Physics of Magma

Magma is composed of three main components

The way that these three components interact is one of the main focuses of petrologists and volcanologists.
Also, the way that these components interact is govered by some of the basic laws of physics.

In layman's terms viscosity is a fluids resistance to flow and is important in many volcanic processes. Viscosity is defined as the internal resistance to flow by a substance when a shear stress is applied. Many factors affect a magma's viscosity. Temperature is one of them, as a with all fluids as a the temperature increases the viscosity also increases. This example can be seen when examining a rhyolitic melt, when the temperature decreases from 1300°C to 600°C the viscosity increases by more than eight orders of magnitude. The silica content and the water content also affect a magmas viscosity. The more silica is in a magma the more viscous the magma will be due to the strong silica-oxygen bonds. When water is added to a magma it has the ability to break the silica-oxygen bonds and therfore, the viscosity decreases.

Rheology is the study of how materials flow and it is very important when examining volcanogenic processes. Two main types of flows can be defined: laminar and turbulent. Laminar flow occurs when the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the walls. Turbulent flow occurs when the streamlines or flow patterns of a fluid are disorganized and there is an exchange of fluid between these areas. The Reynolds number (Re) can be calculated to determine whether a fluid is turbulent or laminar.

image of turbulent flow
image taken from:

When the Reynolds number is less than 10 .... it is considered laminar, when it is greater than 100 it is considered turbulent. The areas in between are defined as transitional and can go either way.

Rheology and viscosity are imporatnat to volcanologists because it will determine if Magma Chambers will convect and overturn. When the Rayleigh's and Reynolds numbers are high enough it is possible for these chambers to over turn which can lead to volcanic eruptions