Physical Geology

Intrusive and Plutonic igneous rocks

igneous rock classification

Intrusive / plutonic

granite, gabbro, diorite

batholith, stock, dike, sill, laccolith, neck

Palisades

Review Igneous Rock Classification

Intrusives and Plutons

Magma from the mantle or derived from melting of the crust collects in huge underground chambers. Because the molten rock is less dense than the surrounding solid rock and because it is under tremendous pressure, it has a tendency to rise upward through the crust over time. Eventually, some magma breaches the surface to become lava, forming volcanic or extrusive features.

Most magma, however, never reaches the surface. Instead, it crystallizes within the crust, forming a variety of underground igneous bodies called plutons and intrusives. Plutons are igneous bodies that crystallize deep underground, therefore,. intrusive igneous rocks are known as plutonic.

Types of Plutonic and Intrusive Igneous Rock

Plutonic and intrusive rocks tend to be either phaneritic or porphyritic in texture. This is because they cool relatively slowly underground.

Beneath the continents, large plutons are usually rich in silica because they have melted and mixed with the rock of the continental crust. This, combined with the fact that they have long, slow crystallization histories, tends to make them enriched in complex silicates such as quartz, K feldspar, plagioclase feldspar and mica. Phaneritic rock with this composition is called granite, which is a very common type of continental igneous rock.

Intrusives that are associated with batholiths tend to be granitic. Others, however, are derived from mantle magmas and have compositions dominated by darker minerals such as pyroxene, amphibole and olivine. These minerals form phaneritic rocks such as gabbro and diorite.

Igneous Plutons

Batholiths: huge bodies of igneous rock (greater than 40 square miles of surface exposure) that are the remains of large magma chambers formed from partial melting of the continental crust. These magma chambers did not intrude the country rock, rather they melted their way into it by the process of stoping.

Stocks: A small batholith (<40 square miles) or extended part of a larger batholith.

Igneous Intrusives

Dikes: tabular sheet of igneous rock that intrudes and cuts across the surrounding ‘country rock’.

Sills: tabular sheet of igneous rock that intrudes between and parallel to the layers of country rock.

Laccoliths: a sill that warps the country rock upward to form a dome.

Volcanic pipes and necks: vertical columns of igneous rock that are the erosionally exposed remnants of the solidified magma inside extinct volcanoes.

Igneous Plutons and Intrusives in the New York area

Batholiths and stocks - the New England region, from Mass. to Maine, is underlain by a number of stocks and batholiths. New Hampshire calls itself the ‘Granite State’ because of the granite rocks exposed in the White Mountains. Coastal and interior Maine is also famous for granite rock, including the rock underlying Mt. Katahdin and the pink granites of Acadia National Park on Mt. Desert Island.

The high peaks region of the Adirondack Mts. in New York State are underlain by very ancient (more than 1 billion years old) plutons composed of an unusual igneous rock called anorthosite (pure labradorite feldspar). This is the same rock that makes up the lunar highlands.

Dikes - the bedrock of New England and the New York City area is cut by many small dikes that can be found wherever bedrock is exposed.

Sills - the most famous sill on the east coast is a thick layer of diabase rock that was emplaced 200 million years ago beneath the sedimentary layers of New Jersey. Today the edge of this sill is exposed along the Hudson River, where the hard igneous rock resists erosion and forms the Palisades Cliffs.