By Crystal Gammon | LiveScience.com
Several winters ago, a team of geophysicists from Missouri flew to the eastern edge of Africa, strapped on bulky backpacks and began walking. They were looking for a set of huge stripes in the Tendaho Graben, a place within the Afar Depression of Ethiopia, where Africa’s continental crust is stretching thin and a new ocean will eventually form.
But the stripes they sought — and eventually found — aren’t visible to the naked eye. They’re magnetic stripes, similar to the ones lining the ocean floor at mid-ocean ridges. David Bridges, a geophysicist from the Missouri University of Science and Technology, and his colleagues sniffed them out using a bit of geological detective work, lots of walking and the hulking magnetometers strapped to their backpacks.
The Tendaho Graben’s magnetic stripes are important because they’re the first ones scientists have documented on land, Bridges said. Even more importantly, because these stripes have formed before the area becomes a water-covered basin, they may change the way researchers interpret the planet’s oceans.
“The really interesting thing is that some of the oceanic basins may perhaps be a little bit younger than we currently believe,” Bridges told OurAmazingPlanet.
Stripes and flips
The underwater relatives of Tendaho’s magnetic stripes were first documented in the 1950s by geophysicists who set sail to take thousands of seaboard magnetic readings. The researchers eventually began to see that their readings sketched out distinct sets of stripes running parallel to mid-ocean ridges, and that each stripe’s magnetic alignment was the reverse of neighboring stripes.
The striped magnetic pattern develops because, as oceanic crust pulls apart, magma rises to the surface at mid-ocean ridges and spills out to create new bands of ocean floor. Ferromagnetic minerals in the hot magma align themselves with the Earth’s magnetic field, which completely reverses its north-to-south polarity every now and then, and freeze in that alignment as the magma cools. Later, after the planet’s magnetic field flips again, the next stripe of new ocean floor aligns its polarity in the opposite direction.
“For many ocean basins, the timing of their openings has been based on the appearance of these magnetic stripes,” because scientists long believed that the stripes first appeared when seafloor spreading started, Bridges said.
Tendaho breaks the trend
But the stripes that Bridges’ team found in Tendaho may prove that conventional wisdom wrong.
Tendaho’s magnetic bands, which measure 6 miles (10 kilometers) wide, are embedded in continental crust, not oceanic crust. And unlike magnetic stripes on the ocean floor, Tendaho’s formed through diking: as the African crust stretched thin, streams of magma intruded the continental crust and hardened. Like in the oceanic stripes, ferromagnetic minerals in the dikes aligned with the planet’s magnetic field as the magma hardened. Their magnetic signals are very similar to those of ocean-floor stripes.
This all happened sometime between 1.8 million years ago, when the region’s continental crust began to break apart, and 780,000 years ago, when the Earth’s magnetic poles last flipped, Bridges said.
Scientists predict it could be as many as 2 million years before the crust in the Tendaho Graben ruptures and begins to form an ocean basin. Altogether, this means that Tendaho’s magnetic stripes could predate the future ocean basin by nearly 4 million years.
And magnetic stripes may predate other ocean basins, too.
“Other groups have found evidence suggesting that perhaps the Atlantic basin opened up a little later than what’s currently believed,” Bridges said. “It’s sort of an interesting time in this field.”
The team’s findings appear in the March issue of the journal Geology.