URBAN EARTH - Fire Rocks EarthCache

Introduction

The word igneous is derived from the Latin word ignis which means fire - which is why igneous rocks are sometimes called fire rocks. But why should rocks be associated with fire? Because while these rocks are cool enough to touch today, their physical structure and chemical properties were forged by great heat - as high as hundreds of degrees celsius.

This EarthCache takes you to three locations where you can see examples of fire rocks up close and then asks you, based on what you've observed and learned, to correctly identify one of them which while it might commonly be described, in the general terms used in building, as granite actually isn't granite at all. So what is it? Read on and learn .

Logging Tasks

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Based on your observations at the three locations and the information on the cache page, please tell me:

1. At the first location (published coordinates) you can see cylindrical columns of a pink granite. It has phaneritic texture and most of its crystals are roughly the same size. While it looks pink from a distance there are crystals of more than one colour - what colours do you see?
   
   
2. Whoever produced these columns took the rough quarried rock and gave it a very high polish - which helps to make all the crystals really easy to see with the naked eye, but how does this polished granite feel to the touch?
   
   
3. At the second location you'll be in front of a branch of a high street bank. The front of the building is clad in a granite similar in some ways to that used in the columns at the first location. This granite though is unpolished. Is it as easy to see the crystals? Does it feel different to the touch?
   
   
4. At the third location you'll find a place to sit - on the dark rock that the stepped amphitheatre is made from. Apart from me telling you earlier that it wasn't granite, what evidence can you see which proves that?
   
   
5. Based on your evidence and everything you've learned so far, is this rock basalt or gabbro?
   
   
6. 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

Granite is a well known igneous rock which many people recognise because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, kerbing, stair treads, building cladding, and cemetery monuments. Granite is used all around us - especially if you live in a city.

There are though numerous other igneous rocks which are used in similar ways and which are generally, but incorrectly, described as granite.

In the commercial stone industry, a "granite" is a rock with visible grains that are harder than marble that will take a polish. Under this definition, gabbro, pegmatite, gneiss, syenite, monzonite, anorthosite, granodiorite, diabase, diorite, and many other rocks will be called "granite."

So let's look at ways that we can distinguish between igneous rocks of various types and, ultimately, classify them correctly in a petrological sense .

Textures of Igneous Rocks

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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  Phaneritic - coarse grained - from the Greek phaneros meaning visible.
  
  Igneous rocks with a phaneritic texture contain crystals which are clearly visible to the naked eye and all roughly the same size.
  
  Rock crystals tend to grow very, very slowly - which means it takes a long, long time for the crystals to get to this size. This tells us that the magma which these rocks were formed from must have cooled very, very slowly at considerable depth beneath the Earth's surface.
  
  Igneous rocks which form in this way are known as intrusive igneous rocks or plutonic igneous rocks (plutonic from Pluto, the Roman god of the underworld, as opposed to Mickey Mouse's dog).
  
  
  
  
• 
  Aphanitic - fine grained - from the Greek aphaneros meaning unseen.
  
  Igneous rocks with an aphanitic texture are made up mostly of crystals which are too small to be seen with the naked eye (typically less than 1mm).
  
  The tiny crystals in this rock tell us that the magma it was formed from must have cooled relatively quickly and solidified before the crystals had time to grow large enough to be seen with the naked eye, which in turn tells us that the magma this rock was formed from cooled at or close to the Earth's surface.
  
  Igneous rocks which form in this way are known as extrusive igneous rocks.
  
  
  
  
• 
  Porphyritic - a mixture of fine and coarse - from the Greek porphúra meaning purple.
  
  In the past the term porphyry applied exclusively to a specific type of purple coloured rock. Purple was the color of royalty, and the imperial porphyry was a deep purple igneous rock with large crystals of plagioclase feldspar.
  
  Porphyry later came to be used as a term to describe rocks with the same sort of texture as that purple rock i.e. large crystals of feldspar surrounded by a groundmass of much smaller crystals, regardless of colour. The distinctively larger crystals in a porphyritic igneous rock are known as phenocrysts.
  
  Igneous rocks with porphyritic texture are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains (phenocrysts) with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth, resulting in the much finer crystals of the groundmass.
  
  

Mineralogy

If the approximate volume fractions of minerals in the rock are known, the rock name and silica content can be read off the diagram below. This is not an exact method, because the classification of igneous rocks also depends on components other than silica, yet in most cases it is a good first guess - and good enough for our illustrative purposes here because a) I know what this rock is and b) you only need to make a choice from a list of two .

This diagram might look complicated but stick with it as there's a lot of useful information in it .

The diagram can be divided into columns from left to right and I've make the left-hand column a little bit darker just to make it stand out. This is the part of the diagram where we see the mineralogical makeup of two rocks - rhyolite and granite. These two rocks are identical in terms of their mineral makeup. Where they differ is in their texture / crystal size. Rhyolite is aphanitic - has crystals which are too small to be seen with the naked eye. Granite is phaneritic - its crystals are large enough to be seen with the naked eye. The rock names across the top of the diagram follow this same pattern - the uppermost rock has an aphanitic texture and the lower rock has a phaneritic texture.

Or, to look at it another way...

If your rock is made up of approximately 50% Orthoclase, approximately 20% Quartz and approximately 15% Plagioclase and the remaining 15% is made up of Muscovite, Biotite and Amphibole then it's either rhyolite or granite.

If the crystals in that rock are mostly too small to be seen with the naked eye then you have rhyolite. Likewise, if you can clearly see the crystals with your naked eye, your rock is granite.

Taking another example, a rock with a little Plagioclase and large amounts of Pyroxine and Olivine will either be basalt or gabbro. Whether or not you can see crystals in that rock with your naked eye will tell you which it is .

And finally - at least for this diagram . The rocks on the left-hand-side of the diagram are made up of minerals which make those rocks light in colour. As we work from left to right the rocks, due to their mineral composition, get progressively darker.

In summary...

Granite is a common type of igneous rock which has formed deep underground (intrusive) and is rich in the minerals feldspar (hence felsic) and quartz. Typically the amount of quartz is between 20% and 60% and at least 35% of the included feldspar is of the alkali type (rich in sodium and/or potassium).

Granite is made up entirely of mineral crystals (is holocrystalline) which are visible to the naked eye (has a phaneritic texture).

Granites tend to be light coloured.

Basalt and gabbro on the other hand...

If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there .

Please submit your logging task responses before posting your log.