Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup

Magma that reaches Earth's surface comes in three essential compositions that produce the three common volcanic rock types: basalt, andesite, and rhyolite.

Basaltic magma, the most common form of magma, results from the partial melting of rock in the zone of Earth's mantle called the asthenosphere. Basalt generally has the lowest silica content of the three common volcanic rocks, which makes it the least viscous.

Andesite can arise in a variety of ways, but most is produced by the partial melting of wet basalt. This often occurs at subducting plate margins where oceanic crust dives beneath another plate. Andesitic magma can also be generated by the melting of continental crust. Compared to basaltic magmas, andesitic magmas are normally more viscous and contain more dissolved gases.

Rhyolitic lava is rich in silica and therefore quite viscous. It can arise from the partial melting of continental crust. Because of its viscosity, which results in the magma solidifying before it can reach the surface, rhyolitic lava generally is less common than other forms.

[Back to MELTING ROCKS]     [Next: Dynamic Earth]


"Volcanoes" is inspired by programs from Earth Revealed.


© Annenberg Foundation 2017. All rights reserved. Legal Policy