Ellicott City - Granodiorite EarthCache

Granodiorite is a type of igneous rock that formed from the cooling of lava or magma. There are two types of igneous rocks. Extrusive rocks cool outside the volcano. Intrusive rocks cool inside lava chambers in the ground and are exposed by the eroding layers above. Extrusive rocks tend to have smaller crystals than intrusive. This is due to the fact that intrusive rocks take longer to cool than extrusive rocks.

Granodiorite is a type of granite intrusive rock which contains more plagioclase feldspar than orthoclase feldspar. Plagioclase is more white and grey while orthoclase is more orange and pink. This makes granadirorite more grey looking than dark looking due to more felsic (white) minerals inside it.

Ellicott City Granodiorite has been dated to the Silurian or Ordovician periods in earth's history. This is about 485.4 millions of years ago (mya) to 419.2 mya. This would suggest that Ellicott City used to be highly geologically active during this time. So why was this place so geologically active? See the geologic map below from the Maryland Geological Survey. It may shed some light on it that answer.

When comparing the area of exposed Ellicott City Granodiorite to other existing areas, it's very minimal. This small place where it can be found would suggest that a large dike hardened to form all this existing Ellicott City Granodiorite. A dike is like a miniature lava tube, only that inside the tube is solidified rock. When the liquid rock moves up towards the surface, it melts any of the existing rock is in the way in exchange for heat. That melted rock, then becomes part of the liquid rock in the dike, creating a new igneous rock such as Ellicott City Granodiorite. Ellicott City Granodiorite rises to the surface until it lost too much of its heat. It then cannot move anymore up through the existing layers and solidifies. Sometimes it moves outward and that is likely what has happened here. In that case, the dike is now called a sill. Dikes and sills are usually small, about a few feet at most. But, they can get very big very fast depending on the thickness of the rock, the heat of the liquid, depth, and pressure.

Overtime, the sediments above these dikes and sills are eroded away thanks to the Tiber and Patapsco Rivers. Because igneous rocks are usually harder than sedimentary rocks, more time is needed to completely eroded the sills hence why there is so much Ellicott City Granodiorite in Ellicott City.

https://en.wikipedia.org/wiki/Ellicott_City_Granodiorite

http://www.mgs.md.gov/esic/geo/lgdepp.html#pze

https://en.wikipedia.org/wiki/Dike_(geology)

http://www.mnh.si.edu/earth/text/3_1_2_1.html

TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS BEFORE YOU LOG A FIND. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG

1. Ellicott City - Granodiorite on the first line of your email.

2. Describe the texture AND color(s) of the Ellicott City Granodiorite at this location.

3. Describe the erosion processes here AND how they are effecting the exposed Ellicott City Granodiorite.

4. Can you see any crystals that have formed in the rock? If so, about how large (or small) are they?

5. Because Ellicott City Granodiorite is an igneous rock, it has many different types of minerals inside of it. Using the diagram above, what is the most AND least abundant mineral you see in this piece of granite? Explain.

6. Using the map in the description above, where are some other places that Ellicott City Granodiorite can be found at (beside at GZ)?

* NOTE - You will likely not receive an email back from me, unless I need clarification on your answers. Please, don't wait for me to tell you you have it correct or not. I do review your emails within 12 hours of getting them in my inbox and with other of my earthcaches, it has become too hard to do. As soon as you send the email, you can log this earthcache.