Small sea creatures that have lain in pristine condition for eons have given a University of Washington researcher the clearest evidence yet that about 80 million years ago a southern landmass began migrating to the north. And what today are rainy British Columbia and chilly southern Alaska were once the sunny climes of Baja California.
For years scientists have been arguing about the theory of the wandering West, and whether it would have been possible for plate tectonics to have moved massive chunks of the Earth’s crust northward along a gigantic faultline. Now UW paleontologist Peter Ward and his collaborators report in tomorrow’s Science magazine that the discovery of pearly fossil shells of an extinct mollusk on two islands off the eastern coast of Vancouver Island give clear evidence that the region was once nearly 2,000 miles to the south.
“Everything that we now recognize as coastal British Columbia was once off the coast of what is now Mexico or Southern California. It was nowhere near its present latitude,” says Ward.
Ward’s study was aimed at proving that traces of magnetism in rocks on the British Columbia coast were the actual relic of the Earth’s ancient magnetic field. As the layered, or sedimentary, rocks were deposited under the sea, mineral crystals became magnetized, freezing the magnetic field. If the rocks had been formed at the equator, the direction of the magnetism in the formation would have been horizontal. At the poles, it would have been vertical. Thus if rocks in British Columbia show a magnetic field slightly tilted from the horizontal, the implication is they were formed far to the south.
But there is a catch. As the Earth’s crust is heated, particularly by volcanic upheavals, the mineral crystals in the rocks can become remagnetized, leaving no trace of the earlier, ancient magnetic field. So those British Columbia rocks may not have moved north, but simply been remagnetized over time.
Ward calls the 131 rock samples he obtained from the two islands, Hornby and Texada, “critical and crucial” in resolving doubt they were deformed by heating. The evidence comes from the shelled marine animals, called ammonites, which Ward’s team recovered from sites on the islands. The fossil shells, each the size of a man’s hand, still had a pearly luster, indicating the presence of the original aragonite, or mother of pearl. If the surrounding rocks had been subjected to heating, the calcium carbonate in the shells would have turned to black calcite. Indeed, Ward says, all of the ammonites discovered to date on neighboring Vancouver Island are black.
The researchers took their core samples only where they found the shiny fossils. In every case, the sedimentary rocks showed a magnetic field closer to horizontal than would be expected for northerly latitudes. Since there was no evidence of heating, and remagnetization, the conclusion was that the rocks were formed far to the south 70 to 80 million years ago, based on the evidence of fossil dating.
Ward concedes that criticisms are certain to be leveled at the new findings. Some geologists have theorized that the sedimentary rocks in the region of British Columbia have been affected by compaction as one layer is deposited on top of the other, causing the magnetized crystals to flatten. Ward, however, notes that if that were the case, the ammonites would also be flat and deformed. “An ammonite is very squishable. If you had compaction the fossil would not be as pristine as our samples,” he says.
The researcher acknowledges that his calculation of the speed of the northward drift of the rocks is too fast for some. He theorizes that the rocks began migrating from Baja California perhaps 75 million years ago, arriving at their present position about 60 million years ago. That rate of movement, about 2,000 miles in 15 million years, indicates a slippage of about 21 centimeters (about 8 inches) a year, compared with the San Andreas fault’s current movement of 4 centimeters a year. “That is fast, but not science- fiction fast,” says Ward, noting that crustal movements around Indonesia also are rapid.
Such a huge landmass, he says, must have slipped its way north along a giant crustal fault stretching from California to British Columbia. However, critics say, there is no evidence of such a fault. The UW researcher believes the fault is certainly extinct, and because it is buried deep in the Earth’s crust, it may never be detected.
Ward also dismisses observations that fossils in the British Columbia rocks show no evidence of tropical marine life, typical of Baja California. He points out that the rocks were formed in the Cretaceous era when the Earth was uniformly warm, with very little latitudinal differences in animal and plant life.
This study of fossils to confirm the ancient origin of rocks, says Ward, brings new excitement to a theory that gigantic blocks of rock “drifted that far that quickly.”
Peter Ward is away. For more information, contact Darrel Cowan, UW professor of geological sciences at (206) 543-4033, or firstname.lastname@example.org. Also contact Ronald Merrill, UW professor of geophysics and geological sciences,at (206) 543-6686, or email@example.com.