What geologists think, part 2.

As I said before, I'm cleaning up all the unfinished projects, and Not Your Usual Rocks is at the top of the pile.

You'd probably be better off starting with Part 1, but there are two by-the-ways

1. The photos that aren't credited are mine, and they are all
© Peter Macinnis, Creative Commons Attribution 4.0 International.
That means you can use them for non-commercial purposes with attribution, but while I squash thieves like the people at the Charles Sturt Memorial Museum, I will happily provide high-res copies to people who ask.

2. The locations reflect a lot of travel, but you can probably work out roughly where I live, if you live near me. If you do, say g'day!

6. All of the effects we see in the geology can be explained

Basically, all the things that we see in the world can be explained by the forces we see operating today. Geologists call this principle uniformitarianism, and it just means the natural laws and processes that we see shaping the earth today are the same ones that shaped the past.

In other words, we don’t work on the principle that there used to be wizards and witches who moved the rocks around; there were no fire-breathing dragons that made the lava melt. We do not need to assume the existence of pixies driving Stealth Bulldozers, poltergeists with geological interests, malignant mammoths, whimsical aliens or lost civilisations.

Continents move, floating on the surface of the planet; earthquakes happen; rocks form, weather and erode; rocks get pushed up; others get pushed down and buried, and so on. On a smaller scale, sediments get washed by water, blown by winds, and sometimes, pushed by glaciers.

When weight is applied to the existing surface, in the form of glaciers or any other way, the earth’s crust behaves like a small raft that an elephant has boarded: the rocks sink. On the other hand, when glaciers melt, the earth springs back up again, and this is currently happening in Scandinavia which was relieved of a lot of weight, about 10,000 years ago.

The rocks, even the not-your-usual rocks, keep to the following principles.

7. There are standard rules of geology

Sometimes, what you see may appear to be contrary to these rules, but if you think that, it usually means you haven’t thought hard enough. The apparent contradictions emerge only because you are unaware of the other rules that applied in a particular place. With enough thinking, you can generally explain what you see.

Rocks are usually laid down in flat layers.

It is a fairly safe rule that sedimentary rocks form flat, parallel beds, because the sediments are washed or blown into some sort of basin, and the first material fills in the gaps and crevices, leaving a flat surface. The effects of currents (or winds) and gravity keep the top fairly flat.

Horizontal strata, Bungle Bungles, Western Australia

An illustration from Charles Lyell’s The Student’s Elements of Geology (1871), page 17, showing how irregularities in an underlying surface are filled in, slightly contradicting Steno. [Public domain]

Then again, some beds can be laid down on a slope. This is called cross bedding or current bedding, and we will look at it in more detail later. Cross bedding can be distinguished from beds that have been tilted later by looking at the horizontal beds above and below.

 

Cross bedding in Hawkesbury sandstone, Old Man’s Hat, North Head, Sydney, Australia

There can be traps for the unwary when it comes to igneous rocks. If the rock arrives as lava, streaming down the flank of a volcano, some of the lava cools and becomes solid, leaving a sloping skin of rock. Nothing is inexplicable.

Eroded remnants of an old volcano near Cape Palliser, North Island, New Zealand.

Younger rocks usually lie on top of older ones

They are always laid down that way, but there are a couple of notable exceptions. Basalt sometimes pushes up through sedimentary (or other) rocks to form a dyke. If the dyke reaches the surface, it flows out over the landscape (which is why it is called a flow. A flow is always younger than the rocks it lies on top of, and older than any rocks which are found above it.

Sometimes, the basalt pushes in between two layer of rock, forming what is called a sill, but the basalt remains younger than the rocks that lie on top of it. How do we know? We look for contact metamorphism, above and below.

The other exception to youngest-on-top comes when rocks bend, and fold, and sometimes (rarely), overfold, so that the usual age order is reversed in a limited area.

In less extreme cases, horizontal beds may just be tilted up and eroded away, leaving tilted rocks behind. If the land sinks at this point, new sediments wash in to start a new age of rock building.

In an area where there are active volcanoes, lava may pour out and flow across the countryside, laying fairly flat layers—except, as mentioned above, on the flanks of the volcanoes, where sloping beds will form.

The Columbia River forms the border between Washington and Oregon in the USA, flowing through a valley carved through a massive series of basalt flows.

  

There can be gaps in the geological record in any place

On my home territory, near Sydney on Australia’s east coast, the rocks are Triassic in age. If you drill straight down you will come eventually to Permian rocks, the coal measures that are exposed around the margins of what we call the Sydney Basin. You find coal at Newcastle, Wollongong, Lithgow and other places. Coal also used to be mined on the very shores of Sydney harbour, but they had to sink a shaft quite a long way down, all the way to the Permian rocks.

In theory, if we keep going down, we should next move into rocks from the Carboniferous, but these layers are missing in my favourite walking area, in the Budawang Ranges, west of Nowra, south of Sydney. We meet up with tilted Devonian metamorphic rocks instead. It looks as though we are missing 100 million years (or more) of geological history.

Any rock-hound will tell you this is an unconformity, and hazard a guess that the Devonian rocks were deeply buried and covered with Carboniferous rocks, but that the earth and its rocks moved hugely, and any Carboniferous rock was eroded away, leaving ribs of tough Devonian stone across the land in the early Permian era. We really can’t be sure there were ever any Carboniferous rocks, but it is quite likely that they came and went, leaving no trace.

Later, the land all sank deep into a sea of some sort of cataclysm. In the Budawang ranges, the lowest layer of the Permian rocks is a conglomerate containing very large boulders, telling us that the first deposits in that part of the basin were laid down in a huge flood.

At Myrtle Beach, on the south coast of NSW, this conglomerate layer is missing, suggesting that the oldest Permian sediments there were laid down at a different time. It may have been a few years, more probably it was a few millennia—or even quite a few millennia. Geology never scurries.

Myrtle Beach, south coast of NSW. The sloping beds below are pointing to 1 o’clock,

and the hand (top left) spans a gap of about 100 million years in the geological record.

There is also a simpler sort of time gap, much harder to identify, called a disconformity. This happens when sediments stop being delivered for a while, but we can largely ignore these hiccups for the moment. We now have the basic background to understand a bit of slightly more detailed geological history. 

The laws and principles of geology

Nicolas Steno started it. Here is a modern version that conveys his thinking in the language we use today.

* Steno’s Law of Superposition says that in a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it.

* Steno’s Law of Original Horizontality says that strata are deposited horizontally and then deformed to various attitudes later. That is, undisturbed true bedding planes are nearly horizontal, though we need to note here that cross-bedding is possible where sandhills or sandbanks are being formed.

* Steno’s Principle of Lateral Continuity: strata initially extend sideways in all directions. That is, every outcrop in which the edges of strata are exposed demands an explanation, and strata on two sides of a valley represent erosion of the rock between.

* Steno’s Principle of Cross-cutting Relationships: anything that cuts across layers post-dates them. This applies particularly to igneous intrusions such as dykes. Aside from Steno’s principles, geologists accept the following notions:

(1) an intruding rock is younger than the rock it intrudes into;

(2) a fault is younger than the rock which is faulted;

(3) any pieces of ‘foreign’ rock included within a rock must be older than the rock they are found in; and

(4) William Smith’s principle of fossil succession.

We will come to that in a moment, but geology was only possible because of James Hutton. He had made enough money from an ammonium chloride factory to be able to retire from work and study geology.

Hutton was an old friend of Joseph Black, the first scientist to distinguish heat from temperature, and also of James Watt (the steam engine maker), so it is no surprise to discover that Hutton assumed that all earth activity was due to what he called the earth’s ‘heat engine’. But most importantly, he said that “…The past history of our globe must be explained by what can be seen to be happening now”.

He emphasised the igneous origin of many rocks (unsurprisingly, given that he came from Edinburgh, where igneous rocks rear up all around the town). Unfortunately, the French Revolution was happening, so the public in Britain was less than enthusiastic about Hutton’s revolutionary notions. They were not only unready for his ideas, they were unwilling to accept them, but the scene was now set.

John Playfair was probably one of the few people to combine geometry with geography and geology. Trained in mathematics at a time when geology had not yet been invented, Playfair was necessarily largely self-taught. Like James Hutton, Playfair was exposed to the stimulating geology of Edinburgh, which would have assisted him in his work.

He also invented geomorphology, giving us ‘Playfair’s Law’, which states that rivers cut their own valleys. Then he gave us the modern concept of grade when he asserted that the angle of slope of each river shows an adjustment towards a balance between the velocity and discharge of water on one hand, and the amount of material carried on the other.

Playfair also made the work of Hutton more accessible when he published his Illustrations of the Huttonian Theory of the Earth in 1802. He explained the rock cycle of repeated weathering, erosion, deposition and solidification in simple terms: notice, with a modern eye, how he covers weathering, erosion, sedimentary rocks forming in the sea and uplift.

The series of changes which fossil bodies are destined to undergo, does not cease with their elevation above the level of the sea; it assumes, however, a new direction, and from the moment that they are raised to the surface, is constantly exerted in reducing them again under the dominion of the ocean. The solidity is now destroyed which was once acquired in the bowels of the earth; and as the bottom of the sea is the great laboratory where loose materials are mineralized and formed into stone, the atmosphere is the region where stones are decomposed, and again resolved into earth.
—John Playfair, Illustrations of the Huttonian Theory of the Earth, 1802, 109.

The idea of igneous rocks came later. Playfair’s ideas only gained wide acceptance after Charles Lyell added Playfair’s ideas into his Principles of Geology, but we have left out William Smith, an orphan who was set to work early as a surveyor for the new canals that were beginning to cross the British countryside, so industrialists could haul goods from place to place.

 

These canals required digging into the ground, and they had to cut tunnels through hillsides. This all gave Smith first-hand chances to observe and classify the many rock types as they are seen in fresh unweathered exposures. Most importantly, he noticed how strata were typified by fossils, and he pointed out that the same stratum could be identified at a considerable distance by the fossils it contained.

In 1816, Smith published his ideas, accompanied by a coloured geological map, and made the point that, given the law of superposition, the fossils in the strata gave us a view of the history of life on earth. Now the way was fully prepared, and Charles Lyell’s Principles of Geology could be released in the early 1830s, just in time for Charles Darwin to take them with him on the voyage of HMS Beagle. That meant he was prepared to unravel in full detail the reasons why life actually possessed a history on earth.

That is how science weaves itself into a web, but it also involves cycles.

Geological science is also science, and there are some principles of science, as well. I will get to those in part 3.