Aerial view from above the uppermost southwest rift zone looking toward the northeast. Rim of Halema`uma`u is at very top of image; note Crater Rim Drive below Halema`uma`u.>
The large ground cracks developed over time when magma moved underground from the summit reservoir into the rift zone. The cracks indicate that this area was forced apart or widened as a consequence of magma entering the rift zone. Such ground cracking above a blade-like magma intrusion (whether rising or moving laterally) is typical at Kilauea, Mauna Loa, and other volcanoes.
The dark, shiny lava flows (near side of Crater Rim Drive in image) were erupted on September 24, 1971. The initial fissure broke out between Haleama`uma`u and the southwest caldera rim (top left). Then a new fissure cut across the floor of Halema`uma`u to form a single fissure zone nearly 2 km long. By the next morning, these fissures had stopped erupting and new ones developed along the southwest rift zone 4-12 km from Halema`uma`u. When the eruption ended four days later, the rift zone in this area had widened 1-3 m, as indicated by geodetic surveys and by tape measurements of cracks not covered with new lava.