The EXPERIMENT

Overview

To help you to understand the processes that might go on within a single magma chamber to produce such mixing a simple experiment was performed. A container is used to simulate a compositionally stratified magma chamber. The bottom opening allows fluid at a constant pressure to enter the system. This causes the fluid already in the chamber to be forced out the top opening. The top opening is analogous to the volcanic neck of an erupting volcano.

The layers within the container are stratified with a lower density, higher viscosity layer on the top, (eg: rhyolite), and a lower viscosity, higher density layer on the bottom, (eg: basalt). By controlling the density and viscosity contrast between the two layers, magmas with different compositions in the same chamber, can be simulated. i.e: a rhyolite overlaying a basalt.

The experimental setup is illustrated below.

  1. Top layer: this layer consist of water mixed with CMC, an organic polymer. Mixing with CMC increases the viscosity with little to no effect on the density. Different ratios of water to CMC can be used to obtain the desired viscosity whilst keeping the density constant. Metal filings are added to this layer so that motion can be detected.

  2. Bottom layer: this layer is composed of water mixed with salt. The addition of salt increases the density while having little effect on the viscosity. Thus the density of the fluid can be controlled. Red food coloring was added so that the two layers could be easily distinguished.

Experiment Stages

As you watch the movie, you will several stages which correspond to changes in geologically significant processes. We have extracted two stills from the movie to illustrate these stages.

Stage 1
As fluid enters the bottom of the chamber, fluid is expelled through the conduit at the top of the chamber. As this process proceeds convection cells develop in the top layer.

Note the semicircular arrangement of the metal filings. In the movie you will see that some parts of the chamber are effected by this convection, and others are not.

Stage 2
A critical level is reached where the denser bottom layer, being less viscous, is more readily forced up through the top layer, and a cone structure develops.

At this point both magmas are being tapped at the same time and mixing of the magmas can proceed in the conduit. The extent to which the two magmas mix in the conduit relates to differences in the densities and viscosities of the fluids. If the viscosity contrast is high, blending is retarded and mingling dominates. If the fluids have similar viscosities then blending is facilitated.

Now, watch the movie again!

Forward to Concluding Remarks

Intro | Movie | Overview | Factors | Example | Experiment | Conclusions |

Fundamental Concepts | Igneous Concepts | EOSWR Home