The Ossipee volcano
towered 12,000 ft high, or more, in this location - about as high as Mount Shasta in California.
130,000 years ago, that is. What we see and call the Ossipee mountains are merely the last remains of that once great volcano that was 10 miles in diameter at its base, 40-mile circumference at the base, and rising 12,000 ft.
Mount Shasta in California
This ring dike is special around the world because very few volcanos generated a Ring Dikes and this one is as near perfect as we can tell. The base of the volcano had a 10-mile diameter and a 40-mile circumference. The remnants, called the Ossipee Mountains, can be seen here:
View from space via Google Earth
View from the bridge.
Classic Dike Intrusion (not found here).
How a volcano works:
25-30 miles beneath the ground we have molten liquid rock -- magma.
130,000 years ago, a crack developed in the Earth's crust and the magma errupted and exploded. This lava took thousands of years to cool.
A big cone formed and then fell back down into the hole it sprung from. The lava remains we see are called Conway granite as is only seen in this area. We call this Granite Type-A. As magma cools it undergoes reactions that form minerals. The rate of cooling is very important. If the magma cools slowly then the reactions proceed for a longer period of time and the resulting mineral crystals (grains) become large and are readily visible to the unaided eye.
90,000 years ago, the second lava flow came up very quickly from deep in the Earth and coated the inside of the lava dome and cooled quickly -- in days. This is different from other types of magmatic dikes that force their way up through cracks in earlier, cooled, lava flows. We call this Granite Type-B. When lava cools quickly, it is often darker and has not had time for crystals to form and develop.
80,000 years ago, the third, and final so far, eruption occurred. Then the volcano began to be eroded by weather and a retreating glacier to what we see today as the Ossipee mountains. But these are just remnants of the once mighty volcano that stood here. We do note that the west side is higher than east side. Maybe a gaseous eruption, like Mount Saint Helens when it exploded, tore away one side of the mountain towards Ossipee center.
Granite Type B - Ring dike, darker in color, probably cooled in a couple of days compared to Granite Type A (Conway granite mantel) which took thousands of years to cool.
Granite B can be fractured by smashing two Granite Type B rocks together. The shards that are the result are very sharp-edged and were used by native Americans as knives, arrow heads, and other tools.
If you want to drive over the edge of the caldera, take Canaan Road North from 171. This road crosses the edge of the caldera and the spot where it does this is unmistakable. This is a separate journey from this Earth Cache.
Follow the GPS to the posted coordinates for this Earth Cache. Park at the bridge that crosses Mountain Road. There are three parking areas off the side of the road. Be respectful of the signs and do not cross into private property that abuts this wonderful and open site. Everything you need for this earth cache can be seen from the bridge. Look in the direction of 348-degrees on your compass for some great views of the brook and both types of granite.
There is no need to go down the embankments to Cold Brook, unless you are up for that challenge and want to get up-close-and-personal with the brook. Here you will see examples of both Granite Type-A and Granite Type-B and you will be transported back in time 90,000 years and will be standing on the Ossipee Volcano caldera's edge on the ring-dike! The mean temperature would be 1,2500 degrees Celcius (2,200 degrees Fahrenheit)! But it is a dry heat!
A Scenic Overlook location is in the coordinates section and is about 16 miles away. From there you get a great view of what remains of the volcano and an excellent spot for leaf-peeping during foliage season! N 43° 43.547 W 071° 7.514
Logging Requirements:
- Describe the color and texture of both Granite Type-A and Granite Type-B.
- Is the width of the dike consistent throughout the entire length?
- Looking at the rock that the dike is located in (granite type-A), is the granite type-B in the cracks between layers of granite type-A, or just running through the stone, or is the granite type-B a coating to the granite type-A?
- Which type of granite, A or B, is best to make tools from? Give examples of the types of tools that could be made.
- Upload a photo taken from Cold Brook that shows both Granite Type-A and Granite Type-B fused together. You do not need to be in the photo, though it is strongly encouraged. Please do not re-use another Cacher's pictures.
As always, Cache-In, Trash-out. Be respectful of the neighbors, and cache safely!