Well-preserved examples of long lava flows exist in the young volcanic
provinces of eastern Australia.
Most voluminous are the flows of north Queensland, with the largest, the
190,000 year old Undara lava flow, travelling 160 km and covering anarea
of 1550 km.
The Undara Lava Tube System, North Queensland, Australia, is remarkable
not only for its geology, but also for unique flora and vertebrate and
Primarily pahoehoe in morphology, the Undara flow is characterised by
extensive single level lava tubes that aided the development of this long
A prominent morphological feature of the flow is the 40 km long, 5 to 20 m
high, 200 m wide "Wall" that is considered to lie above a lava tube
(conduit) which occupied a former stream course at this location.
More than 60 caves and arches have now been discovered in the system. Most
caves are less than 200 metres long but the system includes Australia’s
longest lava tube, over 1,350 metres.
More than six kilometres of tubes have been surveyed and the first profile
ever to depict a source volcano in addition to representative caves and
arches is presented.
190,000 years ago, the Undara volcano erupted 23 cubic kilometres of
basaltic lava at temperatures ranging from 1,170° Celsius to 1,220°
Celsius, covering an area of 1,150 square kilometres.
With an average gradient of only 0.3°, one of the flows extended more than
160 kilometres to become the world’s longest ‘recent’ flow from a single
This great length is attributed to very high effusion rates, favourable
topography, and lava tube efficiency.
The lava tube system extends more than 110 kilometres and includes caves,
arches, and an almost level ridge that is 35 kilometres long and is known
as “The Wall.”
The Wall is considered the best Earth volcanic feature analogous to the
smaller basaltic ridges on the Moon. Adjacent to, or aligned with, the
caves and arches there are oval and elongate depressions.
Most of these depressions are much wider than the caves and arches and
appear to have formed contemporaneously by the draining of lava ponds.
Darker green “rain forest” type vegetation within the wider depressions
contrasts sharply with that of the surrounding eucalypt woodland and is
indicative of former greater areal extent of rain forests, now confined to
coastal and near-coastal areas. Comparison of features of the Undara tubes
with those of currently active and Recent Period tubes elsewhere in the
world, indicates that the tubes of the Undara System were formed by the
draining of roofed lava channels, whose locations were determined by
A flow of runny basalt pahoehoe lava flows down a drainage. The edges and
top, exposed to the air / rain cool, crust over and develop a roof
and walls. The lava beneath continues to flow, now with a protective
insulative barrier from the elements. At some point , the flow suddenly
ceases and the lava drains its path - a lava tube is formed. The tube can
be reactivated numerous times by subsequent flows provided that the
source of the lava cuts off quickly and the liquid lava has time to exit
and drain the tube. Once formed the tube facilitates the flow of lava
further and further from the source, many kilometers is possible.
There are a number of ways for the roof to grow. The semi-solid surface
crust may thicken and stop (think of the filmy layer that forms on the
surface of hot chocolate drinks) If the crust cools slowly ,
hexagonal shrinkage blocks will form the roof (thing of the
shrinkage cracks in a mud puddle). Semi-solid blocks floating down the
flow may pile up, jam and form a roof. Splashing and splattering add to
roof thickness. As the walls and roof thicken the flow becomes
concentrated and as long as the flow fills the tube the crossection
As the flow diminishes and no longer fills the tube, volcanic gases from
the flow may ignite at considerably higher temperatures causing re-melting
and erosion of the roof and drips. The tube then takes on a more
rectangular crossection often tending toward being narrower and taller.
Tubes with a small flow only partially filling the tube can develop their
own roof crust and when they drain produce false floors that are hollow
underneath. When a tube drains quickly the last coating of lava on the
sides cools quickly and can take on a glassy polished surface. If the
sides of the tube are too cool this final lava coating can take on the
form of thin crusty layers that peel off in curved sheets.