The learning point of this EarthCache is to get the geocacher to become familiar with igneous rocks and identify which type a kerbstone artwork in Southampton city centre is made from.
Everything you need to answer the questions is available by attending the co-ordinates and 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 artwork ‘Canal Shore’, by artist Christopher Tipping, runs for 205 metres along the South side of Blechynden Terrace in the centre of Southampton. The dark kerbstones there are inset with white text in a different rock referencing the site. It was completed in September 2015 as part of the redevelopment of the area, to build a new ‘Station Quarter’ in the city.
Mr Tipping, the project artist, was asked to create a contextual response to the site, which would exert a positive influence on the design process and outcomes, with a particular emphasis on the interpretation of the public realm. He undertook a detailed investigation into the neighborhood’s social history, geography, ecology and culture to uncover contextual information, which inform the area’s current form, identity & reputation. This site-specific and research-led activity assisted in driving the creative concept & rationale, which now underpins the general spatial layout, character and interpretation of the new public realm proposals.
An understanding of the site’s past physical condition drives the overall theme and character of the landscape & public realm interventions: a wooded valley, a meandering stream, the curve of the historic shoreline & the ill fated Salisbury to Southampton Canal.
The text of the artwork further explores and narrates the events that have shaped this site for almost a thousand years. Its functionality is also critical to the scheme; its length articulated with elegant but engineered and robust dropped kerbs, transitions setts & crossing points.
The text and the kerbstone are both made of igneous rocks, though they are different rocks. Commerically, both are marketed as ‘granite’ but only one would be regarded as such geologically. For this EarthCache, we’ll examine the main (darker) rock of the piece, rather than the whiter writing.
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.
Underneath the earth’s surface and closer to the heat source deep underground, the magma cools slowly. Because of this, when you have rocks that cooled from magma, you can easily see the different mineral crystals in the rock. The crystals are relatively large. These rocks are intrusive igneous rocks because they cooled slowly 'inside' the earth. These type of rocks are called (defined as) phaneritic because of their crystal texture. In other words, phaneritic rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope.
Lying on top of the earth’s surface, lava cools more quickly, so there is less time for mineral crystals to form and grow. This means extrusive igneous rocks have much smaller crystals, which sometimes can only be seen with a microscope. Rocks that cooled quickly 'outside' the Earth are called (defined as) aphanitic because of their crystal texture.
Rocks that are have large and small crystals mixed together (mixed phaneritic and aphantic) are called porphyritic.
The colour of an igneous rock is related to the colours of the minerals present. Rocks that are rich in silica usually contain considerable quartz and light-coloured feldspar. In general, the darker the rock, the more coloured minerals they contain. Rocks rich in magnesium and iron tend to contain olivine, pyroxene, amphibole, and biotite. These minerals tend to be much more darkly coloured. A rock that contains less than 30% coloured minerals is considered light and may be called felsic (from feldspar and silica). With 30-60% colored minerals, it is called mafic (from magnesium and ferro, meaning iron) and with greater than 60%, ultramafic. If there are holes evident in the rock, the rock is referred to as vesicular.
Key words from this lesson:
• phanertic - rocks containing relatively large crystals
• aphantic - rocks containing relatively small crystals
• porphyritic - rocks containing a mixture of large and small crystals
• vesicular - rocks with visible naturally-occurring holes present
• felsic - a light coloured rock that contains less than 30% coloured minerals
• mafic - a dark rock containing 30-60% coloured minerals
• ultramafic - a dark/green rock containing more than 60% coloured minerals
To log this cache, please visit ‘Canal Shore’ at 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, for your answers, examine the darker kerbstone and not the whiter writing.
1 Look at the crystals in the kerbstone. To identify the speed the rock cooled, please measure the average length and width of the crystals.
2 Describe the colour and type of mineral in the kerbstone that has been used.
3 Are the crystals in the kerbstone phanertic, aphantic or porphyritic?
4 Is the kerbstone felsic, mafic or ultramafic?
5 With reference to the diagram, identify which type of igneous rock the kerbstone is made of.
6 Name three of the creatures mentioned in 'Canal Shore.'
7 Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.
Have fun, and good luck!