Abstract:

The current view on the formation and architecture of ocean crust is a cornerstone of the plate tectonics theory, yet this theory remains incomplete because several fundamental questions remain unresolved: What rocks actually constitute the oceanic crust? To what extent does its thickness vary? And what is the petrological nature of the crust-mantle boundary, i.e., the Mohorovičić discontinuity (Moho)? Much of our present understanding relies on assumptions that have never been rigorously tested, and some widely held misconceptions stem from selective interpretations of convenient hypotheses. One of the most pervasive is that the crust inferred from seismic velocities above the Moho is entirely of magmatic origin. Testing this idea was the primary motivation for “Project Mohole” (1957-1966), which aimed to drill through the entire ocean crust, penetrate the Moho, and sample the mantle. Although the project was terminated due to escalating technical challenges and rising costs, it laid the technological foundation for subsequent international ocean drilling programs. The central hypothesis, that the ocean crust above the Moho is entirely magmatic, remains untested. Because the global Moho lies at a nearly uniform depth of (6.0±1.0) km, Earth scientists have long assumed that this “seismic crust” corresponds to a magmatic crust of globally uniform thickness, independent of spreading rate. However, at slow- and ultraslow-spreading ridges, serpentinized mantle peridotites are exposed on the seafloor and in places dominate the crustal rock association, contradicting this view. To critically and objectively evaluate prevailing models of ocean ridge magmatism, it is therefore essential to drill through intact oceanic crust into the mantle. Since only fast-spreading ridges are likely to generate a complete magmatic crust, drilling should target geologically simple areas of the Pacific seafloor. With the decommission of the U.S. drillship JOIDES Resolution in late 2024, China’s D/V Meng Xiang has become the world’s only vessel currently capable of drilling through the ocean crust; its advanced technology and lower operational costs now make the longstanding goal of penetrating intact ocean crust a realistic prospect.

• Drilling through intact ocean crust is essential to resolve fundamental scientific questions and complete plate tectonics theory.

• The assumption that “seismic crust” equals “magmatic crust” has misled Earth science for decades and must be corrected.

• At slow- and ultraslow-spreading ridges, “seismic crust” is not purely magmatic but a chaotic mixture of magmatic rocks and serpentinized mantle peridotite.

• Only direct drilling through intact crust in the Pacific can reveal the true petrological nature of the Moho.

• China’s D/V Meng Xiang is now the world’s only drillship capable of penetrating the intact ocean crust, making this longstanding scientific dream achievable.

Key words: ocean crust formation, composition and thickness variation of the ocean crust, Hess-type ocean crust, petrological nature of oceanic Moho, International Scientific Ocean drilling, The mission of drilling through a complete magmatic ocean crust using China’s D/V Meng Xiang