Melted rock is a mixture of many elements, not a pure liquid like water. In the early 1900s Norman Bowen melted rock powders in a laboratory furnace, cooled them step by step, and looked at which minerals grew at each temperature. His result, now called Bowen's reaction series, links the silicate families to the way magma freezes.
Why it works
A mixture freezes over a range of temperatures, not at one point. As magma cools, the minerals with the strongest structures, built from iron, magnesium and silica-poor recipes, become stable first and crystallize. Each crystal takes its ingredients out of the melt, so the liquid left behind keeps changing: poorer in iron, magnesium and calcium, richer in silica, sodium and potassium. Minerals at the cool end form from that leftover melt.
| Temperature | Discontinuous branch | Continuous branch |
|---|---|---|
| Hot (about 1,200 °C) | olivine | calcium-rich plagioclase |
| pyroxene | ↓ | |
| amphibole | ↓ | |
| biotite mica | sodium-rich plagioclase | |
| Cool (about 700 °C and below) | potassium feldspar, then muscovite mica, then quartz |
Key idea
As magma cools, minerals crystallize in order: olivine and calcium-rich plagioclase first, then pyroxene, amphibole and biotite, and potassium feldspar, muscovite and quartz last. The leftover melt grows richer in silica.
Worked examples
Worked example: First to crystallize
Which crystallizes first from a cooling magma: amphibole, quartz, olivine or biotite?
- Olivine sits at the hot top of the series.
- Order: olivine, amphibole, biotite, quartz.
Answer: Olivine.
Worked example: The melt left behind
A magma is 50% silica. 40% of it crystallizes as olivine and calcium-rich plagioclase averaging 44% silica, and the crystals sink away. What percent silica is the remaining melt?
- Take 100 g of magma: 50 g of silica.
- The 40 g of crystals hold $40 \times 0.44 = 17.6\,\text{g}$ of silica.
- Left: $50 - 17.6 = 32.4\,\text{g}$ of silica in $60\,\text{g}$ of melt: $32.4 \div 60 = 54\%$.
- Removing silica-poor crystals drives the melt toward andesite and, eventually, rhyolite composition.
Answer: $54\%$ silica.
Common mistake
Many people picture a magma freezing all at once, like a pond turning to ice. In fact different minerals form over several hundred degrees of cooling, and crystals can grow while plenty of liquid remains. Check with a real rock: in a slowly cooled granite, quartz fills the odd-shaped spaces between earlier feldspar and mica crystals, because it was the last to grow.