El Capitan represents 8 separate igneous events assembled over nearly 3 million years. The largest and oldest of the events seen on this map are the formation of the large El Capitan and Taft granites. These 2 units formed about 106 million years ago as distinct bodies of igneous rock; the El Capitan granite had mostly solidified when the Taft Granite was emplaced next to it, and the boundaries where those units come into contact with each other are sharp showing that the older rock really wasn’t molten when the younger one came in. Those initial large granites were followed by a series of intrusions that broke across the granites one after another. Several of these were more mafic than the original granites, and one of them did intrude along the boundary between the 2 oldest granites suggesting that was a weak spot. The “aplite dikes” are the youngest unit and they probably formed as other granites of the Yosemite Intrusive Suite were crystallizing to make the rest of what we know of today as the park. This single wall of rock should give you some idea of what conditions are actually like at the bottom of a volcanic arc. As magmas are generated, one batch after another intrudes its way upwards, sometimes moving as a single large body and other times fracturing the older rocks to move through. These rock probably formed at a depth of about 12-15 kilometers in Earth’s crust, and all of the rocks above have been eroded away to expose these granites today. A version of this map overlain on an image of El Capitan itself can be purchased from the US Geological Survey, and may already be available in your nearby university library. -JBB Image source and original paper: http://doi.org/10.1130/GES01133.1 Full version for sale: http://rock.geosociety.org/Store/detail.aspx?id=MCH106F