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What is a Rock?

Updated on January 1, 2011

The two largest monoliths in the world

Uluru or Ayer's Rock.  In Central Australia
Uluru or Ayer's Rock. In Central Australia
Burringgurrah or Mount Augustus.  In Western Australia
Burringgurrah or Mount Augustus. In Western Australia

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.


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    • stars439 profile image

      stars439 5 years ago from Louisiana, The Magnolia and Pelican State.

      Thank you for sharing interesting knowledge on the age of rocks. 500 million years is such a long time. I use to like to wash them, and just look at them when I was a child. I wondered how old they were, but knew they were old. A lot of kids use to collect them, and I imagine the Smithsonian probably has all kinds. They are just fascinating to look at. Brings back memories of my little Slider turtle that use to take bits of food from my finger tips. He had a neat rock garden with soil, and greenery. He preferred live minnows. God Bless You.

    • prasetio30 profile image

      prasetio30 6 years ago from malang-indonesia

      Wow....No wonder that uluru become one of the greatest natural wonder in the world. I hope I can visit this place one day. Thanks for share with us and I learn many things from you. Voted up and useful!


    • diogenes profile image

      diogenes 6 years ago from UK and Mexico

      Thanks Karolina...Why aren't you writing a few hubs on art for us?


    • profile image

      Karolina 6 years ago

      Fantastic hub, looking forward to come back and see your new posts. Thank you.

      My art gallery

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      Thanks for the visit, glowingrocks...Bob

    • glowingrocks profile image

      glowingrocks 7 years ago from New York

      Nice write.Thanks for sharing.

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      Thanks for that, I'm hoping to marry a rich women, know any!??

      Just kidding, dear Bob

    • QudsiaP1 profile image

      QudsiaP1 7 years ago

      Ah this hub definitely took me back to the geography and science class.

      You write so randomly and still make them as interesting as ever. Perhaps you should write course books. :)

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      Hi HH. Thanks for your heartening comment...Bo

    • Hello, hello, profile image

      Hello, hello, 7 years ago from London, UK

      A fantastic hub and I learned so much from it. Thank you, diogenes, for the pleasure of reading it.

    • Austinstar profile image

      Lela 7 years ago from Somewhere in the universe

      Well, what's wrong with beach combing? I moved to Maui for the sole purpose of becoming a beach bum. Then along came my Bob who I met on a blind date and never got over. He would not love to be a beach bum unless you could lock the door. I didn't know he was so paranoid until after we tied the knot :-) Whatever. Someday, I may just park my carcass on a beach somewhere and stay until I return to the sea from whence I came. (I think).

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      Alaways something new from you. Of course, these wouldn't be Uluru-sized! Unless they were friends of Harry Potter...Bob

      Thank you for the compliment, I am a bit jaded with hubpages and writing in general at the moment, so the article was pretty basic; am considering bank robbery of beach combing...Bob

    • Austinstar profile image

      Lela 7 years ago from Somewhere in the universe

      Everyone loves rocks. The U.S. Post Office probably ships the most rocks worldwide. Yes, they ship a lot of rocks! When I was in Hawaii and went to Haleakala State Park, the ranger showed us the ton of rocks they receive back in the mail from people who picked them up and took them home to the mainland. They shipped them back because of all the "bad luck" that removing the rocks from Hawaii brought to them.

      Hawaiians believe that spirits live in the rocks and they don't appreciate being disturbed! LOL.

      So we can certainly see why the Aboriginals would want to protect their Rock!

      Nice hub, Bob

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      Thank you, dear. Yes, enjoying the peace and quiet. Rocks, especially combined with trees and cactus, can be so lovely. I lived in Mexico for many years and these is a place in Baja California where ancient glaciers left huge piles of rounded rocks, some as big as houses. They remind me of eternity...the rattlers like 'em as well! I love Mexico and am planning to get back there soon...I hope 2011 will be kind to you....Bob

    • profile image

      Pachuca213 7 years ago

      That was cool. It reminded me of when I drove through Yosemite last summer and most of the mountains there in the park are actually the biggest boulders, they are massive! Its so amazing how big rocks can be. Nice hub! Hope you are enjoying the first day of the new year!

    • diogenes profile image

      diogenes 7 years ago from UK and Mexico

      So kind of you to say that, medor. May you have a great 2011...Bob

    • medor profile image

      medor 7 years ago from Michigan, USA

      so cool, great explenation of an old subject... thanks..youare a great writer... i look forward to each and every one of them...