The building is made primarily of Portland Stone, a type of limestone. The floors, including the floor on the entranceway is made from Brescia Marble
What is limestone?
Limestone is a sedimentary rock formed most commonly in shallow, warm marine waters. It is usually composed of an accumulation of shell, algal, coral and fecal debris. It can also form from the precipitation of calcium cabronate (CaCO3) from ocean or lake water.
Portland Limestone
Portland Limestone was formed organically in shallow marine waters on the continental shelf of what is now Western Europe. The deposits were formed in the latest stage of the Upper Jurassic period. Its name is derived from the place it was first found, on the Isle of Portland in Dorset. It is found in various locations, not just in the South of England. The stone is white in colour.
It is composed of ooliths (calcium carbonate spheres) and shell fragments and weathers mainly by dissolution, often leaving shell fragments protruding from the surface. Its pale colour also leaves it prone to rapid soiling, particularly in polluted urban environments.
Ooliths are small, round grains consisting of calcium carbonate, silica, or dolomite. Ooliths have concentric layers that form around a nucleus, such as a shell fragment, a sand grain, or a pellet of alga. They typically have diameters of 0.25 to 2 mm. If you look closely at the limestone with a magnifying glass or loupe, you should be able to make out the tiny spheroid particles held tightly together. These are the ooliths. The well-sorted, fine, even-grained structure of the oolitic limestone provides the perfect freestone (meaning it can be easily carved in all directions) making it an ideal stone for decorative carvings as well as for building.
Uses of Portland Limestone
Portland Limestone is widely used as a building stone in the British Isles, despite its relatively expensive cost. It is used at St. Paul's Cathedral and Buckingham Palace in London, City Hall and the National Gallery in Dublin and of course, at Stormont and City Hall in Belfast.
The stone can also be used to make a type of cement when mixed with clay. This is known as Portland Cement and was patented by Joseph Aspidin in 1824.
Weathering at City Hall
Solution is the most direct type of stone decay in urban environments. Solution damage is not only restricted to pure limestones, and in quartz sandstones held together by CaCO3 cement, the sand grains can be loosened and eventually become detached as the cement is dissolved. Thus, chemical activity can indirectly promote physical disintegration of the stone.
The effects of solution are commonly seen where it attacks intricate carvings and statues, and smoothes out and rounds off protuberances and sharp edges. In such cases it is often difficult to assess just how much removal of stone by solution has occurred.
What is marble?
Marble is a metamorphic rock that is formed when limestone is subject to the pressure and heat of metamorphism. This means that its main constituent is the same as limestone, along with a variety of other minerals in lesser volumes.
Various marbles are used on the interior of Belfast City Hall. The floors are made of Brescia marble and this can also be seen outside at the main entrance.
Brescia marble comes from the Italian city of the same name, sitting at the foot of the alps.
To log this EarthCache as a find, please send me answers to the following via the message centre:
1. What type of rock is limestone?
2.At the round pillars at the main entrance to City Hall, can you identify some effects of soluble weathering here (as described above)?
3. Examine the marble on the ground at the main entrance and compare it to the texture of the Portland Stone in the pillars. Describe and contrast the two.
4. How many black triangles appear on the ground at the entrance?
HAPPY EARTHCACHING!
| I have earned GSA's highest level: |
 |