For the first time, scientists have produced images of theoceanic crust and found that the upper and lower layers of the crustare likely formed from different magma pools. The images begin toanswer some lingering questions about where new ocean crust comes fromand whether it is all formed the same way.
Geophysicists RobertDetrick and Juan-Pablo Canales of Woods Hole Oceanographic Institution(WHOI) and colleagues used reflected seismic, or sound, waves tosuccessfully image the structure of the lower crust across the flanksof the Juan de Fuca Ridge, a spreading plate boundary off the PacificNorthwest coast. Their study, co-authored by researchers at ColumbiaUniversity’s Lamont-Doherty Earth Observatory and Scripps Institutionof Oceanography, appears in the August 25, 2005 issue of Nature.
Byrecording the reflection of seismic waves off the lower crust at thecrust-mantle boundary, a technique common in oil exploration, theresearchers found evidence strongly suggesting that the base of thecrust forms much differently than its overlying layers.
“Seismicreflection is a powerful tool to image the sub-surface detailedstructure of the Earth down to several kilometers or miles below thesurface," study co-author Canales said. “Scientists studying theformation of the ocean crust have been debating over the past decadewhether all of the crust is formed from magma that accumulates in asingle pool or lens a mile or two deep, or if it forms from multiplemagma sills at different levels.”
Detrick, Canales and colleaguesanalyzed about 1,500 kilometers (935 miles) of data collected on theJuan de Fuca Ridge off the coast of Washington, Oregon and northernCalifornia. The images are the first of their kind showing solidifiedmagma lenses and sills, narrow lateral intrusions of magma, embedded inthe boundary between the mantle and the overlying crust, a region knownas the Moho transition zone. The existence of these magma lenses near amid-ocean ridge suggests that the lower oceanic crust is formed fromseveral smaller sources of magma rather than a single large poollocated in the middle of the crust.
Unlike continental crust,which is very old and thick, oceanic crust averages 6-7 kilometers (3-4miles) thick and is constantly being recycled at tectonic plateboundaries on the seafloor. Crust is destroyed at subduction zones,where plates come together, and created at mid-ocean ridges, whereplates are pulling apart, like the Juan de Fuca Ridge. At these ridges,also known as seafloor spreading centers, molten rock, or magma, risesfrom deep within the earth and solidifies to become new crust. But theexact source of that magma—particularly the magma that forms the lowerlayers of the crust—was not well understood until now.
Previously,geophysicists knew that the topmost layer of the crust cooled frommolten rock supplied by a single pool, or lens, of magma located in thecrust’s middle layers. What was not known was whether the lower crust,which lies just above the mantle, solidified from the same melt lens orfrom many smaller magma bodies in the deeper crust-mantle transitionzone. The new study found evidence of multiple pockets of molten rocknow frozen, lending strong support to the latter theory.
Geophysicalstudies along mid-ocean ridges to date using seismic reflection havebeen able to image only one single crustal melt lens, supporting thefirst model of crustal formation. However, other remote-sensinggeophysical methods that are used to infer the mechanical properties ofthe crust indicate that magma must also accumulate at deeper levels, inparticular at the base of the crust or the Moho transition zone.
Themultiple-lens model comes from field observations at ophiolites wherethe remnants of the multiple melt sills can be mapped. Ophiolites areslabs of oceanic crust long ago thrust up onto dry land and are easilyaccessible to geologists seeking clues to what new crust might looklike.
“It is exciting that different observational approaches,marine seismology and ophiolite studies, that look at the same problemat different spatial and resolution scales are converging towards aunified geological and geophysical model of how the ocean crust isformed," Canales said.
The study was funded by the National Science Foundation.
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