This is an earthcache, meaning that you have to do more than sign a log sheet to claim a find.
- 1. You must read the cache page information before or while you are visiting the physical location.
- 2. You need to send the answers to the questions to the me when you post your found log. I suggest the message center, due to the ease of communication between us, and because we'll have a permanent record. My response to you will be through the message center and include copies of any messages sent as e-mails. If you are unable to send answers immediately, put a note in your found log telling me when I can expect them.
- 3. You have to post a photo with you found log either of yourself or something that clearly identifies you, such as your name written on a piece of paper, with the rock in the background. I need to know you were actually there.
Let's concentrate on some simple basics about this smaller erratic rock you are looking at. I'd rather not get into technicalities and just think about some basics that you might be able to remember in the future. A true geologist might be dismayed at the lack of technical information!
First things first - to be erratic, it simply has to have traveled from elsewhere to get where it is now. Grains of sand can be erratic. We notice the huge erratic rocks because they stand out; however, you probably have small erratic rocks in your yard or local park.
Glaciers, ice sheets and ice floes are typically responsible for the transport of rocks from one place to the another. There are two basic types of glaciers. Alpine glaciers form on mountains and flow only to the base of the mountains. Continental glaciers (commonly called ice sheets or floes) are large, continuous masses of ice that flow in multiple directions. Did the erratic rock you are looking at get here under or atop an ice sheet? Or on top of an ice floe?
Question 1: Given its location and knowing past history of the geology of the area, what are your thoughts on how this erratic got to its current location? If your answer is a glacier, state which type. Explain why you made your choice.
Studying this rock, with certainty due to its composition, we know it is an igneous rock. There are three types of rocks - igneous, sedimentary, and metamorphic. Each type can transfer into either of the other two.
Accepting that this rock is igneous, we still have choices. Igneous rock may be volcanic/extrusive rocks, or plutonic/intrusive rocks. Extrusive/volcanic rocks are exposed and cool quickly, not allowing time for crystals to grow greater than microscopic size, so they are fine grained. Intrusive/plutonic rocks cool and solidify slowly within the earth and have time to form larger, coarse grained crystals.
General information: The name "plutonic" refers to Pluto, Roman god of wealth and the underworld. Plutonic rocks are formed in the underworld and often contain precious metals.
Question 2: Look at the rock. Can you see crystals? What type of igneous rock is this? Plutonic or volcanic? Extrusive or intrusive? Why?
Three of the basic crystals that form in plutonic/intrusive rocks are varieties of quartz, feldspar and mica/biotite. Quartz is clear to white; feldspar is typically pink to pinkish brown (sometimes other colors) depending on its chemical makeup; mica/biotite tends to be rather black in appearance.
For now, let's stick with three basic examples of plutonic rocks - granite, granodiorite, and diorite. Granite has a high percent of quartz, so tends to be light in appearance. Granodiorite has a closer to equal amounts of quartz, feldspar and mica/biotite type of crystals. Diorite has more of the mica/biotite type of crystals and less quartz, so appears darker in appearance. Diorite is formed at a much higher temperature than granite or granodiorite.
Question 3: Look at the rock - the best place is on the back side toward the top. Select some spots on the rock without moss or other white material covering them. Based on the simplified description above, what type of plutonic rock do you think this is? Why?
Spalling is when fragments of the rock are broken off the main portion. This may be caused by things such as corrosion, weathering, and impacts from other objects. You can see the results of spalling by pieces of rock fallen to the ground, or by finding places on the rock that look as if they might split off in the future.
Question 4: Look at the rock and surrounding area. Do you think spalling has occurred, is still occurring, both or neither? Give reasons for you opinion.
Question 5: Did you remember to post your photo with your found log? It needs to be of you, or something that positively identifies you, with the rock in the background.
Unless I am on vacation, I will respond to your answers within a day or two. You are stating your thoughts, and as long as they are within reason, they can't be wrong.
Sources are geology books by Lutgens, Tarbuck and Tasa, and by Tucker.