First Author

Earth's ocean crust covers most of the planet's surface. But geophysicists don't have efficient or effective methods by which to understand its formation. As a graduate student at Woods Hole Oceanographic Institution in Massachusetts, Gail Christeson participated in one of the first attempts to drill through the oceanic crust and map the geological boundaries between the distinct layers of lavas and dykes that overlie volcanic rocks known as gabbros. Unfortunately, drilling beyond 100 metres proved too difficult. But many continued to question the long-held assumption that seismic maps, which use 'vibrations' to generate a broad-scale picture of the underlying structure, correlate with the crust geology. Now a research seismologist at the University of Texas Institute for Geophysics in Austin, Christeson and her colleagues decided to bypass drilling and directly compare the geological and seismic features of two crusts ripped open by natural phenomena. On page 418, she and her team show that seismic data cannot be used to reliably map the geological boundary between lavas and dykes in young oceanic crust.

How does oceanic crust formation affect other Earth processes?

More than two-thirds of Earth's surface is covered by crust formed through the mid-ocean ridge system. Anything we can do to understand how this formed will help us to work out how much heat is released during the process, which may affect the biological communities close to the vents. In addition, understanding how sea water circulates through these geological structures could explain how heat is transported from the crust to the sea floor. We speculate that there is a zone of some sort in which minerals can precipitate out of the sea water as a result of increased temperature and decreased porosity, possibly creating giant ore deposits.

How do your findings refute past beliefs?

Our work shows that although their seismic structure is similar, two types of spreading ridge — fast and intermediate — look different geologically. So we can't reliably use the seismic method to map the geological boundary between lavas and dykes.

What is the next step?

The key question now is what the seismic differences that we've mapped are. The next step is to determine exactly what the upper 400–600-km layer of ocean crust does correspond to, now that we know it's not always the top of dykes. Then we'll be able to interpret all the data we've acquired. We plan to drill a hole through such a layer in young oceanic crust.