Crystals or No Crystals?

The landscapes of the mountains surrounding the Tweed Valley are very spectacular. I have discussed some of the facets of the Tweed Volcano and Mount Warning area in previous posts. However, I have not covered much on the main rock type that is mainly responsible for the rugged steep cliffs and valleys of the Nightcap National Park World Heritage Area. This rock is the Nimbin Rhyolite, a quartz rich lava that was dominant in the final phases of the Tweed Volcano. Because of its resistance to weathering it results in inspiring cliffs and rugged ranges.

Rhyolite is a volcanic rock that contains a high volume of silica (quartz) in it. Because of the silica content rhyolite lavas tend to be “sticky” and slow moving. This also causes gases to be trapped in the lava or magma chamber feeding the lava flows. The release of trapped gases can cause explosive eruptions. Therefore, accompanying the lava flows there are also deposits of volcanic ash and glass caused by the rapid cooling of lava during explosive eruptions. All of these features are present in the Nightcap Ranges and surrounding areas.

In a future post I will show a picture of a Nimbin Rhyolite lava which exhibits flow banding. There are many examples of flow banding in lava near Minyon Falls. It is a tricky lava to look at in hand specimen because it is very fine grained. You can only see occasional tiny specks that are crystals but most of the time it is just a grey mass. In outcrop you might see some flow structures like the one pictured, but generally it is a boring looking rock! The same rock is in the Mount Matheson area. Smith and Houston (1995) referred to this rhyolite as crystal-poor rhyolite. It compares very differently to the crystal-rich rhyolite identified elsewhere in the area.

As for the crystal rich rhyolite, I was lucky enough to go for a walk in a property that has just been purchased by the NSW National Parks and Wildlife Service. It is located in the valley between the Goonengerry and Nightcap National Parks. While inspecting the excellent work done to remove exotic weeds from this property and celebrate the inclusion of an important vegetative link between National Parks. I came across some good examples of the crystal-rich rhyolite. In these samples the rock contains large quartz crystals which are very evident (see the picture below). The more crystalline form of rhyolite occurs in about a third of the total area mapped as rhyolite. This includes the area from the Koonyum and Goonengerry ranges in the east to Whian Whian in the west.

Quartz crystals in Nimbin Rhyolite - upper Coopers Creek area

Smith and Houston (1995) observe the crystal abundance is related to the vent (or group of vents) from which the lava was erupted. Only occasionally do crystal rich and crystal poor varieties occur on top or under each other indicating a high degree of lava mixing. The relationship between specific vents and crystal richness shows the vents must have been tapping different magma sources (different magma chambers). Alternatively the vents may have erupted magma from a single, somewhat heterogeneous magma chamber.

However, it is worth noting there is a third major form of rhyolite in the area and is known as the volcanic glass, obsidian. This volcanic glass occurs around the bases of the major lava flows and is often referred to as perlite. The glass is rarely a massive unit but tends to appear brecciated and as an agglomerate. I will discuss this obsidian further in a future post as many interesting features and textures are preserved showing the way that rhyolite lavas move across the lands surface. In the mean time, it is worth remembering that lavas ain’t just lavas. There can be many differences which provide a window into how the landscape was formed.