女生小视频

THE Earth鈥檚 magnetic field was three times its present strength when
dinosaurs roamed the planet, supporting theories that it is generated by an
internal dynamo rather than static magnetic material. Ancient magnetic fields
leave their imprints on volcanic rocks, but field strength is much harder to
measure than direction. John Tarduno of the University of Rochester in New York
state has now used a superconducting device to measure field strength in
crystals that formed 113 million years ago, during a 37-million-year interval
when the magnetic field was exceptionally stable and did not flip direction
(Science, vol 291, p 1779). The unusually strong field matches models
which predict that a 鈥済eodynamo鈥� field should grow in strength during long
intervals when the field direction does not flip every few hundred thousand
years.