# Volcano Watch — When did Mokuaweoweo (the summit caldera of Mauna Loa) form?

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The summits of volcanoes are part of an ever-changing landscape. The summit of Mauna Loa has undergone considerable change in the short period since the first explorers began documenting their visits to the volcano.

Summit calderas usually form over a shallow subsurface reservoir, or magma chamber. The size of the depression governs whether we call the geographic feature a caldera or a crater. In general, if a feature is larger than 1.6 kilometers (1.0 miles) in diameter, we call it a caldera, and, if it is smaller, a crater. In addition, calderas form as a result of subsidence while craters form via either subsidence or explosions.

The magma chamber compartment fills with magma before an eruption and empties when magma evacuates due to an eruption or intrusion. An intrusion can be defined as a "failed eruption". It has all the signs of an eruption, such as increased seismicity and changes in tilt, and includes the subsurface movement or injection of magma. Since no lava reaches the surface, it is called an intrusion.

When magma evacuates a magma chamber, there is often no support for its roof rocks. The roof of the magma chamber literally caves in and creates a depression. The floor of the caldera is rapidly covered with new lava to create relatively flat and smooth topography. Pit craters form in a similar manner, but on a smaller scale, and they commonly develop over rift zones.

Summit calderas are indicative of the shield stage of Hawaiian volcanism. In contrast, Mauna Kea's summit is more pointed than Mauna Loa's and is dotted with cones. Mauna Kea is in the post-shield stage, and its summit caldera has been filled by later eruptions.

Mauna Loa's summit caldera measures 6.2 km (4 miles) long and 2.5 km (1.5 miles) wide. For those of you who have never been to the summit of Mauna Loa, envision three roughly circular depressions aligned in a northeast-southwest direction. Starting in the north, the first circular region is roughly 1 km (0.6 miles) across. The middle section is oblong and is approximately 4.2 km (2.6 miles) by 2.5 km (1.5 miles). The southernmost region is just less than 1 km (0.6 miles) in diameter. The three regions overlap slightly and form the summit caldera called Mokuaweoweo. The caldera rim on the west side stands 170 m (560 ft) above the caldera floor, while on the east, where the cabin is located, is approximately 50 m (160 ft) high.

When did the summit caldera on Mauna Loa form? Geologic evidence uncovered through field mapping indicates that Mauna Loa has formed several calderas in its life cycle. The most recent caldera formed during the interval between 1,000 and 1,500 years ago. The geologic evidence includes radiocarbon dates from lava flows that originated from the summit region. These ages span several hundred years and imply that the summit area was active over a long period of time.

What caused the subsidence? A large eruption on the flank of the volcano did it. From the work on the geologic history of Mauna Loa, we have identified an immense lava flow erupted from the northeast rift zone. We call this flow the Panaewa; its age coincides with the summit overflow lavas. The Panaewa flow forms the coastline from Haena (Shipman's Estate) to the Bay Front. Its source vent is near Kulani prison. Near the coast, the flow is over 10 m (33 ft) thick. This flow covers 172 square km (66 square miles) and has a computed eruptive volume of 1.72 cubic km (0.41 cubic miles). To put this into perspective, we could say that this eruptive volume would sustain the current eruption of Kīlauea for 6,278 days or over 17.2 years!

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Puu Oo vent during the past week and provided visitors with an occasional glimpse of surface flow activity on Pulama pali and on the coastal flats. Lava is pooling in the coastal flats and not entering the ocean at this time. The active flow front extends 0.6 km (0.36 mi) to the west from the end of the Royal Gardens private access road, which was shortened by 230 m (750 ft) by the latest flow activity. A lobe of the flow is 600 m (2,000 ft) from the sea coast in the old Kupapau area near the eastern boundary of Hawaii Volcanoes National Park.

Two earthquakes were reported felt during the week ending on March 22. Residents of the Volcano Golf Course subdivision and the Volcano Hazards team's Chief Scientist, staying at the Kīlauea Military Camp, felt an earthquake at 7:40 p.m. on Wednesday, March 21. The magnitude-3.5 earthquake was located 4 km (2.4 mi) west of the summit of Kīlauea Volcano at a depth of 5.7 km (3.4 mi). A resident of Glenwood reported feeling an earthquake at 9:24 a.m. on Thursday, March 22. The magnitude-3.3 earthquake was located 17 km (10.2 mi) southeast of the summit of Kīlauea Volcano at a depth of 5.7 km (3.4 mi).