The north and south poles of the earth’s magnetic field do not align exactly with the earth’s true rotational poles so that a compass direction will be different than the true geographic direction. This can be observed in Valencia County.
If you were to aim your compass straight east, for example as aiming from Los Lunas towards Meadow Lake, your compass will indicate a bearing of 82 degrees from north, while the true bearing is 90 degrees.
The difference between the true geographic bearing and the compass bearing is called magnetic declination. The current local declination in Valencia County is 8 degrees east of true north, and it changes to the west about 1/2 degree per year.
The declination can be checked for any location in the United States at the federal website NCEI Geomagnetic Calculators (noaa.gov). The local declination was 13 degrees east in 1850, the first time this data was available. Across the continental U.S., the declination currently varies by about 20 degrees east and west from true north.
The magnetic north pole is wandering about 30 miles per year. The poles are also known to have switched during geologic time, with a geologically sudden reversal of the north and south poles. During the past 100 million years, the poles reversed at least 150 times, and the reversal rates have varied considerably.
Consecutive reversals were spaced 5,000 years to 50 million years apart. The last time the magnetic field reversed was about 750,000 years ago. It cannot be predicted how soon in the future the poles will reverse again.
When rocks are formed, they may register magnetic fields at that time. This remanent magnetization in rocks from when they were formed has been used by geologists to estimate the location of the continents over time; magnetic measurements in ocean floor lavas also confirmed the theory of plate tectonics. Other planets have magnetic fields: the four gas giant planets (Jupiter Saturn Uranus and Neptune) have extremely strong magnetic fields, Earth has a moderately strong magnetic field, Mercury has an extremely weak field, but Venus and Mars have almost no measurable magnetic field.
Both terrestrial and marine animals rely on the magnetic pole for navigation, and we can expect significant disturbance to wildlife during pole reversals.
(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geology Landscapes of Valencia County are featured monthly.)
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April 25, 2024
