NOTE: This is an earthcache; there is no physical container to find at the coordinates. To get your smiley for the cache, e-mail your answers for the questions below to the cache owner.
1- Describe the overall color of the cylindrical rock at the coordinates.
2- Estimate the percentage of dark color vs. light color minerals.
3- Based on the presence or absence of quartz, the colors present and percentage of dark minerals, is the rock diorite or granite?
4- Taking note of its condition, do you think it was a good choice of rock for the survey marker and area?
Welcome to beautiful Rawson Square, named after Sir Rawson W. Rawson (yes, his first and last names are the same) who was the governor of the Bahamas in the 1860s. The Square highlights many different parts of history and people along with their significance to the island. For this earthcache we would like to call attention to the cylindrical survey marker next to the Christopher Columbus monument. Erected in 1992, they both serve as a memorial to the quincentennial anniversary of Columbus’ landfall. On the survey marker you will see the letters B, O, and an arrow carved into it. The B and O stand for Board of Ordinance, and the arrow points north.
When making monuments and survey markers, durability and weather resistance is a main factor. In this particular area, chemical erosion is prevalent from the large amounts of rain water and sea spray. Two types of durable rock that weather well are diorite and granite, which both have a Mohs hardness scale of 6 to 7.
Both diorite and granite rocks are classified as intrusive igneous, which means they are made of magma which has cooled slowly underground. This cooling gives them a course, crystalline texture. Diorite and granite have a lot of similarities, but hopefully after learning a bit more, you can distinguish them.
Diorite
Diorite rock has a salt and pepper speckled “dalmatian” appearance with a mostly even (50%-50%) mix of white plagioclase feldspar and black minerals such as hornblende and biotite. It is classified as an intermediate rock, or a rock composed of a balanced mix of light and dark minerals. It contains little to no amount of crystal looking quartz and a moderate amount of silica. The grain size is typically medium to coarse grained, and the minerals are easily seen (phaneritic) and tightly packed together.
Granite
Granite rock often has a speckled look variation of orange/pink/white/cream/black. It is classified as a felsic rock or rich in light colored minerals. The cream and white colors come from quartz (10%-50%), pink and orange from potassium feldspar (65%-90%), and black colors from biotite mica or hornblende (5%-10%). The grain size of granite is course with a crystalline texture. The minerals of granite are not so tightly packed together, and it is less dense than diorite.
Questions:
Now it is time for the questions. To get credit for the cache, please answer the questions below. When answering the questions, do your best. It is more important that you learn a new concept about our earth and can apply your knowledge rather than have a precise answer.
1- Describe the overall color of the cylindrical rock at the coordinates.
2- Estimate the percentage of dark color vs. light color minerals.
3- Based on the presence or absence of quartz, the colors present and percentage of dark minerals, is the rock diorite or granite?
4- Taking note of its condition, do you think it was a good choice of rock for the survey marker and area?
Thank you for visiting this earthcache. Please send your answers to the above questions to the cache owner. In your "found it" log, feel free to write about your experience, but don't include the earthcache answers. We hope you enjoy beautiful Nassau and Rawson Square!
Sources:
https://www.bahamas.com/plan-your-trip/things-to-do/rawson-square
https://www.geologyin.com/2023/12/igneous-rocks.html
https://www.geologyin.com/2024/03/diorite-composition-properties.html
https://rockhoundresource.com/granite-identification-characteristics-pictures-more/
https://www.bahamasairtours.com/destination/rawson-square/
https://www.nps.gov/articles/mohs-hardness-scale.htm