School geology notes part 2

There appears to be a limit to the number of pics, so I had to split the entry in two.

  Once seen, never forgotten. Two examples of joints near Fairlight.

Geologists don’t really know how joints are formed, but they think it has to do with stresses being released as rocks above weather and erode away. And THAT brings us to weathering.

Weathering

All rocks break down in what geologists call “weathering”. This involves the decay of the rocks, combined with all the ways the planet has found for moving rock debris, erosion in other words, are both necessary for the rock cycle to operate. A lot of the most spectacular scenery emerges because some parts of the rock resist weathering, like Drawing Room Rocks near Berry, down the coast.

Here, the sandstone has accumulated an iron-rich layer near the top, but at a guess, water was able to get in through the joints, rock chipped away, and we ended up with this sort of pattern, with ‘occasional tables’.

In geology, nothing is completely permanent. For starters, there is no such thing as insoluble. Many of the minerals in rocks resist being dissolved, but over time, given enough time, no mineral is ever totally insoluble. Some minerals are rather more soluble, and if one mineral in a rock breaks down and washes out, it will only be a matter of time before the hard rock begins to crumble.

Air, heat and cold also play a major part in this breakdown, which is referred to as weathering. Geologists recognise two types of weathering: physical weathering, though this is sometimes called mechanical weathering, and this name probably tells us more about how it works.

All of the sandstone around the school shows clear signs of weathering. The sandstone face below and east of the library is a good example of one form of physical weathering caused by feet.

In 1969, just after major fires in the Royal National Park, I was sent out, in full ranger uniform, partly to see if the old tracks were visible, but also to ‘show the flag’. Where a track passed over sandstone, the path to walk was much lighter than the other rock: human feet had weathered the rock.

Lightning

There used to be a poor example of a lightning strike in the old nature area, but I think we lost that. No matter, it was unimpressive unless you knew what to look for.

   

Around the world, there are about 100 lightning strikes, somewhere, each second. That adds up to a lot of energy hitting things.

When lighting fails to hit a building, a tree, a foolish kite flyer or an unwise golfer, it usually hits rock. Lightning often comes with rain, and when the water has already soaked into a rock, the instant heat of a lightning strike turns that water to steam, flaking off a surface layer. Once the rock is in small pieces, other weathering effects can take over.

Note that these blasts happen on high places in storms. The traces are best sought for in good weather. If you are on a high place in lightning, move away! The danger signal comes when long hair starts to float up into the air, but by then, it may be too late…

And now for my favourite rock forms, which appear in the school grounds only in minor and beginning forms, visible only to a prepared eye.

Honeycomb weathering

Geology shapes our scenery, sculpting the rocks around us, and one of the delights of my home area is honeycomb weathering, sometimes called alveolar weathering by people of French background, while others call it fretting, stone lattice, or most poetically, stone lace.

Honeycomb weathering in Hawkesbury sandstone, some of it cross-bedded, near Box Head, north of Sydney.

Much of the best-exposed sandstone near Sydney is close to the coast, and around the world, it is common to blame salt spray for honeycomb weathering. The idea is that salt spray lands on and soaks into the stone, but when the water evaporates, salt crystals are supposed to wedge sand grains off.

There is definitely more to the picture and that, and while salt spray probably plays a part, as a young man, I saw honeycomb weathering in the Budawang Ranges, 40 km from the nearest sea coast. I have no surviving photographs from that time, but I do have something rather similar from the flanks of Uluru, on the far side from where the climbers used to start.

A curious but entirely natural weathering effect on the side of Uluru.

   

Ant lions

Ant lions were the first insects I ever studied, and they make neat pits in sandy soil. There used to be lots of them under the old demountables, and there should be some under the trees. They are the larvae of lacewings, alias Myrmeleontidae (Neuroptera). The name ‘lacewings’ describes their pretty wings quite well, but ‘ant lion’ is a good name for the larval stage. Instead of hunting like lions though, they dig pits in the sand and sit at the bottom, waiting for an ant to fall in.

I once saw one of these animals capture a small weevil, but usually, they eat ants. Whatever the prey is, once the unlucky animal reaches the bottom, the ant lion seizes it in its pincers and sucks it dry. In the end, it flicks the empty husk of the prey out of the pit. Ant lions are neat!

To find these curious creatures, look for a small conical pit, 1–3 cm across in dry sandy soil. The soil may be close to one of those gum trees with sap that kills grass, or inside a hollow tree, along the edge of a building or under a rocky overhang. Sometimes, you can even see ant lion pits, right out in the open in the dry season on Cape York, in the summer around Myall Lakes in NSW and in dry areas. All they need is dry sandy soil.

A large ant lion can be 6 mm long, but 1.5 mm of that length may be the nippers that it uses to seize its prey. It digs a pit by backing into the sand and moving in a circle, flicking sand out with its head. Recent research on fossils in amber suggests they have made pits for 100 million years.

Dry sand only piles up to a certain slope, called the angle of rest, and this is the slope of the sides of every pit. At this angle, the sand is unstable and ready to tumble down if a small animal walks near the edge. As soon as sand grains hit the bottom, the ant lion starts flicking sand up from the bottom of the pit. Some sand falls down again, knocking its prey down the slope, but if the ant lion flicks enough sand out from below, the whole slope begins to slide down, carrying the food animal with it. Ant lions are easy to keep but they aren’t geological.

A moral tale for kids:

Whenever people in the outback dug a well in a sandy river bed or climbed a dune, they were in the same position as ants, except that there was no monster waiting to grab them and suck them dry. The real danger came as they dug down close to water, because damp sand will hold together, and they could dig a steep-sided hole. Then when the sand dried, it would collapse.

Each year in Australia, one or two children are killed when a sand cave collapses on them. No explorers were ever killed that way, but probably a few needed their companions to dig them out.

As you can see, in science, everything is connected. We teach them to read the rocks, and weave a web around them!

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A small advertisement: some of the illustrations appearing here will be in my upcoming Not Your Usual Rocks. This is now moving into final editing, and will either be the subject of a contract for a coffee table book by mid-June, or it will be issued as a delicious e-book by April 2020.