Andesite Block from Mt. St. Helens - Menlo Park, California - Rock and Mineral Displays on Waymarking.com

This andesite block from Mount St. Helens is one example of a series of large blocks of rocks from various locations, which are on display around the USGS Campus in Menlo Park. There is a self-guided tour of the campus available at the visitor center.
Text of Sign: “The David Johnston memorial rock is a ‘bread crusted’ block from the deposit of an andesitic pyroclastic flow (hot avalanche of rocks, ash and gas) that descended the southwest flank of Mount St. Helens between 450 and 500 years ago and extended as far as 9 kilometers (6 miles) from the pre-1980 summit. The interiors of some of the abundant andesite blocks in this deposit are rich in vesicles (gas bubbles), indicating, that the stiff molten interiors were releasing gas as the flow was being emplaced some of the blocks clearly deformed plastically to accommodate themselves to the shapes of underlying blocks in the deposit. This memorial block is representative of block size where it was collected, but blocks several meters (10 feet or more) in diameter are common elsewhere in the flow.
The blocks contain small crystals mainly of the minerals pyroxene and plagioclase set in a grounmass of volcanic glass. The bulk composition includes about 60 percent silica (SiO2).”