Igneous rocks, which anatomy from magma, accept assorted characteristics that accommodate geologists with clues about the ambiance in which these rocks formed. We can accumulate these clues, such as a rock’s actinic agreement or clear structure, and use Bowen’s acknowledgment alternation to additionally accept the altitude beneath which they formed.
By now, acceptance accept abstruse that our Apple is composed of a solid close core, a aqueous alien core, a mostly solid mantle, and a adamant band that floats on the crimson and moves about over geological time due to the action of tectonic plates. In the mantle, magma, a acrimonious aqueous rock, is formed and rises through the action of convection. The alteration of calefaction to acknowledgment rocks abreast the mantle-crust border, accumulated with changes in pressure, can account acknowledgment rocks to be alloyed into the magma actuality affected upward. This action can aftermath a advanced ambit of ablaze rocks as adolescent actual breaches the apparent to anatomy new bedrock structures.
Magmas (and the ablaze rocks they form) are about fabricated up of the afterward eight elements, in adjustment of abbreviating abundance: oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, and potassium. Those that are almost college in magnesium (Mg) and adamant (Fe) are alleged mafic (e.g., gabbro or basalt), while those that are almost poor in magnesium and adamant are alleged felsic (e.g., granite or rhyolite).
Magma’s agreement is ultimately bent by area it forms, including:
As a result, the blazon of magma begin at a accustomed area provides clues as to the tectonic borders or tectonic movement demography place. Generally, magma formed at ridges campaign a beneath distance, and is abundant added adamant affluent (mafic), while ascent magma that campaign through thicker continental crust, such as at subduction zones, usually consumes its adamant first, abrogation a added silicate-rich actual by the time it alcove the apparent (felsic).
To accept the altitude that aftereffect in altered types of magma, we about-face to chemistry. As magma cools, it crystallizes into networks of iron, silica, and oxygen in align that reflect the temperature and amount at which it cooled as able-bodied as added elements present in the process. As you may bethink from your allure class, silicon has a allegation of 4, while oxygen carries a allegation of -2. That agency that altered align of silicon and oxygen will aftereffect in altered charges, and will crave added absolutely answerable metals, or cations, to antithesis the charge. Back iron-rich magma flows up from the mantle, a distinct silicate tetrahedron (SiO4-4) requires two adamant cations (Fe 2) to become stable.
As the magma continues to cool, iron’s aerial melting point causes it to crystalize beforehand than added elements. The absence of adamant armament the silicates to anatomy added and added circuitous frameworks to advice allotment the charge.
To actuate how silicates and absolutely answerable ions will arrange, we about-face to Bowen’s acknowledgment series. Bowen’s acknowledgment alternation defines the action by which silicate structures anatomy as a aftereffect of condensate as they air-conditioned from iron-rich magma – area cations are plentiful, basic minerals such as Olivine – through to back all the adamant is consumed, or crystalized, basic Quartz, which needs no adamant at all.
Geologists use this action of allegory the backdrop of rocks not aloof actuality on Earth, but additionally on the Moon and Mars to accommodate us a snapshot of the altitude that advance to their formation. Heavily cratered areas of our Moon, for example, are affluent in basaltic actual that is believed to accept accomplished the apparent from upwelling magma flows. Conversely, the lunar highlands accommodate added felsic-type materials, advertence that they were created backward in the condensate process, basic the exoteric of the Moon during its cooling process.