Trick question

The first rock identification lesson taught to introductory geology students is identifying whether a rock is igneous, sedimentary or metamorphic. This first step is no less important for advanced students or seasoned professionals. In some cases, this distinction is not apparent at distance, and sometimes not even apparent under close inspection. Some rocks require microscopic magnification for basic classification. Unfortunately, close examination is not always possible. The rocks may be in inaccessible terrain, in a politically hostile region, or perhaps on another planet. Identification from a distance is tricky.

Here is a sequence of rocks south of Boyce Thompson Arboretum east of Phoenix. This sequence of rock is over one thousand meters thick, shows some horizontal layering, as well as what appears to be tilting and folding. Is this rock sedimentary, igneous, or metamorphic?

Based upon this limited information, most people would initially hypothesize that these rocks are sedimentary. However, closer rocks within the arboretum show similar layering, but are clearly volcanic tuff. The nearby Superstition mountains are more clearly volcanic. With this additional evidence, a more likely hypothesis is that these rocks south of the arboretum are igneous. In fact, the Arizona Geological Survey classifies them as “middle Miocene to Oligocene volcanic rocks.”

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