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.
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