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 the outside wall of a central London office block. The learning point of this EarthCache is to get the geocacher to become familiar with pseudotachylites. These are sometimes formed by faulting and seismic disruption , but in this case they have been formed by meteorite impacts.
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:
orthogneiss: gneiss derived from igneous rock, such as granite
felsic: rocks that are rich in minerals that form feldspar and quartz
mafic: igneous rocks that are rich in magnesium and iron
deformation: the alteration in the size or shape of a rock, caused by stress
Irongate House is an office building that was built in 1978. It was designed by the architects Fitzroy, Robinson & Partners. It is located very close to Aldgate tube station, and is a short walk from Liverpool Street station.
The cladding of the building uses a very unusual type of rock, that in 1978 was brand new on the market. It is the only such use of this rock in the UK. The rock is an orthogneiss from South Africa. Specifically, it is called Parys Gneiss. This rock is quarried near the town of Parys. The quarries where this rock came from were operated from the late 1970s to the 1990s. and are located close to the centre of a major meteorite impact crater called the Vredefort Dome. Whenever large meteorites hit the Earth’s crust there will be massive implications, including for the geology of the area.
An examination of the gneiss on the building will show formations in the rock that look like dark cracks. These are, in fact, what geologists call pseudotachylites.
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.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma (molten rock.)
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.
All three types of rock make up the Earth’s lithosphere, the outermost layer. The lithosphere averages about 100 kilometres in thickness.
More on metamorphic rocks
Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic rocks that are now inside the Earth. Sometimes they are within the crust and sometimes the upper mantle. At that depth, rocks are subject to great pressure and temperature. Although very great, the temperature is still not high enough to melt the rocks, because otherwise igneous rock would form. The pressure is much greater than that required solely to break the rocks up. In fact, the pressure is so high that it changes the chemical make up of the rocks by forcing the elements in the minerals to change position. This may take thousands or millions of years.
Metamorphic rock needs either pressure or temperature, or both of these occurring together, to form.
Metamorphism is an isochemical process, which means the chemical composition is mostly unchanged from that of the protolith (original rock.) The main difference is the recrystallisation of the minerals into a new form. New structural features are often found in the metamorphosed rocks, such as slaty cleavage or schistosity.
Different grades of temperature and pressure will cause the same original rock to form very different metamorphic rocks.
Gneiss
In certain geological conditions, such as slate or schist being subjected to very high temperature and pressure below the Earth’s crust, rock could turn into gneiss. This is a type of metamorphic rock, which shows visible bands (foliation) of minerals. These bands are light and dark coloured, and are made from alternating felsic and mafic layers. Gneiss is a medium to high grade metamorphic rock, meaning it forms in temperatures greater than 320 degrees Centigrade.
The Parys Gneiss
The Parys Gneiss that you see here formed over 2 billion years ago. The rock that you see here represent the lower levels of the Earth’s crust from that area. Metamorphism and deformation has created textures that are typical for this kind of rock.
Looking at the gneiss here, you can clearly see it was once a granite, from the constituent visible minerals. You will see the orthoclase feldspar (pink,) the plagioclase feldspar (white,) the quartz (shiny,) the biotite mica, and the hornblende (both black.) The deformation that changed it into gneiss is evident from the banding that alternates between dark and light colouring, (which are orthoclase-rich and plagioclase-rich bands, respectively.)
The stone on this building was quarried in the Leeukop, Salvamento and Kudu Quarries, on Leeukop Hill, which is 2 miles north of the town of Parys.
The Vredefort Crater
From this map, you can see how massive this impact was!
The Vredefort meterorite impact crater is located about 100 miles south west of Johannesburg and is one of the largest and oldest impact events known on Earth. The impact occurred about 2 billion years ago. It is 90 kilometres in diameter. When a meteor hits the Earth’s crust it makes a large hole (crater.) The force of the collision causes the centre of the crater to rebound, and so brings up rocks that have been deeply buried. The rocks found in the centre of the Vredefort crater are the oldest, and get younger towards the edge of the crater. The highest graded rock also occurs in the middle, and decreases towards the edge. The impact of a massive meteorite like this would cause much of the impacted crust, as well as the meteorite itself, to vaporise on impact.
Pseudotachylites
A tachylite is a black, volcanic glass associated with melted basalt, (in other words the basaltic version of obsidian.) Pseudotachylites (literal translation: false tachylites) also appear as a glassy rock. The huge majority of them are formed when meteorite impacts melt the Earth’s rocky crust. The rock is super heated by the energy exchange, and then is cooled quickly again by the Earth’s atmosphere. Although this is the basic idea of their formation, the exact mechanism isn't fully understood. The current school of thought suggests the rock is melted by friction at the same time cracks (faults) in the Earth’s crust are moved rapidly. The molten rock then pours, or is forced into, these faults, forming pseudotachylites.
In the rock here, they can be clearly seen, looking like long veins in the rock. Fortunately, the location of the quarries is exactly in the right position for the best examples of pseudotachylites from this impact event. Since pseudotachylites are a geologic glass, and glass is somewhere between a solid and a liquid, they might be still moving ever since that massive meteorite collided with the Earth over 2 billion years ago!
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.
- Look at the orthogneiss rock. Is it fine grained or coarse grained?
- Look at the crystals in the rock. How big are they, on average?
- Look at the banding in the rock. Describe the pattern of the alternating colour bands, (for example, flat layers, a swirly formation, etc.)
- Look up at the pseudotachylites. What colour are they on this rock?
- In which direction do they go along the rock, horizontally or vertically?
- 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!
