Heat from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years (see Geology of the Pacific Northwest) and is also responsible for activities in the Lassen volcanic area.
Between 3 and 4 million years ago, volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation.
Alternatively it blast out solid pieces of rock that once formed the volcanic edifice.
All of these fragments are referred to as Pyroclasts = hot, broken fragments. Depending on size, tephra can be classified as bombs. Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff.
adaptive radiation: The diversification, over evolutionary time, of a species or group of species into several different species or subspecies that are typically adapted to different ecological niches (for example, Darwin's finches).
Scientists now have accurate methods (see below ) for dating fossils.
methods rely on characteristic faunal and geological patterns to bracket the period when the fossil existed.
acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter).
adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment.