Types of Magma

Basaltic Magma - When you envision lava in your mind, you likely think about rivers of molten rock flowing over a landscape much like Hawaii. This idealized vision of lava is actually basaltic lava. Basaltic lava is the most abundant lava on earth and is the lava most commonly depicted in pictures and media. In this lesson, we'll learn what defines basaltic lava and some of the different forms it takes.

- Basaltic lava is another term for mafic lava. Mafic lava is molten rock that is enriched in iron and magnesium and low in silica. When mafic lava cools on the earth's surface, it forms basalt, which is why mafic lava is commonly called 'basaltic lava.' - Basaltic lava flows easily because of its low viscosity (low gas content). The low viscosity is due to low silica content. Andesitic Magma - Andesitic magma erupts explosively because it tends to have high gas content. It is viscous and therefore traps gas, builds pressure and explosively erupts. High viscosity is related to high silica content.

Rhyolitic Magma - Rhyolite is a felsic extrusive rock. Due to the high silica content, rhyolite lava is very viscous. It flows slowly, like tooth paste squeezed out of a tube, and tends to pile up and form lava domes. - If rhyolite magma is gas rich it can erupt explosively, forming a frothy solidified magma called pumice (a very lightweight, light-coloured, vesicular form of rhyolite) along with ash deposits, and / or ignimbrite. In certain situations extremely porous rhyolite lava flows may develop. The extreme porosity of such flows allows degassing and subsequent collapse of the flow, forming obsidian (dark coloured volcanic glass). - Rhyolite is the volcanic equivalent of granite. - Rhyolitic magma erupts catastrophically because it has high gas content. It is viscous and therefore traps gas, builds pressure and explosively erupts. High viscosity is related to high silica content – an abundance of silica polymers (chains etc.) leads to the high viscosity.