The new “Geologic map of the northeast flank of Mauna Loa volcano, Island of Hawaii,” the culmination of many years of work by Hawaiian Volcano Observatory geologists, was recently published by the U.S. Geological Survey. The work was spearheaded by John P. Lockwood (affectionately known as “Mr. Mauna Loa”), who is now retired from USGS and HVO, and Frank Trusdell, HVO’s current Mauna Loa Project geologist.
For the northeast region of Mauna Loa, this updated map supersedes the “Geologic Map of the Island of Hawai‘i” (1996) and the “Geologic Map of the State of Hawaii” (2007).
Encompassing 440 square miles of the northeast flank of Mauna Loa, the new map comprises an area equivalent to the islands of Molokai and Lanai combined! The mapped area extends from an elevation of 10,880 feet to sea level, from Puu Ulaula (“Red Hill”) on the southwest to Hilo on the northeast.
Mauna Loa, the largest active volcano on Earth, is known to have erupted 33 times since written descriptions became available in 1832. Some eruptions were preceded by only brief seismic unrest, while others followed several months to a year of increased seismicity. Since 1832, seven eruptions occurred within the area covered by the map: 1852, 1855-56, 1880-81, 1899, 1935-36, 1942, and 1984.
The Northeast Rift Zone of Mauna Loa is about 25 miles long and 1.2-2.5 miles wide. It narrows at Mokuaweoweo, the volcano’s summit caldera, but becomes diffuse (3.4–4.3 miles wide) down rift near Puu Makaala Cone, about 7.4 miles west of Mountain View. The rift zone is marked by low spatter ramparts and spatter cones as high as 197 feet.
Eruptive fissures and ground cracks cut volcanic deposits and flows in and near the crest of the rift zone. Lava typically flows from the Northeast Rift Zone to the north, east, or south, depending on vent location relative to the rift crest. For instance, during the 1880-81 eruption of Mauna Loa, flows initially traveled south towards Kilauea, but later, northeast toward Hilo.
Although most of the Northeast Rift Zone source vents are more than 19 miles from Hilo, one branch of the 1880-81 flow nearly reached Hilo Bay. In fact, Hilo is built entirely on lava flows erupted from the Northeast Rift Zone, most of them older than 1852.
The map shows the distribution of 105 eruptive units (flows) — separated into 15 age groups ranging from more than 30,000 years before present to 1984 — as well as the relations of volcanic and surficial sedimentary deposits. The color scheme adopted for the map is based on the age of the volcanic deposits. Warm colors (red, pink, and orange) represent deposits from recent epochs of time, while cool colors (blue and purple) represent older deposits.
From the geologic record, we can deduce several facts about the geologic history of the Northeast Rift Zone. For example, in the past 4,000 years, the middle to uppermost sections of the rift zone were more active than the lower section, perhaps due to buttressing (compression) of the lower northeast rift zone by the adjacent Mauna Kea and Kilauea volcanoes.
Other interesting tidbits gleaned from the new map include: The Halai Hills near Hilo High School are 15,000- to 20,000-year-old eruptive cones from Mauna Loa, which are among the oldest geologic units found in Hilo. The lava flow at the top of Rainbow Falls is 10,000 years old. The largest flow in the map area is called the Panaewa picrite, an olivine-rich lava flow that erupted from a vent near the Kulani Correctional Facility, and which, at the coast, extends from Haena (Shipman Estate property) through Keaukaha to Mokuola (Coconut Island).
The geologic map provides fundamental information on the long-term eruptive behavior of Mauna Loa Volcano. In addition, it offers a valuable foundation from which collaborative studies in geology and biology can be launched. The map can be viewed or freely downloaded from USGS Publications at https://doi.org/10.3133/sim2932A.
Volcano Activity Updates
This past week, Kilauea Volcano’s summit lava lake fluctuated in concert with summit inflation and deflation, with levels ranging 85-141 feet below the vent rim. On the East Rift Zone, the 61g flow remained active, with lava entering the ocean near Kamokuna and surface flows downslope of Puu Oo and on the coastal plain. The 61g flows do not pose an immediate threat to nearby communities.
Mauna Loa is not erupting. During the past week, small-magnitude earthquakes occurred beneath the volcano, primarily in the south caldera and upper Southwest Rift Zone, at depths less than 3 miles. GPS measurements continue to show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone. No significant changes in volcanic gas emissions were measured.
One earthquake with three or more felt reports was recorded on Hawaii Island this past week. At 5:01 a.m. on July 9, a magnitude-2.5 earthquake occurred 1 mile southwest of Kahaluu at a depth of 8 miles.
Visit https://volcanoes.usgs.gov/hvo for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more. Call for summary updates at 967-8862 (Kilauea) or 967-8866 (Mauna Loa). Email questions to askHVO@usgs.gov.