Volcanic and Plutonic Processes

Magma that reaches the surface erupts as volcanic rock, while magma that solidifies at depth forms plutonic bodies; the two record contrasting emplacement processes.

Definition

The physical processes by which magma is transported, erupted at the surface, or emplaced and crystallized within the crust to form volcanic and plutonic rocks.

Scope

This topic covers the ascent of magma through the crust, the controls on eruptive style from effusive lava flows to explosive pyroclastic eruptions, the morphology of volcanic edifices and deposits, and the emplacement of intrusive bodies such as dikes, sills, laccoliths, and batholiths. It links magma viscosity and volatile content to eruptive behavior.

Core questions

• What controls whether an eruption is effusive or explosive?
• How do magma viscosity and dissolved volatiles govern eruptive style?
• What are the principal forms of intrusive igneous bodies?
• How does magma make room for itself during pluton emplacement?

Key theories

Volatile-driven explosive volcanism

Dissolved gases, chiefly water and carbon dioxide, exsolve as magma decompresses; in viscous silicic magmas the gas cannot escape easily, leading to fragmentation and explosive pyroclastic eruptions, whereas low-viscosity basalts erupt effusively.

Mechanisms of pluton emplacement

Intrusive magma creates space by stoping, ductile wall-rock deformation, roof uplift, and floor subsidence, producing bodies ranging from thin dikes and sills to vast batholiths.

Clinical relevance

Understanding these processes is central to volcanic hazard assessment and eruption forecasting, to interpreting ancient volcanic and intrusive terrains, and to locating magmatic and hydrothermal mineral deposits associated with plutons.

History

Twentieth-century volcanology, advanced by workers such as George Walker, established quantitative links between magma properties and eruption dynamics, while detailed mapping of intrusive complexes clarified the long-debated space problem of how large plutons are emplaced.

Key figures

• George P. L. Walker
• Ray Cas
• John D. Winter

Seminal works

• winter2013
• cas1987

Frequently asked questions

Why are some volcanoes explosive and others gentle?

Explosivity depends mainly on magma viscosity and gas content: viscous, gas-rich silicic magmas erupt explosively, while fluid, gas-poor basaltic magmas produce gentle lava flows.

What is a batholith?

A very large plutonic body, typically of granitic composition and exposed over more than 100 square kilometers, formed by the coalescence of many intrusions deep in the crust.