What geologists think, part 1

A small note up-front: this is a sample from one of my coming attractions: completed or nearly ready works that don't suit my current publisher, and haven't yet been sold.

Note added April 2020, Not Your Usual Rocks is now Mistaken for Granite, and available as an e-book. In some markets, it is also available as a book.

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Beach pebbles, Gerringong New South Wales.

In my closing-out process, I am getting well into Not Your Usual Rocks. Here's a small taste of what is to come in the series.

Every cliff, every rock face, every pebble, will carry a story of the past written on it—if you know how to read it.

The rules of thumb we use to read the rocks are scientific geology, but science is variable in its quality and in its actions. Think of a grassy mat, sitting in shallow sandy soil on sandstone. 

In the middle, there is less struggle for life, but out on the fringes, the struggle is intense, because it’s drier out there, with less soil. Yet, if one grass runner reaches across to some neighbouring soil, the whole species will advance.

Science works in a similar way. For most of the time, most of the scientists in any branch of science agree, about most things, just as the grass in the middle of the mat agrees that all is well. Out on the fringes, we find patches of dead and decaying science.

Even in the centre not all scientists agree all the time, because every now and then, somebody does a bit of tweaking. They adjust a few of the assumptions for some purposes. Once in a while, a scientist does the equivalent of there must/might be better soil out there, somewhere.

The more general a scientific rule-of-thumb is, the more scientists agree about it. But no rule is sacred: if somebody finds even one contradiction, people need to think, and toss out a part, or even all, of the rule.

Sometimes, the two models rest happily, side by side. If we are sending rockets to another planet, the 330-year-old physics of Isaac Newton is good enough, but for some parts of physics, we need the more modern insights of Albert Einstein.

These days, the tweaking seems to happen less often, but it always remains possible that one of the foundations, one of the basic assumptions of geology may need to be altered or even discarded. The standard propositions that guide geological science are also principles that explain why the world looks as it does. 

Hawkesbury sandstone cliff, North Head, Sydney.

 How much story can you see in this picture? I can see lots, but then I was trained by clever observers and I’ve been looking curiously at rocks for six decades—and I know the things that geologists agree about, things like the following principles work well, because they explain the weird shapes that the earth we live on can take.

How do we explain this, without geological science?

Principle 1. We can see where the world has changed.

The world does not abide forever, even if, in our short life spans, the world really is fairly permanent. An occasional volcanic eruption or a massive earthquake can make a difference to what we see. 

Floods can move huge amounts of sediment as mud, and the occasional collapse of a delicate eroded arch may be seen, once the erosion goes too far or in a flood, but that’s about it for changes in the human lifetime.

On a larger time scale, continents shift around, pushing and jostling. As they do, they shove the Swiss Alps, the Himalayas, the Andes and all the other mountainous parts of the world up into the sky. At the same time, weathering destroys the rocks and erosion carries the remnants down to lower reaches. Without rocks being pushed up, there would be no mountains left.

Tilts and folds, Mt Pilatus, Switzerland.

Sometimes, land sinks deep beneath the sea, and new material gets washed in and laid down on top of it. Later, some of those new underwater rocks may be shoved back up into the air. All geological-scale things are slow, and we know this because we can work out the dates of the rocks.

Principle 2. We can tell the age of rocks.

Our various dating methods can give slightly different results, because geochronology, the dating of rocks involves inexact measures. Sometimes the assigned date relies on inference or assumptions, like the cases where we find fossils that come from a species that only lasted a short while, so we say that when similar layers in two cliffs contain that index fossil, they are the same age.

There are a few potential traps. We may have misidentified one of the fossils, or we may be wrong about how long that species lived. The good news is that we keep finding new methods and new data, so that over time, the picture becomes clearer. The good news is that the adjustments are generally small, because the different methods all give a consistent picture.

At times, we may use the half-lives of radioactive minerals, or other measures: once again all of the methods give consistent results. The order of formation given by different methods is the same, and over time, we have got much better at putting precise year-counts, on things. Our planet formed around 4.6 billion years ago, though the sandstone I walk on most days is Triassic in age, and roughly 200 million years old, but the sandstone won’t last forever.

Principle 3. We can see that rocks break down.

Geologists know perfectly well that rocks don’t abide forever: they are by no means everlasting. Their minerals break down, mainly under the combined effects of water and air, and the rocks come apart under the mechanical effects caused by heat, cold, grinding rocks carried by rivers and glaciers, and even from sand-blasting in deserts when strong winds blow.

Then there are the biological influences on the rocks—and I don’t exclude my walking on sandstone from this. Tree roots grow into cracks in rocks and expand, splitting the rocks, while at the other end of the plant scale, some mosses can drill neat holes in quartz, one of the toughest of minerals.

An echidna, Tachyglossus aculeatus, North Head Sanctuary, Sydney, Australia, about 10 km from the centre of Sydney.

Burrowing animals from ants to echidnas drag grains of partly-weathered material to the surface where the grains are more exposed to sun, water and rain. On the surface, large grazing animals make pads. These are tracks along the sides of hills, pushing sediment down and providing a path for water to run off downhill when it rains, carrying surface sediment away, In bulk, the Earth abideth well, and nothing is ever lost—it just bobs up in a new hat.
 

Principle 4. Geology involves reusing old material

None of the material coming from rocks is ever wasted. Calcium from basalt will eventually be dissolved and carried to the sea, where corals, snails and other marine life will extract it and use it to make skeletons, shells or something else.

Later, these dead animals may fall down and over time, their shells become limestone. Sometimes, the shells just dissolve once more, and over time, the limestone dissolves and washed down to the sea. Sand becomes sandstone, mud becomes shale, and so on.

If rocks get buried deeply enough, they may be changed by heat and pressure, so sandstone becomes quartzite, limestone becomes marble, and shale becomes slate. If the rock is covered enough, it may end up as molten lava that spews out onto the surface of the earth again. Rocks aren’t all “just rocks”, because the world’s rocks have different origins.

Principle 5. Rocks come in three main types

You may have missed it, but in the last paragraph, I touched on the three different sorts of rock: the ones made from sediments, the ones shaped by heat and pressure, and the ones that were melted before they formed rocks.

Sedimentary rock: Triassic sandstone, Blue Mountains, west of Sydney.

Sedimentary rock forms when sediment (bits and pieces of almost any sort) falls to the bottom of a lake or sea (or sometimes the bottom of a vast sand dune). Later, it is buried and a complex combination of pressure, water washing through and maybe mild heat, turns it into a rock. Sedimentary rocks are the ones that sometimes contain fossils, though a few deformed fossils can be seen in slates, while many show up in marble.

Weathered granite, Freycinet Peninsula, Tasmania. Obviously granite, if you are trained!

Igneous rock is any sort of material that was once a liquid, because at high temperatures, all rocks will melt. Granite is laid down, far below the earth’s surface, and only shows when a whole load of other rock erodes and weathers away. Granite cools slowly enough for big mineral crystals to form. Basalt, on the other hand, is oozy stuff that flows and spreads. Some basalt comes out of volcanoes as lava, some gets out and flows across the land, making a flat sheet that cools fast, so the crystals are very tiny.

North Bondi, Sydney, contact metamorphism: a volcanic neck erupted through the sandstone here, changing it to quartzite.

Metamorphic rock (the name comes from Greek, and means changed shape) forms when another kind of rock is subjected to heat and/or pressure. Large metamorphic regions are usually formed by extreme heat and pressure, and this is called regional metamorphism.

When melted basalt flows over other rocks, or when a volcano pushes through other rocks, the heat may travel a few metres or tens of metres, producing contact metamorphism. That is what happened at Bondi, and you can read all about it here.

In short, there are very few inexplicables when it comes to looking at the rocks, because in the end, the rocks are attacked and wiped out. Still, you can tell when an inexplicable shows up, because the scientists start flapping their hands until they work out an explanation.

Part 2 is about explaining.

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