As Volcano Awareness Month 2016 comes to an end, so, too, does our series of articles exploring the geology of the Hawaiian Islands. This week, we conclude our journey with Hawaii Island.
As you likely already know, the Big Island is made up of five volcanoes. From oldest to youngest, they are Kohala, Hualalai, Mauna Kea, Mauna Loa and Kilauea.
Less well-known is a sixth volcano — Mahukona — located just off the island’s northwest coastline. This submarine volcano is the oldest of the volcanoes that form the mass of the island. Mahukona last erupted about 300,000 years ago, but its geology is not as well-studied as its taller neighbors because it is not easily accessible.
Kohala began erupting just more than 1 million years ago. The southeast rift zone of this volcano gives it substantial length, extending beneath Mauna Kea and continuing as the offshore Hilo Ridge. Kohala is capped by postshield lava as young as about 120,000 years. While its postshield volcanism is probably done, rejuvenated eruptions might occur in the future — perhaps even millions of years from now, as has occurred on the islands of Oahu and Kauai. In the meantime, Kohala will continue to erode, from rain and catastrophic collapse, causing the volcano to become more rugged as time passes.
Hualalai and Mauna Kea, the next oldest volcanoes, share many characteristics. Both formed less than 1 million years ago and now are in the postshield stage. Their surfaces are dotted with cinder cones — the remnants of mildly explosive postshield eruptions. Mauna Kea is slightly younger, with its most recent eruption about 4,500 years ago, but Hualalai erupts more frequently. Although Mauna Kea probably will erupt again in the future, Hualalai is of more concern because it erupted just 215 years ago (in 1801) and looms above numerous towns along the island’s Kona Coast. Both volcanoes are in the waning stages of their lives, and their ages are beginning to show in the valleys carved into their flanks — especially along the Hamakua Coast on Mauna Kea.
In contrast, Mauna Loa and Kilauea are in the primes of their lives.
Mauna Loa is the largest active volcano on Earth. Its most recent eruption was in March 1984, but the volcano’s almost-32-year-long slumber is deceptive, as Mauna Loa’s long-term history reveals it typically erupts every five to six years. Mauna Loa will certainly erupt again, and the odds are many of us will live to see it. Even now, the volcano is inflating as magma accumulates beneath it.
What Kilauea lacks in size (compared to Mauna Loa), it makes up for with persistence of eruptive activity. The volcano has erupted more often than not for the past several hundred years and has produced a nearly steady stream of lava since 1983.
Geologic investigations of Kilauea’s past reveal the volcano alternates between periods dominated by explosive activity and periods dominated by effusive eruptions (lava flows). In some ways, Kilauea might be analogous to a volatile teenager — prone to occasional fits of temper and overall unsettled behavior, all while growing rapidly. This maturation process takes time, and Kilauea’s shield-building stage likely has hundreds of thousands of years to go before it ends.
Although not yet part of the island, another submarine volcano — Lo‘ihi — bears mentioning. If Kilauea is a teenager, Lo‘ihi is a mere toddler. The volcano is currently 30 kilometers (20 miles) off the south coast of the Big Island and about 1,000 meters (3,300 feet) below sea level. Seaward growth of Kilauea and Mauna Loa, coupled with the growth of Lo‘ihi as the volcano matures, eventually might connect the volcanoes above sea level.
We hope you’ve enjoyed our geological tour of the Hawaiian Islands during the past four weeks and that you were able to attend one or more of the Volcano Awareness Month talks offered by the USGS Hawaiian Volcano Observatory in January. Although the month is ending, volcano awareness can (and should) continue all year.
We invite you to check out HVO’s website (http://hvo.wr.usgs.gov) throughout 2016 for updates and more information about Hawaii’s active volcanoes.
Volcano activity updates
Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level was relatively stable, varying between about 35 and 38 m (115–125 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, scattered lava flow activity remains within about 6 km (4 mi) of Pu‘u ‘O‘o, and is not currently threatening nearby communities.
Mauna Loa is not erupting. Seismicity remains elevated above long-term background levels, with earthquakes occurring mostly beneath the volcano’s upper Southwest Rift Zone at depths less than 5 km (3 mi) during the past week. Continuously recording GPS instruments show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone of Mauna Loa, with inflation recently occurring mainly in the southwestern part of the magma storage complex.
There were no earthquakes reported felt on the Big Island during the past week.
Visit the HVO website (http://hvo.wr.usgs.gov) 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 808-967-8862 (Kilauea) or 808-967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey`s Hawaiian Volcano Observatory.
