January 28, 2014 – HAWAII – In our January Volcano Watch articles — Hawaii Island’s fifth annual Volcano Awareness Month — we are exploring important questions about how Hawaiian volcanoes work. Last week, we discussed how Hawaiian Islands grow; this week, we talk about how they fall apart. In 1964, irregular submarine topography north of Oahu and Molokai was identified in newly available maps of the sea floor made by the U.S. Navy. James Moore, then Scientist-in-Charge at the USGS Hawaiian Volcano Observatory, suggested that this odd bathymetry might reflect massive landslides originating from those islands. Moore’s interpretation was disputed for more than 20 years until comprehensive mapping of the sea floor around the entire state of Hawaii was completed in the late 1980s. It turned out that Moore was right. Large — even catastrophic — submarine landslide structures litter the sea floor around the Hawaiian Islands. In fact, 17 major landslides have been identified off the shores of the main Hawaiian Islands. Fortunately, these slides are exceedingly rare — occurring, on average, only once every 350,000 years. The largest landslides constitute significant portions of the islands from which they originated. Imagine if 10 percent of one of the islands suddenly collapsed into the ocean. Such an event would displace a huge amount of water and cause a large tsunami.
Deposits of coral and sand have been found approximately 1,000 feet above sea level on several of the Hawaiian Islands. Catastrophic landslides are believed to have generated gigantic tsunami waves that washed ashore and left these deposits behind. Evidence across the Hawaiian Islands suggests that landslides occur during all stages of a volcano’s life. The submarine volcano Loihi — the youngest in the Hawaiian chain, located southeast of Hawaii Island — is characterized by a number of small landslides, even though the volcano hasn’t yet breached the surface of the ocean. On the other hand, large landslides from Oahu and Molokai clearly occurred well after the islands were established above sea level. We also know that not all landslides in Hawaii are catastrophic. The south flank of Kilauea is sliding continuously into the ocean at a rate of about 3 inches a year. This motion is punctuated by large, devastating earthquakes that can cause tens of feet of seaward motion in just a few seconds — as when the magnitude 7.7 temblor struck Hawaii Island in 1975 — as well as “slow earthquakes” that are associated with a few inches of seaward motion over the course of one to two days.
Will Kilauea’s south flank ever collapse suddenly? Since the shape of the south flank indicates that the slide has been active for thousands of years, there is no reason to expect that its behavior will change any time soon. Although most evidence suggests that it will continue to sag gradually, this question remains open to interpretation. What, then, causes large landslides in Hawaii? Models suggest that magma pressure alone is not adequate to produce a massive landslide. One can imagine a scenario, however, in which a large eruption weakens an already unstable volcano, allowing gravity to pull the volcano apart. Future scientific research must focus on the mechanism for giant landslides in Hawaii, which represent a major, infrequent hazard. Since other volcanic islands — such as the Canaries and the Azores — are also subject to catastrophic collapse, lessons learned from the Hawaii example might be fruitfully applied to mitigating hazards for the benefit of citizens elsewhere around the world. Next week, our annual Volcano Awareness Month Volcano Watch series will conclude with an examination of questions related to volcano monitoring. –WHT