FORMATION, CLASSIFICTION AND IDENTIFICATION IGNEOUS ROCK

FORMATION, CLASSIFICTION AND IDENTIFICATION

The inside of the earth is very hot - hot enough to melt rocks. And the deeper you go the hotter it gets. Below the surface the molten rock is called magma; at the earth's surface it becomes lava, although nothing has changed except the name.
     The fresh magma is white hot, brillant enough that you would have trouble looking at it. But as it cools it turns yellow, and then various shades of red. Eventually it cools enough to solidify completely and form an igneous rock, such as the granite and basalt below. Granite and basalt are the two most abundant igneous rocks at the earth's surface.

Magma/lava is a mixture of elements such as silica, iron, sodium, potassium, etc. As the magma/lava cools these elements chemically combine, or crystallize, in geometric patterns to form the eight rock forming minerals. For example, in the granite above the pink is orthoclase, the black biotite, and clear to gray mostly quartz.      These eight minerals form the bulk of igneous rocks. They are arranged in Bowen's Reaction Series (BRS) by temperature of formation, high temperature ones at the top and low temperature ones at the bottom. Although it is useful to know these minerals they are not essential for a basic understanding of igneous rocks. Constituent Percent of Total Volume H2O CO2 N2 SO2 SO3 S2 H2 CO Cl2 Ar 67.7. 12.7. 7.65. 7.03. 1.86. 1.04. .75. .67 .41. .20      Cooling is progressive in a magma/lava, some minerals becoming solid at high temperatures (top of BRS) and others at lower temperature (bottom BRS), so that part way through the cooling the magma/lava is a mixture of minerals and still molten rock.      Magma/lava also contains lots of gasses such as water, sulfur dioxide, carbon dioxide, etc., and these are driven off into the atmosphere during cooling. (See Table to right of gasses from the Hawaiian volcano.) If cooling is "slow" (thousands to millions of years) below ground the minerals grow large enough to see with the eye, as with the granite to the left.       These are "coarse grained" (or phaneritic). Any rock in which the grains can be seen by eye are coarse grained. If cooling is "quick" (days to weeks) as at the earth's surface, the minerals do not have enough time to grow, and so are microscopic in size. These are fine grained (or aphanitic). For example, the rhyolite to the left.      If cooling is "very quick" (hours to days) the elements and compounds are frozen in place, no minerals form, and the result is a glass. For example the scoria to above left and the obsidian above right.      Basalt and granite are two of the most common igneous rocks found at the earth's surface. They illustrate the diversity of properties igneous rocks have. Basalt a mafic rock  Granite a felsic rock  1. Dark color 2. High specific gravity 3. Olivine/pyroxene/Calcic plagioclase rich 4. Fine grained - crystals only seen under high power 5. Forms at the surface, principally in the ocean basins, but also in isolated "hot spots" on the continents. 6. Forms high in Bowen's Reaction series 1. Light color 2. Low specific gravity 3. Quartz and orthoclase and sodium plagioclase rich 4. Coarse grained - crystals large enough to see by eye 5. Forms on the continents deep underground 6. Forms low in Bowen's Reaction series      Igneous rocks are classified in several different ways (link), but all rock classifications are a combination of texture and color/composition of the rock. The variety of igneous textures is in the table below Texture Cooling History Example Glassy Vesicular (cellular) Aphanitic (fine     grained) Phaneritic     (coarsegrained) Porphyritic (two     grain sizes) Very fast cooling; non-crystalline. Very fast cooling with rapid gas     escape forming bubbles in the     non-crystalline rock. Slow cooling; microscopic crystal     growth. Very slow cooling; crystals grow     to visible size. Two stage cooling; one slow     underground creating visible     phenocrysts, the second fast at    the earth's surface producing a     fine grained groundmass. Obsidian Pumice, scoria Rhyolite, andesite,     basalt Granite, diorite,     gabbro Any aphanitic rock with    the adjective porphyry      The color/composition of the rock is at its simplest divided into dark colored rocks (mafic), intermediate colored rocks (intermediate), and light colored rocks (felsic). If we combine texture/cooling history and color/composition in a grid we get the classification in the table below.  If classification and identification was all there was to igneous rocks there would not be much use studying them. We classify rocks to learn what they can tell us about the earth.      There are two ideas about igneous rocks that are geologically important. The first idea is that igneous rocks evolve - they change from one kind of rock into another.      The second idea is that rocks are not randomly distributed across the earth. Specific kinds of rocks are always found in specific places for specific reasons, all tied into plate tectonic processes.      These ideas of igneous rock evolution and distribution are related, and eventually we will explore their relationships. For now, we just introduce the basic ideas.