Toomba volcano is one of the youngest in the Nulla Volcanic Province. The province is about 90 miles (150 km) west of the coastal city of Townsville and just east of the crest of the Great Dividing Range. At least 45 volcanic centers have been active in the province during the last 5.2 million years.
The Toomba flow is well exposed at Basalt Corner where the Burdekin River
has cut under the flow. The flow was confined in the Burdekin River
valley but there is no evidence it interacted with water. The flow was
probably emplaced during the dry season. The Toomba flow is about 13,000
years old. Photograph by Steve Mattox.
Inflation is a common process with pahoehoe lava flows. In
Hawaii, pahoehoe flows are initially about 1 foot (30 cm) thick. Over
the following weeks, more fluid lava is injected into the interior of the
flow and the rigid outer crust inflates to thicknesses as much as 23 feet
(7 m). In some places the Toomba flow inflated to a thickness of 65 feet
(20 m). In this photo a geophysicist is climbing the steep margin of the
inflated Toomba flow. The geologist in the top left corner is standing
beside a deep crack that separates the nearly flat, inflated center part
of the flow from the sloping margin of the flow. Photograph by Steve Mattox.
Some examples of inflation are very dramatic. This photo shows a near
vertical slab of pahoehoe. The slab was originally a near horizontal
sheet. It was pushed up as the interior of the lava flow inflated.
Photograph by Steve Mattox.
Another feature of inflated lava flows are lava-rise pits. This photo
shows a large lava-rise pit that developed on the surface of the Toomba
flow. Lava-rise pits form when areas within the flow do not inflate. The
flow inflates to a thickness equivalent to the depth of the pit. Note
geologists for scale and the near horizontal surface of the lava flow
behind the geologists. Photograph by Steve Mattox.
This photo shows another lava-rise pit. The geologists are climbing a
steep slab of pahoehoe that was originally nearly flat. The lava in the
pit to the right did not inflate. The lava in the background inflated.
Photograph by Steve Mattox
Cross-section of the Toomba flow at Lowes Basin. The 130,000 year old
Toomba flow is resting on fossil-rich Devonian limestone.
The Toomba flow was visited during the field excursions of the American Geophysical Union's Chapman Conference on long lava flows.
Hon, K.J., Kauahikaua, J., Denlinger, R., and McKay, K., 1994, Emplacement and inflation of pahoehoe flow sheets: Observations and measurements of active flows on Kilauea volcano, Hawaii: Geological Society of America Bulletin, v. 106, p. 351-383.
Stephenson, P.J., and Whitehead, P.W., 1996, Long lava flows in North Queensland: abstract volume: Economic Geology Research Unit Contribution 56, James Cook University, Queensland, 110 p.
Stephenson, P.J., and Whitehead, P.W., 1996, Long lava flows in North Queensland: excursion guide: Economic Geology Research Unit Contribution 57, James Cook University, Queensland, 40 p.
Walker, G.P.L., 1991, Structure, and origin by injection under surface crust, of tumuli, "lava rises," "lava-rise pits," and "lava inflation clefts" in Hawaii: Bulletin of Volcanology, v. 53, p. 546-558.
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