ROCK LANDFORMS of Australia & NZ - IGNEOUS LANDFORMS

This site will visit landforms developed upon a range of extrusive (lava and ash) igneous rocks.

Rock Type: BASALT (SITE: 4) Feature: basalt cooling columns Location: Fingal Head, northeastern NSW
KEY POINT: Vertical cooling columns (mainly hexagonal) on basalt
This headland is an outcrop of Tertiary basalt aged 22 - 24 m.y. - part of the widespread volcanic activity in northeastern NSW and southeastern Qld which produced many well known volcanic features. The fresh basalt appears dark in colour due to its mineral content (high in plagioclase feldspar and pyroxene, low in silica). The lava formed vertically-jointed columns through contraction due to cooling while stationary (rather than flowing). The joints (cracks) are exposed today as the outlines of mostly 6-sided columns, but some appear to have 3, 4, 5 and 7 sides. Similar columns also occur in basalt at nearby Burleigh Head, SE Qld, where large columnar blocks have tumbled downslope to the sea and are being broken up into rounded boulders by wave action. The columns of rhyolite at the Glasshouse Mtns and Mt French, Qld, and of dolerite at Cradle Mtn, Tas, are also similar in origin and form. top of page
Rock Type: TUFF (SITE: 5) Feature: Ash layers Location: Mt Gambier, southeastern SA
KEY POINT: Layers in volcanic tuff
This photo shows a pyroclastic igneous rock called tuff, formed from ash erupted in violent volcanic events. The site shown is several blocks from the well known Blue Lake crater. The area is part of an extensive volcanic province occupying much of western Victoria and southeastern SA Certain layers are softer and more easily weathered than others; these layers are 'etched' out at a faster rate than those which are more resistant. Volcanic ash particles are regarded as being smaller than 2 mm diameter; the further from an eruption centre the finer the particles found. In places the tuff layers appear disrupted, probably from volcanic bombs (airborne blobs of lava or original rock) landing with considerable force in the soft ash beds prior to the consolidation of these beds. The Mt Gambier eruption sites are geologically recent. It is believed that these materials were deposited approximately 4,800 years ago. The aboriginal inhabitants of the area could well have witnessed these volcanic events. Animation: an ash eruption (Site 5 above and Site 6 below experienced violent eruptions involving ash clouds) top of page
Rock Type: IGNIMBRITE (SITE: 6) Feature: Jointing in welded volcanic ash Location: Kangaroo Pt, Brisbane, southeastern Qld
KEY POINT: Welded and vertically-jointed volcanic ash quarried for building
These cliffs are formed of ignimbrite (also called welded tuff). The composition is of rhyolitic nature and the formation looks like a rhyolite lava flow - but this thick bed has resulted from the deposition of very hot fragments from an ash flow. The ash flow took place in the lower air layer rather than at ground level, and travelled rapidly away from an ancient volcano (which is believed to have erupted violently about 220 m.y. ago). The brown appearance is from external weathering; the fresh rock comes in a range of attractive light colours, including pink. The rock appears as large and hard chunky blocks with vertical joints that developed as the deposits welded and cooled. A major activity of this area is rock climbing; these cliffs are often busy with many climbers and abseilers. Ignimbrite was quarried around Brisbane in the past as porphyry. top of page
Additional Site: Rock Type - BASALT Feature: agricultural area on basalt Location: Orange district, Central Tableland, NSW
	KEY POINTS: A. Basalt forms high quality agricultural soil. B. 'Hot Spot' volcano (Mt Canobolas) C. Variety of igneous rock types
A. In humid areas, basalt breaks down to form a fine-grained red-brown soil that supports many agricultural activities. For example, the Orange district is well known for cool climate fruits (stone fruit and wine grapes) - grown mainly on volcanic soils. B. The igneous rocks of the Orange district originated from the Canobolas Volcano (of similar origin to the Warrumbungle Volcano to the north). A part of the crust became volcanically active about 13 m.y. ago as it passed over a 'hot spot' below. C. The cross-section below (from the Pinnacle to Orange City) ilustrates that there a wide range of rock types is associated with the Canobolas 'hot spot' Volcano. These include lavas of several types and tuff (from ash). Lava chemistry varies considerably. Basalt, when molten, is of low viscoscity - it can flow readily. However, it is relatively easily weathered. Thus it is reasonable to expect that basalt occupied a larger area in the past than it does at present. Basalt from the Canobolas Volcano covers an area of about 40 X 25 kms at present.
(from NSW NPWS information board on Mt Canobolas summit) top of page

to Igneous Page		Igneous landforms occur in a wide range of areas The key point to note is that volcanic rocks (e.g. basalt) having formed on the surface from cooling lavas, are fine-grained (although pyroclastic rocks may contain large 'chunks'), whereas intrusive rocks (e.g. granite), cooling slowly underground, are coarse grained. Grain size is important to the development of many landforms. Structural features, such as jointing, also influence subsequent rock weathering and landform development.
IGNEOUS LANDFORMS		- General Study (B) © C.Grant (Sites 4-6) (Sites 1-3 make up General Study A)

map of Igneous Sites	go to Igneous Landforms	go to Main Landform Page

columns ash layers jointing