The Earth is really a mass of igneous rocks with a thin layer of sedimentary rocks on the outside and a small iron-rich core. The mantle was molten magma during the formation of Earth, so it is igneous rock.
What is magma made of?
Magma is formed by partial melting of Earth’s mantle (or lower crust), with occasional input from gases and water carried by subducting tectonic plates. Different minerals have different melting points. The melting points of some common minerals are summarised in the diagram of Bowen’s Reaction Series.
Bowen’s Reaction Series shows the melting temperature for a range of common minerals and how this relates to type of magma. (Colivine 2011, Wikimedia Creative Commons)
Silicate minerals with a high silica content, such as quartz, melt at lower temperatures (800° C). A low temperature magma will be rich in these minerals and is called a felsic magma. These minerals are typically light in colour.
Silicate minerals with a lower silica content, such as amphibole, melt at higher temperatures. These higher temperature magmas are called mafic magmas. The minerals that melt at a higher temperature are often darker in colour.
Water and carbon dioxide are often dissolved in magmas. As the magma draws nearer to the surface, these gases bubble out. Felsic magmas are stickier (more viscous), so the bubbles containing gases may be trapped, resulting in explosive eruptions. Mafic magmas typically have lower gas contents (as they are less viscous) and tend to ooze as they reach the surface and become lava.
This lava in Hawaii hardens into a dark rock because it is high in mafic minerals. It oozes because the gas content is low.
Evidence of mingling
Large areas of magma underground may take thousands of years to cool and become rock. Granite is the most common rock formed by the cooling of magma under our continents. During cooling, new magma may ooze into the chamber. The new magma often has a different composition, so we see evidence of this as different coloured spots, called globules, within the main rock. Evidence of this process can be seen on the Sydney Harbour Bridge.
The granite blocks on the outside of the pylons supporting the Sydney Harbour Bridge (left) have dark spots (globules) from magma that oozed into the chamber (right).
The magma that mingles into the chamber may have a different mineral composition and may cool at a different rate from the main magma body. The mineral differences are reflected in a different colour of rock. The different cooling rate is reflected in the size of the crystals in the globule. Do you think the globules shown in the photo below were all from the same new magma or that different types of magma may have oozed into the chamber?
The globules in this granite were caused by magma mingling.
Model magma at home
- Model magma mingling at home using oil and water as shown in this video.
- Try to classify igneous rocks from photos.
- Relate magma types to volcano shape and explore the effect of magma viscosity on volcanic eruptions in these WASP activities.