Volcanic Fabric at the Seashore
This is a short note to show what we found along the rocky coast at the mouth of the Yarra River, on our return from Organ Pipes National Park. We didn’t have to go far as shown in Fig. 1.
The rocks on the south side of the Yarra River are slightly higher than on the north side and are exposed at all but the highest tide. Figure 2 shows what the shoreline looks like and Fig. 3 shows the sediment texture of poorly sorted sand and gravel.
A Lava Blister is defined as: “The surficial swelling of a plastic lava flow crust in response to the puffing up of gas or vapour from beneath the flow. Blisters may also form through hydrostatic or artesian forces in the lava. They are usually 1–150 m in diameter, with a maximum height of 30 m, and are hollow. Compare tumulus.” (Oxford Dictionary)
Here’s what the one we found looks like at mid-tide (Fig. 4). I have outlined the rim. Note the smooth surface of the fractured basalt lining the rim.
Apparently, this volcanic field is one of only three locations known where these features occur. I admit this isn’t a very good example, but it has been subjected to coastal erosion form almost a million years. Here are a couple other photos, now that you know what you’re looking at.