A short review of pyroducts (lava tubes) (original) (raw)
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The Complexity of Pyroduct ("Lava Tubes") Genesis: Bird Park Cave, Hawaii Volcanoes National Park
Erupted basaltic lava flows downhill either in channels or conduits. The latter, termed “pyroducts” or colloquially “lava tubes”, can form by different processes. Pyroducts function as self-insolating conduits allowing lava to flow for long distances across low-slope terrain, thereby building shield volcanoes. Studies of roof structure serve to understand the initial roofing of the flow and studies of internal features can unravel processes that reshape the original conduit during prolonged activity. Therefore, even a “mono-trunked”, “simple tube” may be of complex development. In spring 2016, we surveyed and studied Bird Park Cave, a 200-m-long, unbranched pyroduct section underlying the Kipuka Puaulu (kipuka = land surrounded by later lava flows) in Hawaii Volcanoes National Park (HVNP). The kipuka is an outcrop of the Pāhala ash. The underlying cave in Mauna Loa lava is therefore stratigrafically one of the oldest in the HVNP. Geologically, the cave is interesting because of its overall good preservation, its many features and the fact, that it cut through an underlying ash layer. Access is through a cold puka (i.e. sinkhole), filled with ash and the cave ends at another cold puka, also filled with ash. A third puka opened during activity (hot puka) and, by allowing air flow, not only caused the freezing out of a secondary ceiling, but also caused other interesting features such as ripple-marks on the walls. The primary roof of the cave is composed of pāhoehoe lava sheets either emplaced by inflation or by surface flows. In the upper part the underlying ash-layer, hematized to a bright red by the heat of the transgressing lava, is situated near the ceiling, suggesting immediate downcutting after the first emplacement of the conduit. In the lower section tree casts mark the initial lava sheet while the underlying ash layer is found lower down in the passage, suggesting less down-ward erosion but more upward and side-ward enlargement by breakdown. These differences may be due to a change in slope that is overall 5°. The upper section of the cave shows a series of lining (and glazing) events, possibly associated with an oscillating process of down-cutting and ceiling accretion in the sections of lower slope. Two lavafalls are present, providing a mechanism of rapid down-cutting.
Lunar and Martian Lava Tube Exploration as Part of an Overall Scientific Survey
2009
Andrew W. Daga, M.M. Battler, J.D. Burke, I.A. Crawford, R.J. Léveillé, S.B. Simon, L.T. Tan. 1 University of North Dakota and Andrew Daga & Associates, LLC, 111 Mountain Laurel Lane, Malvern, PA 19355, Centre for Planetary Science & Exploration, University of Western Ontario, 1151 Richmond Street, London, ON, Canada, N6A 3K7 mbattle@uwo.ca, The Planetary Society, 65 North Catalina, Avenue, Pasadena, CA 91106 jdburke@caltech.edu, Department of Earth and Planetary Sciences, Birkbeck College London, Malet Street, London, WC1E 7HX, i.crawford@ucl.ac.uk, Canadian Space Agency, 6767 route de l’Aéroport, Saint-Hubert, QC, Canada, J3Y 8Y9, richard.leveille@asccsa.gc.ca, Department of the Geophysical Sciences, The University of Chicago, sbs8@uchicago.edu, University College London Chadwick Building, Gower Street Lo don,WC1E 6BT, UK l.tan@ucl.ac.uk.
2002
Caves are known to occur in the Phanerozoic limestones of northern and eastern Saudi Arabia. They formed in past intervals of moist climate rather than the present hyperarid one. Few caves are known in the west because the predominant rocks are metamorphosed, tectonized and crystalline, comprising the Precambrian shield. There are however two geological units younger than the crystalline basement in western Saudi Arabia, where caves might be found. These are: (1) raised Quaternary coral limestones along the Red Sea coastline; (2) the extensive Cenozoic basalt lava fields. The present report describes the first and successful search for lava-tube caves in the Cenozoic basaltic lava fields of Harrat Kishb (area 5,892 km2). Cenozoic lava fields, or harrats, occupy 80,000 km2 of western Saudi Arabia, where they mainly occur as 12 large lava fields in a chain extending from Syria in the north, across easternmost Jordan, southward for the entire length of western Saudi Arabia; and into Yemen in the south. The investigation reported here was largely confined to an area of central Harrat Kishb. Here, six lava-tube caves were located in three closely spaced areas in central Harrat Kishb in basalt lavas of four different stratigraphic units. All occur in lavas of the Cenozoic Hil basalt, which is less than one million years old. The Hil basalt is subdivided into six subunits. By far the largest and longest lava tube occurs on the western side of the scoria cone of Jabal Hil, which erupted a few thousands of years ago and belongs to the youngest subunit Qh6 of the Hil basalt. A detailed geologic map was made of the surface features of a major arterial lava tube extending for 3 km to the west of the parent cone. Basalt lava flows were extruded from five points along the arterial lava tube to build a ridge comprising a chain of low rootless shield volcanoes. Later, when the eruption ceased, the arterial lava tube drained and subsidence occurred into it. The interior of the lava tube is visible in several of these collapses. It has a height of over 20 m and the floor has a maximum below-surface depth of 42.5 km. Nearby, in stratigraphic subunit Qh4, a single lava tube cave (First Cave) is visible at the bottom of a 26.5 m deep shaft. The sides of this shaft are dangerously loose and the cave was not explored. Farther east, in subunit Qh3, three closely spaced lava-tube caves were found and are here named Ghostly Cave, Bushy Cave and Kahf al Mut’eb caves. The largest of these, Ghostly Cave has two passages extending 143 m and 140 m from the entrance. Nearby, in the same stratigraphic subunit, a small erosional cave (Window cave) only one meter deep is present in weakly lithified agglomerate. This is the only example of an erosion cave known on Harrat Kishb. A sixth lava-tube cave (Dahl Faisal), some 22 m long, was located on northern Harrat Kishb in subunit Qh1 of the Hil basalt. The lava-tube caves are elliptical in cross-section and have short lava stalactites on the roof and lava levées on the walls. They contain evidence of ancient man in the form of defensive walls, and two throwing sticks of possible Neolithic age were found. These tubes are half-filled with fluviatile sands and wind-blown dust, which may preserve a stratigraphic record in pollen and spores. Lava tubes form in basalt lavas after eruption ceases to extrude molten lava. At this stage, basalt lava may continue to flow downslope away from the volcano by draining the main feeder tube inside the flow at its highest point. Theoretically, every lava flow can have a lava tube. There are over 2,000 basaltic volcanoes standing in western Saudi Arabia, many of which have produced multiple lava flows. This fact and the success of the first search for lava-tube caves suggests that numerous such caves may exist in the Cenozoic lava fields of western Saudi Arabia, and further searches are recommended.
Observations of active lava flows has shown that there are two distinct ways in which lava tubes or caves form: 1: Roofing of surface lava channels. This can happen in three ways (e.g. Peterson et al, 1994), see panel 2. 2: Sub-crustal drainage within thin lava lobes or sheets. (e.g. Peterson et al, 1994 & Hon et al, 1994), These evolve from isolated chambers and proto-tubes into multi-chamber and multilevel systems. See panels 3-5 . Both Types (roofed channels and sub-crustal systems) evolve over time into large linear feeder tubes, and the evidence of their original formation may be destroyed. Draining of the liquid lava from these tubes will leave open caves. Most tubes never drain and become blocked with solid basalt. * Open Volcanic Vents are a rare type of cave formed by the draining of the lava back into the source vent. * Caves can also form in tectonic fissures. * Weathering of ash and lava can also form secondary caves.