Another huge problem for proponents of an ancient Earth is the magnetic field. Indications are that the Earth's magnetic field is actually quite young, and attempts to explain away the plain evidence are creative. They defy the laws of physics, but they are creative nonetheless. Creationists have proposed a much more realistic model.
The earth is surrounded by a magnetic field that protects living things from solar radiation. Without it, life could not exist. That’s why scientists were surprised to discover that the field is quickly wearing down. At the current rate, the field and thus the earth could be no older than 20,000 years old.You can read the rest of this layman's-level article at "Rapidly Decaying Magnetic Field", here. A more technical article is available at "The Earth's Magnetic Field is Young".
The earth’s magnetic field is wearing down so quickly that it could be no more than 20,000 years old.Several measurements confirm this decay. Since measuring began in 1845, the total energy stored in the earth’s magnetic field has been decaying at a rate of 5% per century. Archaeological measurements show that the field was 40% stronger in AD 1000. Recent records of the International Geomagnetic Reference Field, the most accurate ever taken, show a net energy loss of 1.4% in just three decades (1970–2000). This means that the field’s energy has halved every 1,465 years or so.