Welcome to the Angel at the Angel!
This EarthCache is located within an area frequently patrolled by Police. Please avoid acting suspiciously whilst completing the Tasks, and, if challenged, please freely explain about geocaching. It may be worth pointing out that it is not a physical box you are looking for, but are simply studying the rocks visible to everyone.
This is an urban EarthCache in which geocachers are invited to examine some pavement setts in central London. The learning point of this EarthCache is to get the geocacher to become familiar with igneous rocks, in particular foyaite.
You'll find the pavement setts at the published co-ordinates, in the pedestrian area on the corner of St. John Street and Pentonville Road.
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.
Here are some keywords for this lesson:
sett - a broadly rectangular quarried stone used for paving
feldspathoid - any of a group of minerals chemically similar to feldspar but containing less silica, such as nepheline.
nepheline - a feldspathoid mineral that occurs in intrusive and volcanic rocks with low silica
comagmatic - igneous rocks that have mineral or chemical peculiarities indicative of a closely similar magmatic source
laccolith - a mass of igneous rock that has been intruded between rock strata causing uplift in the shape of a dome.
matrix - a mass of fine-grained rock in which gems, crystals, or fossils are embedded.
The Angel Building, by the Angel Islington, is the reinvention of an unloved early 1980s commercial building located in the historic district of Clerkenwell. Commissioned by Derwent London and designed by architects Allford Hall Monaghan Morris, the Angel Building opened in 2010. designed by award-winning architects AHMM, the building was shortlisted for the RIBA Stirling Prize in 2011.
The fully refurbished building now sits more comfortably within its immediate context and has been a catalyst for regeneration in the area.
On the corner of St. John Street and Pentonville Road, this building hosts extensive office space with shops and restaurants at ground level in the heart of Angel, Islington.
The original concrete frame has been reused and wrapped with a highly energy-efficient glass and steel skin. The new glazing utilises oversized curtain walling, with 3-metre wide openable windows, so minimising air-conditioning and allowing people to control their own environment.
Sky terraces on the top floor can be enjoyed by all occupants, offering stunning 360-degree views of London.
The Angel Building has won many awards (RIBA, BCO, New London Award, AIA, 3RAwards, Concrete Society and a Civic Trust commendation, BCO Test of Time award) and was shortlisted for the prestigious RIBA Stirling Prize 2011.
The façade of the building, rendered in glass and dark grey aluminium, does not immediately look like the subject of an EarthCache, but there is indeed something interesting going on with the paving setts which form the pedestrian areas on St John’s Street and round the corner onto Pentonville Road.
These are an example of pavement setts, but they are definitely not the pavement setts at GZ!
OK, so let’s get started...
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 lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
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.
Foyaite
And so to the pavement setts on the corner of St Johns Road and Pentonville Road, outside the Angel Building…
These setts are made out of a rock that is marketed as ‘Portuguese Granite’, but this is not entirely correct. It is true that this rock was mined in Portugal. It is from the Monte Foia, near the town of Monchique in the Algarve, (hence the name, foyaite.) Foyaite may superficially resemble a granite, but it does not contain the defining mineral of granites, which is quartz.
The Monchique Intrusive Complex is a large, elliptical (63 x 16 km) laccolith which forms the upland Serra do Monchique of the Algarve. It is predominantly composed of nepheline syenite and was emplaced into Cretaceous marine sediments around 72 million years ago. The main quarry is at the village of Nave, a few kilometers south of Monchique.
As mentioned, foyaite is a nepheline syenite. Nepheline is a feldspathoid mineral that occurs in intrusive and volcanic rocks with low silica. Syenite is a coarse-grained intrusive igneous rock that is chemically similar to granite, but entirely/mostly lacking quartz. The feldspar component of syenite is mostly alkaline.
The foyaite here contains only one feldspar (perthite or antiperthite) and is defined as a Hypersolvus Syenite. These are distinct from syenites that contain both potassium feldspar and a separate plagioclase, which are called Subsolvus Syenites. Syenites usually occur as relatively small independent intrusions, often being comagmatic with granitic intrusions.
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.
1 Please describe the grain of the foyaite setts, (for example, coarse / fine.)
2 What colour is the matrix of the foyaite setts? What mineral is causing this colour?
3. Apart from the matrix, the alkali feldspars are a distinctive and decorative feature of the foyaite. Choose one sett and describe the feldspars (shape, size, colour, texture, orientation.)
4 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!
