For this EarthCache, geocachers are invited to examine a marble sculpture in the centre of Valletta. The learning point of this EarthCache is to get the geocacher to become familiar with the properties of marble.
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 Knot is one of Valletta’s most recent sculptures. It was designed by Maltese artist Vince Briffa and consists of a marble sculpture upon a steel pedestal. It was installed in November 2015 in Castille Square to commemorate Malta’s hosting of a summit on migration. It cost €277,152 and has been the source of controversy, as its modern design is not to everyone’s taste. According to the artist, the sculpture symbolises the geographical link between Malta, Europe and Africa.
The artwork took several months of work at the Margraf plant in Chiampo, near Vicenza, where a team of company technicians and a sculptor shaped a single block of marble that originally weighed 34 tons. The first stage of processing (roughing) was done with a cutting-edge machine that used slack diamond wire cutting technology. After that, the block was hand-worked by by the sculptor. The sculpture you see today weighs 24 tons. The marble was quarried in the Apuan Alps n the Lunigiana, the northernmost tip of modern-day Tuscany, Italy.
For the purposes of this EarthCache, we will be looking at the marble knot rather than the steel pedestal.
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. They are formed by the accumulation of sediments.
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 crust.
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.
There are two basic types of metamorphic rocks: foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure. Non-foliated metamorphic rocks such as hornfels, quartzite and novaculite do not have a layered or banded appearance.
Marble is a non-foliated metamorphic rock that forms when limestone or dolostone is subjected to the heat and pressure of metamorphism. When it is formed from a limestone with very few impurities, it will be white in colour. Marble that contains impurities such as clay minerals, iron oxides, or bituminous material can be bluish, grey, pink, yellow, or black. Marble is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallises to form a rock that is a mass of interlocking calcite crystals. A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.
Most marble forms at convergent plate boundaries where large areas of the Earth's crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.
Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallises and the texture of the rock changes. In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small.
As metamorphism progresses, the crystals grow larger and become easily recognisable as interlocking crystals of calcite. Recrystallisation obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation, which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary.
Recrystallisation is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals become larger as the level of metamorphism progresses. Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases.
Being composed of calcite, marble has a hardness of three on the Mohs hardness scale. As a result, marble is easy to carve, and that makes it useful for producing sculptures and ornamental objects. The translucence of marble makes it especially attractive for many types of sculptures.
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.
- Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. Please describe the marble here.
- Is the stone a light, medium or dark colour? Why do you think this is the case?
- What per cent of calcite or dolomite would you guess is in the marble here?
- Other than the main colour of the marble, what colour are the veins and impurities in the stone?
- 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!