Open Adit™ - Igneous Rocks

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Igneous rocks (from the Greek word for fire) form from when hot, molten rock crystallizes and solidifies. Below the surface of the Earth, 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, brilliant 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 granite or basalt, 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. 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.

Cooling is progressive in a magma/lava, some minerals becoming solid at high temperatures (top of BRS) and others at lower temperature (bottom of 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.

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.
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.

Groups:

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Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.

Extrusive:

Extrusive , or volcanic, igneous rock is produced when magma exits and cools outside of, or very near the Earth’s surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. Pumice, obsidian, and basalt are all extrusive igneous rocks.
Intrusive:

Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth’s surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture.

Classification:

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Igneous rocks are classified in several different ways, but all rock classifications are a combination of texture and color/composition of the rock.

Color/Texture Igneous Classification:
Mafic magmas produce dark colored rocks made of dark minerals (such as basalt), intermediate magmas intermediate colored rocks (e.g. diorite) and felsic magmas light colored rocks (e.g. granite). Because of these fortuitous conditions it is natural to classify igneous rock on color and texture. As a first approximation, a classification based on color and texture is ok, but can lead to great mistakes and ultimately a color/texture classification is inadequate.
Modal Igneous Classification:
A modal classification classifies igneous rocks on the relative abundance of five minerals they may contain: (1) Quartz, (2) Alkali feldspars (orthoclase, but including albite if anorthite content does not exceed 5%), (3) Plagioclase, (4) Feldspathoids (silica poor minerals) (5) Mafic minerals (such as pyroxene and amphibole).
Suites / Normative Igneous Classification:
The normative classification arranges igneous rocks into suites, each suite characterized by a particular chemistry. The four major suites are the komatiite, tholeiitic, calcalkaline, and alkaline. Once we become familiar with these it is possible to talk about earth history in terms of the four suites alone, and largely avoid reference to specific rocks.
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Suite Chemistry: Suites are characterized by three chemical signatures: silica saturation, iron enrichment, and the alkali index. Silica saturation is a measure of the amount of SiO₂ available in a magma or rock. Silica under saturation is when SiO₂ is low enough there is not only not enough to form quartz, there is insufficient to form other minerals such as feldspars. The result is silica poor feldspathoid minerals, such as nephaline and sodalite. Over saturation is when enough SiO₂ exists for quartz to crystallize out. If SiO₂ is high enough it is possible to have a basalt with quartz, an association not commonly thought to exist. The alkali index measures the amount of Ca (calcium) from the top of Bowen's Reaction Series (BRS) relative to the amount of Na+K (sodium+potassium) from the bottom of BRS. Alkali indexes greater than 1 indicate high Ca content typical of the top of..BRS. Indexes less than 1 indicate low Ca and high Na+K typical of the bottom of BRS. Since in the fractionation process elements low in the reaction series are "sweated" out first we expect the first fractionated melts to be higher in Na+K than the unmelted residue. Since the tholeiitic, calcalkaline, and alkaline suites have alkali indexes (>1) (1) (<1), they form a fractionation sequence. Iron enrichment declines steadily with fractionation. This is a measure of the decrease in the importance of ferromagnesium minerals down the reaction series. Iron is low in the Komatiite suite because the ultramafic components Mg, Ni, and Cr are so high.
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Suite Fractionation: Igneous rock evolution can occur both within and among the suites. Within suite evolution occurs when, for example, a calcalkaline suite evolves from a diorite to a granite, or a komatiite suite evolves from a peridotite to a basalt to an andesite. Among suite evolution occurs in volcanic arcs, and other places, when the first igneous activity begins silica over saturated with alkali indexes >1, and evolves to silica under saturated with alkali indexes <1. That is, tholeiitic, followed by calcalkaline, and finally alkaline suites. Another evolutionary process occurs when one fractionated igneous rock is re-fractionated at a later time. This would occur, for example, if a fractionated diorite magma emplaced and solidified into a batholith. If this batholith is later heated, a second, more felsic, fractional product (granite) could be sweated out of it, leaving behind a more mafic residue. Also, a rock of one suite may re-fractionate to a melt with the characteristics of another suite.
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Suite Tectonic Association: One important feature of the suites is their association with particular tectonic regimes. This knowledge is valuable in understanding and reconstructing ancient tectonic events when most of the evidence is destroyed or otherwise unavailable. By analyzing the chemistry of the rocks we can reconstruct the processes by which they formed. Fractionation takes place in two primary tectonic regimes. First is at rifting centers. Silica over saturated parent rocks (komatiites in the Achaean, other ultramafics since then) rise to the surface and fractionally melt. The melt is tholeiitic and rises to the surface to form the pillow basalts and sheeted dikes of ocean crust. The unmelted residue is usually silica under saturated ultramafics which stay in the mantle as layer 4 in the ophiolite suite. (Note that the ophiolite "suite" is not a suite in the same sense as calcalkaline, etc. suites. The second fractionation takes place at convergent boundaries. The tholeiitic oceanic crust moves away from the rifting center until it is subducted. It heats up during subduction and fractionally melts. Typically the first melts erupting closest to the trench are still tholeiitic, but in time the melts evolve to the calcalkaline suite, which build most of the volcanic arc. Later melts become alkaline. These may come from the secondary fractionation of a melt or rock of the calcalkaline suite. The residue of the fractionation occurring along the subduction zone is ultramafic (peridotite) and continues to descend into the mantle, where it is permanently stored.

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