The mountain is heavily eroded to the east, exposing a large amphitheater that is approximately 5 square miles. Estimates vary about how high the mountain was at its highest. An extreme estimate places it near , but it is more likely the volcano was never higher than . The mountain is surrounded by a great volume of volcanic debris, but the debris field and the natural amphitheater in the central part of the mountain are attributed to erosion rather than a late stage explosion similar to Valles Caldera, Mount St. Helens or the San Francisco Peaks. Downcutting has been great enough to expose Cretaceous sedimentary rocks at the core of the cone. Geology The Mount Taylor Volcanic Field is a series of Upper Cenozoic volcanic beds that overlies Cretaceous sedimentary rocks in central New Mexico. This volcanic field is part of the Jemez Lineament, which is associated with relatively young volcanism (< 5 Mya) due to an ancient suture in the Proterozoic basement rock. The general stratigraphy of the volcanic field starts with a more intermediate (rhyolitic) lava composition and transitions to a more mafic (basaltic) composition as more eruptions occurred. The transition of this magma from alkalic to basaltic eruptions is thought to have resulted form multiple, short-lived magma chambers within Mount Taylor, instead of one long-lived chamber that experienced fractional crystallization. Volcanology Mount Taylor was active during the Pliocene to Pleistocene, and is surrounded by a field of smaller inactive volcanoes. Potassium-argon (K-Ar) dating showed that a large majority of the volcanic activity from Mount Taylor occurred between 4.3 and 1.5 million years ago, with the peak of the volcanism occurring between 3.0 mya and 2.5 mya. Mount Taylor volcano was the leading source of volcanism in this area for this time period, aside from some local basalt eruptions. The trend of Mount Taylor eruptions can be divided into four sequences or phases. The first phase, from 3.74 to 2.93 million years ago, was mostly basanite with some alkali basalt which formed mesa caps. This was accompanied by small volumes of more silicic volcanism, producing trachyte, trachydacite, and rhyolite tuffs, domes, and plugs. The second phase, from 2.88 to 2.78 million years ago, built up much of the stratovolcano and consisted of trachyandesite, trachydacite, and dacite. The uppermost flows of this phase are mostly a rock previously identified as plagioclase basalt, but now reclassified as basaltic trachyandesite to