Density measurements

Although we refer the user to Houghton and Wilson (1989) for a full description, we give here the main guidelines to perform density measurements and ultimately obtain sample vesicularity (extracted from Shea et al., 2010a).

Go to the downloads section to obtain a template spreadsheet for density measurements.

Prior to performing density measurements, collected samples are cleaned and dried at T>100ºC for 24 hours. For pumice or scoria samples, a subset of clasts are usually ranked by decreasing size and numbered from 1-100 (Fig. 2). For other specimens, especially large ones, the rock is cut into halves, one half being kept for scanner imaging and the other serving for density measurements and microscopic imaging. Within the half serving for density, several subsamples corresponding to distinct textural units are carefully removed (Fig. 2). If the sample is homogeneous to begin with, no further subdivision is required, provided that its size is large enough for weighing in the laboratory.
The density measurement methods described here are derived from Houghton and Wilson (1989).

Clasts or sample subsets are weighed in air (mass eq 1 in g), and either individually wrapped into polyethylene film (of wet weight eq 2), or made impermeable using water-proofing spray. They are then weighed once more immersed within water (eq 3). Specific gravity, and thereby density is expressed as:


eq 4                                                                                           (Eq. 1)


For buoyant particles, e.g., pumice, the clasts are forced down using a ballast of known wet weight and volume.
Finally, the dense rock equivalent (DRE) density of the magma is used to obtain porosity (f) or vesicularity (f×100):


eq 5                                                                                           (Eq. 2)

This technique is rapid and yields large arrays of data.  Other alternatives include measuring density/porosity directly using a He-pycnometer, yielding both connected and isolated vesicle fractions (Rust and Cashman, 2004). For pumice and scoria sample data sets, density is plotted on a histogram to choose only a few clasts that represent the different endmembers from the entire distribution (Fig. 2). In this manner, 3 to 8 clasts are typically chosen to represent low (1 to 2 clasts), modal (1 to 4 clasts) and high (1 to 2 clasts) vesicularities. For larger samples from lava flows, domes and bombs, tephra clasts showing substantial internal variability, density/vesicularity measurements are done on the sub-samples prepared for each textural zone. The chosen clasts/subsamples are made into thin sections with, in the case of pumice or reticulite, impregnation with resin to avoid breakage of thin glass walls.