The Brooklyn Bridge EarthCache

The Brooklyn Bridge is an iconic symbol of New York City and spans the East River, connecting the boroughs of Manhattan and Brooklyn. Completed in 1883, it is renowned for its elegant Gothic architecture and distinctive twin towers. Designed by John A. Roebling, the bridge is a marvel of engineering innovation, utilizing a hybrid cable-stayed and suspension design. The construction of the iconic Brooklyn Bridge in the late 19th century involved the use of materials sourced from various locations, including granite from Vinalhaven Island, Maine, and limestone from Essex County, New York. The granite, known for its durability and strength, was primarily employed above the water line in the construction of the bridge's towers and elevated structures. In contrast, limestone was utilized below the water line.

The granite found on Vinalhaven Island, formed approximately 400 million years ago, is a product of the cooling of a magma chamber associated with volcanic activity during the Acadian Orogeny. This geological process occurred as microplates of continental crust (terranes) accreted onto the North American craton. Over 100 different plutons, or magma chambers, are scattered along the southern Maine coastline, including Vinalhaven, revealing the roots of ancient volcanoes.

The Vinalhaven pluton, exposed around the island, consists of four main units:
Coarse-Grain Pinkish Biotite Granite (Unit 1):

• This is the largest and most extensive rock unit on Vinalhaven Island, forming the main body of the magma chamber.
• Composed of coarse-grained pinkish granite, it represents the original silicic magma that cooled over time.
• Biotite (Dark-colored, shiny sheetlike mineral), feldspar (abundant, often light-colored crystalline mineral), and quartz crystals are prominent components in this granite, creating a durable and visually distinctive rock.

• A coarse-grain, dark, ferromagnesian-rich (contains iron and magnesium), and silica-poor igneous rock.
• Sheets of gabbro-diorite mixed in with the granite rocks, forming a complex geological structure.
• Exhibits distinctive "pillow" structures with cores of gabbro/basalt surrounded by coarse-grain granites, suggesting intricate interactions between mafic and silicic magmas (more on that later).

• This unit represents a replenishment of silicic magma into the inner core of the magma chamber.
• Characterized by a finer grain compared to the coarse-grain pinkish granite, indicating different cooling conditions or compositions.
• Intrudes across the coarse-grain granite and gabbro-diorite units, showing the dynamic nature of magma chamber processes.

• Large to small pieces of metamorphosed country rocks, locally referred to as the Calderwood Formation.
• These rocks include metavolcanics (rocks that were originally volcanic in nature but have undergone metamorphism), resembling rhyolites, which are fine-grained, pinkish igneous rocks high in silica.
• The Calderwood Formation rocks have been metamorphosed to the greenschist facies, and minerals such as cordierite, andalusite, and hypersthene are present.

The magma chamber experienced a complex history of emplacement and replenishment. The geological features indicate that the magma chamber has been tilted, with rocks along the southern and western edges representing the bottom of the chamber. The coarse-grain pinkish granite represents the main magma body, followed by the finer-grain whitish granite as a replenishment of silicic magma. Later, the mafic basalt intrusions seeped into the chamber. The mafic magma flowed into the bottom of the chamber, forming distinctive pillow structures with cores of gabbro/basalt surrounded by coarse-grain granites. The magma chamber's roof collapsed at the end of the eruptive cycles, forming a caldera. The granite's unique characteristics and the geological features on Vinalhaven make it a significant area for study. The exposed shoreline allows observation of the structure of a magma chamber, showcasing the roots of a once towering volcano.

Silicate minerals and mafic minerals differ notably in their properties and appearance. Silicate minerals, characterized by a silicon-oxygen structure, encompass a diverse range. Quartz, often clear to milky white, and feldspar, presenting in various colors including white and pink, are common silicates. Mica minerals, exhibiting hues from colorless to brown or black, and amphibole minerals further exemplify this diversity. Silicate minerals generally possess lower density; quartz, for instance, is relatively lightweight. Conversely, mafic minerals, found in mafic rocks like basalt and gabbro, display distinct characteristics. These minerals, such as pyroxene, olivine, and amphibole, contribute to the dark coloration of mafic rocks, ranging from black to dark green. Mafic minerals are denser compared to silicate minerals due to their iron and magnesium-rich composition.

While we can't see the limestone (since it is below the waterline), it is worth mentioning a few things. Limestone, predominantly made of calcium carbonate, forms through the deposition of marine organisms or chemical precipitation in sedimentary settings. While limestone can be susceptible to dissolution in acidic water, the water in many aquatic environments is not typically strongly acidic. Therefore, limestone can maintain its structural integrity underwater. Additionally, limestone is known for its load-bearing capacity and stability.

1. Describe the color, texture, and grain size of the Vinalhaven granite. Are you able to identify any minerals within the rock? Do you think it is predominantly made of silicate, or mafic minerals, and what led you to that conclusion?
2. Based on your observations and the description, which of the four Units of Vinalhaven granite would you categorize the stone in the towers as and why?
3. Based on what you have learned, would you classify the granite as anorogenic, or orogenic?
4. Why do you think Vinalhaven Granite was used for the towers of the Brooklyn Bridge (as opposed to let's say Barre Granite, which is a popular building material)?
5. Why do you think limestone was used below the waterline, and granite above the waterline in the construction of the towers?
6. Upload a photo taken with the Brooklyn Bridge. You don't have to be in the photo, though it is strongly encouraged.