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Across 3. A boundary along which tewo tectonic plated push together 6. The time required for one half of the atoms of a radioactive substance to decay 7. The remains or traces of an organism preserved from the geologic past 8. A boundary along which two tectonic plates pull apart 9. A basic unit of the geologic time scale that is a subdivision of an era, periods may be divided into smaller units called epochs 10. The process by which an oceanic plate sinks under another plate 11. A unit of the geologic time scale that is a subdivision of a period 12. A switch in the direction of Earth's magnetic north pole becomes the magnetic south pole 13. A major division on the geologc time scale, eras are divided into shorter units called periods 14. Process by which rocks are placed in their sequence or order 15. The concept that processes that have shaped Earth in the past that are essentially the same as those operating today

Down 1. A fossil that is associated with a particular span of geologic time 2. A law that states that in any undeformed sequence of sedimentary rocks, each bed is older then the layers above and younger than the ones below 3. a circulation in which material is heated and rises in one area, then cools and sinks in another area 4. The largest time unit on the geologic time scale, next in order of magnitude above era 5. The proposed supercontinent that 200 million years ago began to break apart and form the present landmass 6. An area where a column of hot material rises from deep within the planet's mantle