Geology Reference
In-Depth Information
Somehow the rift valleys and earthquakes are related, so ridges must be dynamic, change-
able features.
Ocean floor rocks also surprised many geologists, who had predicted that the Mid-At-
lantic Ridge was a typical mountain range capped by resistant marine limestone, just like
the Canadian Rockies. But extensive dredging along the ridge, coupled with observations
of the Atlantic's many islands, produced nothing but basalt, and relatively young basalt at
that. It turns out that, other than a veneer of soft sediments, the ocean's crust is made al-
most entirely of volcanic basalt. From east to west, spanning more than twenty-five hun-
dred miles of ocean floor, basalt forms the pavement.
What's more, careful dating based on the steady decay rates of radioactive elements re-
veals a simple pattern in the ages of these rocks. Basalt collected from the rift valley right
at the center of the Mid-Atlantic Ridge is newly minted, less than a million years old. The
farther you get from the rift valley, east or west, the older the basalt is, until the rocks near
the continental margins are more than one hundred million years old. Why should rocks
at the center of the ocean be young, while those on the outskirts are so much older? One
logical conclusion is that the Mid-Atlantic Ridge is a line of volcanoes that are spewing
out new basaltic crust. But where did the much older rocks at the edges of the ocean basin
come from?
Key data, the smoking gun of plate tectonics, came from a second submarine-hunting
technology called the magnetometer. World War II submarines are big hunks of iron-rich
alloys, so they are magnetic. Thanks to the development of magnetometers, submarine-
hunting airplanes could fly over the ocean surface and pick up the magnetic anomaly of a
nearby enemy submarine. Following the great conflict, geophysicists invented new types
of magnetometers with greatly increased sensitivity to small changes in the magnetic field.
They adapted these instruments to be towed behind research ships, just above the sea bot-
tom.
Their target was ocean-floor basalt, which carries a weak magnetic signal in the form
of minute crystals of the iron mineral magnetite. Earth's magnetic field is known to vary
slightly from year to year, in what is called secular variation. As basalt magma cools, these
crystals freeze in the direction of Earth's magnetic field like tiny compass needles. Ocean
floor basalt thus preserves the orientation of Earth's magnetic field on the exact date when
the rock hardened. The thriving field of paleomagnetism studies these invisible magnet-
ic force fields that are locked into basalt and other rocks. (On dry land, a hodgepodge of
magnetic signals, the consequence of folding, faulting, and other geological contortions of
continental crust over time, confuses such patterns.)
Starting in the early 1950s, oceanographers deployed magnetometers close to the sea-
floor, sweeping them on long transects across ocean ridges. They hoped that their paleo-
magnetic measurements might yield a better picture ofsecular variation onthe ocean floor.
