Three Dimensional Magma Dynamics in Large Sills
Principal Investigator: Bruce D Marsh, Johns Hopkins University
I worked in the Dry Valleys region of Antarctica as part of a research team investigating igneous rocks - the type of rocks formed as molten magma cools and solidifies. The research team is parcticularly interested in why the igneous rocks occur where they do, and how long it took for the magma to cool. The rocks we studied are in structures called sills, which are horizontal, sheet-like intrusions of igneous rocks formed below the surface of the Earth. Sills do not cut across the layers of rocks they intrude - rather, they have borders that are parallel to them. Erosion subsequent to the sills' formation has left some of them exposed at the Earth's surface where we can study them. We want to understand their formation because much of the crust (outer shell of the Earth) may have been formed by the solidification of sheet-like bodies of magma.
We are working in Antarctica because there are excellent exposures of an important sill in the Dry Valleys, called the Basement Sill. It is thick (300 meters or 1000 feet) and quite extensive (greater than 5,000 cubic kilometers or 1200 cubic miles). Best of all, it contains a feature that allows us to examine how the sill filled with magma - a layer of large crystals (0.01 to 0.06 centimeters or 0.003 to 0.001 inches long) that varies in thickness, crystal size, and position within the sill.
The Dry Valleys offer views into Antarctica's past. Photograph courtesy of the National Science Foundation.
The crystals, which are made of the mineral orthopyroxene, form an excellent marker layer because they are large and distinct. They crystallized out of the magma before the sill was created. As the magma moved into the sill, it carried a trail of these crystals with it. Variations in their distribution helps us determine how fast the magma filled, whether filling was episodic or continuous, and whether magma moved into the surrounding rocks at distinct points or along fissures. Some other large sills around the world contain layers of crystals, but the Basement Sill of Antarctica is one of the rare ones where natural exposures allow a marker layer to be traced in three dimensions.
By studying the Basement Sill, the researchers hope to learn details of processes controlling the structure of both oceanic and continental crust. Its formation will be compared to that of similar sheet-like magma bodies on Easter Island and Reunion Island. Discovering which features these large sills have in common will tell us which processes are most important in volcanic systems.
Bill Philips will be working close to McMurdo Station.
The Dry Valleys (yellow) offer a unique opportunity to observe rock formations in Antarctica because they are not covered by thick ice!