Day 9:  September 14, 2004

Reporting:

Vince Morton, Jamie Russell, Rachel Wood

South Sulfur Banks; Graben Structure in Hilina Pali Region; Hilina Pali Trail overlooking South Shore

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

From the vans, we crossed the 1982 shelly pahoehoe lava flow on the south side of Kilauea Caldera, to a region highly altered by volatiles from several fumaroles. An obvious feature was the yellowish appearance of the massive sulfide deposits in the wall. Also apparent was the acrid SO2 fumes. Around the fumarole openings are abundant large crystals of native sulfur, which is a product of the reduction of SO2. Hot gases of magmatic origin emerging from openings in the ground have altered portions of the 1982 lava that overlie them, as well as the older tephra beneath the lava flow. We recognized units of the Keanakakoi Ash member among the tephra deposits being chemically altered here, including the lapilli stone (layer 4) and vitric tuff (layer 3). In some locations the vitric tuff deposit has undergone complete alteration to pure white SiO2, while maintaining its structure and individual grain boundaries. Other lithic layers show a reddish color, indicating alteration of tephra layers to iron oxides and iron sulphates. Microscopic studies of these deposits have shown that this alteration follows a sequence, with the iron sulphates as the last alteration product.

An unconformity is present in the Keanakakoi ash, which was not present in the type locality for the deposit. Layers of lapilli stone were found with unconformities where vitric tuff cut down into the lapilli tuff. A layer dipping into the caldera cuts across the horizontal layers of the ash member. Some competing hypotheses for the unconformity are that a channel was eroded into the ash member by stream channels or possibly by lahars prior to deposition of the unconformable layers; alternately, it may have been formed by pyroclastic density flows from eruptions that followed the deposition of the Keanakakoi ash. Scour structures would be present at the unconformity contact in the latter case, and paleosols in the former.

Moving away from the wall, we saw the alteration appeared in parallel linear patterns across the outcrop. This suggested the existence of faults beneath the fumaroles, provided a conduit for volatile gases. About 100m to the south of the fumaroles we encountered a steam vent. No evidence for fumarolic volatiles were found here; only H2O vapor, indicating a meteorological source for volatiles, and not a magmatic source. We werenÕt able to obtain a reliable temperature measurement in the vent. It appeared that some polagonitization of the deposits had occurred in this vent. It is striking that faults in such close proximity could produce such different activity from one another.

This area is south of Kilauea in the Koa'e Fault Zone, which connects the Southwest Rift with the East Rift where the latter makes a pronounced change in trend from Southeast to east.

From the parking area we walked across old pahoehoe flows toward the south. Abundant cracks in the preserved flow resemble extensional fissures. There is no evidence of magmatic activity associated with the cracks. Pelee's hair aplenty, reticulite and Keanakakoi ash layers in the cracks dates the flow before 1500. In this area the flow is massive, not shelly or lobate. Flow structures in the pahoehoe suggest the lava originally ran down a gentle slope toward the south. But at present the area is tilted steeply to the north due to an impressive normal fault scarp. The fault strikes 075. The exposure is approximately 3-4 meters wide at the surface, 5 meters deep and extends for hundreds of meters. The south side of the fault is higher by 1-2 meters. Again, no magmatic activity is evident in the fault or elsewhere in the immediate area around the fault. Other surface expressions of normal faults are developing to the south of the main fault. About a kilometer to the north of the fault scarp another similar structure is visible, possibly another scarp. The two scarps bound a graben structure Š a typical extensional feature seen in other settings, for example in mid ocean spreading centers. The area has features of a rift valley. Cracks trend roughly parallel to the boundary between Kilauea and Mauna Loa.

Road was reportedly displaced by earthquake in 1965 by 8 feet, and the south side of the fault rose 2 feet. This event might be related to a tsunami which caused some deaths on the island.

Other structures nearby include some tumuli.

The faulting feature in the Southwest rift zone presented quite an anomaly in comparison to the magmatic East Rift zone. The volcanic edifice of Kilauea is building up on the southern flank of the massive structure of Mauna Loa. Because of this configuration, eruptions and mass movement from Kilauea tend to proceed southward rather than in a radially symmetric pattern. This results in the main extensional features south of Kilauea, the Southwest rift zone and the East rift zone. The present fault location is between these two main rifts.

In the East rift zone, extension is accompanied by magmatic intrusion and eruption, but in the Southwest rift it is amagmatic. In the East Rift Zone, the strain is accommodated by magma filled dikes intruding at depth, and lava erupting within vents. In the latter, grabens (rift) develop via normal faults and the spreading is amagmatic. In extensional environments, normal faults curve to more shallow dips at depth, and are referred to as listric. These listric faults combine to form a detachment surface or d-collement.

(Pali, Hawaiian word for cliffs.) These cliffs are a series of 5 or 6 major fault scarps descending in a step-like formation from our location at 731m elevation to sea level.
As we travel down the Pali, we are going down in geologic history. The Halina Basalt exposed here is 50,000 years in age. Puna Basalt is Holocene age. As the trail descends to the south shore it traverses the scarps, providing excellent exposures of cross-sections of Mauna Ulu lava flow and the Pali lava flow. At the bottom of the first scarp, we saw a layer of ash, presumably erupted from Kilauea.

Further down in the second scarp we observed excellent exposures of a lava flow in transition from pahoehoe to a'a. Ropy pahoehoe flow structures are intermixed with a'a blocks. Overturned exposures and lava tube interiors were revealed, many coated with white amorphous silica. The amorphous silica is similar to that seen on Day 2 in Kilauea Iki. Those deposits resulted from a dissolution of glassy surfaces by hot volatiles, followed by re-deposition in this amorphous form.

The transition from one flow regime to another can result from a steepening of the slope such as in this steep cliff setting. Pahoehoe flows proceed with a thin insulating skin over a molten interior. The skin is easily stretched and extended, allowing the interior to flow smoothly and retain heat more effectively. However, if the force pulling the flow downward increases beyond some critical point, the skin layer can break, disrupting the smooth flow of the interior and subjecting a greater surface area of the lava to radiative cooling. The surface of the flow then breaks up into a chaotic jumble of discrete blocks and is thereafter an a'a flow.