A caldera* wall is formed by successive volcanic activity across the same area over long periods of time. In this case, Sedimentary rocks were laid down some 135-200 million years ago, sandstones mostly. And then about 23 million years ago the Mount Warning volcano erupted, and over the next 3 million years or so a series of separate eruptions occurred, punctuated by long periods of inactivity. This built a large and complex volcano, the northern end of which you are standing in.
*Caldera: Large depression commonly formed by collapse of the ground following explosive eruption of a large body of stored magma. (USGS).
These eruptions are best described in three phases, an initial extrusion of basalt, followed by a more explosive phase where acid material, mainly rhyolites, were thrown out, and a final quieter extrusion of basalt. The first group of lavas have been called the 'Lismore basalts'. This series of lava flows travelled a considerable distance and covered a large area, extending from the Lismore district in the south to the Beechmont district in the north. These can be seen as the lower terraces of the range opposite, below the cliff lines. Basalts weather down into relatively neutral and highly fertile red loamy soils called 'krasnozems'. The basalt soils support huge stands of subtropical rainforest.
The volcano then became more violent and acid rocks were extruded, sometimes explosively, from the vents. Rhyolites, volcanic 'glass', agglomerates and tuffs were included in this group. The rhyolites are particularly resistant to erosion in this environment and can be seen as cliffs on the high plateaux forming the caldera rim.
The final phase of volcanic activity was a relatively quiet series of basaltic lava flows. The rocks extruded during this period of activity have been called Blue Knob basalts and form a capping on the high plateaux of Springbrook, Lamington, the Tweed Range, and the Nightcap Range. Like the Lismore basalts on the lower slopes these rocks weather to fertile red soils that support dense stands of subtropical rainforest. Small relic patches of temperate rainforest exist on some of the higher peaks further south.
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A thinner spot of the earth’s crust passes over a particular hot part of the earth’s mantle, it’s core. Tectonic plates shifting along. This takes a while – 3 million years in this case. It causes a period of oozing lava, then a period of more explosive eruptions and then another period of oozing lava on top. Ooze – bang – ooze. Soft erodible basalts becoming highly fertile soils – protected from drying out at the bottom. Harder, rockier, more barren rhyolites and then more of the basalt on top, this time more exposed to the elements. You can see this in many places around here, the northern end of this huge Caldera bowl.
What’s different in front of you here?
Stand at the posted coordinates and look out in a generally South-South-East direction. In the very far distance in front of you is Mount Warning - this is the centre of the ancient volcano.
Question 1:
The rounded formation of the top of the exposed cliffs opposite (to the left of Mount Warning) are formed of what?
Question 2:
Can you see the Eucalyptus stands to the right of Mount Warning and below the exposed mountain side (this is known as Ship's Stern)? These grow on a different type of soil to the temperate rainforests more prominently around. Which of the rock types discussed is dominant where these trees are?
There is a rather spectacular Sandstone sculpture some 40m south of you. This has its own geological story worth reading on the plaque, and is a great spot for a photo if you would like to upload one here.