As spectacular and awe-inspiring as the caldera and lake are, The Pinnacles, to me, is the most illuminating geological feature at Crater Lake National Park. The story told in these deposits is awe-inspiring in its own right. When I was an undergraduate, I was under the impression (I don't recall whether I was told or pieced together my own knowledge and assumptions) that this represented magmatic segregation , with the felsic component on top, and the mafic component on the bottom. This sequence was then subsequently inverted as the magma erupted from the top down when the magma chamber was breached during the catastrophic eruption. There is, however, a serious problem with that conjecture: that's simply not how magmatic segregation works! I'll talk a little more about that and the theory that emerged in the 1980's that may help explain the trigger for this and other large eruptions, tomorrow.