Recent advances in geochronology, or rock dating, allow geologists to measure radioactive decay and assign very precise absolute ages to geological events. In another study, scientists found isotopic evidence of a sudden switch to primitive plate tectonics dating back about 3.8 billion years. The discovery suggests that a simple form of subduction had begun at least in one place on the planet by that time. And a tree-lined African lake has provided data on the Earth’s magnetic field for the past 150,000 years.