This is an urban EarthCache in which geocachers are invited to examine paving outside a hotel in the centre of Guildford. The learning point of this EarthCache is to get the geocacher to become familiar with granite, the minerals that form it, and the phenocrysts that may appear within it.
Everything you need to answer the questions is available by visiting the location and by reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.
The gate on the high opens at 16:00 on week days and closes at 23:00. On Saturdays it is closed between 09:00 and 18:00 and on Sundays 12:00 until 17:00.
The road here is pedestrianised between 1000-1630hrs, but if you're caching outside of these hours, please be careful around the road.
Keywords for this lesson
sett - a broadly rectangular quarried stone used for paving
matrix - the geological material in which something is embedded
phenocrysts - a large or conspicuous crystal in a rock, distinct from the matrix.
The published coordinates lead you to the outside of a hotel in central Guildford. For this EarthCache, you’re invited to look at the paving setts on the road, and the kerbstones outside the hotel. There’s no need to walk into or across the road - they’re clearly visible from the pavement. The setts also go across the pavement, under the Angel Gate. Also, there’s no need to examine the larger square pavement slabs themselves - this EarthCache is about the road paving and the kerbstones.
Unfortunately, the date and provenance of these setts and kerbstones is unknown.
The composition of the kerbstones is unclear. The road setts are made of one stone, granite, but you will notice they come in many different colours.
Introduction to rocks
Minerals make up rocks. Rocks are formed in many different types of environment. These can be on, or within the Earth's crust. There are three types of rock, and each is formed in a different way.
Sedimentary rocks are formed on the Earth’s surface from the products of weathering which then becomes cemented or deposited.
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
All three types of rock make up the Earth’s crust.
More on igneous rocks
Igneous rocks, as opposed to sedimentary and metamorphic rocks, are formed through the cooling and solidification of magma or lava. Volcanoes contain molten rock - above the surface (exterior) this is called lava and below the surface (interior) this is called magma. Pockets of magma come up from the mantle/main vent and push their way through weak spots and vents in the earth's crust, often under volcanoes. When magma is pushed up through a volcano that's erupting, the magma becomes lava as it exits out of the crater.
Rocks that come from magma become intrusive igneous rocks, and the rocks that come from lava become extrusive igneous rocks. The differences are due to many factors, including the geochemistry and cooling rate, among many other factors.
Since magma is located inside the Earth, the overlying rock and sediment act as a blanket that keeps the magma warm and allows it to cool slowly. There are many other factors that keep the magma warm including radioactive decay, the great pressure upon it, and time. Lava, on the other hand, exits the volcano and cools much faster on the surface.
When looking at igneous rocks with the naked eye, you can often see crystals within their matrix. Sometimes, these are only visible with a microscope. The rate of cooling affects the speed these mineral crystals grow. Generally speaking, the longer the rock is allowed to cool, the bigger the mineral crystals that are formed.
The colours of granite
Granite is a coarse-grained intrusive igneous rock that contains large grains that can easily be seen with the naked eye. Granites contain at least 20% quartz, and often contain up to 60%.
Since granite is an intrusive rock, it formed from magma that gradually cooled beneath the Earth’s crust, and was never expelled as lava. This gradual cooling is the reason the phenocrysts in granite are often large enough to see with the naked eye. Over millions of years, different sorts of granite lithify (solidify) in different ways depending on their chemical makeup and the specifics of the environment they form in. If granite had been an extrusive igneous rocks, i.e. one that had formed from lava above the Earth’s surface, it might have cooled a lot faster and created a more homogenous looking rock with no discernible grains.
Granite is made up of many different minerals, including quartz, potassium feldspar, mica, amphiboles, and trace other minerals. Granite typically contains 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite). The different colours of granite provide evidence of the different mineralogy and origins of each type. The minerals that make up granite give it the unique colours we see in different types of granite:
Granite that is predominately coloured white is mostly made up of quartz and feldspar minerals. Sometimes it contains black specks, which might be small crystals of amphibole or pyroxenes. This might be because the chemistry of the magma that formed the granite or the conditions it formed in were insufficient to form amphibole. If the rock is milky white, quartz might predominate, though if the rock is more off-white, feldspar might be responsible. Often, it will be a mixture of both of these, (potentially, plus other minerals.)
Pink coloured granite is often the result of an abundance of potassium feldspar within the magma that formed it. You might be able to see small specks of milky semi-transparent quartz and dark brown/black amphibole..
Red coloured granite is a variation of pink potassium feldspar-abundant granite, where the k-feldspar takes on a more red than pink colour. It can also occur when iron oxide appears in hematite grains or is included within the feldspar.
Granite that is speckled black and white, or dark green is often formed from magma that was abundant in quartz and minerals that make up amphibole.
As a very general indicator, biotite-rich granite typically has a black or dark brown colour and muscovite-rich granite typically has a metallic gold or yellow colour.
Please remember this colour guide is extremely imprecise, at best.
Probably not granite
Black ‘granites’ are often another type of rock altogether, since granite must contain at least 20% quartz. Gabbro and basalt, which are dark-coloured igneous rocks, are often mistaken or sold on the market as ‘black granites.’
Blue ‘granite’ is similarly often not actually granite. Sometimes larvikite, an igneous monzonite rock, is described as ‘blue granite,’ as is anorthosite, an intrusive igneous rock which contains abundant blue labradorite. However, neither rock is true granite.
Rock sold as green ‘granite’ might be all sorts of different rocks, such as marble or soapstone. Granites typically do not contain many green coloured minerals, though there are plenty of other rocks that do. Nevertheless, granite occasionally does obtain a green colouration if it contains amazonite, which is a green-coloured feldspar.
Porphyry and phenocrysts
A phenocryst is a large crystal that stands out in the rock from the matrix (surrounding groundmass.) When this happens, geologists call the texture porphyritic. A phenocryst typically is at least 0.5mm in diameter, in other words it is visually discernable. A porphyritic texture is the texture of a rock with large phenocrysts and a background matrix of smaller crystals. A granite rock with a porphyritic texture is called a granite porphyry. Porphyry is common in many granites.
There are two important factors that affect the crystal size of minerals within a cooled and lithified rock. One is the rate at which the magma cools and the other is chemical composition. The predominant factor is the rate of cooling, where the magma either cools rapidly or slowly. This will drive the size of individual mineral crystals.
In the context of intrusive igneous rocks like granite, porphyritic textures can develop when the magma cools happens relatively quickly. The minerals that were formed earlier will have formed slowly and remain as large crystals whilst the sudden cooling of the remainder of the rock will form an aphantic (not visible to the naked eye) matrix.
Porphyritic textures might also occur, for example, when magma crystallises below a volcano but is erupted before completing crystallisation, forcing the remaining lava to crystallise more rapidly with much smaller crystals.
Granite porphyry has the same chemistry and mineralogy as ordinary granite, but differs in the size range of its crystals. Granites are coarsely crystalline igneous rocks, having a phaneritic (visible) texture. Porphyritic granites often have a mix of large & small crystals. To form this crystal contrast of smaller and larger minerals the rock may have gone through different stages of cooling that formed different sizes of minerals. During formation of phenocrysts, the magma was cooled slowly giving time for large crystals to grow. Then, during late stages of lithification, the magma was rapidly cooled, forming smaller crystals as the matrix surrounding the phenocrysts.
If a geologist can gather evidence from a rock about its mineral consituents, as well as the relative size of crystal grains compared to one another, they can piece together the larger evolution of an associated volcanic structure or magma chamber. A look at the granites here is a good example of how small-scale details can provide large-scale conclusions on volcanic history of a region.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
Please remember not to look at the larger square slabs on the pavement. This EarthCache asks you to examine the paving setts on the road, and the kerbstones.
- Please look at the setts on the road outside the hotel and under the Angel Gate. What colours of sett can you see?
- The inclusion of which minerals might be responsible for the colours you have found in the road setts?
- Now, please look at the double line of cobbles that continue the kerb line from the pedestrian pavement where the hotel gateway crosses the pavement. It is made of a different rock to the paving setts on the road. Which of the two contain particularly large feldspar phenocrysts, the setts or the kerbstone?
- What colour are these phenocrysts you have found?
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Good luck, and thanks for visiting this EarthCache!
