When Granite Meets the Ocean

This is the last post from Tasmania, so it’s fitting that we’re going to examine a dynamic modern environment that gives an idea of the past erosion of all those rocks we’ve seen. We’re going to visit the Hazards Granite mentioned in the last post. I think it’s the second mountain from the right in Fig. 1.

 Fig. 1

We went to the top of the second peak from the left last time. That’s the Coles Bay Granite. Each mountain is a pluton of these igneous rocks, which were emplaced during the Devonian period (between 390 and 360 MY). From the lighthouse on top of the Coles Bay pluton we have a good view of the Hazards pluton (Fig. 2).

 Fig. 2

Note the vertical streaks, which are probably a staining phenomenon rather than compositional. However, the orange-red color in the lower third of the pluton is real, being due to high concentration of orthoclase we found in the lower parts of the Coles Bay Granite. From Fig. 2 it is safe to say that the Hazards Granite is going to be a fractionated alkali granite.

We can get a good view of the shape of the magma chamber from Fig. 3, which reveals elongate protrusions in the upper part of the pluton. This is very likely the form it took as it pushed against the surrounding rock, which has all eroded away, leaving the magma chamber available for examination.

Fig. 3

 A  B

We walked about a half-mile along the edge of the steep sided pluton, through microenvironment such as the thick ferns shown in Fig. 4. This looks to be the rainy side of the mountain.

 Fig. 4

Figure 5 is looking down obliquely at the water about 60 feet below us. This view shows how the joints are weak points at which waves can hurl rock fragments to chip away at the granite, widening the joints in a very robust and orderly process.

 Fig. 5

We dropped down an easy slope to an inlet called Sleepy Cove (Fig. 6).

 Fig. 6

The tide was low so the beach was accessible. We’ll take a look at how granite turns directly into sand and forms a beach with practically no transport. First, we see that the rock has the same composition as the Coles Bay Granite (Fig. 7).

 Fig. 7

We see a lot of orthoclase (red crystals) and quartz (Gray) with albite/microcline (white minerals) and a little hornblende and biotite (dark minerals). The assemblage changes however when the grains are weathered from the original rock (Fig. 8).

 Fig. 8

Where has the reddish orthoclase gone? There are also no dark mineral grains, the hornblende and biotite. These minerals are susceptible to chemical weathering. They are as strong as quartz and albite, but they aren’t as stable. In fact, the albite and microcline crystals (white grains in Fig. 8) will break down quickly as well, leaving only quartz as the final mineral. This is the reason that almost every beach in the world is primarily if not entirely composed of quartz.

Let’s look at some ways the rock (Fig. 7) turns into sand (Fig. 8). First, joints widen under the pounding of grains already weathered from the rock (Fig. 9). This process can be seen to be important at every scale in Fig. 6.

 Fig. 9

The constant blasting by wave-carried sand grains (especially the slightly harder quartz) works on every weak or exposed part. For example, the two boulders in Fig. 10A are being removed from the inside in a modified version of spalling.

Fig. 10

 A  B

 C  D

Figure 10B shows the removal of the inside of the boulder on the right in Fig. 10A and Fig. 10C shows the beginning of this process on the left boulder. This is a common process for erosion of granites, but it doesn’t usually advance so rapidly. Figure 10D shows a hole worn in a slab thinned by erosion along a horizontal joint.

The rocks surrounding Sleepy Cove are being simultaneously weathered by abrasion and chemical breakdown in a microcosm of the complex erosion of mountains into layers of sedimentary rock like we’ve seen all across Tasmania (Figs. 11 and 12).

 Fig. 11

 Fig. 12

A synapsis of the geological history of Tasmania preserved in the rocks we’ve discussed in these posts would require a separate post. Instead, I’d like to end with a note on uniformitarianism: The granite mountains seen in Fig. 1 will be worn down by the processes demonstrated in Fig. 12, in as little as 10 million years (a wild guess), and a thick layer of sand will replace them. Of course, plate tectonics may have a different future in store for Tasmania. Only time will tell.



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