Figure 6.11: A ship sends out sound waves to create a picture of the seafloor below it. The
echo sounder pictured has many beams and as a result it creates a three dimensional map
of the seafloor beneath the ship. Early echo sounders had only a single beam and created a
line of depth measurements. ( 13 )
nental margins or in the sea near chains of active volcanoes. Trenches are the deepest places
on Earth. The deepest trench is the Marianas Trench in the southwestern Pacific Ocean,
which plunges about 11 kilometers 35,840 feet (35,840 feet) beneath sea level. Near the
trenches, the seafloor is also especially deep.
Besides these dramatic features, there are lots of flat areas, calledabyssal plains, just as
the scientists had predicted. But many of these plains are dotted with volcanic mountains.
These mountains are both large and small, pointy and flat-topped, by themselves as well as
in a line. When they first observed the maps, the amazing differences made scientists wonder
what had formed these features.
Seafloor Magnetism
In the previous lesson, you learned that magnetometers used on land were important in
recognizing apparent polar wander. Magnetometers were also important in understanding
the magnetic polarity of rocks in the deep sea. During WWII, magnetometers that were
attached to ships to search for submarines discovered a lot about the magnetic properties of
the seafloor.
Infact, usingmagnetometers, scientistsdiscoveredanastonishingfeatureofEarth’smagnetic
field. Sometimes, no one really knows why, the magnetic poles switch positions. North
becomes south and south becomes north! When the north and south poles are aligned as