For most of my life I believed, as most everybody believes, that the earth is billions of years old. A couple months ago I inquired to the Minnesota Geological Survey as to how it knows the rocks are 3.6 million years old, which it references on a bronze plaque located in Ramsey Park in Redwood Falls.
I was sent two geochronology documents that held lots of fine detail regarding the usage of high-precision equipment (i.e. Sensitive High Resolution Ion Micro Probe) with various techniques for determining age via interpretation of isotope ratios in zircon crystals within the granite rock sample.
Then I asked how they know: 1. the rock formation has remained a closed system during its entire history; 2. the initial concentration of parent or daughter isotope; 3. decay rates have not varied throughout billions of years; 4. all parent and daughter isotopes are uniformly distributed in the rock formation.
I received an answer saying that geochronology tends to be built on multiple assumptions. Those multiple assumptions cannot be upheld by the scientific method, and should not be presented as "fact" and cast in bronze for general public indoctrination.
Geochronology was used to analyze lava rock from Mt. Ngauruhoe in New Zealand known to be 60 years old. The various isotope methods came up with dates ranging from 270,000 to 3.9 billion years.
If the earth were billions of years old, then its magnetic field it should be gone by now, and we would be unprotected from harmful radiation.