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Breaking Down Rocks in the Deep Ocean

When I witness adults cooing over Eocene-era rocks, or tasting 15 million-year-old ocean sediments, I instantly wonder what their childhood was like. Were they kids that didn’t want to leave the sandbox after recess? Were they shy and looked at the ground more than they looked at the sky? Why curiosity for inanimate objects over, say, plants or something with eyes and a heart?

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A sample “party” gathers to determine what parts of the rock will be sampled. Photo by Amy West

How the earth is layered beneath the seafloor infiltrates daily activities on board; it can lead to rewritten lyrics for Ring of Fire to fit our current exploration in the actual Ring of Fire. At dinner, the slice of carrot cake, with different dots of color throughout, can resemble “ophiolites” (rare rocks formed around the start of subduction) to co-chief scientist Julian Pearce. As he cuts into the slice of chocolate cake and its graham cracker crust, it reminds Pearce of our difficult attempt that day of coring past soft, mushy sediment and into hardened sediment and altered rock. As Pearce and sedimentologist Alastair Robertson reminisce when they first met each other more than four decades ago, they can recall what geological feature they were standing on at the time. They see the details of our world through a hand lens.

Handing over the first part of the core, “ the core catcher”, which keeps the rest of the core inside the long core barrel as it’s pulled to the ship. Photo by Amy West
Handing over the first part of the core, “ the core catcher”, which keeps the rest of the core inside the long core barrel as it’s pulled to the ship. Photo by Amy West

Which can be pretty neat when you get into the nitty gritty of what transpires to create our continents. Conversing with these rock and sediment lovers is like entering a rabbit hole though. Once you start down the path of igneous rocks (formed from lava), it divides into several kinds of igneous rocks, such as basalt or andesite. Yet, those two lavas can be composed differently depending on where they form in tectonic settings (e.g. a subduction zone or mid-ocean ridge). Within those settings lavas can cool faster than others enabling them to form different-sized crystals, or form glass. Those crystals (minerals) will have different names like olivine or feldspar depending on their elemental makeup. Stripping a rock down to its bare elements, like titanium or chromium, helps to resolve a rock’s origin, or what melted in the first place to create it.  However, if any fluids came into contact with it, that rock may now be “altered”, which takes you down a winding path of secondary minerals such as calcite or zeolite.

Zeolite?

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A rock showing where fluids sat in the rock for a long time. Photo by Amy West
Rock core that has been altered by fluids. Photo by Amy West
Rock core that has been altered by fluids. Photo by Amy West

 

 

 

 

 

 

 

 

We’ll stop at that intersection in the rabbit hole, but it’s important to note that zeolites and other alteration minerals actually trap water when they form, and turn into a different mineral if they lose the water. So ultimately what this expedition really boils down to is tracing that trapped water. Water changes everything. It’s what makes volcanoes near subduction zones so explosive, it alters a rock’s chemistry, and provides a challenge to studying the ins and outs of subduction.

In a nutshell: when new seafloor forms from volcanic activity, minerals and fluids remain in the rocks until that seafloor eventually sinks into a trench millions of years later. As the sinking slab of rock gets warmer and softer, minerals and fluids escape and head straight up, hitting the overlying rock layers. That rush of fluids actually lowers the temperature at which the earth’s dry interior melts, thus helping to create some of the lava that will eventually bubble up to form new ocean crust in the “forearc”—where we are now. Taking cores of these deep rocks in the forearc can tell us how much water came from the subducting plate, thus taking us further, quite literally, down the rabbit/borehole.

Different faces of the many rocks collected near the edge of the Bonin Trench. Photo by Amy West
Different faces of  many rocks collected near the edge of the Bonin Trench at depths from 200-1200 ft below the seafloor. Photo by Amy West

One science meeting here can turn into pages of new vocabulary— a crash course in petrology, geophysics, geochemistry and sedimentology.

The overall picture is that this team of scientists can first look at a lava and tell how it cooled and what it eventually became. From there, they focus more narrowly on the rock’s details (e.g. type and shape of minerals) to explain how the rock began its life. Unravelling the ocean story is more complicated than land-based geology, simply because of the extremely challenging process to acquire these rocks (a future post).

But to form any theory about subduction, several experts must work together to organize the few pieces they have of a 1000-piece puzzle.

Structural and alteration geologists can look at tiny cracks or veins in the rock to explain what stresses and alterations the rocks have faced.

Petrologists group the rock according to its size, shape and abundance of minerals, looking for clear delineations between different rock types.

Barely visible fracture lines highlighted in the middle of this sediment core can tell structural geologists the direction of stresses. Photo by Amy West
Barely visible fracture lines highlighted below the crack in this sediment core can tell structural geologists the direction of stresses. Photo by Amy West

Geochemists can “digest” rock and get the chemical composition by measuring the abundance of elements in a rock.

Paleomagnetists can determine the direction of the Earth’s magnetic field when the rocks formed to help assign ages to the rocks.

Physical property specialists can highlight a rock’s physical traits like density or naturally occurring radiation and how each changes with depth.

In the end, childhood fascinations with the ground beneath us drove many scientists, such as co-chief scientist Mark Reagan into the careers they have today. If it weren’t for, perhaps, that hand lens and rock hammer he had as a kid, he might not have cracked open a world harboring endless geological mysteries. It reminds us to always stay curious.

Co-chief scientist Mark Reagan pointing out interesting features to petrologist, Tim Chapman. Photo by Amy West

 Follow Amy and the JOIDES Resolution on Twitter. Past blogs about this expedition are here.