The following passage offers a good description of the Ossipee Ring Dike: "One hundred million years ago, the area of today's Ossipee Mountains was a very different place. What appear to be individual peaks are actually what is left of a large, now extinct volcano. It is thought that the volcano may have been as high as 10,000 feet, and would have looked something like Mt. Vesuvius or Mt. Fuji. Three major eruptions, each about ten million years apart, changed the volcano into what is seen today. The second eruption, around 90 million years ago, created the famous ring dike.
Useful Terms about Volcanoes
Before any discussion can take place, it would be useful to define some of the terms related to volcanoes and to geography in general. Some of these terms are:
- Ring dikes:Ring dikes are thick, nearly vertical igneous bodies that form concentric circles around a central intrusion. They are usually caused by the repeated subsidence of the cauldron. A ring dike can also be thought of as a long mass of igneous rock that cuts across the structure of adjacent rock.
- Caldera:A cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption. It is caused by the emptying of a magma chamber.
- Subsidence:Subsidence is the motion of a surface as it shifts downward.
- Igneous Rock:Rocks that are formed from molten rock that has cooled.
- Intrusion: The forcing of molten rock into an earlier formation.
Formation of the Ossipee Ring Dike
Viewed from above, the Ossipee Mountain range appears as a nearly perfect circle, with the area in the center being relatively flat. The Ossipee Mountains that are seen today are really the subsurface remains of the old magma chamber that has become exposed over the years, and the flat area in the center is the bottom of the old caldera. The diameter of the range is ten miles and the distance around the base is forty miles. The original volcano was thought to be around 10,000 feet tall, and the highest peak, Mt. Shaw, is today around 3,200 feet.
One hundred million years ago, there was a fracture in the volcano and an eruption occurred forcing molten magma to the surface. As the magma emerged, circular sections collapsed into the empty magma chamber. The molten magma cooled forming the type of igneous rock called Conway granite. This type of granite is found in New Hampshire's Carroll and Belknap counties. This granite and other volcanic rock subsided and later, deeper magma would push through earlier fractures during the second major eruption.
The second eruption took place around ten million years after the first eruption. Magma was forced up along the edges of the earlier plug, ringing the plug with more eruptions. Dikes of molten magma intruded the earlier Conway granite, and formed the ring-dike that is seen today. This ring-dike is famous with geologists world-wide due to its almost perfect completeness.
The Ossipee Mountains Today
Evidence of the early volcanic activity of the Ossipee Mountain area can be found throughout. Connor Pond and Dan Hole Pond, two deep water ponds found in Ossipee, NH, are the remains of what had been gas vents on the early volcano. Remains of the ring-dike can be easily seen in an area called Cold Brook in South Tamworth, NH. The New Hampshire Geological Society has set up an excellent field trip for anyone interested in the ring-dike or in the unique geology of this very special area of New Hampshire,"