Route 200, the Daniel K. Inouye Highway, crosses Humuula Saddle, which separates Mauna Kea and Mauna Loa, the two largest volcanoes on Hawaii Island. This Saddle showcases outstanding volcanic geology and is easy to reach for “roadside geologists.”
For the first 19 miles, Highway 200 ascends almost a mile above sea level through mostly young native forest. The road crosses lava flows and patches of ashy soil ranging in age from 150 to 4,000 years old. Rainfall is so abundant here — up to nearly 300 inches per year — that vegetation covers most outcrops.
Around mile marker 19, Highway 200 emerges onto young-looking, lightly-vegetated a‘a lava from the 1855-56 Mauna Loa eruption. This flow is one of seven that traveled from Mauna Loa toward Hilo in recorded history. The 1855-1856 eruption lasted a year and a half. Imagine the commuter’s nightmare were this to occur today.
Between mile markers 19-20, tall trees occupy numerous kipuka growing on Punahoa pahoehoe lava erupted from Mauna Loa 3,100-3,200 years ago. Near mile marker 21, the road drops into a kipuka floored by the 400-year-old Mauna Loa Ainahou a’a flow.
Near Saddlehouse Road, the road rises onto Mauna Loa a‘a lava erupted in 1935-36. This flow was the first-ever in the United States that authorities tried to “manipulate” to protect property downslope. The U.S. Army Air Corps, advised by Thomas Jaggar, Director of the Hawaiian Volcano Observatory at the time, dropped 600-pound bombs on the active flow about 10 miles southwest of the intersection. The goal was to disrupt the lava channel, causing the flow to spread laterally rather than continue downhill. The impact was negligible.
On the right side of the highway, just after Saddlehouse Road, the 1935-36 lava is rumpled into a spectacular series of low compressional ridges (“lava ogives”). These features are easiest to spot in early morning or dusk light. Each ridge indicates that you are near the end of the flow, where the lava became too stiff to continue. The actual terminus is about 2 miles away.
South of the road between mile markers 22-23, a pullout marks the start of the Puu Oo Trail, which crosses a landscape of diverse kipuka and younger flows. Spectacular old-growth ohia and tree ferns appear in scattered groves. The trail, which used to continue all the way to Volcano ranchlands, now ends after 4 miles at the edge of the 1984 flow, the youngest lava to head toward Hilo.
Past mile marker 22, road builders deliberately constructed Highway 200 atop the 1935-36 a’a flow. That’s because rubbly a’a is much easier than solid pahoehoe to grade.
Near mile marker 23, Highway 200 approaches the base of Mauna Kea. The lumpy, grassy landscape to the right of the road results from the eruption of hawaiite, a unique type of lava that is notably stiffer and richer in silica and potassium than Mauna Loa flows. The numerous upslope cones belong to the Puuloa volcanic field, which last erupted about 4,600 years ago and was the source of the hawaiite. Puuloa vents extend to the top of Mauna Kea.
A well-loved local landmark, Puuhuluhulu, is at the crest of the Saddle. This densely-forested Mauna Kea cinder cone is entirely surrounded by younger Mauna Loa flows. An abandoned quarry at its eastern end exposes the cone’s interior, showing that sometime after eruption of loose, porous cinders, narrow dikes of basaltic lava worked their way into the edifice. The dike rock matches the chemistry of Mauna Loa’s lavas, suggesting that they intruded much later. But how and why they did remain a head-scratcher for geologists.
Near the northwestern foot of Puuhuluhulu is a stone wall, built by hand in the late 19th century by local cattle ranchers. The 1935-36 lava lapped around and overtopped the wall in places. Stunning inflation features illustrate that the flow inflated like rising bread crust on both sides of the wall after initially coming to rest against it.
Other interesting features may be discovered in the Humuula Saddle, such as ancient dune fields, glacial deposits, and rocks from Earth’s deep interior. We’ve described only a few here, hopefully instilling a new appreciation for our spectacular aina.
Volcano Activity Updates
This past week, Kilauea Volcano’s summit lava lake level fluctuated with summit inflation and deflation, ranging about 102-157 feet below the vent rim. On the East Rift Zone, the 61g lava flow remained active downslope of Puu Oo, with scattered breakouts on the coastal plain. The 61g flows do not pose an immediate threat to nearby communities.
Mauna Loa is not erupting. A few small-magnitude earthquakes occurred beneath the summit caldera and upper Southwest Rift Zone, mostly at depths less than 3 miles, with a few more on the volcano’s west flank at depths of 3-8 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.
No earthquakes were reported felt on Hawaii Island during the past week.
Visit https://volcanoes.usgs.gov/hvo for past Volcano Watch articles, volcano updates and photos, recent earthquake info, and more. Call for summary updates at (808) 967-8862 (Kilauea) or (808) 967-8866 (Mauna Loa). Email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.