Hopi Buttes

Within an area of about 50x55 km are concentrated numerous volcanic necks, remnants of lava flows, now capping mesas, dykes and many craters of the maar type. Shoemaker et al. (1962, p. 329) estimate that about 300 diatremes or maar type volcanoes are present in the Hopi Buttes region. The area constitutes a shallow basin and much of the volcanic ejecta is intercalated with Pliocene sediments. Variable degrees of erosion have revealed all levels in the volcanic structures allowing detailed reconstruction as described and illustrated by Williams (1936), Shoemaker et al. (1962) and others. At many vents a central lava neck, which may be traceable into horizontal flows, is surrounded by inward-dipping tuffs and breccias representing an earlier explosive phase of volcanism, but many other types of structure are manifest. Some flows can be traced to dyke feeders. The bulk of pyroclastic rocks, which are more voluminous than the lavas and vent fillings, are interbedded with lavas but more characteristically with river or lake deposits. Most pyroclastic rocks comprise angular pieces of limburgite and monchiquite in a sedimentary matrix with lapilli and rounded bombs the exception. The rocks forming the necks, dykes and flows are all petrographically similar and consist of phenocrysts of zoned titanaugite and olivine in a groundmass of pyroxene and opaques with minor apatite, analcime, sanidine and glass, and rare amphibole and biotite. The majority of rocks were called limburgites by Williams (1936, p. 124) but these grade into monchiquites which contain substantial analcime. The scarcity of feldspar is characteristic. Sr isotopes and Rb data are given by Powell and Bell (1970).