ABSTRACT Near Moku‘āweoweo, Mauna Loa’s summit caldera, there are three fans of explosive deposits. The fans, located to the west, northwest, and east, are strongly arcuate in map view. Along ‘Āinapō Trail, 2.8–3.5 km southeast of the caldera, there are several small kīpuka that expose a fourth explosive deposit. Although these explosive deposits have been known for some time, no study bearing on the nature of the explosive activity that formed them has been done. By analyzing cosmogenic exposure age data and the physical properties of the debris fans—lithology, size distributions, and clast dispersal—we conclude that the lithic deposits are the result of five separate phreatic events. The lithic ejecta consist of fragments of ponded lavas, pāhoehoe, gabbroic xenoliths, and “bread-crust” fragments. The exposure ages indicate that the explosive deposit on the west caldera rim was erupted 868 ± 57 yr B.P.; for the northwest fan, the age determination is 829 ± 51 yr B.P.; and on the east rim, ejecta deposits are younger, with ages of 150 ± 20 and 220 ± 20 yr B.P. Lavas underlying these deposits have exposure ages of 960–1020 yr B.P., consistent with the stratigraphy. Near ‘Āinapō Trail, the explosive deposit is much older, overlain by flows dated with a pooled mean age of 1507 ± 19 yr B.P. From the cosmogenic dating, we have three reliable and unambiguous dates. At a much earlier time, a fourth explosive eruption created the ‘Āinapō Trail deposit. We conclude there were at least five explosive episodes around the summit caldera. These deposits, along with recent work done on Kīlauea’s explosive activity, further discredit the notion that Hawaiian volcanoes are strictly effusive in nature. The evidence from the summit of Mauna Loa indicates that it, too, has erupted explosively in recent history.

We have organized ten National Aeronautics and Space Administration (NASA)–sponsored planetary volcanology field workshops on Hawai‘i since 1992, providing an opportunity for almost 140 NASA-funded graduate students, postdocs, and junior faculty to view basaltic volcano features up close in the company of both terrestrial and planetary volcanologists. Most of the workshops have been thematic, for example, concentrating on large structural features (rift zones and calderas) or lava flows, or features best viewed in high-spatial-resolution data, but they always include a broad set of topics. The workshops purposely involve long field days—an appreciation of scale is important for planetary scientists, particularly if they are or will be working with slow-moving rovers. Our goals are to give these young scientists a strong background in basaltic volcanology and provide the chance to view eruptive and volcano-structural features up close so that they can compare the appearance of these features in the field to their representations in state-of-the-art remote-sensing images, and relate them in turn to analogous planetary features. In addition, the workshop enables the participants to start a collection of field photographs and observations that they can use in future research and teaching. An added benefit is that the participants interact with each other, forging collaborations that we hope will persist throughout their careers.