ROCK LANDFORMS of Australia & NZ - IGNEOUS LANDFORMS

This site will visit landforms developed upon rocks of the Warrumbungle Volcano.

Case Study: WARRUMBUNGLE VOLCANO, northwestern NSW Rock Type: Trachyte Features: Assemblage of plugs, flows, domes, dykes and other landforms GEOLOGICAL MAP
	SITES ON MAP: 1 - Breadknife, 2 - Crater Bluff 3 - To Bluff Mtn, 4 - Belougery Spire 5 - Balor Peak, 6 - To Park HQ KEY POINT: Variety of features developed upon the former complex 'hot spot' volcano
A portion of the Warrumbungle Volcano is shown. The volcanic rocks were intruded, extruded and deposited over sedimentary rocks about 17-13 m.y. ago. The basement, Pilliga Sandstone, has been exposed by Spirey Creek's downward erosion through relatively soft pyroclastic rocks. The main volcanic rock is trachyte, which occurs in a variety of forms, including spectacular domes, plugs and dykes. The plugs and dykes stand out now because weathering and erosion have removed the surrounding softer rocks. Trachytic magma, being viscous, is often associated with explosive eruptions which deposit breccia, tuff and pumice over their surrounds. Some of these features will be shown in this case study. WARRUMBUNGLE VOLCANO TIMELINE - Key Dates top of page
Case Study: WARRUMBUNGLE VOLCANO, NW NSW (SITE: 1 on map) Rock Type: Trachyte Feature: The Breadknife - volcanic dyke
KEY POINT: Fine example of a narrow resistant dyke exposed by weathering and erosion
The Breadknife, a trachyte dyke, is the best known landform of the Warrumbungles. It formed when magma was intruded vertically into volcanic tuff and breccia, and set into a hard, narrow mass. Upon cooling, numerous contraction joints appeared. The dyke was later exposed by erosion of the surrounding softer materials to leave a spectacular narrow wall of hard rock hundreds of metres long and only several metres wide. The hiking track offers excellent views, as well as the opportunity to study the contact surface with surrounding pyroclastic rocks. National Park rules prohibit climbing the Breadknife to protect both the landform and people. The areas between the peaks of the foreground and middle distance are lower due to erosion of softer pyroclastics - tuff (mostly ash) and volcanic breccias (larger fragments). The valleys in the distance are lower still, with basement rocks exposed. top of page
Case Study: WARRUMBUNGLE VOLCANO, NW NSW (SITE: 2 on map) Rock Type: Trachyte Feature: Crater Bluff - volcanic plug
KEY POINT: Resistant volcanic plug exposed by removal of softer surrounding material
Crater Bluff is a 'classic' volcanic plug (or neck) of hard trachyte and its faces are used regularly for rock climbing. The plug formed below the surface from cooling magma which blocked the vent; pressure from below being insufficient to clear the solidifying mass away. The resistant plug is prominent now as adjacent softer materials (tuff and breccias) have been removed by erosion, leaving the plug surrounded by a bowl-shaped depression. The rock making up Crater Bluff has been shown to be similar in composition to that of the Breadknife; the Crater Bluff magma mass thus most likely to have also formed the Breadknife (dyke). top of page
Case Study: WARRUMBUNGLE VOLCANO, NW NSW (SITE: 3 on map) Rock Type: Trachyte Feature: Bluff Mountain - lava dome
click on photo for enlargement KEY POINT: Resistant, igneous extrusive feature (lava dome)
Bluff Mountain is a lava dome, an igneous extrusive landform, built from trachyte lava which was viscous enough to pile up around the vent rather than flowing away. It is a single mass of trachyte, with somewhat coarser crystals than other nearby trachyte features - probably due to slower cooling. During formation the lava dome swelled and shed blocks of trachyte breccia from its outer cooling and expanding surface. This material piled up around the dome base giving scree slopes that formed a cover over much of the dome. Shrinkage joints are visible on parts of the mountain. Bluff Mtn offers marvelous views in all directions, including the many peaks of the Park, the surrounding plains and distant ranges. Rock climbing is popular on the steep north face. top of page

to Igneous Page		Features and processes of the Warrumbungle Volcano Volcanic rocks are igneous rocks that have cooled at or near the Earth's surface from lava, ash, and/or other materials associated with volcanic activity. Many former volcanoes are found across Australia. New Zealand's North Island is volcanically active. The Warrumbungle Volcano is found in central northwestern NSW. Some features formed on its rocks are described in this case study. It is one of many sites associated with the former 'hot spot' volcanic activity of eastern Australia.
IGNEOUS LANDFORMS		- Case Study (A) © C.Grant

Map of Igneous Sites	go to Igneous Landforms
Case Study (B) ('Hot Spot' volcanoes)	go to Main Landform Page

Breadknife Crater Bluff Bluff Mtn