In the early 20th century, geologist N. L. Bowen and his co-workers first proposed a theory that has since come to be known as “Bowen’s reaction series.” This theory, which explains the order in which minerals in igneous rocks crystallize from magma as it cools, is based both on field studies and laboratory experiments. In simple terms, Bowen’s reaction series lists the order in which minerals crystallize: Those with the highest melting point form first, followed by those with lower melting points. With this theory, Bowen attempted to explain how basalt and granite might crystallize from a single pool of molten rock, or magma. Of course, it’s not that simple.
The theory is usually illustrated by a Y-shaped graphic, with the dark iron- and magnesium-bearing (“mafic”) minerals on one arm of the Y; sodium and calcium (plagioclase) feldspar on the other arm of the Y; and quartz, muscovite and potassium feldspar (orthoclase) on the vertical bar. Temperature of mineral crystallization decreases toward the bottom of the Y.
Bowen called the plagioclase arm the “continuous branch.” This is because, as the magma cools, the ratio of calcium to sodium in the feldspar changes continually: Calcium-rich plagioclase forms first and sodium-rich forms last. Plagioclase is the only mineral formed, and it continues crystallizing until all available sodium and calcium are gone.
The other arm of the Y is more complicated, because several different mafic minerals are formed by crystallization along this branch. Not only do different minerals form at different temperatures, but existing crystals can react with the magma, rearranging their internal structure to form a new mineral.
As the magma cools, olivine forms first. This simple mineral – two atoms of iron or magnesium for each atom of silicon – uses so much iron and magnesium that the remaining liquid becomes silicon-rich. As the molten rock cools, crystals of the pyroxene group begin to form; but at the same time the olivine crystals react with the silicon and other elements. The crystal lattice of the olivine rearranges itself so that there is only one atom of iron or magnesium for every silicon atom. As the molten rock cools even further, minerals of the amphibole group (hornblende) begin to form, and pyroxene crystals react with silicon and aluminum remaining in the magma to alter to amphiboles. If iron and magnesium are still present, the sheet silicate biotite will crystallize and amphibole crystals will react to form biotite. That is the discontinuous series: olivine, pyroxene, amphibole and biotite.
Once all iron and magnesium have been trapped in the minerals of the discontinuous series, and sodium and calcium have been removed by the plagioclase feldspars, any remaining magma will be enriched in potassium and silicon, along with less common elements. At this point in cooling, orthoclase feldspar forms to use the potassium and pure silicon dioxide forms crystals of quartz. With sufficient water, muscovite mica also forms, as well as more uncommon minerals that take up elements present only in small amounts.
Bowen’s theory presumed that all rock types can crystallize from one magma pool. He believed that basalt and gabbro, which contain high-temperature minerals but no quartz, form after high-temperature crystals settle to the bottom of the magma chamber, where they can’t react with the cooling melt. According to the theory, once settling had occurred, quartz-rich rocks such as granite crystallized high in the magma chamber. Settling like this has been observed on a small scale in some igneous bodies, but never to the extent Bowen believed. Instead, present geological thought calls for magmas of different composition to form basaltic vs. granitic rocks.
This does not mean that Bowen’s reaction series is completely unfounded. His experiments showing reaction of crystals with minerals have explained some knotty questions in high-temperature studies. More importantly, Bowen was the first to examine the order in which minerals crystallized as magma cools, and his findings are still taught more than a century later.
