In the late 1940s and 1950s,
in order to track Soviet submarines and hide U.S. subs, the Navy funded
echo sounding to study the bathymetry (underwater topography)
of the ocean floors.
The stunning results revealed
features very different from those on the continents. Generalized
results were published as National Geographic maps, showing:
- submerged mountains,
called oceanic ridges
- cracks perpendicular
to the ridges,called fracture zones
- narrow, deep gashes,
- vast flat areas, called
- drowned undersea islands,
called seamounts and oceanic plateaus
While these ships were
cruising around the world's oceans, the scientists onboard requested
and received permission to carry out additional "piggy-back" experiments.
- Using a dredging bucket,
they periodically hauled up samples of the seafloor. They
collected lots of mud, but also rocks such as basalt, gabbro, and
serpentinite. No granite, no schist, no sandstone, no andesite....none
of the rocks common on the continents.
- With a magnetometer
towed behind the ship, they measured the magnetic field,
and compared the results to values predicted from a model of Earth-as-dipole.
Measured values usually differ from predicted values due to the
effect of magnetic rocks.
Magnetic minerals align
themselves with Earth's magnetic field when the rock forms (e.g.,
a lava flow cools), preserving a magnetic vector that can be measured
with a magnetometer.
The difference between
the predicted and measured values is a magnetic anomaly. Magnetic
anomalies on land (widely available by the 1950s) exhibited irregular,
studies on land showed that about half of the samples had "internal
compass needles" that pointed south, instead of north. Geophysicists
concluded that Earth's magnetic field has reversed its direction many
times. Most geologists rolled their eyes at the idea.
studies helped clear up the mystery. Marine magnetic anomalies
form linear bands of alternating positive and negative magnetism (see
Fig. 2, above left, click to zoom in); i.e., rocks in half
the bands (white on Fig. 3, below) are reversely magnetizeda
N arrow on a compass would point south.
In the early 1960s, Bay
Area geologists decided to test the hypothesis that the Earth's magnetic
field reverses itself. They collected young volcanic rocks from all
over the world, measured the magnetism of each, and determined the
age of each (the latter with the recently-developed K-Ar radiometric
- all normally magnetized
rocks formed at certain times (Fig. 3: e.g., 0.0-0.7 mya)
reversely magnetized rocks formed at other times (e.g., 3.4-4.0
Scientists now conclude
that the earth's magnetic field has reversed hundreds of times.
findings were applied to marine magnetic anomalies:
- The linear anomalies
on seafloor are symmetrical across oceanic ridges (Fig. 4,
at right, click to zoom in).
- Pattern of bands correlates
exactly with onland reversal sequence of Figure 3, above.
- Band over center is
youngest, other bands are progressively older to the sides.