MT WARNING: Matthew Flinders & the Coastal Landforms of SE Australia

Hot Spot volcanoes are found in a long chain along eastern Australia. They are believed to have formed when the portion of the crust containing Australia drifted across a stationary 'hot spot' in the upper mantle. The effect was to cause a sequence of volcanic centres above the hot spot. North-east Australia passed over this spot first, and so contains older volcanoes. South-east Australia passed over it more recently, and so has younger volcanoes. The area of the Tweed Volcano crossed this hot spot about 20 million years ago; at this time widespread volcanic activity took place at many local centres. This page looks at three sites formed by the massive Tweed Volcano; Mt Warning, centre of the huge Tweed Volcano, and the coastal remnant lava flow sites of Fingal Head and Burleigh Head. Note: While volcanoes are not coastal landforms as such (i.e. they are produced by volcanic processes rather than coastal processes), they fall within the scope of this Web Site, which aims to examine the landforms of the coasts of SE Australia.
Mt Warning, NSW
'Mt Warning Vis. 22 leagues ... A woody, rising country of a pleasant appearance ... Capt. Cook's soundings ... C.Byron Vis. 9 leag. ... July 13 1799 ... Fine ... Rainy' - from Flinders' chart (East Coast, Sheet II) Taking soundings as he went, Flinders passed close by Cape Byron, the most easterly point of the Australian mainland. For the production of his chart Flinders made use of Cook's work, attributing this appropriately.	enlarge map Extract from Flinders' chart
Mt Warning is the remains of the central plug of the former Tweed Volcano. The massive shield volcano left a gently sloping, mainly basalt mass of at least 50km radius. It is one of the largest shield volcanoes in the world. As well as occupying a very large area, it was much higher than at present; it is likely that it used to be between 2 and 2.5 times its present height. Materials ejected from the Tweed Volcano, in addition to basalt lava, include rhyolite lava and perlite (rhyolite glass) and the pyroclastic materials of tuff and agglomerate. The plug, Mt Warning, is made up of more resistant rock, solidified from the last magma in the vent.	Photo 1: Mt Warning and part of the surrounding Tweed Valley
The Tweed Volcano includes Mt Warning itself (an eroded volcanic plug), and the mainly basaltic slopes of the surrounding mountains: Lamington, Springbrook, McPherson, Border, Tweed, Nightcap and Burringbar. These ranges have been eroded and 'separated' from the original volcano by the formation of the Tweed Valley through erosion by running water. The photo (right) shows the huge, steep-sided caldera or basin carved out by the Tweed River (and its tributaries), and parts of the surrounding border ranges. [ Note: a small part of the river appears as a silver band on the left-hand edge of the photo.] The area around the border of NSW-Qld owes much of its present-day character to the volcanic materials of the Tweed Volcano. In the background is the coastline, made up mainly of a series of long beaches.	Photo 2: Mt Warning, Tweed Valley, and Border Ranges (from the air)
Mt Warning often appears to 'be making its own clouds'. This is known as the orographic effect. As an air mass rises to pass over a mountain the air temperature drops and condensation (and rain) may result. Many mountainous areas, including the Great Dividing Range of eastern Australia, are wetter than adjacent lower regions for this reason. The photo shows features to the north of Mt Warning. The plateau areas to the left and right of the gap are mainly basalt, and with high rainfall, support significant areas of rainforest. Some of the cliffs are composed of more resistant rock such as rhyolite. The border between NSW and Qld follows the highest land (i.e. the northern limit of the Tweed watershed).	Photo 3: Numinbah Valley, Mt Warning (centre), Springbrook Plateau (left), Lamington Plateau (right)
Fingal Head, NSW
Also part of the Tweed Volcano's flows are many widespread basalt lavas. As basalt is non-viscous (ie. 'runny'), it forms low-angle flows, and readily runs long distances. Basalt from the Tweed Volcano can be found 50 kms from the central source. Fingal Head is composed of basalt that has cooled within a relatively deep flow. While cooling, contraction took place, cracking the rock into vertical columns (or cooling columns). The headland (near the photo) is enjoyed by people fishing and sightseeing. Surfers and dolphins are often seen making the most of the Pacific Ocean's waves.	Photo 4: Basalt cooling columns, Fingal Head, NSW
Burleigh Head, Qld
This headland of the Gold Coast is of interest; it is easy to identify basalt lying above the much older metasedimentary basement. The basalt is fine grained (it cooled rapidly), and has little internal structure. Like much of the basalt of northeastern NSW and southeastern Qld, these flows can be attributed to the Tweed Volcano of about 20 million years ago. However, the basalt, while hardening from lava, formed cooling columns similar to those at Fingal. These columns indicate that the flow was originally a thick layer of lava. As it cooled and set it cracked, producing vertical joints that defined tall columns of mainly six sides (although columns with varying numbers can be found). With time, weathering has exposed the columns to the surface. Some have cracked, and many have tumbled down to the base of the headland. Here they are gradually rounded and reduced in size by wave and current action.	Photos 5A & 5B: Basalt cooling columns, Burleigh Headland, Qld Left: Exposed columns near the top of the headland (lichen growing on them) Right: Basalt boulders at cliff base

Visit the aboriginal story of The Durrangan
Formation of landscape features and processes,
Yuraygir National Park (south of the Tweed Volcano)
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