City Hall
Philadelphia City Hall is an iconic architectural landmark located in the heart of Philadelphia, Pennsylvania. Construction of Philadelphia City Hall began in 1871 and took approximately 30 years to complete, making it one of the most notable architectural and construction achievements of its time. It is also the largest free standing masonry building in the world.
The building's facade features extensive use of granite and marble, which not only enhance its grandeur but also have interesting geological compositions. The combination of Wissahickon schist and white marble gives the building its distinctive appearance and contributes to its architectural grandeur. Philadelphia City Hall is known for its impressive masonry work and architectural details, and the choice of these materials played a significant role in achieving that.
Granite:
Use: Granite is predominantly used in the construction of the exterior walls of Philadelphia City Hall. It is employed for its durability and resistance to weathering, making it an ideal choice for a building that is exposed to the elements. The granite used in City Hall primarily comprises the lower course of the exterior facade, the massive base, and some decorative elements. The columns at the entrance of Philadelphia City Hall are made of red granite. These columns are a prominent feature of the building and add to its grandeur
Red Granite Columns
Geologic Composition: The granite used in City Hall is likely sourced from quarries in nearby states like Pennsylvania or Vermont. Granite is an igneous rock composed of primarily three minerals: quartz, feldspar, and mica. The specific mineral composition can vary, giving rise to different colors and patterns. In City Hall's case, the granite is typically gray or pinkish-gray and exhibits a coarse-grained texture. The chart below shows how different makeups of minerals informs the colors. Granite with a lot of quartz is more whitish and color and granite with potassium feldspar are more reddish in color.
Granite Mineral Composition
Wissahickon schist
Use: Wissahickon schist was used extensively in the construction of the building, particularly for the exterior facades. It was used for the walls, columns, and various decorative elements on the building's exterior. The stone was carefully cut and shaped to fit the architectural design of the building, resulting in a grand and imposing structure.
Geologic Composition: The Wissahickon Schist is a type of metamorphic rock found in the eastern United States, particularly in the region around Philadelphia, Pennsylvania. Wissahickon Schist is primarily composed of metamorphosed sedimentary rocks, such as shale and siltstone. These sedimentary rocks have undergone a process of metamorphism, which means they have been subjected to high temperature and pressure, causing them to recrystallize and change in mineral composition. The formation of Wissahickon Schist is attributed to the regional metamorphism of the original sedimentary rocks, which were deposited during the Paleozoic era. The metamorphic process occurred deep within the Earth's crust over millions of years, resulting in the rock we see today.
The minerals commonly found in Wissahickon Schist include mica minerals (biotite and muscovite), quartz, feldspar, and various amphibole and garnet minerals. The presence of mica minerals often gives the rock a characteristic schistose texture with shiny, platy mineral flakes. Wissahickon Schist exhibits a schistose texture, which means it has a foliated appearance with aligned mineral grains. The foliation is typically due to the parallel arrangement of platy minerals like mica. This alignment is a result of the metamorphic process and provides the rock with a distinct layered appearance.
The color of Wissahickon Schist can vary, but it is often gray to bluish-gray, with variations in hue depending on the specific mineral content and any impurities present.
Marble:
Use: Marble is primarily used for decorative elements, including sculptures and ornate carvings found on the interior and exterior of Philadelphia City Hall. It is utilized for its aesthetic appeal and workability, allowing intricate designs to be carved into it. Notable examples include the Alexander Milne Calder sculptures and the decorative detailing on the building's upper portions.
Marble Columns
Geologic Composition: Marble is a metamorphic rock, which means it started as a different type of rock (usually limestone or dolostone) and underwent a transformation under heat and pressure. The marble used in Philadelphia City Hall may have been sourced from various quarries. Its composition mainly consists of calcite or dolomite minerals, giving it a crystalline appearance with various colors and veining patterns. The specific type of marble used can vary, with different shades of white, gray, or other colors. Marble is usually a light-colored rock when it is formed from limestone with very few impurities. The marble that contains impurities such as clay minerals, iron oxides, or bituminous material can be bluish, gray, pink, yellow, or black in color
Both granite and marble not only contribute to the architectural elegance of Philadelphia City Hall but also reflect the geological diversity of the surrounding regions. The combination of these two types of stone in the building's construction serves both functional and aesthetic purposes, making it a lasting and beautiful landmark in the city
References
Information above was referenced from the following sources:
Activities
Stand in front of the west entrance and observe the different types of rocks; you can also walk into the courtyard for more examples and then send me answers to the following questions.
1. Granite - Find three examples of granite that are different colors and identify what you think the most abundant minerals are in them
2. Marble – Find several examples of marble. Describe the texture and color. Do you think that this is pure marble or does it have impurities and how can you tell?
3. Why do you think these stones were selected for the building of City Hall?
4. Look for signs of weathering and describe them. What do you think caused the weathering and do you think it will continue?
5. Post a picture of yourself (face not required) or your personal item at City Hall