The granite formations of New Hampshire were so historically important to the economy of the state and well known in the region that they gave rise to the state’s nickname ‘The Granite State’. The moniker is attributed to the significant granite number of granite mines in the state and its use in the building of towns, municipal buildings and monuments in the 19th century.

	Concord state capitol building built from the state's famous granite deposits: © Karmafist

The formation of the granites date back to the Acadian Orogeny, an extensive mountain building event which commenced in the Late Silurian (~375 million years ago) and concluded in the Late Devonian Period. It was the third of four mountain building episodes that contributed to the formation of the Appalachian Mountains, as a result of the north-westerly collision of the Avalonian island arc with the eastern flank of the Laurentian continent (present day North America, north-west Ireland, Scotland and Greenland) and the subduction of the Southern Iapetus oceanic crust.

As with the Cairngorms, the granites found in New Hampshire are part of a series of pluton bodies of intrusive igneous rock, known as the New Hampshire Plutonic Suite, that were emplaced throughout New England during the Acadian Orogeny: the Bethlehem Granodiorite, Kinsman Granodiorite, Spaulding Tonalite and the Concord Granite. The first three of these members were syn-tectonic, meaning they were concurrent with the Acadian Orogeny; however the Concord Granites (~365Ma), a series of post-tectonic plutons similar to the Cairngorm granites in Scotland, were emplaced 30-50 million years after the Acadian tectonism.

Granites in the river bed at Albany Bridge: © Anthony Quintano

The Concord Granites are non-foliated (layered) as they were not affected by deformation events in the Acadian Orogeny. They are medium-grained, light grey in colour and composed of plagioclase, microcline, orthoclase, quartz, biotite, and muscovite minerals. The generation of plutons requires the melting of source rocks to create magma. The source rocks for the Corncord Granites are thought to have been greywackes, dark coloured sandstones with poorly sorted angular grains usually formed from submarine avalanches.