Andesitic magma

Definition

Andesitic magma is a type of intermediate volcanic rock that is primarily composed of silica (SiO2) and contains between 52% to 66% of silica by weight. It is associated with subduction zone volcanism, where oceanic plates descend into the mantle beneath continental plates, leading to the generation of this specific type of magma. The unique composition of andesitic magma influences its viscosity and the explosive nature of the eruptions it produces.

5 Must Know Facts For Your Next Test

1. Andesitic magma forms at convergent plate boundaries, particularly where oceanic lithosphere is subducted beneath continental lithosphere.
2. The presence of water in subducting oceanic plates helps lower the melting point of rocks, resulting in the formation of andesitic magma.
3. The high viscosity of andesitic magma contributes to the formation of stratovolcanoes or composite volcanoes, which can produce highly explosive eruptions.
4. Andesitic magma typically results in eruptions that are more violent compared to those produced by basaltic magma due to its higher gas content and viscosity.
5. Common examples of volcanoes formed from andesitic magma include Mount St. Helens in Washington and Mount Fuji in Japan.

Review Questions

• How does the composition of andesitic magma influence its eruptive behavior compared to other types of magma?
  • The composition of andesitic magma, which contains a higher percentage of silica than basaltic magma, leads to increased viscosity. This higher viscosity traps gases within the magma, resulting in greater pressure build-up before an eruption. As a result, when andesitic magma erupts, it tends to produce more explosive volcanic activity compared to basaltic magma, which has lower viscosity and allows gases to escape more easily.
• Discuss the role of water in the formation of andesitic magma at subduction zones.
  • Water plays a crucial role in the formation of andesitic magma at subduction zones. When an oceanic plate subducts beneath a continental plate, water trapped in sediments and minerals is released into the overlying mantle wedge. This addition of water lowers the melting point of surrounding rocks, facilitating the partial melting needed to generate andesitic magma. Thus, the presence of water is essential for the production of this type of magma in convergent plate boundary settings.
• Evaluate the implications of andesitic magma for volcanic hazard assessment in regions with composite volcanoes.
  • Evaluating andesitic magma is vital for volcanic hazard assessment because its properties can lead to highly explosive eruptions. Regions with composite volcanoes, such as those formed from andesitic magma, face significant risks due to potential pyroclastic flows, ash fall, and lahars following an eruption. Understanding the characteristics and behavior of andesitic magma allows scientists to better predict eruption events and implement effective risk mitigation strategies for nearby populations.

Related terms

Subduction Zone:

A region of the Earth's crust where an oceanic plate sinks beneath a continental plate, leading to volcanic activity.

Viscosity:

A measure of a fluid's resistance to flow; higher viscosity in magma can lead to more explosive volcanic eruptions.

Composite Volcano: A type of volcano that is characterized by steep profiles and periodic explosive eruptions, often formed from andesitic magma.