Local Geology of the Monadnock Region (Cheshire County):
The rocks of the Monadnock Region, in general, are metamorphic rocks called schists and gneisses (derived from pre-existing volcanic igneous rocks and marine sedimentary rocks) and associated intrusive igneous rocks (mostly granites or various granite-like rocks such as the granodiorite that underlies Pisgah State Park). Analysis of the textures and mineral compositions of the rocks of our region tell us about the conditions of temperature, pressure, and tectonic activity to which they were subjected in the course of their formation. These analyses, therefore, help us to reconstruct the geologic and tectonic history of the Monadnock Region.
Mafic vs. Felsic:
A mafic rock is rich in in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro.
A felsic rock is rich in feldspar and quartz. Most felsic rocks are lighter than mafic rocks and have beige or light brown hues. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, and orthoclase.
Basalt- is a mafic, extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava. Basalt is a dark gray rock. Some minerals that are commonly found in basalt are feldspar and pyroxene.
Granite- is a felsic, intrusive igneous rock. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Most granites consist of a large percentage of quartz and feldspar. Trace amounts of other minerals such as mica and hornblende can alter the color of granite.
What is an Inselberg (also known as a monadnock)?
An inselberg is an isolated rock hill or ridge that rises abruptly from a gently sloping or virtually level surrounding plain. Inselbergs can range from 15 feet tall to several hundred feet tall.
Logging Requirements:
-
Is the rock outcropping at the GZ an Inselberg? Why or why not?
-
What is the composition of the rock? Is it felsic or mafic? Describe the color and appearance of the rock.
-
As of June 2019, earthcaches are now allowed to have a required photo logging task. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
**Please complete all of the necessary logging tasks before claiming your find**
**Confirm your find by answering these questions on this Google Form or by sending your answers to us via message center**
Congrats to BevanQ for the FTF!
Enjoyed finding this EarthCache? Consider awarding it a favorite point!
Sources:
https://en.wikipedia.org/wiki/Inselberg
https://www.nh.gov/nhdfl/documents/pisgah-management-plan-approved-version.pdf
https://en.wikipedia.org/wiki/Mafic
https://en.wikipedia.org/wiki/Felsic#Classification_of_felsic_rocks
https://en.wikipedia.org/wiki/Basalt
https://en.wikipedia.org/wiki/Granite
| We have earned GSA's highest level: |
 |
