RIFT ZONES ON HAWAIIAN VOLCANOES Courtesy: Scott Rowland, University of Hawai‘i Ka Hana ‘ Imi Na ‘ auao – A Science Careers Curriculum Resource Go to: Ka Hana ‘ Imi Na ‘ auao – A Science Careers Curriculum Resource Go to:

Rift zones are major structural features on Hawaiian shield volcanoes

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

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

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

Rift zones extend well offshore 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.

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

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 1-2 km

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

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

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). 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.

Rift zones are pretty obvious from earthquake locations

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.

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

Solidified dikes are exposed by erosion on older Hawaiian volcanoes

If a whole lot of dikes are exposed, it means that erosion has exposed the core of an old rift zone 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

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

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

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.

DIKE DIMENSIONS 10s of km Dike Length: the distance from the magma chamber to the eruption site (can be 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) 1-3 km Dike Width: measured in old, eroded volcanoes (usually ~1 m) ~1 m

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

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

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)

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. Another pit crater formation mechanism has been proposed

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

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

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