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What is a Rock?
The two largest monoliths in the world
Some rocks date to half the age of the Earth
With comments on Uluru, and more.
That may seem like a rather silly question, and if you can answer it to your own satisfaction, well, congratulations, many can't.
I mean, those lumps of material we call rocks are everywhere, so much so, that we often don't appreciate their beauty, much less their origin and composition, along with many other marvels nature provides us with.
One thing we do know is they didn't arrive yesterday, unless they were cooling material from a recent volcanic eruption. Rocks, in fact, have been forming for up to 500 million years: some much older, some are much younger, but few less than some 20 million years.
The upper layer of the planet we call home - Earth - and have christened "The Crust," is made up of elements. You will remember from geology or physics 101, an element is a substance composed of just one type of atom. When I last looked, there were 117 kinds of elements, some known for thousands of years, other discovered recently and scientists postulate there are still a few more out there - perhaps on other planets and stars - still to be identified.
The commoner elements that make up the part of the planet by forming minerals, we - and everything else - lives on (even birds have to come down eventually) are oxygen, iron, calcium, carbon and several more...I don't think long lists of things make particularly entertaining reading and are useless unless you are sitting an exam.
These, as we said, form minerals such as calcite, quartz and feldspar, etc., which themselves form the three kinds of rock: igneous, metamorphic and sedimentary.
Very briefly, and why there’s no lists - there are estimated to be 700 types of igneous rock alone - igneous rocks - the word means “fire” - are formed as magma from volcanoes cools on or under the surface of the planet.
If it on the surface, it is labelled extrusive and below, plutonic.
A few examples you may be familiar with are granite, basalt, obsidian and pumice. (igneous rock can be heavy and hard or lighter and more fragile)
Sedimentary, as the name implies, are formed as sediments of minerals and other material collects on the surface or in water; Sedimentary rock makes up only 5% of all rocks. Some common examples are sandstone, shale, breccia and conglomerate (the last two formed from small pebbles).
Metamorphic rocks - the word means “change in form” - are formed under high pressure and heat, usually in the presence of volcanic or tectonic plate action...when the huge sub-surface plates collide, immense pressure and heat is generated. We see these on the surface due to uplift or erosion. Well known examples are marble and onyx (formed from limestone or dolomite), shale, slate, and quartzite, (formed from sandstone).There are more than a thousand kinds of rock and identifying the rarer types is the province of the geologist .
Rocks can be tiny or enormous. One significant example of the latter is “Uluru,” once known as Ayer’s Rock, in Central Australia, which must be easily the best known monolith on the planet, and is 348 meters high and 9.4 kms around! It is known as an inselberg, an island mountain, like an iceberg, with most of its bulk beneath the ground.
What is astonishing is that it is there at all, as it was part of a range of mountains which have disappeared through erosion. Geologists say Uluru’s relatively smooth surface is the reason “the rock” remains, as it was able to deflect rain (some!) and wind-driven sand blasting, along with the desert climate of alternating heat and cold, while its neighbours succumbed. The local Aboriginal tribes which worship Uluru might give you a more esoteric and metaphysical reason for the survival; they have certainly ensured its surviving the hordes of chipping and climbing tourists by seeing laws to protect Uluru enacted. Incidentally - and of course, Uluru’s popularity has caused it to become commercially exploited and it is hard to see the Rock without staying in the neighbourhood if you can get a high-priced room. (Alice Springs is a four-hour drive away at least). But I must say, along with the Grand Canyon, this is a place that should not missed. Yet Uluru is only the second largest enormity of its type in the world. The first is another sandstone monolith, also Australian, also Aboriginal owned. The is Mt. Augustus, (Burringgurrah). It is approximately the same circumference, but rises to 717 meters! Situated 850 km. north of Perth in WA, it is not so spectacular as its smaller cousin so doesn’t get the same (in fact, hardly any) attention...that is one long, hot, empty drive, or was when I was there in 1989.
People may ask where the oldest rocks are to be found and the answer is Canada which is one old nation in parts. In 1999, the then oldest rocks known were 4.031 billion years old and found in acasta gneiss (nice of them) in the Slave Crayton, NW Canada. Some which may be marginally older have since been discovered in Quebec (The French not letting a stone go unturned, pun intended, to be ahead). Such is the dating system for very old rocks, it can only be pinned-down to within several million years.
So what is the hardest rock? Hands up those who answered “diamond, Mr Bob!” Well...not really, as a diamond is a mineral not a rock, although it is formed, as are igneous rocks, with extreme heat and pressure deep in the earth. Many rocks are very hard, such as quartz, unweathered granite, etc. I ran a rose-quartz mine near Tehuacan in Mexico once and the rock was so hard and abrasive it used to chew the bulldozer-blade teeth and the crusher hammers up in hours! I gave that up after I fell down a mountain and rolled in a lot of cholla cactus! I looked like a porcupine. The real truth was I fell foul of the mine owner, Ignacio P-----, because I objected to the way he treated the peons and I tried to better their conditions. He was of the “let them eat tortillas and sleep on the ground” variety; a spoiled little rich kid.
Which has taken me away from the subject, and to the end of this humble hub which I hope enlightened you on rocks and drunks who fall down mountains.