Guest journal—Why Adak Canyon?

Gene Yogodzinski, of the University of South Carolina, peruses maps during geology dives to study Adak Canyon. In front of him, a large-format printer is creating a map of the seafloor. For several days of the Aleutian research cruise, Yogodzinski led the use of the Jason II to find and sample some of the canyon’s oldest volcanic rocks. Photo by SONYA SENKOWSKY

Location: Adak Canyon

Posted 08.07.04 at 12:00 am

Seeking “birthday rocks” in Adak Canyon
Guest Journal by David W. Scholl, geophysicist
Stanford University/Moss Landing Marine Laboratory

Editor’s note: While the coral researchers pursue questions about the interactions of corals and surrounding habitat, a small group of geologists onboard used Jason II over the past weekend for three days of dives on unrelated research, led by chief scientist Gene Yogodzinski, of the University of South Carolina. I asked geologist Dave Scholl, a member of that team, to explain why their group is so interested in the Aleutian region—and specifically, Adak Canyon, the focus of their dives. In response, he wrote us this guest journal entry.

The Aleutian Islands arc their way westward from the end of the Alaska Peninsula to nearly reach Kamchatka. The islands, many with active volcanoes, rise above the summit of a large and mostly submerged mountain range--the Aleutian Ridge. The ridge, roughly 2,200 km in length and 100-200 km wide (1400 miles wide and 60-120 miles wide), is the northern sector of the Pacific’s “ring of fire.”

Scientists don’t know exactly when Aleutian volcanism first began to form its segment of the ring, but estimates range anywhere from 45 to 55 million years ago. We also don’t yet know why the Aleutian Ridge, or arc, formed in the first place.

What happened in the high north Pacific to cause one moving tectonic plate to sink beneath the other—in the process, injecting water into the Earth’s hot underlying mantle 100-150 km below the sea bed (60 to 90 miles down) and causing it to melt? (Water acts as a “flux” and lowers the melting temperature of most rocks.)

The resulting melt, or magma, ascended to the Earth’s surface to erupt on the sea floor and constructed the great pile of volcanic rocks that built the Aleutian Ridge to sea level—and much higher.
To address the fundamental wonderment about why the Aleutian Ridge exists, researchers begin by finding and then dating the oldest rocks constructing it. So, speaking geologically, where do you go to get samples of Aleutian birthday rocks?

Why, to Adak Canyon of course.

Why Adak Canyon? Because on Adak Island, the ridge’s oldest suspected rocks are exposed. Immediately west of the island, the underwater gorge of Adak Canyon cuts nearly 3000 meters (nearly 10,000 feet) deeper into the ridge’s rocky innards. To get samples of older rocks that we suspected were exposed along the canyon’s steep eastern wall, we used JASON II. The first of our two dives successfully recovered some of the oldest volcanic rocks; the second dive recovered more of the oldest volcanic rocks and a somewhat young granite body that invades them. Finding and sampling the granite was a genuine boon, because these rocks can be precisely dated, and their chemistry and mineralogy record important information about how magmatism, or melting, creates island arcs.

It is interesting to note that it was based on studies of Aleutian volcanism that Bob Coats in the late 1950s first described and visualized the process that causes the Earth’s mantle to melt and form the “ring of fire.” In plate tectonics parlance, the name of this process has been dubbed “subduction.”

It can thus be said that, conceptually, the Aleutian Island Arc is the mother of all arcs. After the Adak Canyon rocks are submitted for laboratory dating, we shall see if she will allow us to know her birthday.