The Pawtuckaway Ring Dike complex lies within the Pawtuckaway State Park in southeastern New Hampshire. It is locally known for the day hiking and mountain biking trails of moderate difficulty. Hiking to the top of one of the hills within the park offers 360˚ views in some locations. The park is also known in the rock climbing community as an excellent spot for free climbing the many house-sized boulders (bouldering) around the base of the hills. Topographically the Pawtuckaway Ring Dike complex (PRD) is expressed as a ring of low-lying mountains (see Fig. 1). The tallest, Mt. Pawtuckaway, has an elevation of 1030 feet above sea level, however since the local area is around 300-400 feet above sea level the height of the tallest hill is only 700 feet (local relief).
Geologically the PRD has a very interesting history. Radiometric dating of the rocks indicate they are 128±2 million years old (Eby, 1985) making them some of the youngest in New Hampshire. The rocks of the PRD are igneous, which means they were formed from melted rocks (magma) that solidified. This would suggest that the rocks are volcanic (solidifies above the surface of the Earth) and the PRD is a volcano (nice circular volcano shape) but it is not. The rocks of the PRD are plutonic (cooled deep within the Earths crust) with compositions of monzonites (two feldspars, very little quartz) and gabbros (very iron magnesium rich, usually associated with oceanic crust) both of which are fairly uncommon rocks in New Hampshire. They intrude rocks that are metamorphosed sedimentary rocks (another blog) but seem to have had little to do with them.
It is thought that the PRD was the plumbing for a volcano that was several kilometers above the present day surface 128 million years ago in the Cretaceous. The large magma chamber below was feeding the volcano through a network of conduits that were parallel to the layering of the rocks (sills) or perpendicular to the layering (dikes). The PRD rocks were made from magma that was generated by a mantle plume, which is a place under the crust where very and hot extremely viscous mantle rock is moving upward towards and into the bottom of the crust. Interaction with crust and the mantle plume causes melt generation and volcanic eruptions at places that are known as hot spots. Famous hot spot eruptions are occurring today at Iceland and Hawaii. As the plates move over the mantle plumes over geologic time, they form a chain of volcanoes, which become extinct once they are no longer over the mantle plumes. These extinct chains of volcanoes are called hot spot tracks as they show the direction of plate motion and point to the hot spot from which they were created.
The PRD is hot spot related and lies within a hot spot track that stretches from within Quebec down through New Hampshire and out into the Atlantic Ocean (where it is called the New England Sea Mount Chain) right to the hot spot that created all of the volcanoes in the chain. In the 128 million years since the plate was directly over the mantle plume, the volcano and several kilometers of the Earth’s crust has been eroded away and deposited on the continental shelf and slope, leaving nothing but a ring of rocks that are eroding slower than all of the rocks surrounding them as an interesting topographic high that you should go and see.
Reference cited
Eby, G. N., 1985, Geology and geochemistry of the Mt. Pawtuckaway ring-dike complex, white Mountain igneous province, New Hampshire [abs.]: Geological Society of America abstracts with Programs, v. 17, p. 17.
