Geoscience Reference
In-Depth Information
ic mineral grains aligned with the Earth's magnetic field. So, sail over recent submarine
basalts and the magnetic field of the Earth will be slightly enhanced. But, as we heard in
the last chapter, the Earth's magnetic field sometimes reverses. Magnetism trapped in vol-
canic rocks that erupted when the field was reversed will carry an opposite component to
the present field, lowering the reading slightly. Thus the magnetic stripes on either side of
the mid-ocean ridge build up, with older and older sea floor as you move away from the
central ridge in either direction. The sea floor is indeed spreading.
10. How parallel stripes develop in the magnetization of volcanic rocks on the ocean
floor, as new ocean crust spreads out from an ocean ridge.
Boundary of creation
Overall, the spreading rate is slow but relentless, ranging from about 10 centimetres a year
in the Pacific to 3 or 4 centimetres per year in the Atlantic, about the same rate at which
your fingernails grow. But the eruption of lava to create new crust is not steady, which is
why parts of the ridge get rifted and subside as they are stretched open and others build
up peaks. Beneath the centre line of the ridge, hot mantle material is rising in a mush
