JGE 1996 - Volume 44

Vertical variation in basalt vesicle size and abundance has traditionally been utilized as a relative-age indicator in deformed volcanic terrains. However, there is more to be learned from vesicles than simply which way is up. Recent study of vesicle size-frequency distributions has resulted in quantification of rates of volatile exsolution in pre-eruption magmas, flow motion and cooling processes, as well as the elevation of lava emplacement.

Through analysis of vesicle size-frequency distributions from a series of slabs cut from a single basalt hand-sample, students are introduced to simple rock preparation techniques and subsequent numerical analysis. Additionally, comparison of size-frequencies between sequential slabs allows for introduction of these techniques and the mathematics of bubble growth in fluids.

From our example analysis, it was determined that vertical variation in the size-frequency distribution of basalt vesicles over short distances implies multiple processes of bubble growth. In our sample, buble enlargement occurred largely through coalescence during buoyant assent. Systematic vertical variation in negative exponential distributions of vesicle sizes indicates that initial coalescence of bubbles is followed by minor inflation and selective combination of only the largest bubbles as lava cools.

We have developed a new advanced hydrogeology pollutant-transport simulation apparatus that is widely adaptable and can be used for quantitative experiments. It allows students to readily observe processes that are otherwise unobservable because of temporal and spatial constraints and because they take place in the subsurface. The simulator is a gently inclined, 61 cm x 122 cm terrarium with input and outflow ports on either side. The terrarium is filled with layers of sand and clay to simulate stratigraphy. "Wells" are simulated by clear plastic tubes with screened ports that are spaced at regular intervals and depths. In one experiment, water-based dye is injected as a plume in the top center of the terrarium, creating a slug that moves down the gradient towards the wells. Groups of students pipette samles from the wells at regular intervals and qualitatively estimate dye concentrations using a color-tone calibration set. Flow models and time-integrated displays of plume concentrations can be constructed. A second experiment involves measuring water height in the wells and constructing a flow net for the system using a computer program.