This orientation is not an assumption, because in virtually all situations, it is also possible to determine the original "way up" in the stratigraphic succession from "way up indicators". Thus, if we started with 1 gram of 40K, after 1. The existence of these two clocks in the same sample gives a cross check on each other. And it's impossible to know where a lunar meteorite originated. These geological principles are not assumptions either. Such features as colour, ripple marks, mud cracks, raindrop imprints, and slump structures are directly observable in the field.
This time is the same regardless of the amount of the element present, and this property of radioactive elements leads to the exponential decay law.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
These requirements place some constraints on the kinds of rock that are suitable for dating, igneous rock being the best. Material filling in a cavity left by a organism that has dissolved away. The accumulation of electrons is governed by the rate of background radiation. Many other indicators are commonly present, including ones that can even tell you the angle of the depositional surface at the time "geopetal structures""assuming" that gravity was "down" at the time, which isn't much of an assumption. Natural potassium is mostly not radioactivebut a tiny percent 0. Image showing fossils that connect the continents of Gondwana the southern continents of Pangea. Not quite as old, but still pretty old.