Brandon Boldt's Geology Post Cards

Pumice Terrace

On day three of the New Zealand trip, the group visited an outcrop near Lake Taupo that presented an interesting dynamic to an otherwise traditional depositional environment. The picture (above) shows strata alternating between layers of clast supported, well sorted, small, sub-rounded pumice clasts and layers of larger, rounded, pumice clasts in a fine grained matrix. This outcrop also happens to be one of many river terraces in the area. River terraces are formed when debris fills in valleys during glacial stadials due to ice driven forces and low river transport rates, followed by high river transport rates during interstadials that erode material, creating "V" shaped incisions in the land. The alternation between "filling in" during stadials and "down-cutting" during interstadials produces the recognizable terrace topography. Fluvial processes, like rivers, are the reason for the alternating layers in the outcrop. In normal circumstances larger clasts are evidence of high flow (more power needed to move the clast) while smaller clasts and layers of silt show a decrease in velocity and can even allude to the evolution of rivers to streams and lacustrine environments. The interesting dynamic to this outcrop is that the clasts are pumice stones. Pumice is a highly vesicular (filled with gas bubbles from a "gas blown" origin) form of rhyolite that has a lower density than water. While inspecting this outcrop one must question whether the larger clasts actually mean higher water velocity. The argument can be made that because the layers with larger pumice clasts are more rounded and have some fine grained matrix (while most fluvial deposits have little matrix material and are clast supported due to water removing the materials between clasts), and that larger pumice stones may float better than fine pumice with few vesicles, that the more fine grained layers are evidence of high flow rates and the larger pumice layers of more quiescent periods in the history of this system. In other words, how these layers appear and what they represent is opposite in respect to most outcrops - would you expect anything different from the antipodes?

Volcanic Blocks

Volcanic blocks are pieces of solid country rock or rocks formed from previous eruptions, ejected from the volcano. Unlike volcanic bombs, which are ejected in a molten state as spurts of lava that may solidify in air or on the ground, volcanic blocks are nearly always angular and may be gigantic. The volcanic block above , found near the summit of Mt. Ruapehu, is slightly larger than a backpack (in the picture for scale) and has made a clear impact in the snow. This ejected material is more likely a volcanic block than a volcanic bomb because it is angular, does not have flow textures (common in volcanic bombs due to their molten history) and most interestingly the bottom border is covered in yellow sulphur crystals. Sulphur is commonly precipitated and crystallized in volcanically active areas, and due to this volcanic block's short life span (snow has not yet covered it and the last eruptive event for Ruapehu was within a year) it is more than likely that the sulphur had grown on the country rock before being displaced. Volcanic blocks and volcanic bombs are only a couple geologic hazards associated with volcanoes, along with others like lava flows, lahars, pyroclasitc flows, and corrosive ash falls that can collapse structures and cause airplane jets to clog and malfunction.