Day 1:  September 6, 2004

Reporting:

Hubert Staudigel, Peter Schiffman, Robert Zierenberg

Crater Rim Trail; Hawaii Volcano Observatory,  Southwest Rift Zone, Ka’u desert, Block Size exercise from 1924 phreatic eruption, Halema’uma’u crater, 1982 spatter rampart; floor of Kilauea caldera; tumuli in 1885 flows

Support Files:   Daily Data for clast size exercise; Daily tracks, a plot of the hiking route, and track data, including GPS tracking data as a txt file and as an xls file.

Shortly after the beginning of our hike, we stopped at an overlook, where we viewed the partly obscured Mauna Loa to the West and the very clearly visible Kilauea Caldera in the East.

Key features of Mauna Loa (i.e. the “Long Mountain”) include the very gentle slopes, with a few vents, whereby Kilauea appears to be a rather small “blister” on the flanks of Mauna Loa.  Kilaua, to the south offers a grand view including Pu’u Puai, the vent for Kilauea Iki (“little Kilauea”) which represents the emerging point of the Kilauea’s SE rift (which further downrift turns into the East Rift.  The most prominent feature of Kilauea caldera is Halema’uma’u the substantial pit crater towards the western part of the Caldera and in the distance to the west the emergence of the Southwest  rift.  Lava flows have rather distinct colors where the most recent lava flows appear dark black in color while older flows are more earthy-brown.  We saw a series of recent lava flows, in particular in the western part of the caldera, the Sept 1974 lava flowing towards the south and the Sept 1982 flow flowing into the caldera. 

There are a few important boundary conditions to volcanism of Kilauea Caldera, in particular the predominant wind directions that let pyroclastic rocks (such as from the 1959 eruption of Pu’u Puai) drift towards the SW, and the rift configuration that determines the deformation of the volcano. The Kilauea portion to the south of its rift zone moves (“spreads”) towards the south (towards the ocean), where it can expand into open space while the northern part from these rift zones is buttressed against Mauna Loa and cannot move.  For this reason, the crater walls are higher in the north, and shallower in the south, and most of the lavas are issued and flow towards the south.

We recognize three main units, an underlying pahoehoe flow, an overlying, thinly layered pyroclastic deposit, and some very apparent and large blocks overlying the ash/fine lapilli volcaniclastic.  The layered pyroclastic deposits ebelong to the Keanakakoi Ash member of the Puna Basalt, and the overlying blocks  are from the 1924 explosive eruption of Halema’uma’u.  We discussed age relationships between in situ rocks at the crater rim and the crater collapse and concluded that the crater collapse must have happened after the pahoehoe lava erupted.

We discussed some basics about grain size variation in pyroclastic rocks, whereby volcanic  ash is less than 2mm grain size, lapilli are between 2 and 64 mm, and bombs and blocks are above  64 mm.  Bombs are comagmatic and blocks are wallrock that is not related to this particular eruption. 

Visit at the USGS HVO (Hawaii Volcano Observatory)

Don Swanson, HVO scientist in charge offered some insights into the HVO operations.  Don explained basic operations of the HVO, where the main charge is to make scientific observations and to put these into use for society’s benefit, for understanding hazards and predicting volcanic eruptions.  Don mentioned a series of monitoring techniques, including in particular seismology, but also deformation, and volcanic gas emission monitoring.  He elaborated on seismology that is probably the most useful monitoring technique, to determine the location of earthquakes and motion of magma underground.  Mauna Loa, is particularly carefully studied, because it appears to be continuously accumulating magma in its plumbing system but it is not erupting any, suggesting that a major eruption may be on its way.  He also pointed out a tracing of a seismic crisis of the magma plumbing system leading to Pu’u O’o when apparently a stoppage in the conduit resulted in accumulation of magma half way down the rift, with the potential threat of an upstream new eruption.  The blockage was relieved in the end and no new eruption site was established, but it was very clear that the eruptive pattern can change almost from one hour to the next.

We observe the Keanakakoi ash covering the ground, and the section is thicker than we observed at event 2.  Wavy and cross-bedded bedforms can be observed The deposit mantles the hummocky topography of the pre-caldera pahopehoe flows.

At this location, the Keanakakoi ash member is substantially  thicker (approximately 3-4 m ).  We discussed in particular two major layers, the bottom layer consisting of many thin layers of very fine vitric ash, and we noted the abundance of larger clasts in thicker, lithic-rich layers near the top.   We compared the lower vitric layer to deposits from the 1980 Mt St, Helens eruption, where,  volcanic eruptions caused dust-bearing “surges” that may be compared with hurricane force winds that were,  in the case of Mt St. Helens strong enough to flatten trees with several feet thick trunks.  The presence of abundant vitric material indicates very effective fragmentation and glass formation by water-rock interaction of a phreatomagmatic system, where magma interacts with groundwater to produce extremely explosive eruptions.

We also discussed the age of this eruption that apparently ceased in about 1790, but charcoal was used to ¹⁴C-date earlier parts of this eruption indicating that this eruption may have started as early as the 15^th century,  lasting several 100 years.

Keanakakoi tephra is exposed at the edge of this fissure and the edge of the fissure is overlain by welded spatter (agglutinate).  The welded spatter forms a nearly circular pattern (two half circles on both side of the fissure) of about 10 m diameter, beyond which it stops being welded together and made up of outward fining bombs and lapilli.  Neighboring to this location,there is one similar occurrence to the north and to the south of the same fissure.  We interpret these occurrences as minor bursts of lava fountaining along this fissure that is parallel to another major fissure that is south from there and that has been filled up with lava during the Sept. 1982 lava flow.

UTM  05 259028E    214806795N

A 8-10 m wide major fissure with Keanakakoi tephra  exposed along its edges, and with Sept. 1982 lavas draping over the sides.

We stopped at a relatively narrow point of this fissure and looked towards the west, where  lava spectacularly drapes over the edges of the fissure, with “bathtub” rings that define apparent levels of lava ponding in the fissure,  At our location, there was an apparent bridge across this fissure that had partly collapsed.

Keanakakoi tephra section, with encrustations.  We discussed the local weathering environment that appears to be very acidic (we measured a soil pH of 4.5) and where  some yellowish encrustations are composed of amorphous silica and jarosite, a potassic,Fe (III) sulfate that can form only during rather acidic environmental conditions.  We compared the local alteration environment to the recent discoveries on Mars, where the presence of jarosite also appears to indicate a rather acidic alteration environment.    The acidity is created by constant “fall-out” of sulfuric acid-bearing aerosols.  These aerosols are created from the continuous outgassing of SO2 from the magma chamber beneath Halema’uma’u.  The acid precipitation leaches the fine-grained, glassy matrix of the Keanakakoi tephra.   The resulting solutions are “wicked” to the walls of the fissures by capillary action.  Ev aporation then results in precipitation of the encrustations.  

pH measurements in the previous year were made following some major rainstorm giving near neutral measurements that indicate that a major rainfall can actually remove much of the (soil-) acidity, and later smaller amounts of acidic precipitation will re-establish the acidic conditions.

Group exercises to measure block sizes from the 1924 Halema’uma’u eruption on top of the 1920 Kilauea flow.   The purpose of the exercise is to use the distribution of maximum block size to create a model for the dynamics of the 1924 phreatic eruptions.

A north-south elongated ridge, approximately 8 m high, displays the results of a Curtain of Fire, with agglutinated/welded spatter, and a well exposed open fissure that allows the study of the deposition of spatter in such an event.   The spatter rampart is surrounded by flows of shelly pahoehoe, a proximal form of this lava type, which is characterized by thin brittle crusts which were inflated by rapidly exsolving gases, prior to solidification.

Towards the north,  light conditions allowed very easy identification of the Uwekahuna Lackolith, an intrusive body exposed near the base of the Uwekahuna  Bluff.

We climbed one of two, approximately 10-15 m tall and 100m wide tumuli, hills with an inverted saucer shape, and prominent extension cracks on the top.  These tumuli are inflation structures that are common at the end of pahoehoe flows.