by USGS Hawaiian Volcano Observatory

This week, we continue our series of articles describing the most scientifically important volcanic eruptions in Hawai`i in recent times. Today’s article covers the 1969–1974 eruption of Mauna Ulu, on Kilauea’s east rift zone.

The eruption began from a series of fissures, as do most other Hawaiian eruptions, on May 24, 1969. And, like the Pu`u `O`o eruption (described earlier this month), the first stage of activity at Mauna Ulu included 12 episodic lava fountains, some reaching as high as 540 m (1,770 ft), and short-lived lava flows. After December 1969, fountains gave way to almost continuous quiet effusion of lava. The eruption paused in October 1971 but restarted in February 1972, and lava continued to flow until a series of eruptions near Kilauea’s summit spelled the end for Mauna Ulu on July 24, 1974.

The Mauna Ulu eruption was, at that time, the longest-lived rift zone eruption at Kilauea since at least the 15th century, and it occurred near Chain of Craters Road, so access for scientists was relatively easy. As a result, detailed, on-the-spot observations of eruptive activity could be made on an almost daily basis.

As was true for eruptions described in this column over the past two weeks, some of the most important new insights concerned lava flows. For example, observations from Mauna Ulu resulted in a better understanding of the development of pahoehoe and `a`a lava flows, how lava can flow long distances through tubes, and how broad pahoehoe fields form. The lava flow field buried several pit craters, including Alo`i and Alae—an excellent demonstration of the fleeting existence of Kilauea’s topography, and how Kilauea grows over time.

In addition, scientists watched as a lava shield (Mauna Ulu) was constructed before their eyes, providing a sense of how other lava shields on Kilauea and Mauna Loa were built. Interestingly, the crater at the summit of Mauna Ulu formed as the shield around it grew higher, and not due to collapse of the shield’s summit.

A lava fountain during the opening episode of the Mauna Ulu eruption feeds lava flows that cover Chain of Craters Road. Both `Alo`i Crater (lower left) and `Alae Crater (upper right) were filled with lava later in the eruption. Photo taken on May 28, 1969.

Although `a`a lava had entered the ocean during numerous other eruptions from Kilauea and Mauna Loa, the Mauna Ulu eruption was the first time that scientists were able to observe pahoehoe forming lava deltas and undersea flows. In fact, the observations and video recordings of pillow lavas forming as Mauna Ulu lava flows entered the sea were the first recorded anywhere in the world.

The Mauna Ulu shield hosted a lava lake in its summit crater for much of the eruption, and HVO volcanologists observed that the lava level rose slowly and fell abruptly as gas accumulated and escaped from beneath a surface crust, a process called gas pistoning. Interestingly, the same process is occurring right now within Kilauea’s summit eruptive vent. Our understanding of the current activity is based in large part on what was observed during the Mauna Ulu eruption over 40 years ago.

The Mauna Ulu eruption also provided an opportunity to document the interactions between eruptive activity on the east rift zone and at the summit. When the rate of lava eruption at Mauna Ulu began to slow, the summit inflated and earthquake activity increased as magma backed up in summit reservoirs. Deflation at the summit accompanied surges in lava effusion. These observations allowed HVO scientists to forecast when changes in the eruption were likely to occur; they are the basis for our current understanding of the relation between Pu`u `O`o and Kilauea’s summit magma reservoir.

Next week, in the final part of our series on scientifically noteworthy eruptions, we will discuss the 1959 Kilauea Iki eruption, renowned for its high lava fountains.
Until then, you may want to attend some of this week’s Volcano Awareness Month activities, described on our Web site (http://hvo.wr.usgs.gov) or by calling 808-967-8844.