New Hampshire Geology New Hampshire Bedrock Map

Jurrasic Period
(206-144 million years ago)


The Jurassic Period [jer-AA-sick]  was a very geologically active and exciting period in New Hampshire. The rifting of the North American Plate from the African and European Plates was starting to open up the brand new Atlantic Ocean. Molten rock, or magma, would sometimes rise up through the crust and erupt as a volcano and other times it just got close to the surface. Keep in mind that "close to the surface" still meant 3 to 5 miles below the surface, because on average, the Earth's plates are about 20 miles thick. New Hampshire was still tropical, and dinosaurs reigned supreme. If you attempted to go for a Jurassic walk in the park, you would likely have ended up as some dinosur's light snack.

Getting To The Root Of The Matter

Knowing that granite forms 3 to 5 miles below the surface, that means all of the existing granite outcrops on the surface today, whether on an exposed mountain top or in a road cut, used to be that far below the surface. (See the Devonian Period for more information.) If what we are seeing today used to be that far underground, what happenned to all of the Earth's crust above the granite since then?

As mentioned in the Triassic Period, there had been no mountain building episode, or orogeny, on the eastern edge of North America once the breaking up of Pangea started nearly 180 million years ago. Since that time, only the forces of weathering and erosion have been at work. Weathering and erosion are slow processes, in terms of human lifespans, but 180 million years is a very long time. Enough time, apparently, to wear away 3 to 5 miles of solid bedrock and carry it away somewhere else. Where did all of this bedrock go? Much was carried away by rivers, wind and gravity to lowlands and the continental shelf or scoured and bulldozed by glaciers and deposited elsewhere. (For more on the effects of glaciation in New Hampshire, see the Pleistocene Period.)

Conway Granite

Some of the blobs of magma, or plutons, that didn't make it to the surface during the Jurassic Period, formed a unique salmon colored granite known as the Conway Granite. It is called Conway granite because it was identified there first. However, it is not limited to the Conway area. In fact, Conway Granite can be found in Franconia Notch, among other places. It happens to be the bedrock that Cannon Mountain is made of, which means that The Old Man Of The Mountain was made of Conway Granite. Referring to the recipe for granite (found on the Silurian Period page), we know that granite generally contains the three minerals of quartz, mica and feldspar. The feldspar in this case is pinkish/orange, or salmon in color, which is what distinguishes this kind of granite from others.

The Ossipee Mountain Ring Dike Complex

During the Jurassic Period, the land that would one day be New Hampshire experienced significant volcanic activity. Some of the 'yellow blobs' discussed above represent some classic, world famous (among geologists anyhow) nearly circular volcanic mountains known as ring dikes. A dike is a crack in the bedrock that is filled in with intruding material, in this case, magma. A ring dike is a circular crack formed by the collapse of a magma chamber's ceiling underground. The magma blobs tended to be rounded in shape, not unlike the "lava" blobs in a lava lamp. Magma erupted through these circular structures throughout the state yielding several textbook examples of circular ring dike mountains. The best known and largest example in New Hampshire are the Ossipee Mountains in Moultonborough, Sandwich, Tamworth, Ossipee and Tuftonborough seen in the box at right. Another excellent example, albeit much smaller, is Mt. Pawtuckaway in Pawtuckaway State Park.

Yellow Blobs

nh geo mapDuring the Jurassic Period in New Hampshire, magma from the Earth's interior did a fine job of taking advantage of the weak spots created by the rifting of Pangea into what would become today's tectonic plates.  Take another look at the Bedrock Geologic Map of New Hampshire below and look for the yellow blobs.  These blobs mark the places where the magma either came close to the surface, or broke through it.  Notice that the blobs are rounded in shape and roughly lie in a straight line just a little off of due North to South.  Geologists  disagree on the cause for this pattern.  Some think that New Hampshire slid over a 'Hot Spot' as Pangea was rifting. The hot spot would act like a blow torch and allow the magma to find its way close to, and up to, the surface.  Others believe the weak spots were created by rifting which happened to be in a straight line in this case.  As in all scientific theories, more evidence needs to be gathered to determine which, if either, of the theories is more likely.

(Click on the image for a closer look.)|
Image credit:New Hampshire Geological Survey


Textbook Volcanics In
New Hampshire

The Ossipee Mountain Ring Dike Complex is clearly evident in 3D on the map below.  Geologists the world over are familiar with this New Hampshire mountain complex due to its nearly circular structure.

ossipees topo

Below is a view of the Ossipee Mountains as viewed from the Southwest at Geneva Point overlooking Lake Winnipesaukee in Moultonborough.

oss mtns (Click on the image for a closer look.)|
Image credit: Dan Reidy