This is an Earthcache – as such, there is no physical cache. Instead you will partake in a geology lesson by making observations and answering 3 questions about the intrusive igneous rock at the posted coordinates. Please obey posted signs about not climbing on the stone.
Ellicott City Granodiorite
Ellicott City granodiorite formed inside the Earth 20 million years ago. The granite you see here along the sidewalk intruded during the last stages of the Taconic mountain building event. As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present-day Appalachians and midcontinental North America.
Ellicott City granodiorite is largely composed of quartz, plagioclase, and biotite. Other minerals making up the granodiorite matrix are epidote, hornblende, allanite, sphene, and sulphides.
Why is Ellicott City Granodiorite INTRUSIVE instead if EXTRUSIVE?
Intrusive igneous rocks form from Molten rock called MAGMA below the surface in sills. Extrusive igneous rocks form LAVA above the Earths surface. Even though magma and lava may have the same chemical structure, the rock that is formed when they cool has very different properties. The length of time it takes for lava to cool above the surface is relatively short compared to Magma deep underground. Lava can sometimes solidify in minutes while rock insulated by the earth can take centuries. Slow cooling magma gives the distinct minerals in the rock time to consolidate into large crystals.
Uses of this stone:
Stone quarried from Ellicott City is mostly used for Belgian blocks, curbing, and macadam. Examples of use of it in building architecture can be seen on the exteriors of the Basilica of the National Shrine of the Assumption of the Blessed Virgin Mary in Baltimore and at or just down the street at Ellicott City Station.
LOGGING REQUIREMENTS:
To log this Earthcache: Read the geology lesson above. Answer all three questions posted below. Answers can be sent via e-mail or messenger contacts on my Geocaching profile. Do not post the answers to the questions in your logs. Photographs of you at the rock are encouraged.
QUESTION 1. Touch the Ellicott City granodiorite. Describe the texture?
QUESTION 2. List two differences between how “intrusive” and “extrusive” rocks are formed.
QUESTION 3.What are the size and color of the crystals that have formed in the stone?
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REFERENCES:
1. Roadside Geology of Maryland, Delaware, and Washington, D.C., Mountain Press Publishing Company, John Means, 2010, p. 207-208.
2. How are extrusive and intrusive rocks formed?, Pratik Santra, M.Sc Geology, University of Calcutta (2020), quora.com
3. Ellicott City Granodiorite , wikipedia.com
4. Ellicott City - Granodiorite, Earthcache by bluesnote,GC69C3G,geocaching.com
5. Granodiorite, The University of Auckland, flexiblelearning.auckland.ac.nz