STRESSES DURING CINDER CONE COLLAPSE AS RECORDED BY DISPLACEMENT OF CRATER-RIM BLOCKS, STRAWBERRY CRATER, ARIZONA

Strawberry Crater is one of several hundred cinder cones in the San Francisco volcanic field of northern Arizona. The process of cinder cone formation and deconstruction are not well understood despite the fact that they are found not only on Earth, but on other terrestrial planets in our solar system. Strawberry Crater is a unique cone in that variable gas content of magma caused cinders during late stages of the eruption to be welded together upon deposition. These agglutinated cinders broke into large blocks that were displaced as late-stage eruption of lava breached the walls of the cone. Study of these dismantled blocks is important in order to further understand the processes that accompanied the breaching.

Research has included identification of facies in block layers and measurements of block sizes and displacements. Facies analysis shows that many of the blocks represent formerly continuous horizons and reflect a variance in gas content of the magma that affected the temperature and accumulation rate of pyroclasts, and therefore the degree of welding. Faults between blocks are dominantly vertical to subvertical regardless of block size. This evidence suggests that breaching of the cone occurred abruptly as magma pressure exceeded the strength of the cone walls, releasing the remaining magma trapped in the cone during the late-stage lava flow.