Why Mapping Matters

Superior Beginnings

Dr. Finlayson has been thinking geologically for a long time. Growing up in Michigan, Val was first introduced to deep geological thoughts about the Midcontinent Rift, the billion-year old feature that led to the formation of  Lake Superior . 

Caption: The part of the Earth that later became North America started to pull apart, allowing magma to emerge on the floor of a huge rift valley. However, instead of splitting into a new continent, the rift closed. Credit:   National Park Service 

Marie Tharp's Marvelous Maps

It is no exaggeration to say that seafloor maps are the foundation of Dr. Finlayson’s work.  “Without detailed maps of the ocean floor, my entire field of study wouldn’t exist,” says Val. “It goes back to the groundbreaking work of Marie Tharp and Bruce Heezen. People were aware of seafloor differences and features here and there, but Marie Tharp’s  maps  brought us the first comprehensive idea about the ocean floor and complex, connected features.” 

Image Left: Marie Tharp with one of her groundbreaking physiographic maps. Her maps were first published in the late 1950s and 1960s. Image Right: Marie Tharp at work. Credit:  Columbia University’s Lamont-Doherty Earth Observatory 

Tuvalu

Mapping on a smaller scale matters a lot during cruise planning and sample collection. Val’s first at-sea experience was a dredge expedition in 2013 to the southwestern Pacific near the Tuvalu chain. Prior to the expedition, there were no detailed maps of the region. 

Maps collected on the expedition were used to select dredging sites right away. “Watching the data come in and seeing how it informed the decision making process in real-time helped me learn what to look for and how to make these decisions for future expeditions.”

“Dredge work is very different from the kind of work that the Nautilus does,” says Dr. Finlayson. “The dredge has some basic information about how far off of the bottom it is and the tension on the cable, but it is mostly flying blind. A dredge can return a lot of material or none at all.” Luckily, samples from that expedition provided a treasure trove of material and were the basis of numerous geochemistry and geochronology (the dating of geological structures) papers for Val and other colleagues.

Image Top Left:  Tuvalu region before detailed mapping data was available. Image Lower Right: Detailed mapping data after several expeditions to this area, including AVON02, RR1310, KM1609, FK171005, FK171224, and MR07-06.   Credit:  From KMZ file compiled for Benyshek et al. (2019 )

Image Right: A very full dredge just brought on board, and about to be lifted up to be emptied.  Credit: Val Finlayson

Rock Sampling with Hercules

Collecting geological samples using ROV Hercules during the  NA138 Luʻuaeaahikiikekumu expedition  to Papahānaumokuākea Marine National Monument was quite a different experience.  “Using the ROVs allows us to successfully target sample areas and understand the context where samples are collected.”   Numerous samples  from seamounts in the region were collected.

Image: After scientists identify a promising rock sample, ROV pilots use Hercules' robotic arm to pluck the rock from the ocean floor.  Credit: OET / Nautilus Live

Loudoun Seamount

During the NA138 expedition, a novel approach was used for the ROV Hercules dive on Loudoun Seamount. Instead of proceeding up the crest of the main ridge, the dive track proceeded up the side.  This allowed Val to ground-truth the detailed mapping information by getting eyes on the geology, including fractures and rift zones. Rounded pillow lavas near the start of the dive track gave way to columnar jointing in the middle section, with massive boulders near the top.

Image: For this Hercules dive on Loudoun Seamount (white line), the dive started at deeper depths and proceeded up the side of this ridge.  Video footage collected during the dive provides insights into how the seamount formed millions of years ago (2x vertical exaggeration). Image credit (map): OET / QPS Fledermaus Image credit (photos): OET / Nautilus Live

Collection Is Just The Beginning

Rock samples collected by the Nautilus go to the  sample repository at the University of Rhode Island .  Sampling submarine lavas for geochemistry back in the lab takes a few steps. "We want the least seawater-altered material possible, and usually that means cutting out the interior of a piece of rock. We clean that up, crush it, and then either finely powder it or work with small crushed pieces, depending on the type of analysis. That sample undergoes leaching, which removes any seawater contamination and leaves behind the original isotopic signature of the lava, which is also the isotopic signature of the mantle it melted from. We can then dissolve the sample and separate out the elements whose isotopic signatures we want, and measure those with a mass spectrometer. The combination of isotope compositions of different elements like strontium (Sr), lead (Pb), neodymium (Nd), and hafnium (Hf), tell us about the kind of mantle these lavas came from, and its history in the dynamic Earth system."

Images Left: Just some of the many rocks that Dr. Finlayson and colleagues collected during the  NA138 expedition . Image Top Right: Finally, a sample is in Val’s hands. Image Lower Right: Cut-away of one of the samples, showing what the unaltered interior of the rock looks like.  Complex geochemical and geochronological analyses of each rock will take place after the samples reach shore. Credit: OET / Nautilus Live

Putting It All Together

Once sample analyses are returned, sometimes many months later, the new information can be combined with geochemistry and geochronology data from other samples to try to understand regional and planet-scale history. One of these stories is about absolute plate motion, meaning how all of Earth’s plates have moved in the past relative to a fixed point inside the Earth. Another big goal is to understand  hotspot  activity over time, including subtle changes in mantle chemistry and periods of activity and inactivity. Each sample contributes new data points and clarifies Val’s understanding of this problem.

Image: This map shows known locations of hotspots around the world. A “hotspot” is a (mostly) fixed location in the mantle where material is often anomalously hot, and thermally distinct compared to typical upper mantle. These locations produce melt that erupt out of “intraplate” (not near a tectonic plate boundary) volcanoes. As tectonic plates move over these hotspots, chains of volcanoes form on the crust that get progressively older the further they are from the hotspot that formed them. Credit: Val Finlayson

A Complex Story in the Central Pacific

When asked where they would like to do future mapping and sampling, the answer came readily. “Everywhere!  Samples from all over the Pacific are needed to really understand complex plate motions and hotspot behavior over time.”  However, the Line Islands stand out as one area where Dr. Finlayson would like to see additional sampling and geochemical and geochronological analyses. “The area has such a complex pattern of volcanism, and it is one that we don’t fully understand yet.” 

Image: GEBCO mapping of the Pacific, with more detail showing the Central Pacific.   Credit: Bathymetry data reproduced from the GEBCO_2021 Grid,  www.gebco.net