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Slip-Sliding Away—Disassembling Hawaiian Volcanoes

January 24, 2014

Black dashed lines delineate 17 distinct landslides that have occurred around the Hawaiian Islands over the last several million years. The largest slides originated from the north sides of Oʻahu and Molokaʻi. Colors ranging from pink to purple indicate the water depth around the islands, while shades of gray show land topography above sea level. Red areas on the Island of Hawaiʻi indicate lava flows erupted within the past 200 years. (USGS)

The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:

In our January Volcano Watch articles during Hawaiʻi Island’s 5th 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 Oʻahu and Molokaʻi 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 up to several hundred meters (approximately 1,000 ft) 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 Lōʻihi—the youngest in the Hawaiian chain, located southeast of Hawaiʻi 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 Oʻahu and Molokaʻi clearly occurred well after the islands were established above sea level.

We also know that not all landslides in Hawaiʻi are catastrophic. The south flank of Kīlauea is sliding continuously into the ocean at a rate of about 8 cm a year (3 inches a year). This motion is punctuated by large, devastating earthquakes that can cause several meters (approximately tens of feet) of seaward motion in just a few seconds (as when the magnitude-7.7 temblor struck Hawaiʻi Island in 1975), as well as “slow earthquakes” (described in past Volcano Watch articles) that are associated with a few centimeters (inches) of seaward motion over the course of 1–2 days.

This false-color composite image of Hawai’i Island was constructed from data gathered between 1999 and 2001 by the Enhanced Thematic Mapper plus (ETM+) instrument, flying aboard the Landsat 7 satellite. (NASA)

Will Kīlauea’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 Hawaiʻi? 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 Hawaiʻi, which represent a major, if fortunately infrequent, hazard. Since other volcanic islands—such as the Canaries and the Azores—are also subject to catastrophic collapse, lessons learned from the Hawaiʻi 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.

Until then, you’re invited to attend a talk about Kīlauea Volcano’s gas emissions and vog on January 28 in Hawaiʻi Volcanoes National Park. Details are posted at http://hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 808-967-8844 for more information.

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