RIFT ZONES ON HAWAIIAN VOLCANOES

1. Ka Hana ‘Imi Na‘auao – A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana RIFT ZONES ON HAWAIIAN VOLCANOES Courtesy: Scott Rowland, University of Hawai‘i

2. Rift zones are major structural features on Hawaiian shield volcanoes

3. Most Hawaiian volcanoes, especially when they’re young, are elongate rather than round in map view

4. Ko‘olau rift zone axes and caldera Wai‘anae rift zone axes and caldera Rift zones on O‘ahu are identified most easily by mapping dike swarms

5. Rift zones have the form of broad topographic ridges, and are where flank eruptions occur Topography of Mauna Loa’s SW rift zone (dark flows post-date 1778)

6. The Hāna ridge is the incredibly-long offshore extension of East Maui Volcano’s East rift zone The Hilo ridge is closest to Mauna Kea, but some believe it is a rift zone of Kohala The Puna ridge is the offshore extension of Kīlauea’s east rift zone, and is longer than the on-land part. Rift zones extend well offshore

7. rift zones filled caldera(?) Even Lō‘ihi, the baby of the Hawaiian volcano family, already has rift zones (and a caldera, maybe) Bathymetry of Lō‘ihi, compiled by the Hawai‘i Mapping Research Group

8. 1-2 km A portion of Mauna Loa’s NE rift zone - each of the gray areas is a lava flow, and following them upslope leads to a 1-2 km-wide band, which is the rift zone axis

9. It is pretty clear that almost all recent eruptions of Kīlauea, Mauna Loa, and Hualālai have occurred from rift zone vents

10. Hawaiian volcanoes look kind of like big slugs snuggled next to each other

11. Here is a scenario for how the Big Island’s volcanoes and rift zones formed. Note that the rift zones avoid forming in a direction that would point them at an existing volcano. We understand pretty well why rift zones develop in a young volcano that is growing next to an older neighbor that already has rift zones. We don’t understand why rift zones form in the first place (e.g. on Kohala).

12. Rift zones are pretty obvious from earthquake locations

13. If an eruption is going to occur along a rift zone, magma has to get there from the magma chamber. It does this as a blade-shaped body of magma called a dike (think of a knife cutting though cake). - Tracking the rock-breaking earthquakes allows geologists to determine that most dikes propagate at 1-2 km/hour. - Harmonic tremor tells geologists that magma is continuing to flow underground. Next time you are near a large water pipe, put your hand on it and you will feel harmonic tremor.

14. A “curtain of fire” (actually a curtain of lava) occurs when a dike intersects the volcano’s surface. Most Hawaiian eruptions begin this way. 1971 eruption viewed from the Hawaiian Volcano Observatory, photo by Hawai‘i Volcanoes National Park staff

15. Solidified dikes are exposed by erosion on older Hawaiian volcanoes

16. Here are a whole bunch of dikes in a roadcut near Windward Community College (unfortunately they’re now covered by a wall). Photo by F. McCoy If a whole lot of dikes are exposed, it means that erosion has exposed the core of an old rift zone

17. Farrington Hwy. The axis of one of the Wai‘anae Volcano rift zones is exposed in the cliff at Kāneana. old sea cave

18. Pre-dike flows that the dike propagated through Edge of the dike (arrows show propagation direction) Side of the dike There is even a place nearby where erosion has exposed the side of a dike, not just the edge.

19. Dike rock is usually more resistant to erosion than the lava flows that the dikes are cutting through. They end up standing above the more eroded flows to form narrow blade-like ridges.

20. Dike Length: the distance from the magma chamber to the eruption site (can be 30-40 km, or more) Dike Height: the distance from the deepest to the shallowest rock-breaking earthquakes during a dike propagation event (usually 1-3 km) Dike Width: measured in old, eroded volcanoes (usually ~1 m) 10s of km 1-3 km ~1 m DIKE DIMENSIONS

21. Lines of spatter vents Deep fissure Young lava flows Pit craters Rift zones on the surface of a young volcano are marked by vents, gaping cracks and fissures, young lava flows, and pit craters. ~1 km Vertical air photo of Nāpau crater, fissures, and faults, Kīlauea ERZ.

22. If pit craters were blasted out from below, there would be a pile of ejected material around the rim (there isn’t). Pit craters form by collapse –not blasted out from below person for scale

23. One mechanism that has been proposed to form pit craters involves a process called “stoping”. This involves the repeated collapse of the roof of a cavity until the cavity breaks the surface. This process has been observed to occur in old mines. (diagram from Walker 1988)

24. Another pit crater formation mechanism has been proposed In this scenario, you should expect to find pit craters at the place where the ground fractures come close to each other. This idea was proposed by Chris Okubo, at the time a UH undergrad.

25. Fractures Here is a small pit crater, “Devil’s Throat”, very near to Chain of Craters Rd., in Hawai‘i Volcanoes National Park. photo by P. Mouginis-Mark

26. Where do you live!! Why should you care that eruptions on Hawaiian Volcanoes almost always occur from rift zones?

27. and provides a mechanism for large South-Flank earthquakes A cross-section from Mauna Loa, across Kīlauea and offshore shows Kīlauea resting on Mauna Loa, and both resting on ocean sediments