Bowen's series showed that melts range from silica-poor to silica-rich. Geologists name magmas, and the rocks they make, by that one number: the percent of silica ($\text{SiO}_2$) by mass. Silica controls how hot a magma is, how dark its rock will be, and above all how sticky it is.
| Class | Silica | Erupts at about | Rocks (color) |
|---|---|---|---|
| Ultramafic | below 45% | rarely erupts today | peridotite, the mantle (green-black) |
| Mafic | 45–52% | 1,000–1,200 °C | basalt, gabbro (dark) |
| Intermediate | 52–63% | 800–1,000 °C | andesite, diorite (salt-and-pepper) |
| Felsic | over 63% | 650–800 °C | rhyolite, granite (light) |
Why it works
Silica tetrahedra don't wait for the magma to freeze before linking up: even in the melt they share oxygens and form chains and networks. The more silica, the more links, and the harder the liquid is to stir. Felsic magma can be millions of times more viscous than basalt. Felsic magma is also cooler, because the minerals it makes sit at the cool end of Bowen's series, and cooler liquids are stickier still.
Key idea
Magmas run from mafic (low silica, hot, runny, dark rock) to felsic (high silica, cooler, sticky, light rock). Silica links into networks in the melt, so more silica means much higher viscosity.
Worked examples
Worked example: Classify a magma
A magma is 58% silica. How is it classified?
- 58% is between 52% and 63%.
- So it is intermediate, the kind that makes andesite.
Answer: Intermediate.
Worked example: Mixing two magmas
3 parts of basalt magma (50% silica) mix with 1 part of rhyolite magma (74% silica). What is the mixture?
- Weighted average: $\dfrac{3 \times 50 + 1 \times 74}{4} = \dfrac{224}{4} = 56\%$.
- 56% is intermediate. Mixing is one of the ways andesite forms at subduction zones.
Answer: $56\%$ silica: intermediate.
Common mistake
It is easy to think the hottest lava must be the most explosive. Basalt is the hottest magma, and it usually erupts gently because it is runny and its gas escapes easily. Cooler, silica-rich magma traps gas and can explode. Check with two famous volcanoes: Kīlauea's hot basalt flows quietly; Mount St. Helens' cooler dacite blew apart in 1980.