VIMS

Piecing Together the Plate Puzzle

The theory of plate tectonics postulates that Earth's surface is covered by a mosaic of rigid, constantly shifting plates. Reconstructing the movement and position of these plates through time has been a goal of geologists ever since the plate tectonics revolution of the 1960s. Solving the plate-tectonics puzzle provides a needed context for nearly all fields of geology— from mineral exploration to paleoclimatology.

But plate reconstruction presents a puzzle of great complexity. Plates lie on a sphere, move relative to one another around independent poles of rotation, and change shape as continents rift apart, new ocean basins form, and old ocean crust subducts into deep-sea trenches.

The geologists aboard the Palmer, led by Dr. Steve Cande of the Scripps Institute of Oceanography, are working to resolve a long-standing enigma of plate reconstructions in the Southern Ocean—the gap that results when they try to fit the Pacific, Antarctic, and Australian plates together during the early Tertiary (~45-50 million years ago). This was a time of significant change in Southern Ocean tectonics. It coincides with the onset of uplift in the Transantarctic Mountains, deposition of thick layers of potentially oil-bearing sediments in the Ross Sea, the collision of India and Asia, and a shift in the movement of the Pacific Plate that may help explain the bend in the Hawaiian Island chain.

Dr. Cande's crew aboard the Palmer includes graduate students Katie Phillips of Scripps, Elisabeth Nadin and Carl Tape of Cal Tech (home of Dr. Cande's collaborator Dr. Joann Stock), and Stuart Schmitt of the University of Wisconsin. Their job while onboard is to analyze data from the ship's multibeam sonar and from a magnetometer that we're towing about 1,000 feet aft (to keep the ship's metallic bulk from interfering with the instrument's sensitive detectors).

The magnetometer measures the strength and direction of magnetic signals locked within the rocks of the seafloor. These basaltic rocks becomes magnetized during seafloor spreading, the process in which lavas erupt along a mid-ocean ridge, creating new seafloor as the plates on either side of the spreading center move apart. As these lavas cool and harden, their iron-bearing minerals align with Earth's magnetic field like tiny compasses.

Because the Earth's magnetic field periodically reverses and spreading rates vary, the end result of seafloor spreading is a characteristic pattern of magnetic stripes with alternating polarity and differing width, somewhat like a barcode. This barcode is mirrored on either side of each mid-ocean ridge, as spreading carries the newly created seafloor in opposite directions. Radiometric dating of seafloor rocks gives each barcode stripe an age.

By comparing their magnetic data to the existing seafloor barcode, the geologists aboard the Palmer can fix the newly mapped seafloor segments in space and time. Combining the magnetic data with the readings from the Palmer's multibeam sonar, which paints a detailed map of seafloor troughs, ridges, and fracture zones, provides a powerful tool for constraining the motion and position of plates in the Southern Ocean.

Data from earlier cruises by Dr. Cande and colleagues suggest that the apparent gap between the Australian and Pacific plates in the early Tertiary could be closed by rotation around a previously undetected and long-dead spreading center. That feature finds expression today in the Adare Trough, a linear depression in the seafloor just off Cape Adare at the northwestern corner of the Ross Sea.

Rifting along the Adare Trough during the early Tertiary helps explain other evidence for rotation between East and West Antarctica at the time, and implies the existence of a previously unrecognized three-way plate boundary separating the East Antarctic, West Antarctic, and Australian plates.

The rifting also helps resolve a debate concerning "hot spots," plumes of lava that rise from Earth's mantle to form island chains like Hawaii. The "stabilists" in this debate argue that hot spots remain stationary while a plate passes overhead. "Dynamicists" hold that hot spots can move. Data from the Adare Trough support the dynamicists' position. The data suggest that changes in the motion of the Pacific Plate cannot entirely account for the sharp bend in the Hawaiian chain, and therefore that the hot spot itself must be moving.

Dr. Cande's plan for our current cruise is to gather more detail on the geomagnetism and seafloor features of the Ross Sea and Southern Ocean, including another close look at the Adare Trough. These details will help to further constrain plate motion in the region.