Early Earth was a rough place for life to gain a foothold.
One of the questions facing scientists is just when the earth’s protective magnetic field was strong enough to shield its inhabitants from harmful cosmic radiation.
Ancient rocks from South Africa indicate that a magnetic field 50-70 percent the strength of today’s magnetic field was in place by 3.4 billion years ago.
The evidence comes from nanometer-sized crystals of the mineral magnetite embedded in millimeter-sized quartz crystals. Quartz is the most stable mineral in the Earth’s crust and a good time capsule for the included magnetite. A magnetometer is used to detect the magnetic signal preserved in the iron atoms in the magnetite and to measure the strength and orientation of the ancient magnetic field.
[Note: Not just any rocks can be used in analyses in dating ancient geological events, because millennia of deformation and reworking of the Earth’s crust could re-set geological timepieces, so great care is taken in selecting the best candidates for dating.]
Source: Tarduno, John, et al., March 5, 2010 Science.
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