You seek the Gulf Tower at 707 Grant Street in downtown Pittsburgh. However, you will need to be at the Seventh Avenue entrance. You see two brass signs: GULF TOWER, which is overhead, and GULF BUILDING, which is on the granite facade. There are also two brass revolving doors.
Note: Please be respectful of the building and its occupants. All observations can be made from the public sidewalk. No need to enter the building!
The Gulf Tower is one of Pittsburgh's most iconic Art Deco skyscrapers, but its true beauty lies in its skin. While many buildings use local sandstone or brick, the Gulf Tower is clad in a distinctive Pink Granite sourced from New England.
To claim this EarthCache, you’ll need to put on your geologist hat and examine the crystals that make this building shimmer.
The Geology of Granite
Granite is an intrusive igneous rock. This means it formed deep underground from cooling magma. Because it cooled slowly over thousands of years, individual mineral crystals had time to grow large enough to see with the naked eye.
The specific "pink" hue of the New England granite used here comes from a high concentration of Potassium Feldspar (also known as K-feldspar).
The "Big Three" Minerals
When you look closely at the facade, you are looking at a mosaic of three primary minerals:
1. Feldspar (Pink): It has a blocky shape and reflects light along flat surfaces called cleavage planes.
2. Quartz (Gray and Glassy): These look like bits of smoky glass filled in between the feldspar. Quartz has no cleavage, so it looks more like irregular, jagged chunks.
3. Biotite/Mica (Black and shiny): These are the dark specks in the stone. They are often flat and flaky, providing a sharp contrast to the lighter minerals.
Why New England Granite?
The granite used in the Gulf Tower likely hails from the Deer Isle or Milford regions of the Northeast. These quarries are famous for their bi-modal texture—meaning they have very large crystals (phenocrysts) set within a medium-grained background. This stone was chosen not just for its beauty, but for its compressive strength and resistance to the acidic smog that was prevalent in Pittsburgh during the 1930s.
Logging Tasks
To log this EarthCache, please send a message or email with the answers within 1 week to the following questions based on your observations at the coordinates. DO NOT post answers in your log.
1. Crystal Size: Find a section of the granite at eye level. What is the average size (in millimeters or inches) of the largest pink feldspar crystals you can find?
2. Texture: Run your hand over a polished section versus an unpolished or thermal finished section (if visible). How does the color of the pink feldspar change between the two finishes? Alternate question: If having a problem finding the pink feldspar, why do you think this feldspar is difficult to locate?
3. Weathering: Look at the corners of the blocks near the sidewalk. Despite granite’s hardness, do you see any signs of spalling (chipping) or staining from the city environment?
4. The Sparkle Factor: Standing back a few feet, which mineral seems to be responsible for the glitter effect when the sun hits the building? (Hint: Look for the flat, reflective faces).
5. REQUIRED: Post a photo in your log of yourself or a personal item with the GULF TOWER sign in the background near the posted coordinates. This verifies that you were at the earthcache, and is a memorable addition to your log. (No spoilers please)
REFERENCES:
https://www.geologyin.com/2024/09/intrusive-vs-extrusive-igneous-rocks.html
https://buildingsdb.com/PA/pittsburgh/gulf-tower/
https://www.geologyin.com/2014/09/how-does-bowens-reaction-series-relate.html
http://open.maricopa.edu
http://www.msisurfaces.com