Ka Hana ‘Imi Na‘auao – A Science Careers Curriculum Resource
Go to: www.cds.hawaii.edu/kahana
Rift zones are major structural features on
Hawaiian shield volcanoes
RIFT ZONES ON HAWAIIAN VOLCANOES
Courtesy: Scott Rowland, University of Hawai‘i
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
Ko‘olau
rift zone axes
and caldera
Wai‘anae
rift zone axes
and caldera
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
Topography of Mauna Loa’s SW rift zone (dark flows post-date 1778)
The Puna ridge is the
offshore extension of
Kīlauea’s east rift zone,
and is longer than the
on-land part.
Even Lō‘ihi, the baby
of the Hawaiian
volcano family,
already has rift zones
(and a caldera,
maybe)
rift
zones
filled
caldera(?)
Bathymetry of Lō‘ihi,
compiled by the Hawai‘i
Mapping Research Group
It is pretty clear that almost all recent eruptions of Kīlauea,
Mauna Loa, and Hualālai have occurred from rift zone vents
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).
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
Hawaiian volcanoes look kind of like big slugs snuggled
next to each other
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 bladeshaped body of magma called a dike (think of a knife cutting
though cake).
A “curtain of fire” (actually a curtain of lava) occurs when a dike
intersects the volcano’s surface. Most Hawaiian eruptions begin
this way.
- 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.
Solidified dikes are exposed by erosion on older Hawaiian
volcanoes
1971 eruption viewed from the Hawaiian Volcano Observatory, photo by
Hawai‘i Volcanoes National Park staff
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.
There is even a place nearby
where erosion has exposed
the side of a dike, not just
the edge.
Pre-dike flows that the
dike propagated through
old sea cave
Farrington Hwy.
Edge of the dike
(arrows show
propagation
direction)
Side of the dike
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
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
Rift zones on the surface of a young volcano are marked by vents, gaping
cracks and fissures, young lava flows, and pit craters.
Pit craters form by collapse –not blasted out from below
Lines of spatter vents
Deep fissure
person for scale
Young lava flows
Pit craters
~1 km
If pit craters were blasted out from below, there
would be a pile of ejected material around the rim
(there isn’t).
Vertical air photo of Nāpau crater, fissures, and faults, Kīlauea ERZ.
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.
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.
(diagram from Walker 1988)
Here is a small pit crater, “Devil’s Throat”, very near to
Chain of Craters Rd., in Hawai‘i Volcanoes National Park.
Why should you care
that eruptions on
Hawaiian Volcanoes
almost always occur
from rift zones?
Where do you live!!
Fractures
photo by P. Mouginis-Mark
A cross-section from Mauna Loa, across Kīlauea and offshore
shows Kīlauea resting on Mauna Loa, and both resting on ocean
sediments
and provides a mechanism for large South-Flank earthquakes