Australian Capital Territory: Orogenic Volcanism

As I’ve been discussing in the last few posts, the rocks of the ACT record a mountain-building episode between about 485 and 400 million years ago. This post is going to examine some volcanic rocks extruded towards the end of this period.

We drove to the top of Mount Ainsley, Site 16 from the regional guide (See an earlier post for details), from where we looked down on Canberra (Fig. 1), standing on top of a stack of mostly volcanic rocks extruded between 433 and 427 Ma, towards the end of the orogeny. The top of the mountain is approximately 800 feet above the base, which also reveals outcrops of the Mount Ainsley Volcanics formation.  That is a thick pile of volcanic material deposited over a 6 million-year interval.

Fig. 1

 A   B

However, as we’ve learned before, geological time is long. For example, (using SI units for convenience), the mean extrusion rate is 250 m/6 million years, or 40 cm/1000 years. The number is simply too small to comprehend as an annual rate.  This is a good rate, however, because it suggests that on average about 40 cm (16 inches) was extruded from a nearby volcano every 1000 years. So it wasn’t as if hell was raining down daily. Let’s see how our estimate compares to the rocks.

We’ll start at the highest point on Mount Ainsley. It’s typical for volcanic rocks to look like they have bedding because they are often produced in episodes and thus form layers, but they aren’t deposited by water so they don’t have well developed layers. That’s what we see in Fig. 2

Fig. 2

 A  B

 C  D

Figure 2A shows a well-preserved face, and this irregular bedding as well as some fractures are visible.  A close-up (Fig. 2B) shows a gray rock, varying from lighter to darker shades. There is some irregularity in the coloration on this relatively fresh surface and xenoliths (pieces of rock) and phenocrysts (minerals) are visible as lighter color blebs and very small particles in panel B. The variability of the deposit is obvious in Fig. 2C. The left side of the steps is thinly bedded whereas thick layers are visible to the right.  In fact, Fig. 2D (taken on the other side of the knob that defines the highest point of Mount Ainsley), shows massive, rounded appearance.

The only way to identify individual volcanic deposits is if enough time passes between them for the surface to weather. That isn’t obvious here and there was no mention of it in the guidebook. Thus, it’s possible that Fig. 2 represents a single episode, or many scattered over decades or centuries.

Massive layers are exposed in road cuts as we head down the mountain (Fig. 3), through the thick section of volcanics.

Fig. 3

 A  B

Figure 3A is featureless but panel B reveals a different texture in the middle of the photo, as if the material was fractured before it cooled, giving it an irregular appearance.

I haven’t mentioned what kind of volcanic rock this is. First, it’s gray color is intermediate between darker volcanics (e.g., andesite) and rhyolite, which is very light gray. The guidebook identifies this as dacite, which is in fact intermediate between the two. Dacite consists primarily of quartz (gray material in photos) and plagioclase feldspar (white material). Some of the 800 feet of volcanics was probably extruded as a thick substance because magma with lots of quartz tends to be thicker than low-silica melts. However, these magmas also tend to be explosive (remember Mt. St. Helens), trapped gasses being released catastrophically.

At the bottom of Mount Ainsley, we went to Site 26, where blocks of agglomerate are exposed in a small area (Fig. 4).

 Fig. 4

These boulders are in place, being the tips of a larger outcrop covered by the thin soil. A piece broken off by a previous visitor was waiting for me (Fig. 5A). I didn’t have to touch it. Note that we are now at the bottom of the mountain, somewhere near the bottom of the pile of volcanic material. These rocks are certainly thousands, if not millions, of years older than those in Fig. 2.

Fig. 5

 A  B

The sample in Fig. 5A is 18 inches long. The lighter areas are either pieces of country rock, or feldspar (which crystallizes at a higher temperature than quartz). Figure 5B is close enough to see the wide range in the size and shape of the phenocrysts (the image is ~5 inches across).

I’m going to take a moment and look at this rock (not the same sample) even closer (Fig. 6).

Fig. 6

 A  B


Figure 6A is about 3 inches across, and individual phenocrysts can be identified, including both white (feldspar) and black (unknown) minerals. Zooming in on the photo, the crystallization of the phenocrysts is clear, and even the dark gray groundmass takes on an irregular shape, no longer appearing uniform.

Figure 6B is at even higher magnification. It shows the edge of one of the white phenocrysts against the matrix. The contact is not as sharp as one would expect if it was already solidified and then ejected along with the fine particles within which it was trapped. This magma blasted out of the volcano so hot that the material raining down on the land fused together, while retaining individual mineral characteristics. It wasn’t molten. This kind of structure is unique and well known because we can study ignimbrite in the modern world.

I threw Fig. 6C in for the hell of it. This is at the highest magnification my phone could achieve. Zoom in on this photo and you’ll see the assortment of crystal shapes that existed in the magma chamber when the phenocryst (angular, light-colored shape) composed of individual crystals was ejected. Note the sharp contact in the lower-left corner of the photo, so different from Fig. 6B.

Neither the magma chamber nor the external environment was uniform. Imagine a pot of boiling oil, bubbling, spilling out of the pan and making a mess. That what an eruption from this kind of volcano is like.

I’ll wrap up this series of posts next time.









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