Using nothing but a compass as a navigational tool, ancient mariners from China to Sumer navigated the world’s oceans. Even in the modern age of GPS, the magnetic compass is an unfaltering navigational tool for seagoing vessels, airplanes and hikers on the ground.
In high latitudes the dazzling display of aurora, also known as the northern lights, illuminates the night sky. Those stunning displays are a direct result of Earth’s magnetic field and signify that the magnetic field is doing its job deflecting charged particles away from the lower latitudes.
Nearly everyone has seen how iron filings arrange themselves into the pattern of magnetic lines around a magnet. Near Earth, the invisible magnetic lines of force look similar as they envelop the planet.
William Gilbert published a treatise in the 16th century called “The Magnet” in which he summarized all knowledge about electricity and magnetism. As part of his research, he shaped a natural magnet called a lodestone into a sphere and demonstrated that its magnetism affected a compass the same way Earth does.
Around the spherical Earth, magnetic field lines wrap from the North Pole to the South Pole like the segments of an orange. As it extends far out into space beyond Earth’s surface, the solar wind distorts the magnetic field into a teardrop shape that trails out on the lee side.
The source of the magnetic field is not precisely known. Geophysicists think that it is due to Earth’s liquid outer core creating a dynamo effect induced by Earth spinning on its axis.
The Earth’s magnetic field protects us from harmful radiation from solar flares, coronal mass ejections and other charged particles from space.
The magnetic field traps charged particles and channels them along the field lines that converge as they intersect the surface at the magnetic poles. There they encounter the low-pressure gases in the upper atmosphere and glow like the gas in a neon light. The process is the same, except the gases are oxygen and nitrogen instead of neon or other gases used in display lights.
The magnetic field has reversed at random intervals over the past hundreds of millions of years. During times of reversal, a compass would indicate south instead of north.
The reversal, the cause of which is unknown, traps tiny particles of magnetite, a magnetic iron mineral found in volcanic and sedimentary rocks.
In volcanic and other igneous rocks, magnetite crystals form early while the lava or magma is still liquid, and the tiny crystals behave like compasses. When the molten rock cools and solidifies, the crystals retain their orientation to indicate the direction of the magnetic field when the rock solidified.
In sedimentary rocks, the magnetite crystals take on the orientation of the magnetic field as they settle through the water. Subsequent sedimentation and the process of lithification then harden the rock and lock the crystals in place.
This provides a convenient bar code that allows us to correlate the pattern of reversals in a given rock formation with the template of known reversals. In this way, the date of a rock formation can be determined with a high degree of accuracy.
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
