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