Exploring the ‘Anatomy’ of Ocean Crust at Site 1256

In the ‘Superfast campaign’ of the Integrated Ocean Drilling Program (IODP) and the Ocean Drilling Program (ODP) before it, four successive scientific ocean drilling expeditions have come to the same location in the Pacific Ocean (6°44.2’N, 91°56.1’W) to deepen a single “hard rock” drill site: ODP Hole 1256D. Beginning in 2002 with ODP Leg 206, and followed by IODP Expeditions 309 and 312 in 2005, and Expedition 335 in the late spring of this year, these expeditions of the scientific research vessel JOIDES Resolution have combined to deepen this one scientific reference penetration more than 1500 meters into the ocean crust.

This hole was intentionally located in a region of the Pacific Ocean where the ocean crust was known to have formed at a “superfast” rate some 15 million years ago (at a spreading rate of more than 20 centimeters a year – faster than the formation of any present-day crust). The fact that this crust formed so fast made the upper crust thinner than elsewhere, meaning that the lower portions of the crust could be reached at shallower depth.
Fast-spreading crust was also chosen because, in comparison to slow-spreading crust, it is much simpler and more homogeneous. This enables scientists to reasonably view this single hole as being representative of about 50% of the current ocean floor – and by extension, about 30% of the entire surface of the planet.
From the beginning, Hole 1256D was planned and designed to be a deep hole – far deeper than most holes drilled for scientific purposes into the hard rocks of the ocean crust. In fact, whereas the vast majority of such holes extend only several hundred meters down, 1256D has been successfully extended over time to 1500 meters and beyond, making it one of the deepest holes in the history of scientific ocean drilling.
Hole 1256D has provided the second complete sequence of ocean crust lavas in scientific ocean drilling, the first complete sampling of the underlying sheeted dike complex, and the critical first sampling of the dike-gabbro boundary. There also remains much to learn about igneous processes at mid-ocean ridges from the cores that are yet to be recovered from deeper in the hole.
The diagram below, “Schematic Section of Fast-Spreading Ocean Crust,” shows the stratigraphy (layers of rock) of fast-spreading ocean crust…
[Illustration by Sarah McNaboe, Integrated Ocean Drilling Program / US Implementing Organization]
Everything below the current depth of Hole 1256D represents a theoretical model developed through the study of ophiolites (oceanic crust that has been uplifted and exposed above sea level) and marine geophysics, to be tested through drilling. Each layer in the diagram is briefly explained below.
Deep sea sediments are commonly composed of clay, silt, and nannofossils (microscopic creatures that once lived in the ocean millions of years ago, whose skeletal remains have sunk to the bottom of the seafloor). Over time, these sediments become lithified, or hardened into rock.
Lava Flows
Molten basaltic rocks that erupted on the ocean floor and rapidly cooled after coming into contact with seawater.
Sheeted Dikes
Vertical, parallel strips of basaltic rocks. Before cooling and hardening into solid rock, each of these dikes was a fissure that fed the upper lava flows at the axis of the mid-ocean ridge.
A coarse-grained rock that formed by the cooling of magma at depth – at a slower rate than the dikes and lavas.
Mohorovi?i? Discontinuity (Moho)
The Mohorovi?i? discontinuity, or ‘Moho’ for short, marks the point at which fundamental change occurs in the composition and physical properties of rocks above and below this discontinuity, which is presumably the upper boundary of Earth’s mantle. The Moho is named after Andrija Mohorovi?i?, a Croatian meteorologist and seismologist, who first discovered this boundary by analyzing the velocity and refraction of seismic waves recorded from an earthquake near Zagreb in October 1909. He noticed that seismic waves propagate faster below this discontinuity, reaching up to 8.6 km/s (kilometers per second), compared to 6.7-7.2 km/s above the Moho. This marks the beginning of the bulk of Earth’s interior, which extends from the base of the crust to the core about 2890 kilometers below. (See the diagram, “Basic Layers of the Earth,” below).
[Illustration by Sarah McNaboe, Integrated Ocean Drilling Program / US Implementing Organization]

Contrary to popular belief, the mantle is not entirely made of magma. Instead, the mantle is solid, though it may partially melt locally (a few % per unit volume). This melting typically occurs about 60 kilometers below mid-ocean ridges, and the magma generated there becomes the gabbros, dikes, and lavas of the crust at the mid-ocean ridge.

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