My final new plate-tectonic map for the week is a big chart of the age of the seafloor. It's as dramatic as Bozo the Clown, demonstrating the core evidence behind plate tectonics—seafloor spreading. Before Harry Hess had his flash of insight in the late 1950s, the ocean floor was little more than a huge blank space on the tectonic map. Afterward, it was an open book with instructions for restoring a hundred million years of continental drift. Today, mapping the seafloor in ever-greater detail is still paying dividends in geoscience.

Certain dates always ring a bell for me, and 18 May is one of them: It was a Sunday morning in 1980 and the beautiful volcano near Portland blew up, killing 57 people in the thinly populated (and previously evacuated) country to its north. I was among geologists at the USGS's Menlo Park campus in California. We were excited by the scientific opportunity, but devastated to learn that one of our own had died that day: David Johnston. Today he would be retired, or close to it, after an exceptional career, but that was not to be. Read more about that day and that eruption.

The next map I've added to the plate tectonics collection shows the most prominent hotspots—where they sit and what we call them. Some hotspots have obvious names, like Hawaii, Iceland and Yellowstone, but most are named for obscure ocean islands (Bouvet, Balleny, Ascension), or seafloor features that in turn got their names from famous research ships (Meteor, Vema, Discovery). This map should help you keep up during a talk aimed at specialists.

The map shows 49 hotspots, but many more have been proposed, and there is no single set of agreed-upon characteristics that define a hotspot, although all of them involve volcanism in a place or volume that isn't expected from "normal" plate tectonics. Long-time readers know that I'm somewhat jaundiced about the subject. But the need is clear for this map because whatever they really are, these localities have interesting geology.

The next map I've put up shows the geologic features and activities that plate tectonics explains: volcanoes, earthquakes and faults, mainly. These things line up in very specific ways around the world, and those lines are what led scientists to come up with plate tectonics. When geologists get together to talk about plate tectonics, these are the things they refer to. The map is a snapshot of the world that shows only the parts that are active today—if you count the last 1 million years as "today." Find the Tectonic Activity of the World map in the Plate Tectonic Maps category.