they are now, geologists say the polarity is normal. When they are in the opposite position,
they say that the polarity is reversed.
Scientists were surprised to discover that the normal and reversed magnetic polarity of
seafloor basalts creates a pattern of magnetic stripes! There is one long stripe with normal
polarity, next to one long stripe with reversed polarity and so on across the ocean bottom.
Another amazing feature is that the stripes are form mirror images on either side of the
mid-ocean ridges. The ridge crest is of normal polarity and there are two stripes of reversed
polarity of roughly equal width on each side of the ridge. Further distant are roughly equal
stripes of normal polarity, beyond that, roughly equal stripes of reversed polarity, and so on.
The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges
of continents, which are sometimes lined by a deep sea trench (Figure6.13).
Figure 6.13: Scientists found that magnetic polarity in the seafloor was normal at mid-ocean
ridges but reversed in symmetrical patterns away from the ridge center. This normal and
reversed pattern continues across the seafloor. ( 8 )
The scientists used geologic dating techniques to find the ages of the rocks that were found
with the different magnetic polarities. It turns out that the rocks of normal polarity are
located along the axis of the mid-ocean ridges and these are the youngest rocks on the
seafloor. The ages of the rocks increases equally and symmetrically on both sides of the
ridge.
Scientists also discovered that there are virtually no sediments on the seafloor at the axis,
but the sediment layer increases in thickness in both directions away from the ridge axis.
This was additional evidence that the youngest rocks are on the ridge axis and that the rocks
are older with distance away from the ridge (Figure6.14). The scientists were surprised to