The Stone Jetty was enhanced by several pieces of public art around the beginning of the millenium. You'll find statues of birds, mazes on the grounds and plenty of places to sit that compliment other statues and art in the town.

At GZ, you'll notice two different types of granite have been used in the surround of the post - we're going to use these to learn a little more about granite.

1) Look at the inside circle of granite (the one with the text engraving) - using the information below, what kind of granite do you think this is? What type of minerals contribute to this colour?
2) Look at the outer rim - again, using the information below, what type of granite do you think this is - what type of minerals contribute to the colour?
3) Look at the crystal size of the inner and outer circle. Which type of granite do you think cooled more quickly - what makes you think that?
4) Looking at the crystals again, do you think these stones are phaneritic or aphanitic ? What are the reasons for your answer?
5) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions. 

Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.

Formation of granite

Molten rock containing a high content of silica, minerals, and alkali metal oxides seeps into gaps in sedimentary rock layers before slowly cooling and solidifying underground over the course of millions of years, creating granite.

As the molten rock gradually cools over time, crystals form at different periods of time from the various minerals that are present which alters the final appearance of the granite.

One piece of granite typically consists of 20-60 percent quartz, 10-65 percent feldspar, 5-15 percent mica, and minor or trace amounts of amphiboles and other minerals.

What gives granite it's colour?

Below is a list outlining the primary colour of common minerals so that we can understand what colour the granite is likely to be if a high ratio of that mineral is present.

• Quartz: Milky White or Colourless
• Feldspar: Off-White
• Potassium Feldspar: Light Pink to Red
• Biotite: Black or Dark Brown
• Muscovite: Light Gold or Light Yellow
• Amphibole: Black or Dark Green

White Granite

White granite contains a high percentage of quartz and feldspar, which gives it a milky off-white and somewhat opaque appearance.

Many white granite memorials also contain black spots of different sizes which is likely due to the presence of small amphibole grains.

Black and White Granite

Some granite has an equal blend of both black and white, this combination is due to an equal amount of quartz, feldspar and amphibole.

Pink Granite

Pink granite gets its salmon tone from a high percentage of the potassium feldspar mineral.

Within the material of pink granite, you can often see small specks of semi-transparent milky quartz, dark specks of amphibole, and opaque white feldspar.

Crystal Size and Cooling in Granite

In igneous rocks such as granite, the size of crystals is intricately tied to the rate at which the molten magma solidifies. Consider it akin to the process of cooling a liquid to form ice. Swift cooling leads to the development of minute crystals, resembling the rapid freezing of small water droplets in an ice tray. On the other hand, gradual cooling affords more time for crystal growth, resulting in larger, well-defined crystals akin to the formation of sizable ice crystals in a slow-freezing process.

If magma cools quickly, for example when lava erupts from a volcano, then many crystals form very quickly, and the resulting rock is fine-grained, with crystals usually less than 1mm in size.

If magma is trapped underground , it cools slowly because it is insulated by the surrounding rock. Crystals have more time to grow to larger size and crystals 2mm and larger can form.

For example, the picture below is Shap Granite, formed in the Lake District - the coin for scale here shows a slow cooling crystal as it's large in size.

Textures of Igneous Rocks

Phaneritic - coarse grained

Textures of Igneous Rocks

We can use the definitions below to talk about the size, shape and arrangement of the crystals in an igneous rock.

Phaneritic - coarse grained

Igneous rocks displaying a phaneritic texture boast crystals that are easily discernible to the naked eye and uniformly sized.

Aphanitic - fine grained

Igneous rocks exhibiting an aphanitic texture consist of crystals too minute to be perceived with the naked eye, usually less than 1mm in size.