Make your own free website on
(Or, now that we're done with minerals...)

   Rocks are simply - or perhaps not quite so simply - an aggregation of one or more minerals. There are 3 types of rocks: Igneous, Sedimentary, and Metamorphic.

Igneous Rocks

  Igneous  (from latin ignis meaning fire) rocks are formed from molten rock, either at Earth's surface or at depth after cooling. These rocks make up the bulk of the Earth's mass. The parent material for igneous rock is magma - molten rock below Earth's surface.

     Magma buoyantly rises toward the surface because it is less dense than the surrounding rocks.
     Magma which reaches the surface is called lava. These are classified as extrusive, or volcanic.
     Magma which loses its mobility and crystallizes at depth are termed intrusive or plutonic (after Pluto, the god of the lower world in classical mythology).

  Magma usually contains some suspended crystals and dissolved gases - primarily water vapor - confined by the pressure of the surrounding rock.
     Most magma is composed of the mobile ions of the 8 most abundant elements in Earth's crust: O, Si, Al, Fe, Ca, Na, K, and Mg. As magma cools, the random ion movements slows, and they begin to arrange themselves in orderly crystalline structures - crystallization. The mineral grains which form then precipitate from the melt - and may settle to the bottom of the magma chamber creating a visible difference in texture.

     Igneous rocks are classified based upon their texture and mineral composition.

  Texture - the overall appearance of a rock based on the size, shape, and arrangement of its interlocking crystals.

     Texture reveals a lot about the environment in which the rock was formed.

  Factors affecting crystal size....
  1) The rate at which magma cools.
    2) The amount of silica present.
    3) The amount of dissolved gases in the magma.

Glass - unordered ions forming rock.

Types of igneous texture

Aphanitic - fine grained; microscopic or no crystals formed when cooling was too rapid for crystal formation. These fine grained rocks are classified based on color since crystal identification is not possible.

     Voids left by gas bubbles escaping the magma are called vesicles and are most abundant in the upper portion of lava flows where cooling occurs so rapidly that gases are trapped.

Phaneritic - coarse grained texture; formed when magma cools slowly enough to form crystals, possibly very large. Exposure usually only occurs after erosion removes surrounding rock.

Porphyritic - 2 or more distinctly different crystal sizes, one small the other large. This typically signifies a two stage cooling period where magma cooled slowly for some time, then moved toward the surface - changing the rate of cooling - and stopping further crystal growth. The larger crystals in a porphyry are called phenocrysts. The matrix of smaller crystals is called the groundmass.

Glassy - formed when molten rock is quenched quickly and forms - what else? - a natural glass. Magmas with high silica content increase viscosity and slow lava. (Viscosity is a measure of a fluids resistance to flow)

Pyroclastic texture - Individual rock fragment ejected during a volcanic eruption may still be hot enough to fuse together when it lands.

Pegmatitic texture - Exceptionally coarse grained igneous rocks; composed of crystals larger than one centimeter in diameter. Most form in veins near the margins of magma bodies during the last stages of crystallization.

Bowens Reaction Series

Norman L. Bowen, suggested that some silicate minerals tend to crystallize at a higher temperature than others. Olivine, Pyroxene and Calcium-rich Plagioclase are shown near the top of his series; where as, lower temperature minerals, such as quartz is near the bottom. In general, the earliest
minerals to crystallize are relatively low in silica, so that the residual magma remaining after their
crystallization is more enriched in silica relative to its original composition.

There are two different reaction series in Bowen's Reaction Series. The first is called the Continuous Reaction Series:
This is the crystallization series in which early crystals react with the melt without changes in mineralogy. (No abrupt changes in mineralization - smooth transition due to substitution of ions)

The second series or the
Discontinuous Reaction
Series: is the crystallization
sequence in which early crystals are transformed into new minerals by reaction with the melt with abrupt changes in crystal line structure. (One minerals stops forming, another begins)

Crystal settling - if the earlier formed minerals are more dense; separation of solid and liquid components of a magma. When the remaining melt solidifies, it is chemically much different from the original parent magma. The formation of more than one magma from a single parent magma is called Magmatic differentiation.

Assimilation - the incorporation of a host rock into a magma. Accomplished by fractures - pieces fall into the magma (xenolith) - or by melting it.
Magma mixing- when one magma body intrudes into another.

Mafic - heavy, dark colored (Mg, Fe), more dense, low in silicon. Basalt.

Felsic - Feldspar, Silica - lighter colored, high silica, lower density. Granite.

Intermediate - Andesitic - amphiboles and intermediate plagioclase feldspars. Mid-way between basalt and granite.

Gradations exist - granodiorite.

Granitic magma - high silica, viscous, 800 degrees C.
Basaltic magma - low silica, above 950 degrees C.

Magma - generated by 3 conditions:
   1) increase in heat
   2) decrease in pressure
   3) increase in volatiles

Partial melting - produces felsic magma higher in silica content than the original magma.