Rattlesnake Island Geology / Formation

[rgilfert]

I'm a brand new member of this forum (or any forum for that matter) and clearly don't know what I'm doing yet. I'm also a new resident on Rattlesnake Island. I was reading through old posts on this forum and saw some about the history and geology of the Lakes Region. I'm interested in learning more about the geology of Rattlesnake (I was a Geology major in college a bazillion years ago). I came across the following quote and am wondering whether anyone has more information about it? "Rattlesnake Island is geologically part of the Belnap Mountains". I'd love to understand how the island was formed! Thanks.

[BroadHopper]

I was told that the Belknap Mtn range is what was once a volcano. The last ice age erode the volcano to what you see today.

The Ossipee Mtn range is also an extinct volcano. It is much more obvious today.

Find the Central Lakes Region Cartographic map, and you can see the distinction.

After the last ice age the lake was much higher. The outlet was Alton flowing to the ocean where Merrymeeting River is today.

[Slickcraft]

This book may be of interest.
http://www.perpublisher.com/per71.html

The author gave a talk atop Mt Major last fall and later at the Alton Library. I missed the talks but did hear that he is quite knowledgeable.

[rgilfert]

This book is exactly what I was looking for!! At another site (http://books.google.com/books?id=LyilfcDqciQC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false) you can actually view the first 14 pages. In these pages it is shown how Rattlesnake Island was part of a ring dike (page 13) connected with the Belnap Mts...fascinating!! I previously thought that the Ossippee Mts were the only ring dike in NH. I just ordered the whole book (70 pages). Thanks so much for pointing me in the right direction.

[Slickcraft]

Interesting. Looking at figure 1.18 of the book at Google books, right now I am sitting in the "inner core area", the center of that ring dike. I am sorry that I missed his talk.

I do know Dave Roberts, as do many other forum members. Guess that I'll have to order the book as well. Thanks for the Google books link and welcome to the Forum

[mcdude]

scroll down to post #38 where Rattlesnake Gal and Broadhopper fill us in about the Belknap volcanic ring dikes.
http://www.winnipesaukee.com/forums/showthread.php?p=142795#post142795

http://www.winnipesaukee.com/photopost/data/507/176A2_January_05_052_2.jpg

[rgilfert]

mcdude & Rattlesnake Gal...outstanding info! You guys really know your stuff. I have the exact same map....and once you look at it (obviously because I'm now armed with your info)...it's obvious that it's A) a ring dike and b) includes Rattlesnake. I find this fascinating. Thanks you so much!

[Rattlesnake Guy]

I have always been curious about how big the volcano was at the top of the map that McDude posted. The Osspee mountains you likely have a view of from your place. The base diameter is about half the size of Mt Everest. Without that peak and the peak that Rattlesnake is part of, who knows if the glacier would have scratched such a beautiful hole to form the lake.

Welcome to the Island.

P.S. Did not see any tree damage on the 1/3 of the island we saw this weekend. Not many branches down either. (At least until today's wind)

[rgilfert]

Rattlesnake Guy, the views that are offered on the entire north side of the island absolutely drive me crazy! Red Hill & the Ossipee Mtns....how can you possibly beat that?..mesmerizing! My family (1920 to 1985) used to have property in Melvin Village (across the cove from the Wawbeek) and the view looking south-west was nice...but couldn't hold a candle to what we have now. I've got to put a call into WAM to see if I can move my boat launch up from 5/01. Live and learn! Thanks for your info.

[diz]

I have always been curious about how big the volcano was at the top of the map that McDude posted. The Osspee mountains you likely have a view of from your place. The base diameter is about half the size of Mt Everest. Without that peak and the peak that Rattlesnake is part of, who knows if the glacier would have scratched such a beautiful hole to form the lake.

Welcome to the Island.

P.S. Did not see any tree damage on the 1/3 of the island we saw this weekend. Not many branches down either. (At least until today's wind)

There are two DVDs, one about the ring dike and one about the Ossippee aquifer, made by a local geologist. I got my copies at Black's in Wolfeboro. They're really informative - the one about the dike explains the history of eruptions and the different types of granite left behind.

[CaptT820]

Hi guys this is my first post, but being that I am a registered Professional Geologist (P.G.) in the state of New Hampshire I figured I could lend some knowledge. Rattlesnake Island is in fact part of a ring dike that geologically looks like two crescent moons not connected in the middle. Rattlesnake Island, Diamond Island, Ship Island, and Moose Island all share the same exact geology with part of the Belknaps. All of these islands are made up of mafic (dark colored) based minerals such as biottie and are categorized as a Quartz Syenite of Jurassic Age (in this case the rocks have been dated from 170-193 million years old). The middle-upper portion of the Gunstock ski area is also part of the ring dike.

[mcdude]

welcome Capt....and thanks for that informative post :cool:

[Pineedles]

Hi guys this is my first post, but being that I am a registered Professional Geologist (P.G.) in the state of New Hampshire I figured I could lend some knowledge. Rattlesnake Island is in fact part of a ring dike that geologically looks like two crescent moons not connected in the middle. Rattlesnake Island, Diamond Island, Ship Island, and Moose Island all share the same exact geology with part of the Belknaps. All of these islands are made up of mafic (dark colored) based minerals such as biottie and are categorized as a Quartz Syenite of Jurassic Age (in this case the rocks have been dated from 170-193 million years old). The middle-upper portion of the Gunstock ski area is also part of the ring dike.

So why can't we find diamonds like they do in other volcanic activity areas?:)

[rgilfert]

Pineedles, that would be nice....but I'll settle for tonight's winning MegaMillions ticket!

[CaptT820]

Unfortunately the ring dike that is present around the Belknaps and the Winnipesaukee islands does not produce diamonds due to a number of reasons. The main reason is that the ring dike is actually the outer cone portion of the volcano that has been built up over millions of years. Diamonds are formed by extreme heat and pressure at depths up to 6 to 10 miles below the surface and are brought up by the magma that explodes out of the middle of the volcano. The diamonds we find today (such as in South Africa) are from cooled, solid, magma chambers that are rich in diamonds, called Kimberlite Pipes (named after the town of Kimberly where they were first found). Not every volcano explodes with diamonds in the middle, the main factor is the type of rock that is ejected and the depth that it was sourced from.

[rgilfert]

I just finished reading a short paperback book titled "The Geology of Winnipesaukee Quadrangle, New Hampshire" that I purchased on eBay last week (22 pages plus a large folded Geologic Structure Sections map). The book was published in 1941 and provides fascinating (i.e. to us "rock hounds") info about the formation of the Belnap Mountains/ring-dyke (and therefore Rattlesnake Island). The following are a couple of pages from the book. The book will permanently reside at our place on Rattlesnake Island along with the other excellent book mentioned in this thread (i.e. "Stepping Stones Across New England")...if anyone would like to check it/them out, stop by next summer.

[Slickcraft]

Unfortunately the ring dike that is present around the Belknaps and the Winnipesaukee islands does not produce diamonds due to a number of reasons. The main reason is that the ring dike is actually the outer cone portion of the volcano that has been built up over millions of years. Diamonds are formed by extreme heat and pressure at depths up to 6 to 10 miles below the surface and are brought up by the magma that explodes out of the middle of the volcano. The diamonds we find today (such as in South Africa) are from cooled, solid, magma chambers that are rich in diamonds, called Kimberlite Pipes (named after the town of Kimberly where they were first found). Not every volcano explodes with diamonds in the middle, the main factor is the type of rock that is ejected and the depth that it was sourced from.

Rats. Living inside the inner ring dike I was ready to start blasting for diamonds.:D

[Merrymeeting]

It's threads like this that make this forum such a wonderful place to visit! :cheers: