A new mandate for deep-ocean drilling.
A fundamental fact of life for marine geologists is that they know little of the composition and structure of the two-thirds of Earth's crust that underlies the oceans. While it has often been said that the surface of Mars is better known than the seafloor of Earth, the situation is far worse for the oceanic crust. Our knowledge is limited to inference from buckets of gravel dredged from the deep and remote geophysical sensing. The extensive covering of sediment, the technical difficulty of working beneath kilometers of water, the limited rock exposures, and the ubiquitous coverings of black hydrogenous manganese precipitated on the rocks from the water column have largely frustrated conventional geological techniques such as combining surface mapping with geophysical sensing at depth to interpolate Earth's deep structure. It is increasingly apparent that the only way to obtain direct and precise knowledge of the composition and structure of the oceanic crust is to drill into it.
The importance of this gap in our knowledge of the earth became evident with the confirmation of seafloor spreading, the acceptance of plate tectonics, and the completion of global seismic surveys in the 1960s. Despite widespread expectations to the contrary, the surveys proved that continental and oceanic crusts are fundamentally different. Whereas the continental massifs are old (billions of years, on average), thick, and composed of silica-rich rocks such as granites and andesite, the oceanic crust is young, thin, and composed of silica-poor magnesian lavas such as basalt. While continental formation has slowed or stopped, new oceanic crust continuously forms at the ocean ridges, driven by deep mantle convection. As the seafloor spreads and collides with continents and other ocean plates at island arcs, old crust is also continuously destroyed--overridden and mixed back into Earth's mantle at subduction zones. Thus, a direct and complete knowledge of the composition of the oceanic crust is required to understand the evolution and chemistry of the earth and to decipher how Earth's dynamic engine works.
After an initial debate in the late 1950s and early 1960s, a scientific consensus delineated a fairly straightforward "layer-cake" ocean-crust stratigraphy of uniform layers of gabbro, sheeted dikes, and pillow lavas atop the mantle and overlain by marine sediments. This model was based on a match of seismic wave velocities and densities and types of rocks dredged from the seafloor and found in on-land sections of fossil oceanic crust with the physical properties of the three principle ocean-crust seismic layers. Detailed geological models were then constructed by extrapolating the simple, observed, layered, seismic structure to the geology mapped in tectonically disrupted sections of fossil oceanic crust found on land and in island arcs.
Based on this thinking the lowermost, and thickest, crustal seismic layer was believed to be uniform, consisting of coarse, stratified gabbroic rocks laid down on the floor, walls, and roof of a large magma chamber. Thus evolved the extremely attractive model of the "infinite onion (see illustration overleaf):" a large near-steady-state, continuous magma chamber underlying the global ocean ridge system, disrupted only by the largest of ocean fracture zones, from which layers of oceanic crust continuously grew at top, sides, and bottom to form a uniform layer of coarse rocks comprising the lower two-thirds of the oceanic crust (also see Oceanus, Winter 1991/92, page 36).
Two decades of additional observation of seafloor and oceanic crust and study of on-land fossil oceanic crust has thrown this model into question. Most, if not all, fossil oceanic crust on land is now believed to be atypical, formed in young rifts, around island arcs, and in marginal seas above subduction zones. These rocks simply cannot be taken as representative of the oceanic crust at large. Moreover, the structure of oceanic crust is now viewed as three-dimensional, highly dependent on the speed of seafloor spreading in any one region and the rate at which magma erupts from the mantle.
Recent seismic results suggest no large steady-state magma chambers beneath ocean ridges--the linchpin of the layer-cake model. Observed seismic structure has become increasingly complex, with the lower oceanic crust varying in thickness near fracture zones and at small offsets in the ocean ridges. Compilations of dredge results and gravity lows centered over dredge segments also suggest that a continuous gabbroic layer simply does not exist at slow-spreading ridges, and that their internal stratigraphy is governed by ongoing faulting and deformation accompanied by hydrothermal alteration as cold seawater sinks into the crust, heats up, and reacts with the rock as much as by intrusion of new lava at depth and its eruption to the seafloor. Stephen Swift and Ralph Stephen of the Geology and Geophysics Department at the Woods Hole Oceanographic Institution have recently proposed that gabbro, generally believed to be the major constituent of layer 3, attenuates seismic waves too rapidly, raising the possibility that the composition of the lower ocean-crust layer could be radically different than we have supposed.
Deep-ocean drilling was first attempted, with some success, during the ill-fated Mohole Project in the 1960s. The project foundered in budgetary excess and the political process, but nonetheless gave birth to what must be the most successful international scientific cooperation in history: the Deep Sea Drilling Project and its successor, the Ocean Drilling Program. In two decades of drilling these programs produced remarkable results. The theory of plate tectonics was confirmed by demonstrating that the youngest oceanic crust is at the mid-ocean ridges and that it ages progressively toward the continents. Exploration of the continental margins and records of deep-sea stratigraphy provided an explosive stimulus for the relatively new field of paleoceanography, critical to understanding how the global climate has evolved and what controls it.
While crustal drilling in the oceans has dramatically confirmed and defined the nature of the shallowest ocean-crust layers, it has largely been unable to recover rocks from the deepest, largest layer. The composition, internal stratigraphy, and rock history of the lower oceanic crust, then, remains one of the fundamental unanswered questions of Earth science. Despite considerable effort by the drilling-program, the middle layer, composed of fractured, brittle, tough fine-grained basalt has proven technically difficult to penetrate at reasonable cost, postponing the long-standing goal of recovering a complete, intact section of oceanic crust and shallow mantle.
The 1987 drilling of 500 meters of gabbro in only 16 days by the drilling program's JOIDES Resolution at a tectonically exposed section of the lower oceanic crust on the southwestern Indian Ridge drastically changed our approach to ocean-crust exploration. The largely unanticipated success demonstrated that drilling lower-oceanic crust was totally different from drilling shallow-oceanic crust. Not only is it far easier, with nearly 100 percent rock recovery, but it requires no new technology. Moreover, this one long section confounded many in the geologic community. No evidence for a major magma chamber was found. While the compositional diversity of the rocks did closely resemble that found in fossil magma chambers exposed on land, these rocks were formed by a new and physically different dynamic process that involves close interplay of deformation and crystallization in semisolidified crystal mushes. Similar rocks recovered as gravel by dredges and submersibles, without their accompanying key stratigraphic and structural relationships, had led to opposite and entirely incorrect conclusions--demonstrating the inadequacy of studying oceanic crust without drilling.
This galvanized the geologic community. There was growing realization that a costly, single, deep hole through the oceanic crust would be insufficient to understand a laterally heterogeneous oceanic crust whose stratigraphy and composition varied radically from one region to another. This prompted a major international meeting, held in Woods Hole in 1989, to consider a new approach to oceanic deep-crustal and shallow-mantle drilling. The meeting report, Drilling the Oceanic Lower Crust and Mantle, proposed that while drilling a single deep hole through the oceanic crust should be a long-term goal, a major program of drilling directly into the lower oceanic crust and mantle should be done over the next decade. This report was then considered and endorsed by a working group reporting to the JOIDES (Joint Oceanographic Institutions for Deep Earth Sampling) Planning Committee and is now likely to occur if funding for the Ocean Drilling Program continues.
The strategy proposed, called offset drilling, borrows from that used by sedimentologists, using uplifted rock exposures as windows for drilling critical partial sections in the lower oceanic crust and mantle from which to reconstruct its overall composition and structure. This would be done in different regions that represent various conditions of ocean-crust formation, and would permit direct evaluation of models for crustal formation at slow- and fast-spreading ocean ridges and for crustal formation close to and near mantle hot spots. This strategy, while dependent on geologic interpretation of the sections to piece them together into an integrated model of ocean-crust formation, is far more scientifically sound than trying to interpret the whole from a single hole.
The first crustal-information drilling voyage is scheduled for November 1992 at an exposure of oceanic crust originally formed at the East Pacific Rise. This fast-spreading ridge is at the opposite extreme from the slowest spreading ocean ridges previously drilled in the Indian Ocean. We expect the rocks recovered will be strikingly different. They may reveal new processes for ocean-crust formation. This cruise will directly test for the first time the various new models for ocean-crust formation and fast- and slow-spreading ocean ridges. It is the beginning of a new era in the exploration of ocean basins.
Henry Dick is a Senior Scientist in the Department of Geology and Geophysics at Woods Hole Oceanographic Institution. He grew up in Oregon (and therefore considers himself an expatriate of a foreign country in Massachusetts). He obtained a Ph.D. in geology and geophysics at Yale University in 1976 by trekking through the remote Kalmiopsis wilderness in Oregon, which was purported to be a fragment of fossil ocean crust. It was rumored that he made a geologic map of the area while having a good time backpacking, trail biking, and swimming in the many mountain streams. Having debated the origin of the rocks he collected there with numerous other authorities on the ocean crust, none of whom, like himself, had ever actually even held a rock from the seafloor, he thought it might be a good idea to go take a look. The Institution has been stuck with him ever since; though several times it did send him to the bottom of the Atlantic and the Caribbean in Alvin, and to many other strange places. He has kept coming back. His current ambitions include being on the ocean drill ship JOIDES Resolution when it first drills through the ocean crust into Earth's mantle, and to get his 16-month-old daughter to sleep through the night.
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|Author:||Dick, Henry J.B.|
|Date:||Dec 22, 1992|
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