AMCS Bulletin 19 / SMES Boletín 7 — 2002 35 Small Subcrustal Lava Caves: Examples from Victoria, Australia* Ken G. Grimes RRN 795 Morgiana Rd., Hamilton, Victoria, 3300, Australia; [email protected] * Presented in 2002, revised in 2006. Introduction Peterson & others (1994), Hon & oth- them pressure ridges). Unfortunately, ers (1994) and Kauahikaua & others on the relatively old (20-40,000 year) This paper discusses the formation of (1998) and which is illustrated in Figure flows in Victoria weathering and veg- a type of small lava cave that forms by 3. Recently, Halliday (1998a & b) has etation growth has reduced much of the “Subcrustal drainage” of lava flows. Ex- described two types of small lava cave: surface to a cracked and jumbled rubble. amples will be drawn from the Western His “sheet flow caves’ and ‘hollow vol- Thus, definitive axial clefts are difficult District Volcanic Province of western canic tumulus caves’ which he regards to identify and the new (genetically- Victoria, Australia (Figure 1). Caves as being distinct. I will argue that these from that province are numbered in are probably just two of several possible the Australian Karst Index with a “3H” members of a continuum of forms which prefix, abbreviated here to just “H” e.g. have been referred to as “Subcrustal H-70 (Matthews, 1985). lava caves” (e.g. Stevenson, 1999). In a review/study of active volcanoes in Hawaii, Peterson & others (1994) pro- The terminology of surface lava flow posed two distinct models for the forma- features and their caves has become tion of lava tubes: either by the roofing rather complex and confusing in recent over of linear surface lava channels (Fig- years, so I will list here some terms - and ure 2); or by the draining of still molten my intended usage. material from beneath the solidified crust Surface lava features — what is a of pahoehoe flow lobes (Figure 3). The tumulus? The changing usage of “tumu- former process produces relatively large lus” affects the definition of a “tumulus” and simple lava tubes. However, this cave ! Walker (1991) gave the term paper will concentrate on the smaller, “tumulus” a genetic definition which but commonly complex, caves formed both expanded the term to incorporate by localisation of flow beneath the crust all lava rises, including elongated ridges, of thin flow lobes or sheet-flows, and and narrowed its usage to those rises subsequent partial draining - a process that show evidence of inflation, given that has been progressively recognised by opened axial clefts on the crest, but and described by Peterson & Swanson which have no evidence of lateral com- (1974), Wood (1977), Greeley (1987), pression (if there was, Walker would call Figure 1. Location map of the Western District Volcanic Province, and its main lava Figure 2. Three ways of forming large cave areas. lava tubes by roofing a surface channel. 36 AMCS Bulletin 19 / SMES Boletín 7 — 2002 based) usage of the term “tumulus” has mix of his “tumuli”, “pressure ridges”, an example of the latter type. limited use. In Victoria the usage of “lava rises” and “lava rise pits”. In Victoria, speleologists have used “tumulus” has always been restricted For this paper I will use the descrip- the term “blister cave” for the small, to the distinctive, steep-sided, roughly- tive, non-genetic, term “lava mound” simple, isolated chambers found under circular mounds described by Ollier to describe all high areas within a lava the stony rises (Figure 4). However, (1964) in the Harman valley (many of field. Lava mounds are likely places care is needed to avoid confusion with which do have obvious large summit for the formation of small drained sub- another usage of that term for small clefts) and the more general term “stony crustal lava caves, whatever the process chambers formed by gas pressure (Gib- rises” is used for the chaotic complex of mound formation. son, 1974, and Larson, 1993). I suggest of broader hummocks and hollows that Cave types: In any discussion of lava usage of lava blister for those inflated by occur on many of Victoria’s younger caves and their genesis it is important to liquid lava (and later drained), and gas lava surfaces. This local usage of “stony distinguish between active (lava-filled) blister for those generated by gas pres- rises” would seem to correspond to the proto-caves and the drained tubes and sure. “Blister” should only be used on “hummocky pahoehoe” of Hon et al chambers (i.e. caves) which appear at its own where the genesis is uncertain. (1994) but some have relatively flat the end of the eruption – as discussed surfaces that correspond to their “sheet by Halliday (2004). The basaltic Western District Volca- flows” and there are also transitional In an earlier paper (Grimes, 1995) I nic Province (previously known as the forms. In Walker’s (1991) terminology described complex, lateral, levee-breach Newer Volcanic Province) of western the Victorian “stony rises” represent a systems associated with lava channels at Victoria has over 400 identified eruptive Mount Eccles, and distinguished them points and it ranges in age mainly from from smaller, isolated, drained chambers Pliocene (about 5 Million years) up to in the surrounding “stony rises” but very recent times (5ka), though there are did not suggest a formal nomenclature. some volcanoes as old as 7 Ma (Joyce, The terminology of Halliday (1998a,b), 1988, Joyce & Webb, 2003, Price & which is based on the surface lava flow others, 2003). Lava caves are known character (after Walker, 1991), is difficult across the whole province (Figure 1), to apply to the Victorian subcrustal caves but are most common in the younger because of the problem in distinguish- flows associated with Mount Eccles ing “tumuli” (sensu Walker, 1991) from (20-33 ka, Head, & others, 1991, and P. other lava mounds. I also suspect that Kershaw, per comm, 2005) and Mount rather than two discrete genetic types Napier (about 32 ka, Stone & others, of small subcrustal cave as proposed by 1997). Recent summaries of both the Halliday (“sheet flow caves’ and ‘hollow surface landforms and the volcanic caves volcanic tumulus caves’), we have a of the province appear in Grimes (1995, broad continuum of forms with a number 1999); and Grimes & Watson (1995). of distinctive end-members. The earlier literature on lava caves of I suggest that the cave classification the region by Ollier, Joyce and others should not be tied to the surface termi- is reviewed in Webb & others (1982) nology until the processes of cave devel- and Grimes & Watson (1995) and only opment are better known. Also, basing some of those papers are referenced here. the cave nomenclature on the surface The younger lava flows have surfaces lava forms may be confusing cause and ranging from strongly undulating (“stony effect—rather we should be explaining rises”) to flat. some surface mounds and tumuli as a At Mount Eccles the main volcano result of localised subcrustal flow in is a deep steep-walled elongated cra- tubes, not the cause (see conclusion). ter which contains Lake Surprise. At The unifying factor in all these caves is the north-western end the crater wall that they form by drainage from beneath has been breached by a lava channel a broadly-crusted lava flow; hence I that flows west and then branches into will refer to them here collectively as two main channels (referred to locally subcrustal lava caves. as ‘lava canals’) running to the west- In this paper my discussion will northwest and to the south-southwest concentrate on the smaller subcrustal (Grimes, 1995, 1999). Extending to Figure 3. Formation of lava caves by sub- lava caves, those that form originally, the southeast from the main crater there crustal drainage of a series of advancing lava lobes. Step 3a is the situation if the rather than the larger more evolved forms is a line of smaller spatter and scoria source of lava ceases early in the develop- which can develop from them over time cones and craters. Several smaller lava ment; irregular caves form. Steps 3b and and which tend to become closer in channels run out from these. Lava caves 4 indicate the further evolution into more shape to the tunnels formed by roofing occur in a variety of settings. linear feeder tubes as lava continues to of surface channels. In Victoria, the Mt. Beyond this central area of explosive flow through the system. Hamilton lava cave (Figure 14) may be activity, basalt flows form a lava field AMCS Bulletin 19 / SMES Boletín 7 — 2002 37 about 16 km long and 8 km across. From The cone contains one group of complex the western end of this lava field a long lava tubes (Ollier, 1963). flow, the Tyrendarra Flow, runs 30 km In the late Quaternary lava flows of southwards to the present coast and Mount Eccles and Mount Napier, in continues offshore for a further 15 km the Western District Volcanic Province, (sea level was lower at the time of the we find both cave types described by eruption). This long flow must have had of Peterson & others (1994) and also a major feeder tube, but no drained sec- isolated “lava blister” caves - I will tions have been discovered to date. draw my examples from those areas. Mt Napier and the Harman Valley The complex lava cave system at Mount flow: Mt Napier, about 20 km northeast Hamilton appears to be a further-evolved of Mt. Eccles, is a steep cinder cone “feeder” system. Figure 5. Turtle Cave, H-90, at Byaduk, capping a broad lava shield 10 km in Most of the longer caves known at is an example of a simple “lava blister” diameter.