CHAPTER 3 - Igneous Rocks (P. 55-78)

MAGMA - molten material and dissolved gas (in the subsurface). The most important molten materials are the silicate melt composed of Silicon and Oxygen and Other dissolved atoms, particularly aluminum, iron, calcium, sodium, potassium, and magnesium. Note that these 8 elements make up about 99 percent of the Earth's crust.

LAVA - magma that reaches the Earth's surface before cooling (most gasses have escaped).
Magma is like seltzer: It contains "Volatile gases" - H2O, CO2, SO2, N2, CH4 and other dissolved gases.


CRYSTALLIZATION - the formation and growth of a crystalline solid from a liquid or a gas; crystals are the orderly arrangement of atoms in a framework. In IGNEOUS ROCKS, crystal size is a reflection of:

In a confined space, such as IN lava or magma, crystals are interlocking. The crystals we see in museums typically grew on the walls of underground caverns or empty spaces left by fluids (water or gas) in the earth.


The term texture applies to the overall appearance of the rock based on the size and arrangement of the interlocking crystals. Textural terms include:


BOWEN'S REACTION SERIES explains how the composition of magma changes at it cools (See page 58, Fig. 3.2). HIGH TEMPERATURE (Mafic) MINERALS form first and SETTLE OUT (by sinking to the bottom of the magma chamber). As the magma continues to cool all the high temperature minerals form first, leaving a cooler, but still molten rock (magma). As this magma cools to form rock the last minerals to crystallize are the LOW TEMPERATURE (Felsic) MINERALS. This process changes the composition of the magma as it cools: this process is called MAGMATIC DIFFERENTIATION.

The MANTLE (which is ultimately the source of heat rising in the earth's CRUST) is nearly homogeneous in composition, Consists of an ULTRAMAFIC rock called PERIDOTITE composed of the minerals OLIVENE, PYROXENE, PLAGIOCLASE (Ca- Feldspar), plus other minerals and dissolved gases.


Where magma from the MANTLE reaches the surface, such as along mid-ocean ridges and large, hot, igneous plumes rising under certain volcanos, like HAWAII, the composition of the lava has not changed very much. The magma is very MAFIC in composition. MAFIC IGNEOUS ROCKS include:

Mafic rocks (basalt and gabbro) are very dark in color and very dense because of their high iron content. Mafic rocks are abundant in the Earth's crust beneath the oceans.


Magma that has undergone a high degree of MAGMATIC DIFFERENTIATION has a composition consisting of FELSIC MINERALS (low temperature igneous minerals, such as quarts, K-spar, Na-spar, and mica. Felsic rocks include:

ANDESITIC ROCKS or Intermediate Rocks

ANDESITE is a medium gray, fine-grained rock of volcanic origin, named after the Andes Mountains, but is abundant in all volcanic regions associated with subduction zones (where crust is sinking and partially remelting - example: volcanos associated with the Pacific "Ring of Fire"). Andesite is INTERMEDIATE in composition between MAFIC and FELSIC rocks. The mixture of mafic and felsic minerals gives the rock a "salt and pepper" appearance.

Intrusive: Gabbro Diorite Granite
Extrusive: Basalt Andesite Rhyolite


HYDROTHERMAL SOLUTIONS - "volatilize gases" that escape from magma (water, CO2, Methane) plus "dissolved" minerals.

HYDROTHERMAL DEPOSITS - precipitates from solutions escaping from a magma chamber, form VEIN DEPOSITS. Often form economic deposits of gold, copper, zinc, silver, and other metals.

PEGMATITE - very large crystals of igneous minerals form near the top of a gas saturated igneous intrusion.


NOTE: The PALISADES, a long rocky cliff in NJ along the western side of the Hudson River in the vicinity of the George Washington Bridge, is an excellent example of a SILL, a large volcanic intrusion that squeezed in between sedimentary rock layers that were at one time buried deep beneath the Earth's surface.

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