Transcript RIFT ZONES AND CALDERAS ON HAWAIIAN VOLCANOES -

RIFT ZONES AND CALDERAS ON HAWAIIAN VOLCANOES
Scott K. Rowland, University of Hawai‘i at Manoa
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 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)
Rift zones extend well offshore
The Hana
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
Kilauea’s
east rift zone,
and is longer than the
on-land part.
Even Lo‘ihi,
the baby
of the Hawaiian
volcano family,
already has rift zones
(and a caldera,
maybe)
rift
zones
filled
caldera(?)
Bathymetry of Lo‘ihi,
compiled by the Hawai‘i
Mapping Research Group
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 Kilauea,
Mauna Loa, and Hualalai
have occurred from rift zone vents
Hawaiian volcanoes look kind of like big slugs snuggled
next to each other
Here is a scenario for the formation of the big island’s
volcanoes and rift zones. Note that the rift zones avoid
forming in a direction that would point them at an existing
volcano. Note also that this was drawn before Lo‘ihi
was
recognized as an active 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).
Rift zones are pretty obvious from earthquake locations
If an eruption is going to occur along a rift zone somewhere,
magma has to get there from the magma chamber.
It does this as a blade-shaped body of magma called a dike.
-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.
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
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 Kaneana.
old sea cave
Farrington Hwy.
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
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)
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.
Lines of spatter vents
Deep fissure
Young lava flows
Pit craters
~1 km
Vertical air photo of Napau
crater, fissures, and faults, Kilauea
ERZ.
Pit craters form by collapse –
they are 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)
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.
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
IS THERE A NONGEOLOGY REASON
TO CARE ABOUT THE
FACT THAT FLANK
ERUPTIONS ON
HAWAIIAN
VOLCANOES
ALMOSTALWAYS
OCCUR FROM RIFT
ZONES?
Of course !!
A cross-section from Mauna Loa, across Kilauea
and offshore
shows Kilauea
resting on Mauna Loa, and both resting on
ocean sediments
and provides a mechanism for large South-Flank earthquakes
CALDERAS ARE THE OTHER MAJOR STRUCTURAL FEATURE
ON HAWAIIAN VOLCANOES
Moku‘aweoweo
caldera, at the summit of Mauna Loa
Kilauea
caldera:
Complex calderaboundary faults,
and Halema‘uma‘u,
(H) an intra-caldera
pit crater
1954
lava flow
H
Magma chamber processes, as long as there is sufficient
supply to keep the magma chamber from solidifying
Complex migration
of the center of
inflation prior to an
eruption in 1967.
During the ensuing
eruption, the deflation
pattern was not a
reverse of the inflation
pattern.
Fiske & Kinoshita (1969)
The cumulative “collapse”
profile of the Kilauea
caldera
since 1823 is funnel-shaped
rather than piston shaped.
Vertical air photo of
Moku‘aweoweo,
the
summit caldera of
Mauna Loa.
Note flows truncated by
caldera margin.
~5 km
Ryan et al. (1882)
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
Savaii (Samoa) has very prominent rift zones marked by scoria cones
Karthala volcano on Grand Comoro
(about 1/2-way between Mozambique
and Madagascar) is the volcano most
similar to Hawaiian volcanoes with
respect to rift zones and a caldera
PAU