usually limited
to area near the active vent and thus is overshadowed by the hazard posed by lava flows.
Man-made structures that escape other damage in the eruption, however, can be damaged or destroyed by cracking, tilting, or settling of the ground beneath them. Ground cracks will remain after the eruption is over and can pose a threat to unwary people and animals if the cracks are obscured by heavy vegetation.

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