Hilo is situated on lava flows from two of the five volcanoes that form the Island of Hawaiʻi. In the northern part of Hilo near the Wailuku River (that forms the approximate boundary between Mauna Kea and Mauna Loa Volcanoes,) Mauna Loa flows overlie much older ash deposits and flows from Mauna Kea.
Twenty-seven Mauna Loa flows (pāhoehoe and ʻaʻa) have been identified in and near Hilo. The youngest flow is from the historic Mauna Loa eruption of 1880-81, and the oldest flow yet found lies near Hoaka Road, with an age of more than 24,000 years. (USGS)
The 1881 lavas reached just north of the present University of Hawaiʻi at Hilo campus. After crossing the present Komohana and Kumukoa Streets, a very narrow section crossed what is now Mohouli Street, about 300 yards above the intersection with Kapiʻolani Street.
Several hundred homes are now built on pāhoehoe lavas of the 1881 flow and can easily be recognized by their ubiquitous “rock gardens” (no soils have yet formed on this flow). Kaumana Cave was formed at this time and was a major supply conduit for the lavas that threatened Hilo. (USGS)
Lava Caves (more commonly called lava tubes) are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels and pāhoehoe flows.
When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty conduits beneath the ground. (USGS)
Kaumana Cave is located up the hill from the downtown area on Kaumana Drive (Saddle Road,) stretching for almost two miles. When you get to the Cave you can see a concrete stair case which leads through the old skylight down to the entrance to the Cave.
The Kaumana Cave, part of a 25-mile-long lava tube, is the centerpiece of a small park maintained by the County of Hawaiʻi. Above Hilo, near the 4-mile marker along Kaumana Drive, the cave’s entrance – actually a skylight formed when part of the lava tube collapsed – is open to curious visitors who want to explore the inside.
The roof of the tube is 20 to 25 feet thick in most places and most of the rubble on the floor fell during or shortly after the eruption, when the skylight entrance fell.
The tube was initially filled with fast-moving lava then the level dropped and a long period of flow along the floor took place and from time to time slopped over to the side creating the bench-like features seen near the cave entrance. Roof blocks fell and became embedded and coated with basalt. The lava stream later emptied leaving the evacuated tube. (Hostra)
A steep staircase leads into a collapse pit. Here the cave roof collapsed and allows entry into the lava tube. From here you can enter different sections of the cave, going mauka (uphill) or makai (downhill) paths.
Going makai, a short path leads to the entrance. There are a few boulders to step carefully through, after which sections of smooth and mostly level surfaces allow a bit easier access. About 50 yards into the downhill section you reach a choke point, a little scrambling and a bit of duck-walk is necessary to get through.
After the narrow, the cave opens back up again. After another hundred yards there are a series of ledges, old crusts left by cooling lava when it half-filled the cave. To continue from here requires crawling through another very low passage. (Cooper)
“Long ʻōhiʻa tree roots hang from overhead … Sides of Cave have dribbles of lava from above forming odd stalagmitelike objects on floor. (There is a) very noticeable slope which is quite easy to travel.”
“Another junction. This one has three branches. There are two shallow rimmed lava cones filled with water. Wedge-shaped overhang is off to one side.” (1953 Loins Club; DOI)
In periods of normal rainfall, running water sometimes is audible beneath the floor of the cave. Rainfalls of 8 to 12 inches produce waterfalls spouting from cracks high on the wall of one cave section.
They form a small stream that runs on or just beneath the floor for several hundred yads before finally sinking into cracks. Its flow is augmented by several small bubbling springs at or just above floor level and part of its flow also is lost into small floor-level cracks. (Halliday)
Kaumana Cave is an example of a lava tube cave that carries floodwater for over half-a-mile. The lower end of Kaumana Cave opens into a drainage ditch several yards below the roadway of Edita Street.