What you will learn in Volcanoes and Igneous Activity

1. 1. Three ways to melt a rock

   Rock does not melt because it gets hotter. There are three routes across the solidus and only one of them involves adding heat — which is why mid-ocean ridges, with no heat source of their own, are the planet's largest volcanic system.

2. 2. Silica decides everything

   Silica sets viscosity across eleven orders of magnitude, viscosity decides whether the gas gets out, and whether the gas gets out decides what the eruption does. The temperature relation runs the way you do not expect.

3. 3. Volcanoes on the map

   The ring of fire as a consequence rather than a name — and the measurement that shows "volcanoes are on plate boundaries" is not quantitatively true. Most of Earth's volcanism is submarine and builds no cone at all.

4. 4. What actually kills people

   Six hazards by mechanism, with reach, speed and real death tolls beside them. Lava flows are near the bottom of the list, and a VEI 3 with a glacier on top killed more people than any eruption of the twentieth century.

5. 5. Measuring an eruption

   The Volcanic Explosivity Index as the logarithmic scale it is, built the same way the magnitude scale is built two modules later. A VEI 8 is a thousand times a VEI 5, and one step of the scale is not the decade it advertises.

6. 6. Hotspots, and one year without a summer

   The Hawaii–Emperor chain as a record of plate motion, the assumption that reading rests on and the drilling that broke it — and then Tambora, 1816, and why the cooling agent was sulphate rather than ash.