Introduction
The thing about geology is that there seems to be a never ending stream of new words to learn.
The terminology which geologists use to describe things can seem incredibly complex to the the point of being almost impenetrable for those of us who aren't experts - but learning some of it is a fairly essential part of expanding our ability to describe and categorise what we see in the field.
This EarthCache deals with a group of three common terms used to describe the texture* of igneous rocks such as granite. The object of your study is a statue with a plinth (base) made from a nice example of Dalbeattie / Criffel granite which formed around 400 million years ago.
*It's important to remember, when we talk about texture here we're not talking about how the rock feels to the touch, we're using texture in the geological sense to describe the size, shape and arrangement of the crystals in an igneous rock .
The texture of an igneous rock is one of the measures geologists use to classify and name the rock.
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Based on your observations at the EarthCache location and the information on this page you should be able to tell me:
- The box-shaped granite plinth at the base of the column the statue stands on has two intrusive dykes exposed on a single surface - which surface is it?
- How wide are the two intrusive dykes?
- In some detail, describe the two intrusive dykes. Include in your description things like colours and how they are arranged, their aplitic / pegmatitic nature, are they straight? curved? wavy? stepped? and anything else you notice about them.
- In the larger dyke, how may episodes of intrusion would you say took place and how do you arrive at this conclusion?
- Please add a non-spoiler photograph of yourself or a personally identifiable item at the cache location as proof of your visit there. You can include any part of the statue in your photograph which doesn't give away the answers.
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
So you may or may not know that I like words - and I like to understand their origins and thus gain more insight into their true meaning - or what is known as their etymology. This in turn helps with remembering the words and with picturing what those words are describing as well as with making educated guesses at the meanings of other, similarly formed words.
And in learning a bit more about the granite which forms the plinth for this statue - and the igneous veins which cut through it, thanks to some kind and generous people steering me in the right direction, I noticed that words with the same ending kept cropping up - granite, aplite and pegmatite.
-ite
A suffix is a letter, or group of letters, added to a word to make a new word.
Take the word slow and add ly and you have a new word - slowly. In the word slowly, ly is the suffix.
This EarthCache concerns the suffix ite, as found in granite, aplite and pegmatite - and also found in loads of other words that are used in geology such as felsite, perthite, aragonite - the list goes on and on 😄.
And of course let's not forget shopfrontite - although admittedly that's a blanket definition coined by a geological society advertising an upcoming building stones walk, which must have included a fair amount of exotic stone cladding embellishing the facades of various retail emporia, rather than a petrological definition 😄.
Research indicates that -ite is a suffix of Greek origin, indicating origin or derivation from, or immediate relation with, the person or thing signified by the noun to which it is attached.
By way of illustration if we took, say, the word magnetite, knowing that it was a rock or mineral, and took off the ite suffix we'd be left with the word magnet and we could suppose that it had been given this name because it was a rock or mineral with some properties related to magnetism.
In the above example ite is the suffix attached to the noun magnet. Sometimes suffixes and/or nouns are derived from other languages, often Latin or Greek and we need to do a little more translation to arrive with some confidence at an English definition.
Let's Start With Granite / ˈgræn ɪt /
Granite is coarse-grained, light-coloured igneous rock consisting mainly of quartz, mica, and feldspar with a peppering of dark minerals such as biotite or hornblende and is often used as a building stone - and as plinths for statues 😉.
The word granite comes from the Latin granum, a grain, in reference to the coarse-grained nature of the rock - so, as an aid to memory, we might think of granite as grainite.
Geologists would describe granite as being holocrystalline because it is made up entirely of mineral crystals. The crystals in granite are usually visible to the naked eye.
Granite Is Hard - But Not Unbreakable!
Granite starts off as molten rock and molten rock is fluid in nature - it flows.
Obviously the granite as we see it today isn't molten rock - having cooled slowly from its molten state to form hard, solid rock.
As the granite cooled, the mineral crystals grew large enough to be seen with the naked eye, in this case a few millimetres across, and locked together very tightly - imparting the characteristic strength we associate with granite. The colours of the various minerals in the granite result in a coarse, but fairly uniform, speckled appearance.
Granite is hard, but it's also rigid and inflexible and so brittle, which means it can and does fracture or even shatter - especially when subjected to enormous forces such as those present in the Earth's crust when continental plates collide.
Fractures in granite can arise during folding of the rock in mountain-building processes, as a result of the decrease in pressure during the uplift of the rock from deep down in the Earth to the surface, or even just because the rock shrinks as it cools.
I'm Your Density!
Granite is an intrusive igneous rock - which means that it formed from magma (molten rock) deep below the surface of the Earth.
When solid rock melts to form a magma it becomes less dense than the surrounding rock. This difference in density causes the magma to push upwards with great force, for the same reason that oil will rise upwards in a body of water or a helium filled balloon will rise upward in the more dense surrounding air, or a beachball pushed down to the sea bed then released will rise quickly to the surface.
If you've ever seen a violent volcanic eruption - where rising magma erupts through the Earth's crust - you'll have some idea of the potential scale of the upward force of the magma in question 😲.
Intruder Alert!
Fractures in rocks present spaces which can be filled with other stuff - which is something that natural forces are good at.
As magmas rise toward the Earth's surface they tend to force their way into any available spaces in existing solid rock. Geologists call this process intrusion, hence why rocks like granite are known as intrusive igneous rocks.
If rising magma happens to meet an earlier granite which has cracks in it, arising from processes like the ones described earlier, the magma will tend to intrude into those cracks and fill them up with new molten material. That molten material will then also cool to become solid rock, crystallising as it cools.
While the chemical composition of the intruding magma may be very similar to that of the original granite, the size of the crystals formed as the intruded material cools can differ considerably, making it easy for us to see the igneous intrusions later in the solid rock.
Geologists call igneous intrusions that push up towards the surface of the Earth through cracks in solid rocks dykes.
Play It Again, Sam
A crack in a rock represents a weakness in that rock and that weakness can persist even after new molten material has filled it and solidified, especially if the crack continues to grow longer or expand in width. Later igneous intrusions, pushing their way up toward the surface, will often follow these same weak pathways again. As a result of this, being able to see the effects of several episodes of intrusion along a single igneous vein or dyke is not unusual.
We might see aplite and/or pegmatite dikes intruding into granite and even intruding each other - aplite cutting through or running alongside pegmatite and vice-versa - depending on the number of intrusive events and the chemical makeup of the melt on each occasion.
So now that we understand the nature of intrusive igneous rocks and how, in their molten state, they intrude earlier rocks before cooling to become solid rock, crystallising in the process, it's time to look at two more ites - aplite and pegmatite.
Moving On To Aplite / ˈæp laɪt /
Aplite is a fine grained igneous rock which forms narrow dykes generally less than one metre in thickness and often only a few centimetres wide.
Aplites and granites are similar in composition but differ considerably in crystal size. That is to say that the combination of mineral ingredients in the average aplite is much the same as the combination of ingredients in the average granite - mainly quartz, mica and feldspars, but the crystal size in an aplite is typically 2mm or less versus granite's average crystal size of around 5mm.
The tiny crystals in aplite result from comparatively more rapid chilling of the molten material as it comes into contact with the cold granite, leaving little time for crystal growth before the molten material solidifies.
And Finally - Pegmatite / ˈpɛg məˌtaɪt /
The image to the right shows a section of a pegmatite dyke. The image above that shows two vertical aplite dykes cutting through a granite. It's pretty easy to see the huge pink feldspar crystals in the pegmatite, whereas the crystals in the aplite dykes are so small they're pretty much invisible.
Like granite and aplite most pegmatites are composed primarily of quartz, mica and feldspar.
Pegmatites though are considered extreme igneous rocks due to their exceptionally large crystals which are typically at least 1cm in diameter but can grow, in extreme conditions, to metres in length! 😲
Super-large crystals in igneous rocks are normally associated with slow crystal growth arising from very gradual cooling of magma (and additional factors) over very long periods of time, but the large crystals in pegmatites form comparatively rapidly in the latter stages of a magma's crystallisation.
Early on, the magma usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide, which aren't readily taken up and incorporated into the developing crystals. As a result of this, these volatiles become concentrated in the melt (another word used to describe a magma and it's particular mixture of ingredients at a point in time).
As the ratio of volatiles in the melt increases the magma becomes less viscous, or more runny if you prefer, and large crystals form more easily and much quicker than was previously possible.
Etymology / Aide-mémoire
Returning then to the etymology of the words granite, aplite and pegmatite...
Again, The word granite comes from the Latin granum, a grain, in reference to the coarse-grained appearance of the rock - so, as an aid to memory, we might think of granite as grainite...
Aplite gets its name from the German word aplit which, in turn, is derived from the Greek haploos, meaning simple, so we might think of aplite, with the plain, simple appearance arising from its fine mass of tiny crystals as simpleite 🙂.
And then there's pegmatite, derived from the Greek, πήγνυμι (pegnymi), which means to bind together, in reference to the intertwined crystals of quartz and feldspar, although it might also be fair to look at the large, irregularly shaped crystals which look a bit like crystals joined together in clumps and think of pegmatite as joinedtogetherite 🙂.
Grainite, simpleite and joinedtogetherite don't sound quite as scientific as granite, aplite and pegmatite but the fact remains that that's essentially what the scientific names mean, because they are based not on the chemical makeup or atomic structure of the igneous rocks in question, but simply on their visual appearance. The people who named these rocks essentially described what they saw.
So perhaps shopfrontite isn't quite as silly as it first sounds after all 🤔.
A rock with the characteristics of granite can be described as granitic. Likewise, a rock with the characteristics of aplite can be described as aplitic and pegmatite as pegmatitic.
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.
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