To accomplish our mission, we have returned to ODP (Ocean Drilling Program) Hole 1256D (6°44.2’N, 91°56.1’W) in the Pacific Ocean, about 900 kilometers west of Costa Rica. Ours is the fourth scientific research expedition to drill in Hole 1256D, which lies in ocean crust that formed approximately 15 million years ago at a "superfast" rate (at more than 20 centimeters a year, which is faster than any present-day crust formation). The fact that the crust formed so fast in this location makes the upper crust here thinner than elsewhere, making it possible to reach the lower portions of the ocean crust without having to drill as deep as would be necessary in other parts of the world.

 

Hole 1256D currently penetrates about 1500 meters below the seafloor, and reaches the base of the upper, volcanic crust. Expedition 335 continues the mission to understand the formation of oceanic crust by attempting to further deepen Hole 1256D and study the magmatic rocks of the lower crust, which we expect to access for the first time intact in their original position.

 

By recovering and studying samples of the lower crust, we will be able to test working hypotheses about how new ocean crust is constructed, and how the crust reacts with seawater as it forms and as the crust cools and spreads away from ocean ridges.

 

[Co-chief scientists Benoit Ildefonse and Damon Teagle, along with science party member, Marguerite Godard]

 

 

Although oceanic (vs. continental) crust represents almost 2/3 of the Earth’s surface, we know surprisingly little about it. In particular, we want to better understand the formation of new crust, which occurs along Earth’s mid-ocean ridges.

 

The formation of new crust at the ocean ridges represents the first step in Earth’s plate tectonic cycle, where new plates are formed. This provides the principal mechanism by which heat and material are lifted from within the Earth to the surface of the planet. Earth’s surface is made up of 14 tectonic plates – and it is the motion and interactions of these plates that leads to mountains, earthquakes, volcanoes, and the recycling of elements (carbon for example) between the Earth’s interior, oceans, and atmosphere.

 

                                                       

 

Gabbro is a coarse-grained, granite-like material sometimes known as ‘black granite’ that is often used in kitchen countertops and on the front of commercial buildings such as banks (which are now frequently turned into wine bars in the UK!). Gabbro forms when molten rock under the Earth’s surface, or magma, is trapped and slowly cools. The magma that produces gabbros is generated when the solid mantle partially melts about 60 kilometers below the mid-ocean ridges, as the mantle rises within the Earth. (As you can see in the diagram below, the mantle lies just beneath the crust).

 

 

 

 

 

 

 

 

 

 

Drilling a complete in situ section of ocean crust has been an unfulfilled ambition of Earth scientists since the inception of ocean drilling in the early 1960s.

 

Why is this so important?

 

It isn’t that we expect big surprises in terms of the types of rocks we will recover from the lower portion of the ocean crust – we already know what’s down there. The problem is that the samples scientists currently have available for study have all been deformed and chemically altered by the processes that brought them nearer to Earth’s surface (through tectonic mountain building and faulting in slow-spreading crust).

 

Consequently, the thermal, chemical, and perhaps biological exchanges occurring deep in the oceanic crust remain poorly understood because of this lack of direct observations in situ. What we are looking for is to study the structure and composition of these rocks, from the microscopic scale to a scale of several hundred meters, in situ (which means undisturbed, in their original position).

 

Until we have samples taken at depth, there are many scientific questions that simply cannot be answered without a large level of speculation.