The three eras of geologic time after the Precambrian are sometimes called the Phanerozoic Eon – that name means “visible life.” The Paleozoic, which ended yesterday, means “ancient life,’ Mesozoic, beginning today, means “middle life,” and the Cenozoic Era, which we’ll cover in December, means “recent life.” This is a good time to repeat my disclaimer that we are NOT going through earth history at a proper scale. If we were, we’d be in the Precambrian until mid-November, and it would be a pretty boring commentary. So I have arbitrarily assigned January to all of the Precambrian, February through August to the periods of the Paleozoic, September, October, and November to the Mesozoic, and December represents the Cenozoic Era.
So today we begin the Mesozoic, sometimes called the Age of Reptiles or the Age of Dinosaurs, but lots of other living things diversified during this time and we’ll try to touch on many of them in the coming three months.
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On September 1, 1923, at two minutes to noon, a massive earthquake shook Tokyo and Yokohama, Japan. Its magnitude is given as 7.9 on the moment magnitude scale, and the estimated death toll was at least 143,000, making it one of the deadliest quakes in history. A strong typhoon hit the area at about the same time – and it has been suggested that the heavy storm surge from the typhoon might have put enough pressure on an already highly stressed fault that it could have triggered the quake – but in any event, the fault must have been ready to break already. Japan sits at the complex boundaries between four tectonic plates – the Pacific, Philippine, Eurasian, and a branch of the North American Plate called the Okhotsk Plate. At Tokyo, the northern point of the oceanic Philippine plate is subducting beneath both Eurasia and this extension of North America – a real mess, tectonically, so it’s not much of a surprise that the area is prone to great earthquakes.
—Richard I. Gibson