Lecture #8: Intrusive Igneous Rocks

At depth in the earth solid rock melts at high temperatures to form a magma. An igneous rock is one that appears to have crystallized from magma. Two types of igneous rocks are extrusive and intrusive. Lava is magma which works its way to the surface to erupt and, hence, become an extrusive rock. Intrusive rocks are those which appear to have crystallized from magma emplaced in surrounding rock.

As indicated by Figure 3.14 of Hamblin and Howard, extrusive rocks consist of rhyolite, andesite, and basalt. Intrusive rocks include: Granite, diorite, gabbro, and peridotite. Your lab manual also explains various textures such as phaneritic, aphanitic, glassy, and pyroclastic. The classification of igneous rocks is based on the presence of absence of quartz, the composition of feldspars, the proportion of ferromagnesian minerals. Granite and rhyolite are called the felsic rocks, diorite and andesite are the intermediate rocks, and gabbro and basalt are the mafic rocks.

Evidence for intrusive igneous rocks includes: 1.) contacts with country rock which is older rock into which the magma has intruded; 2.) inclusions of country rock within the intrusion; and 3.) chilled margins where the magma has solidified more rapidly.

Plutonic rocks are igneous rocks formed at great depth. A stock is a small discordant pluton whereas a batholith is a pluton of more than 100 sq. km in outcrop area. A dike is a tabular body of igneous rock cutting across bedding and hence, discordant. A sill is a tabular body which is concordant. A blister shaped sill is called a laccolith. A pegmatite is a extremely coarse-grained igneous rock with crystals as large as 10 meters across. Many minerals of commercial value are mined from pegmatites including mica, feldspar, and a number of gemstones.

Much of the surface of the earth is a veneer of sedimentary rock underneath which is basement rock which is either igneous or metamorphic. Much of the Canadian Shield is basement rock exposed at the surface.

How magma forms: A rock becomes entirely molten is the temperature is higher than a melting point controlled by the minerals in that rock. The relative melting temperature of minerals is indicated by figure 3.1 of Hamblin and Howard. Pressure, amount of gas (particularly water) present, and the kink of neighboring minerals can all influence the melting point of a mineral. Sources of heat for melting in the crust include: 1.) the original heat of the earth at the time of formation 4.6 billion years ago. This heat moves upward from the mantle into the crust mainly by conduction ; 2.) Some elements, such as uranium, produce hear through radioactive decay; 3.) Heat transfer by conduction from a nearby body of magma; 4.) Hot mantle plumes may upwell into the crust and thus carry heat. Such mantle plumes account for some igneous activity such as the midoceanic eruptions that built up Hawaiian Islands and the eruptions of Yellowstone National Park; 5.) As rocks on either side of fault zones grind past each other, they generate frictional heat.

Origin of various igneous rocks: Mafic rocks originates from partial melting of the underlaying mantle. Intermediate rocks are generated at a fairly uniform depth along the Circum-Pacific belt possible as a consequence of partial melting of basalt along subduction zones. Felsic are believed to be formed by the partial melting of the lower crust.

Granite Name---------------- Location-------------------- Use

Dark Barre*----------------- Barre, Vermont

Light Barre*--------------- Barre, Vermont------------- Vermont Capital

Kitledge Gray*------------ Milford, New Hampshire

Milford Pink*-------------- Milford, New Hampshire

Conway (like Milford Pink) Conway, New Hampshire

Algerie----------------------- Otis, Mass. ---------------- Curbing Stone

Bethel White*-------------- Bethel, Vermont

Stone Mountain*---------- Georgia

Canada Black*------------- Alma, Quebec

Spartan Pink*-------------- Sparta, South Carolina

Cherry Hill*---------------- St. Peters, Pennsylvania

* - Found on first floor of the Deike Building