In the early 1900s a young petrologist from Canada carried out a series of experiments that changed the way geologists understood igneous rocks. His results help us to understand the origin of magmas, types of igneous rocks and volcanic eruptions.
Bowen’s Reaction Series
Norman Bowen carried out a series of experiments between 1910 and 1914, using new high temperature furnaces, petrographic microscopes and chemical analysis methods. He found that minerals in mafic magma melted at different temperatures. This helped to explain the composition of magma, lava and igneous rocks in different settings. The findings are summarised in the Bowen’s Reaction Series diagram which relates the mineral composition of magma to mineral melting points, arranged by temperature.
The mineral composition of magma is related to its temperature in Bowen’s Reaction Series (Colivine 2011, Creative Commons)
Felsic and mafic
Igneous rocks are divided up based
on the proportions of light and dark minerals. The light-coloured silicates are
the main minerals in granitic rocks. These are called felsic because
they are made of feldspar and silica.
Rocks with at least 45% dark
silicate minerals and no quartz are basaltic. They are called mafic from magnesium
and ferrum (Latin for iron). The higher metal content makes basaltic
rocks darker and denser than granitic rocks.
The mineral content of igneous rocks (Woudloper 2009, public domain)
Partial melting of the mantle
Bowen’s reaction series explains
why mafic magmas are so hot (up to 1,250°C). High heat is required to melt the
mafic minerals in Earth’s mantle. Mafic magma is produced over hot spots in
Earth’s mantle. A familiar example of this is Hawaii. The Hawaiian volcanoes
produce mafic lava flows of dark basalt.
Cooler magmas (as low as 650°C)
are produced when lower temperatures melt mantle rock. Only some of the mantle
minerals melt. This is called partial melting. At lower temperatures many
felsic minerals and very few mafic minerals will become part of the melt that
forms magma.
Differences in pressure and the
addition of water can affect the melting point of minerals, however, the order
of partial melting remains the same.
Partial
melting occurs when some minerals melt at low temperature (yellow circles)
while others remain solid. As temperature increases more minerals melt until eventually
all are molten. (Jullcurie 2014, Creative Commons)
Changing magma composition in
Earth’s crust
Hot mafic magma in Earth’s crust may
also break and trigger partial melting of the surrounding rock. The molten
crustal rock is assimilated into the magma and changes its composition.
Magma also cools and changes
composition within Earth’s crust. As the outer portions of the magma cool,
mafic minerals crystallise first. These mafic minerals solidify around the
edges of the magma chamber or sink to the bottom. The main body of magma
becomes more granitic (felsic) over time.
Knowing the melting points of
minerals helps us to understand how magmas are formed and change over time.
Bowen’s experimental work has been vital to our understanding of how igneous
rocks form and why their composition varies within large structures.
Learn more:
- Learn about rocks and the rock cycle in this ESWA video.
- Experiment with partial melting and develop your own reaction series of model minerals as shown in this AusEarthEd video.
- The UK Geological Society explains partial melting and magma origins on this webpage
- Build tasty models of silicate minerals in this activity
- Learn more about igneous rocks and processes in this presentation